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B. Sc. 3rd Year Single Paper Scheme 2016
Unit: I
i.
Plant Water Relations
Water is the principal, most important and most abundant inorganic constituent of plant
cells and is essential for proper health, growth and development. Plants absorb water from
the soil through roots transport it to other plant parts through xylem and lose about 98% of
absorbed water into the atmosphere by transpiration especially through leaves. Various
physical and physiological phenomena involved in plant water relations include absorption
and movement, diffusion osmosis plasmolysis permeability imbibition and water potential.
Importance of Water to Plant Life:
Water makes up most of the mass of plant cells, as we can readily appreciate if we look at
microscopic sections of mature plant cells: Each cell contains a large water-filled vacuole.
In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is
vacuole. Water typically constitutes 80 to 95% of the mass of growing plant tissues.
Common vegetables such as carrots and lettuce may contain 85 to 95% water. Wood,
which is composed mostly of dead cells, has lower water content; sapwood, which
functions in transport in the xylem, contains 35 to 75% water; and heartwood has slightly
lower water content. Seeds, with a water content of 5 to 15%, are among the driest of plant
tissues, yet before germinating they must absorb a considerable amount of water. Water is
the most abundant and arguably the best solvent known. As a solvent, it makes up the
medium for the movement of molecules within and between cells and greatly influences
the structure of proteins, nucleic acids, polysaccharides, and other cell constituents. Water
forms the environment in which most of the biochemical reactions of the cell occur, and it
directly participates in many essential chemical reactions. Plants continuously absorb and
lose water. Most of the water lost by the plant evaporates from the leaf as the CO2 needed
for photosynthesis is absorbed from the atmosphere. On a warm, dry, sunny day a leaf will
exchange up to 100% of its water in a single hour. During the plant‘s lifetime, water
equivalent to 100 times the fresh weight of the plant may be lost through the leaf surfaces.
Such water loss is called transpiration. Transpiration is an important means of dissipating
the heat input from sunlight. Heat dissipates because the water molecules that escape into
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the atmosphere have higher than- average energy, which breaks the bonds holding them in
the liquid. When these molecules escape, they leave behind a mass of molecules with
lower-than-average energy and thus a cooler body of water. For a typical leaf, nearly half
of the net heat input from sunlight is dissipated by transpiration. In addition, the stream of
water taken up by the roots is an important means of bringing dissolved soil minerals to the
root surface for absorption.
Physical Properties of Water:
Water has special properties that enable it to act as a solvent and to be readily transported
through the body of the plant. These properties derive primarily from the polar structure of
the water molecule.
The Polarity of Water Molecules Gives Rise to Hydrogen Bonds
The water molecule consists of an oxygen atom covalently bonded to two hydrogen atoms.
The two O—H bonds form an angle of 105°. Because the oxygen atom is more
electronegative than hydrogen, it tends to attract the electrons of the covalent bond. This
attraction results in a partial negative charge at the oxygen end of the molecule and a
partial positive charge at each hydrogen. These partial charges are equal, so the water
molecule carries no net charge. This separation of partial charges, together with the shape
of the water molecule, makes water a polar molecule, and the opposite partial charges
between neighboring water molecules tend to attract each other. The weak electrostatic
attraction between water molecules, known as a hydrogen bond, is responsible for many of
the unusual physical properties of water. Hydrogen bonds can also form between water and
other molecules that contain electronegative atoms (O or N). In aqueous solutions,
hydrogen bonding between water molecules leads to local, ordered clusters of water that,
because of the continuous thermal agitation of the water molecules, continually form,
break up, and re-form,
The Polarity of Water Makes It an Excellent Solvent
Water is an excellent solvent: It dissolves greater amounts of a wider variety of substances
than do other related solvents. This versatility as a solvent is due in part to the small size of
the water molecule and in part to its polar nature. The latter makes water a particularly
good solvent for ionic substances and for molecules such as sugars and proteins that
contain polar —OH or —NH2 groups. Hydrogen bonding between water molecules and
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ions and between water and polar solutes, in solution effectively decreases the electrostatic
interaction between the charged substances and thereby increases their solubility.
Furthermore, the polar ends of water molecules can orient themselves next to charged or
partially charged groups in macromolecules, forming shells of hydration. Hydrogen
bonding between macromolecules and water reduces the interaction between the
macromolecules and helps draw them into solution.
The Thermal Properties of Water Result from Hydrogen Bonding
The extensive hydrogen bonding between water molecules results in unusual thermal
properties, such as high specific heat and high latent heat of vaporization. Specific heat is
the heat energy required to raise the temperature of a substance by a specific amount.
When the temperature of water is raised, the molecules vibrate faster and with greater
amplitude. To allow for this motion, energy must be added to the system to break the
hydrogen bonds between water molecules. Thus, compared with other liquids, water
requires a relatively large energy input to raise its temperature. This large energy input
requirement is important for plants because it helps buffer temperature fluctuations. Latent
heat of vaporization is the energy needed to separate molecules from the liquid phase and
move them into the gas phase at constant temperature—a process that occurs during
transpiration. For water at 25°C, the heat of vaporization is 44 kJ mol–1 , the highest value
known for any liquid. Most of this energy is used to break hydrogen bonds between water
molecules. The high latent heat of vaporization of water enables plants to cool themselves
by evaporating water from leaf surfaces, which are prone to heat up because of the radiant
input from the sun. Transpiration is an important component of temperature regulation in
plants.
The Cohesive and Adhesive Properties of Water Are Due to Hydrogen Bonding
Water molecules at an air–water interface are more strongly attracted to neighboring water
molecules than to the gas phase in contact with the water surface. As a consequence of this
unequal attraction, an air–water interface minimizes its surface area. To increase the area
of an air–water interface, hydrogen bonds must be broken, which requires an input of
energy. The energy required to increase the surface area is known as surface tension.
Surface tension not only influences the shape of the surface but also may create a pressure
in the rest of the liquid. As we will see later, surface tension at the evaporative surfaces of
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leaves generates the physical forces that pull water through the plant‘s vascular system.
The extensive hydrogen bonding in water also gives rise to the property known as
cohesion, the mutual attraction between molecules. A related property, called adhesion, is
the attraction of water to a solid phase such as a cell wall or glass surface. Cohesion,
adhesion, and surface tension give rise to a phenomenon known as capillarity, the
movement of water along a capillary tube. In a vertically oriented glass capillary tube, the
upward movement of water is due to (1) the attraction of water to the polar surface of the
glass tube (adhesion) and (2) the surface tension of water, which tends to minimize the area
of the air–water interface. Together, adhesion and surface tension pull on the water
molecules, causing them to move up the tube until the upward force is balanced by the
weight of the water column.Smaller the tube, the higher the capillary rise.
Water Has a High Tensile Strength
Cohesion gives water a high tensile strength, defined as the maximum force per unit area
that a continuous column of water can withstand before breaking. We do not usually think
of liquids as having tensile strength; however, such a property must exist for a water
column to be pulled up a capillary tube. Breaking the water column requires sufficient
energy to break the hydrogen bonds that attract water molecules to one another. Careful
studies have demonstrated that water in small capillaries can resist tensions more negative
than –30 MPa. This value is only a fraction of the theoretical tensile strength of water
computed on the basis of the strength of hydrogen bonds. Nevertheless, it is quite
substantial. The presence of gas bubbles reduces the tensile strength of a water column. If a
tiny gas bubble forms in a column of water under tension, the gas bubble may expand
indefinitely, with the result that the tension in the liquid phase collapses, a phenomenon
known as cavitation.
Water Transport Processes
When water moves from the soil through the plant to the atmosphere, it travels through a
widely variable medium (cell wall, cytoplasm, membrane, air spaces), and the mechanisms
of water transport also vary with the type of medium. For many years there has been much
uncertainty about how water moves across plant membranes. Specifically it was unclear
whether water movement into plant cells was limited to the diffusion of water molecules
across the plasma membrane‘s lipid bilayer or also involved diffusion through protein4
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lined pores. Some studies indicated that diffusion directly across the lipid bilayer was not
sufficient to account for observed rates of water movement across membranes, but the
evidence in support of microscopic pores was not compelling. This uncertainty was put to
rest with the recent discovery of aquaporins. Aquaporins are integral membrane proteins
that form water-selective channels across the membrane. Because water diffuses faster
through such channels than through a lipid bilayer, aquaporins facilitate water movement
into plant cells. Note that although the presence of aquaporins may alter the rate of water
movement across the membrane, they do not change the direction of transport or the
driving force for water movement. We will now consider the two major processes in water
transport: molecular diffusion and bulk flow.
Diffusion:
Water molecules in a solution are not static; they are in continuous motion, colliding with
one another and exchanging kinetic energy. The molecules intermingle as a result of their
random thermal agitation. This random motion is called diffusion. As long as other forces
are not acting on the molecules, diffusion causes the net movement of molecules from
regions of high concentration to regions of low concentration—that is, down a
concentration gradient. In the 1880s the German scientist Adolf Fick discovered that the
rate of diffusion is directly proportional to the concentration gradient (Δcs/Δx),that is, to
the difference in concentration of substance s (Δcs) between two points separated by the
distance Δx. In symbols, we write this relation as Fick‘s first law: The rate of transport, or
the flux density (Js), is the amount of substance s crossing a unit area per unit time (e.g., Js
may have units of moles per square meter per second [mol m–2 s–1]). The diffusion
coefficient (Ds) is proportionality constant that measures how easily substance s moves
through a particular medium. The diffusion coefficient is a characteristic of the substance
(larger molecules have smaller diffusion coefficients) and depends on the medium
(diffusion in air is much faster than diffusion in a liquid, for example). The negative sign in
the equation indicates that the flux moves down a concentration gradient. Fick‘s first law
says that a substance will diffuse faster when the concentration gradient becomes steeper
(Δcs is large) or when the diffusion coefficient is increased. This equation accounts only
for movement in response to a concentration gradient, and not for movement in response to
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other forces (e.g., pressure, electric fields, and so on). Diffusion is rapid over short
distances but extremely slow over long distances.
From Fick‘s first law, one can derive an expression for the time it takes for a substance to
diffuse a particular distance.The average time needed for a particle to diffuse a distance L
is equal to L2/Ds, where Ds is the diffusion coefficient, which depends on both the identity
of the particle and the medium in which it is diffusing. Thus the average time required for a
substance to diffuse a given distance increases in proportion to the square of that distance.
The diffusion coefficient for glucose in water is about 10–9 m2 s–1. These values show
that diffusion in solutions can be effective within cellular dimensions but is far too slow for
mass transport over long distances.
Bulk Flow:
A second process by which water moves is known as bulk flow or mass flow. Bulk flow is
the concerted movement of groups of molecules en masse, most often in response to a
pressure gradient. If we consider bulk flow through a tube, the rate of volume flow depends
on the radius (r) of the tube, the viscosity (h) of the liquid, and the pressure gradient that
drives the flow. Jean-Léonard-Marie Poiseuille (1797–1869) was a French physician and
physiologist, and the relation just described is given by one form of Poiseuille‘s equation:
expressed in cubic meters per second (m3s–1). This
equation tells us that pressure-driven bulk flow is very sensitive to the radius of the tube. If
the radius is doubled, the volume flow rate increases by a factor of 16. Pressure-driven
bulk flow of water is the predominant mechanism responsible for long-distance transport
of water in the xylem. It also accounts for much of the water flow through the soil and
through the cell walls of plant tissues. In contrast to diffusion, pressure-driven bulk flow is
independent of solute concentration gradients, as long as viscosity changes are negligible.
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Osmosis:
Osmosis is a special case of diffusion. Osmosis is the diffusion of water through a
selectively permeable membrane (a membrane that allows for diffusion of certain solutes
and water) from a region of higher water potential to a region of lower water potential.
Water potential is the measure of free energy of water in a solution. 1.Diffusion of a
solvent (usually water molecules) through a semipermeable membrane from an area of low
solute concentration to an area of high solute concentration. 2.Net movement of water
molecules through a semipermeable membrane from an area of higher water potential to an
area of lower water potential. 3. Tendency of water to flow from a hypotonic solution (low
concentration of dissolved substances) to hypertonic solution (higher concentration of
dissolved substances) across a semipermeable membrane. Osmosis can be demonstrated by
a simple experiment:
In this experiment water contained in a beaker is separated from a solution in a thistle
funnel by a semipermeable membrane. Water will continue to move across resulting in the
rise of the solution in the funnel until equilibrium is reached. When two solutions have the
same concentration of solutes, they are said to be isotonic to each other (iso means same, ton means condition, -ic means pertaining to). Water movement can be prevented by
applying pressure – the osmotic pressure.
Now consider two solutions separated by a selectively permeable membrane. The solution
that is hypertonic to the other must have more solute and therefore less water. At standard
atmospheric pressure, the water potential of the hypertonic solution is less than the water
potential of the hypotonic solution, so the net movement of water will be from the
hypotonic solution into the hypertonic solution.
Osmotic pressure is the pressure that must be applied to a solution to prevent the inward
flow of water across a semipermeable membrane. The osmotic pressure is defined to be the
pressure required to maintain equilibrium, with no net movement of solvent. It is also
defined as a hydrostatic pressure caused by a difference in the amounts of solutes between
solutions that are separated by a semi-permeable membrane
The pressure built up inside the cells as a result of the entry of water is called osmotic
pressure. The entry of water into the cells is regulated by the solutes in the cell. Less
concentration of solutes means more concentration of water. Positive osmotic pressure
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means more water inside the cell and negative osmotic pressure means less water inside the
cell. The positive osmotic pressure developed inside a cell as a result of entry of water is
called turgor pressure. When the cells are full of water, they are called turgid and when
they lose water, they are called flaccid. The root cells become turgid after the absorption of
water and this develops a positive potential in them.
Applications
Osmotic pressure is the basis of filtering ("reverse osmosis"), a process commonly used to
purify water. The water to be purified is placed in a chamber and put under an amount of
pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in
it. Part of the chamber opens to a differentially permeable membrane that lets water
molecules through, but not the solute particles. The osmotic pressure of ocean water is
about 27 atm. Reverse osmosis desalinators use pressures around 70 atm to produce fresh
water from ocean salt water. Osmotic pressure is necessary for many plant functions. It is
the resulting turgor pressure on the cell wall that allows herbaceous plants to stand upright,
and how plants regulate the aperture of their stomata. In animal cells which lack a cell wall
however, excessive osmotic pressure can result in cytolysis.
Osmotic Potential
(1) The potential of water molecules to move from a hypotonic solution (more water, less
solutes) to a hypertonic solution (less water, more solutes) across a semi permeable
membrane.
(2) A measure of the potential of water to move between regions of differing
concentrations across a water-permeable membrane
As pure water contains no solutes, thus, it should have zero (0) water potential. And also
for this reason, the value of osmotic potential of a solution is always negative since the
presence of solutes will always make a solution have less water than the same volume of
pure water.
In application, when two solutions are isotonic the osmotic potentials will be equal, and
there will be no net movement of water molecules. When different, the solution that is
hypotonic (diluted solution, less solutes more water) will have higher osmotic potential
(less negative ψπ ) whereas the solution that is hypertonic (concentrated solution, more
solutes less water) will have lower osmotic potential (more negative ψπ). Difference in
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osmotic potentials will cause water molecules to move from a hypotonic solution to a
hypertonic solution
Endosmosis, Exosmosis and Plasmolysis
If a cell is placed in a solution whose total solute concentration is higher than the cells – a
hyperosmolar solution, water will flow out of the cell- this is exosmosis, its volume will
decrease, protoplast and and vacuoles will shrink resulting the plasma membrane to fold
inward and move away from the cell wall, this is called plasmolysis. Crenation is caused
by water movement out of a cell in a hypertonic solution. Continuous loss of water from
plant cells results in wilting and drooping of leaves and stems. If a cell is placed in a
solution whose total solute concentration is lower than the cells, water will flow into the
cell, the process is endosmosis. Hemolysis is caused by water movement into a cell in a
hypotonic solution.
Imbibition
The adsorption of water by hydrophilic surfaces is called hydration or imbibition. It is a
type of diffusion whereby movement of water occurs along a diffusion gradient. An
adsorbent (a hydrophilic surface) for example cellulose matrix or dry plant material like
seeds are required for imbibition to occur. The swollen seeds produce a large pressure; this
pressure makes the seedlings to emerge. This is the imbibition pressure, also referred to as
matric potential denoted by γ in the context of plant water relations. It is a measure at
atmospheric pressure of the tendency for a matrix to adsorb additional water molecules. It
is expressed in terms of water potential. Two conditions are necessary for imbibition to
take place- water potential gradient and the affinity between the adsorbant and the imbibed
liquid.
Water potential
The measure of the relative tendency of water to move from one area to another, and is
commonly represented by the Greek letter Ψ (Psi). Water potential is caused by osmosis,
gravity, mechanical pressure, or matrix effects including surface tension. It is useful in
understanding water movements within plants, animals, and soil.
Botanists use the term water potential when predicting the movement of water into or out
of plant cells. Water potential has two components, a physical pressure component,
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pressure potential psip and the effects of solutes, solute potential psis. Water will always
move from an area of higher water potential (higher free energy; more water molecules) to
an area of lower water potential (lower free energy; fewer water molecules). Water
potential, then, measures the tendency of water to leave one place in favor of another place.
Water potential is affected by two physical factors. One factor is the addition of solute
which lowers the water potential. The other factor is pressure potential (physical pressure).
An increase in pressure raises the water potential. By convention, the water potential of
pure water at atmospheric pressure is defined as being zero (psi = 0). For instance, it can be
calculated that a 0.1 M solution of sucrose at atmospheric pressure (psip = 0) has a water
potential of -2.3 bars due to the solute (psi = -2.3). [Note: a bar is a unit of pressure,
measured with a barometer that is about the same as 1 atmosphere. Another measure of
pressure is the megapascal (Mpa). 1 Mpa = 10 bars.]
Movement of H20 into and out of a cell is influenced by the solute potential
(relative concentration of solute) on either side of the cell membrane. If water moves out of
the cell, the cell will shrink. If water moves into an animal cell, it will swell and may even
burst. In plant cells, the presence of a cell wall prevents cells from bursting as water enters
the cells, but pressure eventually builds up inside the cell and affects the net movement of
water. As water enters a dialysis bag or a cell with a cell wall, pressure will develop inside
the bag or cell as water pushes against the bag or cell wall. The pressure would cause, for
example, the water to rise in an osmometer tube or increase the pressure on a cell wall. It is
important to realize that water potential and solute concentration are inversely related. The
addition of solutes lowers the water potential of the system. In summary, solute potential is
the effect that solutes have on a solution's overall water potential.
Movement of H20 into and out of a cell is also influenced by the pressure potential
(physical pressure) on either side of the cell membrane. Water movement is directly
proportional to the pressure on a system. For example, pressing on the plunger of a waterfilled syringe causes the water to exit via any opening. In plant cells this physical pressure
can be exerted by the cell pressing against the partially elastic cell wall. Pressure potential
is usually positive in living cells; in dead xylem elements it is often negative.
Ψ = P – π ΨW = ΨP + ΨS
1 Bar = 0.1 Mpa = 0.987 Atm. 10 Bar = 1 Mpa
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Absorption and Movement of Water:
Water can enter plants through entire surface; however the bulk of water is absorbed by
plants through the roots. Maximum absorption of water takes place in the root hair zone.
Root hairs are unicellular tubular outgrowths of some epidermal cells which increase the
water absorbing surface area of the roots. From the root hair the water moves through the
root cortex and to the xylem elements. Water in the root moves through two pathways: The
Apoplastic Movement of Water occurs exclusively through the cell wall without crossing
any membranes. The Symplastic Movement occurs from cell to cell through the
plasmodesmata. The symplast comprises the network of cytoplasm of all cells
interconnected by plasmodesmata.
Major proportion of water flow in the root cortex occurs via the apoplast, as the cortical
cells are loosely packed and thus offer no resistance. The Apoplastic water movement
beyond the cortex is blocked by the casparian strip composed of a wax like substance
called suberin present in the endodermis. Thus beyond endodermis water is forced to move
through the cell membranes- this is the transmembrane pathway. In this water may also
cross through the tonoplast surrounding the vacuole, this is vacuolar pathway. Once the
water reaches the root xylem transpiration drives the water to move to the leaves via stem.
Theories of Water Translocation
Root Pressure Theory (Priestly, 1916; Crafts & Broyer, 1938 and Stocking, 1956)
Root pressure occurs in the xylem of some vascular plants when the soil moisture level is
high either at night or when transpiration is low during the day. When transpiration is high,
xylem sap is usually under tension, rather than under pressure, due to transpirational pull.
At night in some plants, root pressure causes guttation or exudation of drops of xylem sap
from the tips or edges of leaves. Root pressure is studied by removing the shoot of a plant
near the soil level. Xylem sap will exude from the cut stem for hours or days due to root
pressure. If a pressure gauge is attached to the cut stem, the root pressure can be measured.
Root pressure is caused by active transport of mineral nutrient ions into the root xylem.
The endodermis in the root is important in the development of root pressure. Without
transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the
water potential. Water then diffuses from the soil into the root xylem due to osmosis. Root
pressure is caused by this accumulation of water in the xylem pushing on the rigid cells.
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Root pressure provides a force, which pushes water up the stem, but it is not enough to
account for the movement of water to leaves at the top of the tallest trees. The maximum
root pressure measured in some plants can raise water only to about 20 meters, and the
tallest trees are over 100 meters tall.
Stocking (1956) defined root pressure as a pressure developing in tracheary elements of
xylem due to entry of water as a result of metabolic activities of roots. It is maintained by
activity of living cells and is affected by supply of oxygen and poisonous substances. The
reasons for the drawback of theory are: (a) Its magnitude is very low which is unsuitable
for tall trees. (b) Root pressure is absent during summer when transpiration is very high. (c)
It is not abserved in plants like conifers. (d) It is not observed in plants growing in cold,
drought or poorly aerated soil while ascent of sap is normal.
Transpiration Pull / Cohesion Tension theory
The cohesion-tension theory is a theory of intermolecular attraction commonly observed in
the process of water traveling upwards (against the force of gravity) through the xylem of
plants, which was put forward by Charles John Jolly and Henry Horatio Dixon, 1894,
1914.
According to this theory leaves loose water into atmosphere by transpiration. This in
turn decreases water potential of mesophy cells. These cells draw water from adjacent cells
having high water potential. This potential gradient extends upto sap of xylem. As a result,
a tension is created in xylem elements of leaf which is transmitted down to the xylem
elements of leaf which is transmitted down to the xylem elements of root. This puts the
water in xylem to a great pull or tension. This is known as transpiration pull or
transpiration tension. The water molecules within a column are joined to one another by a
strong mutual force of attraction called cohesion force. Thus transpiration tension draws
water upwards by a force from roots and continuity of water column in maintained by
cohesive force present amongst water molecules.
Its magnitudes are very high and account for the continuity of water column in xylem
against its own weight and resistance caused by internal tissues in its translocation. This is
the most widely accepted theory.
Transpirational pull results ultimately from the evaporation of water from the
surfaces of cells in the interior of the leaves. This evaporation causes the surface of the
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water to recess into the pores of the cell wall. Inside the pores, the water forms a concave
meniscus. The high surface tension of water pulls the concavity outwards, generating
enough force to lift water as high as a hundred meters from ground level to a tree's highest
branches. Transpirational pull only works because the vessels transporting the water are
very small in diameter, otherwise cavitation would break the water column. And as water
evaporates from leaves, more is drawn up through the plant to replace it. When the water
pressure within the xylem reaches extreme levels due to low water input from the roots (if,
for example, the soil is dry), then the gases come out of solution and form a bubble - an
embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are
present (these have a plug-like structure called a torus, that seals off the opening between
adjacent cells and stops the embolism from spreading).
Water movement within the xylem conduits is driven by a pressure gradient created by
such force, not by capillary action. Specifically, the evaporation and transpiration of water
in the leaves causes water in the xylem to move from the roots, which have a higher water
potential, up the stem of the plant that has a decreasing water potential along its length.
Transpiration:
Transpiration is the loss of water from a plant in the form of water vapor. Water is
absorbed by roots from the soil and transported as a liquid to the leaves via xylem. In the
leaves, small pores allow water to escape as a vapor and CO2 to enter the leaf for
photosynthesis. Of all the water absorbed by plants, less than 5% remains in the plant for
growth and storage following growth. Plants lose gallons of water every day through the
process of transpiration, the evaporation of water from plants primarily through pores in
their leaves. Up to 99% of the water absorbed by roots is lost via transpiration through
plant leaves.
Why do plants transpire?
Evaporative cooling: As water evaporates or converts from a liquid to a gas at the leaf cell
and atmosphere interface, energy is released. This exothermic process uses energy to break
the strong hydrogen bonds between liquid water molecules; the energy used to do so is
taken from the leaf and given to the water molecules that have converted to highly
energetic gas molecules. These gas molecules and their associated energy are released into
the atmosphere, cooling the plant.
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Accessing nutrients from the soil: The water that enters the root contains dissolved
nutrients vital to plant growth. It is thought that transpiration enhances nutrient uptake into
plants.
Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas
exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave
the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for
photosynthesis to operate.
Water uptake: Although only less than 5% of the water taken up by roots remains in the
plant, that water is vital for plant structure and function. The water is important for driving
biochemical processes, but also it creates turgor so that the plant can stand without bones.
A simple equation describing how these factors alter transpiration is
Transpiration = [Water potential (leaf)] – [Water potential (atm)] / Resistance
The units for this equation are mols of water lost per leaf area per time (mol/cm2/s).
This equation makes predicting rates of transpiration easy. For example, any time the
numerator (the value for the driving force) is increased, the rate of transpiration becomes
faster and vice versa. Similarly, if the denominator (the value for resistance) increases, this
means there is greater resistance and thus, slower transpiration.
Factors Affecting Rates of Transpiration
Plant Parameters – These plant parameters help plants control rates of transpiration by
serving as forms of resistance to water movement out of the plant.
Stomata – Stomata are pores in the leaf that allow gas exchange where water vapor
leaves the plant and carbon dioxide enters. Special cells called guard cells control each
pore‘s opening or closing. When stomata are open, transpiration rates increase; when they
are closed, transpiration rates decrease.
Boundary layer – The boundary layer is a thin layer of still air hugging the surface of
the leaf. This layer of air is not moving. For transpiration to occur, water vapor leaving the
stomata must diffuse through this motionless layer to reach the atmosphere where the water
vapor will be removed by moving air. The larger the boundary layer, the slower the rates of
transpiration.
Cuticle – The cuticle is the waxy layer present on all above-ground tissue of a plant
and serves as a barrier to water movement out of a leaf. Because the cuticle is made of
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wax, it is very hydrophobic or ‗water-repelling‘; therefore, water does not move through it
very easily. The thicker the cuticle layer on a leaf surface, the slower the transpiration rate.
Cuticle thickness varies widely among plant species. In general, plants from hot, dry
climates have thicker cuticles than plants from cool, moist climates. In addition, leaves that
develop under direct sunlight will have much thicker cuticles than leaves that develop
under shade conditions.
Environmental Conditions – Some environmental conditions create the driving force
for movement of water out of the plant. Others alter the plant‘s ability to control water loss.
Relative humidity – Relative humidity (RH) is the amount of water vapor in the air
compared to the amount of water vapor that air could hold at a given temperature. The air
in the intercellular spaces of a hydrated leaf would have a RH near 100%, just as the
atmosphere on a rainy day would have. Any reduction in water in the atmosphere creates a
gradient for water to move from the leaf to the atmosphere. The lower the RH, the less
moist the atmosphere and thus, the greater the driving force for transpiration. When RH is
high, the atmosphere contains more moisture, reducing the driving force for transpiration.
Temperature – Temperature greatly influences the magnitude of the driving force for
water movement out of a plant rather than having a direct effect on stomata. As
temperature increases, the water holding capacity of that air increases sharply. The amount
of water does not change, just the ability of that air to hold water. Because warmer air can
hold more water, its relative humidity is less than the same air sample at a lower
temperature, or it is ‗drier air‘. Because cooler air holds less water, its relative humidity
increases or it is ‗moister air‘. Therefore, warmer air will increase the driving force for
transpiration and cooler air will decrease the driving force for transpiration.
Soil water –Plants with adequate soil moisture will normally transpire at high rates
because the soil provides the water to move through the plant. Plants cannot continue to
transpire without wilting if the soil is very dry because the water in the xylem that moves
out through the leaves is not being replaced by the soil water. This condition causes the
leaf to lose turgor or firmness, and the stomata to close. If this loss of turgor continues
throughout the plant, the plant will wilt.
Light – Stomata are triggered to open in the light so that carbon dioxide is available
for the light-dependent process of photosynthesis. Stomata are closed in the dark in most
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plants. Very low levels of light at dawn can cause stomata to open so they can access
carbon dioxide for photosynthesis as soon as the sun hits their leaves. Stomata are most
sensitive to blue light, the light predominating at sunrise.
Wind – Wind can alter rates of transpiration by removing the boundary layer, that still
layer of water vapor hugging the surface of leaves. Wind increases the movement of water
from the leaf surface when it reduces the boundary layer, because the path for water to
reach the atmosphere is shorter.
Stomata:
The stomata are the primary control mechanisms that plants use to reduce water loss and
they are able to do so quickly. Stomata are sensitive to the environmental cues that trigger
the stomata to open or close. The major role of stomata is to allow carbon dioxide entry to
drive photosynthesis and at the same time allow the exit of water as it evaporates, cooling
the leaf. Two specialized cells called ‗guard cells‘ make up each stoma (stoma is singular
for stomata). Plants have many stomata (up to 500 per mm2) on their leaf surfaces. Some
plant species have stomata on both sides of the leaf while others have stomata on the lower
leaf surface to minimize water loss. Stomata are present in the sporophyte generation of all
land plant groups except liverworts. Dicotyledons usually have more stomata on the lower
epidermis than the upper epidermis. Monocotyledons, on the other hand, usually have the
same number of stomata on the two epidermes. In plants with floating leaves, stomata may
be found only on the upper epidermis; submerged leaves may lack stomata entirely.
Structure of the Stomata
A stoma or pore is formed by a pair of bean-shaped guard cells. The guard cells have the
ability to open and close the stoma. The inner walls of the guard cells are thick and the
outer walls thin. Guard cells differ from the translucent epidermal cells in that they contain
chloroplasts. Stomata communicate with the air chambers in the spongy mesophyll. The
size of a typical stomatal pore ranges from 3 – 12 μm or 5 – 15 μm in width and 10 – 14
μm in length. The stomata are responsible for the interchange of gases for respiration and
photosynthesis. The stomata allow for the loss of excess water in the form of water vapour,
which also allows for cooling.
How do stomata open? Stomata sense environmental cues, like light, to open. These cues
start a series of reactions that cause their guard cells to fill with water.
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1. Signal received: The blue light at dawn is the signal that is recognized by a receptor on
the guard cell.
2. The receptor signals the H+-ATPases on the guard cell‘s plasma membrane to start
pumping protons (H+) out of the guard cell. This loss of positive charge creates a negative
charge in the cell.
3. Potassium ions (K+) enter the guard cell through channels in the membrane, moving
toward its more negative interior.
4. As the potassium ions accumulate in the guard cell, the solute potential is lowered.
5. A lower solute potential attracts water to enter the cell.
6. As water enters the guard cell, its hydrostatic pressure increases.
7. The pressure causes the shape of the guard cells to change and a pore is formed,
allowing gas exchange.
Potassium Ion Theory (Levitt 1974)
It was observed by Fujino (1967) that opening of stomata occurs due to the influx of K+
ions into the guard cells. The source of K+ ions are the neighbouring subsidiary and
epidermal cells, there by increasing the concentration from 50mM to 300mM in guard
cells. The increase in K+ ion concentration increases the osmotic concentration of guard
cells thus leading to stomatal opening. ATP helps in entry of K+ ions into the guard cells.
Levitt (1974) observed that proton (H+) uptake by guard cells, chloroplasts takes
place with the help of ATP. This leads to increase in value of pH in guard cells. Rise in pH
converts starch into organic acid like malic acid. The uptake of K+ ions is balanced by:
Uptake of chloride (Cl-) ions, Transport of H+ ions released from organic acid (malic acid),
by negative charges of organic acids when they lose H+ ions
Thus all these factors lead to the opening of stomata. The stomata closure is due to
excretion of K+ ions from guard cells surrounding epidermal and subsidiary cells.
Factors affecting Stomatal Movement
Environmental cues that affect stomata opening and closing are light, water, temperature,
and the concentration of CO2 within the leaf.
- Stomata will open in the light and close in the dark. However, stomata can close in the
middle of the day if water is limiting, CO2 accumulates in the leaf, or the temperature is
too hot.
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- If the plant lacks water, stomata will close because there will not be enough water to
create pressure in the guard cells for stomatal opening; this response helps the plant
conserve water.
- If the leaf‘s internal concentration of CO2 increases, the stomata are signaled to close
because respiration is releasing more CO2 than the photosynthesis is using. There is no
need to keep stomata open and lose water if photosynthesis is not functioning.
Alternatively, if the leaf‘s CO2 concentration is low, the stomata will stay open to continue
fueling photosynthesis.
- High temperatures will also signal stomata to close. High temperatures will increase the
water loss from the leaf. With less water available, guard cells can become flaccid and
close. Another effect of high temperatures is that respiration rates rise above
photosynthesis rates causing an increase of CO2 in the leaves; high internal CO2 will cause
stomata to close as well. Remember that some plants may open their stomata under high
temperatures so that transpiration will cool the leaves.
Antitranspirants: colourless plastics, silicone oils, waxes, fungicides, phenyl mercuric
acetate, ABA & CO2 at higher concentration
ii. Mineral Nutrition
The plants upon which we depend for the food we eat, and for the oxygen we breathe,
depend in turn upon the soil. A good soil supplies the plants with the mineral elements they
use. Vigorous, highly productive plants can be grown in solutions of fertilizer minerals in
the absence of soil. Mineral nutrition thus comprises the study of how plants obtain mineral
elements (either through water, air or soil) and utilize them for their growth and
development.
It is through the roots that the components of the soil are made available and useful to
plants. Root-soil relations are intricate and poorly understood and we need to know more
about roots and their relations with soil. Since the elements are necessary to growth and
reproduction, they are termed as 'essential elements'. By 'water culture', it is possible to
determine which elements are essential and which are non-essential. This method also
indicates the symptoms that arise from deficiency of the element. The essential elements
most commonly used and added to the soil in the form of fertilizer are nitrogen,
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phosphorus and potassium. These are the major nutrients. The absence or deficiency (not
present in the required amount) of any of the essential elements leads to deficiency
symptoms. The symptoms develop as hunger signs in the plants and can be studied by
hydroponics or water culture or soil-less culture. In hydroponics, large volume of nutrient
solution is used to grow the plants. Concentration and pH of the nutrient medium is
adjusted regularly. Air is also supplied to the culture to provide enough oxygen to the root
system. Almost all the elements present in the soil can enter the plant. About sixty
elements have been so far detected to occur in plants. The mineral content of a plant is
determined by the analysis of plant ash.
Macro and Micronutrients and their Role:
Certain elements like hydrogen carbon oxygen nitrogen, potassium, calcium, magnesium,
phosphorus, and sulphur are required by plants in large amounts and are called major or
macronutrients (at least 1 mg/g of dry matter). The elements like chlorine boron, iron
manganese, zinc, copper, nickel and molybdenum are required in minute quantities. Hence
they are called minor, micro, rare or trace elements. They are required by plants in very
small amounts and are called micronutrients (less than 0.1 mg /gm of dry matter).
Essential Nutrients (Criteria for Essentiality)
Of the several elements present in a plant, there are some, which are essential for the
healthy growth of plants. They are called essential elements. An essential element can be
recognized by the following criteria according to Arnon and Stout 1939:
1. It is indispensable for the growth of plants.
2. The element is involved in the nutrition of plants. 3. It may additionally have a
corrective effect on mineral balance and other soil conditions.
4. A plant is unable to complete its vegetative or reproductive phase in the absence of the
element.
5. The absence or deficiency of the element produces disorders. These disorders are a
direct result of the lack or deficiency of the element. The element alone can correct the
disorders produced by its absence or deficiency.
Sixteen to seventeen elements have been found to be essential. The other elements are
called non-essential elements or nutrients. The elements, which take part in the metabolic
activities of plants, whether essential or non-essential, are named as functional elements.
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Sources of essential elements
All elements incorporated into plants are ultimately derived from the atmosphere, water
and soil. Carbon enters as atmospheric carbon dioxide, hydrogen is obtained from water,
and oxygen can come from the air, from water or as inorganic ions. Nitrogen which is
abundant in the air is inert and plants are unable to use it as such. However, it is fixed as
nitrate, nitrite or ammonia by various agencies and taken up by roots from the soil. All
other inorganic elements needed by plants are absorbed from the soil where they are
ultimately derived from the parent rock by weathering.
Functions of Essential Elements in Plants
Carbon, hydrogen and oxygen are absorbed in the form of carbon dioxide and water from
air and soil. These three elements enter the composition of all types of organic compounds
like carbohydrates, organic acids, fats, proteins, enzymes, and hormones etc., which build
up the protoplasm.
1. Nitrogen:
Nitrogen is obtained from the soil in the form of nitrates (NO3-), nitrites (NO2- ) and
ammonium (NH4+) salts.
Functions
Nitrogen is a constituent of amino acids, proteins, amides, nucleic acids, enzymes some coenzymes, chlorophyll and alkaloids. It is, therefore, essential for cell division, full
vegetative and reproductive growth, metabolic activities of cell, photosynthesis, etc.
Deficiency
1. Chlorosis, which starts from older leaves and progresses to younger leaves.
2. The leaves show mottled appearance of purple or red anthocyanin pigments over the
vein.
3. Lateral buds remain dormant.
4. There is little tillering in cereals.
5. Flower formation is either suppressed or only a few flowers are formed, which lead to
the formation of small fruits and less viable seeds.
6. In potato, smaller and fewer tubers are produced.
Excess: Excessive Nitrogen stimulates abundant shoot growth.
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2. Phosphorus
It is obtained from the soil in the form of phosphates (H2PO4- or HPO42-).
Function
It is a component of nucleic acid, phospholipids, ATP, nucleoprotein, NAD, NADP and a
number of co-enzymes. Since the young cells are the seat of maximum metabolic activities,
maximum phosphorus is found in the meristems, fruits and seeds.
Deficiency
1. Stunted growth.
2. Premature leaf fall.
3. Chlorosis appears later, which is of mottled type and is shown first by older leaves.
4. Necrosis in lamina, petiole and skin of soft fruits.
5. Lateral buds show prolonged dormancy, but active buds are not affected.
6. Vascular tissues are poorly developed.
3. Sulphur
It is absorbed from the soil in the form of sulphate (SO42-) though small quantities of
sulphur dioxide can be got from the air by foliar absorption.
Function
1. For the formation of sulphur containing protein.
2. It is a component of two vitamins -B and Biotin.
3. In onion and garlic, sulphur occurs in the form of glucosides, which provide
characteristic odour to these plants. Sinigrin and sinalbin are glucosides in black and white
mustards respectively.
4. Enhances the number of nodules and nitrogen fixing bacteria in legumes.
5. Formation of chlorophyll.
Deficiency
1. Leaves show chlorosis.
2. Reduction in juice content of citrus fruits.
3. Fruit formation, in general is retarded.
4. "Tea yellow" in tea plant.
5. Premature germination of lateral buds, killing of young branches.
6. Smaller and chlorotic leaves.
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7. Necrosis of leaf margin and tip.
8. Inward rolling of leaf margin and rapid defoliation.
4. Potassium
It is absorbed in the form of K+ ions.
Functions
It is the most abundant cation in plants.
1. Required for the metabolic activities of cells in young growing parts like root tips and
young leaves but seeds are an exception.
2. Essential for the functioning of about forty enzymes which take part in glycolysis,
Krebs‘s Cycle, photosynthesis, starch synthesis, synthesis of nucleic acids and chlorophyll
and the activity of ATP in many reactions.
3. It maintains hydration, permeability and reactive state of protoplasm.
4. Produces turgor pressure inside cells for their movements.
5. Stomatal opening and closing is linked to its influx and efflux from the guard cells.
Deficiency
1. Leaves show chlorosis.
2. Necrosis at the leaf tip, margin or irregular patches.
3. Citrus leaves become bronzed and twisted.
4. Growth is stunted due to reduced apical activity.
5. Apical buds may die - Loss of apical dominance is a characteristic deficiency symptom
of K+ ions and it leads to bushy growth.
6. Cereals may show lodging.
5. Magnesium
It is absorbed as Mg2+ions.
Functions
1. Is a component of chlorophyll molecule and participates in the structure of ribosomes.
2. Essential for the formation of carotenoids.
3. Required by a large number of enzymes RUBP Carboxylase, PEP Carboxylase
connected with phosphate transfer.
4. Mg is also believed to be involved in the synthesis of DNA & RNA
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5. Higher concentrations of Mg2+ ions are found in seeds and growing areas of root and
stem.
Deficiency
1. Chlorosis. Yellow, red and purple tints are often found in the chlorotic areas, especially
towards the margins.
2. Followed by necrosis.
3. Defoliation may occur.
4. Reduced vegetative and reproductive growth.
5. Phloem and pith become reduced or remain undeveloped.
6. Tomato fruits having pale orange colour, reduced pulp and woolly flesh.
6. Calcium
It is absorbed as Ca2+ ions.
Functions
1. Calcium is a component of calcium pectate, which is found in the middle lamella.
2. Acts as an activator of enzymes like ATP-ase, some kinases, phospholipases, a-amylase
and succinate dehydrogenase.
3. Takes part in lipid metabolism.
4. Required in the maintenance of cell membrane.
5. Essential for control of carbohydrate metabolism.
6. Plays some role in binding nucleic acids and proteins in chromosomes.
7. Counteracts toxicities of other metallic ions.
Deficiency
1. Fragility of chromosomes.
2. Roots become translucent and stop apical growth.
3. Young leaves show marginal and apical withering.
4. Flower and fruit stalks break - premature falling.
Micronutrients
7. Iron
Absorb from soil in both Fe2+ and Fe3+ ions.
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Functions
1. Iron is a constituent of cytochromes, ferredoxin, nitrogenase, catalase peroxidase,
hematin etc.
2. Essential for the development of chloroplast and chlorophyll.
3. Many of iron containing enzymes take part in electron transfer in both photosynthesis
and respiration.
4. Structure of polyribosomes is dependent upon iron containing compounds.
5. Activator of nitrate reductase.
6. Synthesis of proteins and DNA.
Deficiency
Chlorosis - inter veinal chlorosis.
8. Manganese
It is absorbed as Mn2+ ions.
Functions
1. Activator of enzymes like oxidases, peroxidases, dehydrogenase, kinases and
dicarboxylases.
2. Maintenance of lamellar structure of chloroplast.
3. Essential for photolysis of water and evolution of oxygen.
Deficiency
1. Chlorotic patches are in the form of specks or reticulations in dicots and stripes in
monocots.
2. Necrosis may follow.
3. Leaves show premature fall or do not develop at all.
4. Both stem and root experience stunted growth. Their apices may die back.
5. Flowers are often sterile.
6. Grey spot disease in oat develops due to manganese deficiency.
9. Zinc
It is obtained as Zn2+ ions.
Functions
1. Constituent of carbonic anhydrase - zinc is essential for supply of CO2 to the chloroplast
and for the evolution of CO2 during respiration.
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2. Production of IAA (auxin)
Deficiency
1. Inter veinal chlorosis
2. Followed by necrosis
3. Terminal bud dies and leads to leaf rosettes
10. Boron
It is absorbed in the form of borate ions (BO33- or B4O72-).
Functions
1. Favours absorption of calcium
2. Produces root nodules in legumes
Deficiency
1. Causes disintegration of softer tissues
2. Browning of cauliflower
3. Heart rot of sugar beet
4. Reduced transpiration due to defunctioning of stomata
11. Copper
It is mostly absorbed as Cu2+ ions though the element can also exist as Cu+ion.
Functions
1. It can take part in electron transport system in both respiration and photosynthesis.
2. In photosynthesis the copper containing enzyme is plastocyanin.
3. It is the connecting link between photo system - I and II.
Deficiency
1. Appearance of dark green colour in young leaves followed by chlorosis.
2. In exanthema the tree barks show deep slits from which gum exudes.
3. The reclamation disease is named so because of its widespread presence in reclaimed
lands of Europe. Tips of leaves undergo chlorosis. Hence, also known as the leaf -tip
disease.
12. Molybdenum
Absorbed from soil as molybdate ion (MoO42-).
Functions
Essential for nitrogen fixation.
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Deficiency
1. Yellow spot disease of citrus fruits
2. Whiptail disease in crucifers like cabbage
3. Flowers show premature fall
4. In cauliflower the inflorescence loses its compact form
13. Chlorine
Chloride ion is ubiquitous in nature and highly soluble, readily taken up far in excess of
requirements, thus rarely if ever deficient. Species absorb 10 to 100 times as much chloride
than they require far in excess of their minimal requirements – a common example of
luxury consumption. Absorbed from soil as Cl‾, remains as such without becoming a
structural part of the organic molecules. Chloride is a highly mobile anion with two
principal functions. It is a major counter ion, maintaining electrical neutrality across
membranes and one of the principal osmotically active solutes in the vacuole. Chloride, a
highly mobile anion also appears to be required for cell division in both leaves and shoots.
Along with manganese, chloride is required for photolysis of water in oxygen evolving
complex. Plants deprived of chloride tend to exhibit reduced growth, wilting of leaf tips,
and a general chlorosis. Fruit set is also reduced. Leaves turn bronze, roots stunted, become
club shaped near tips.
14. Nickel
Nickel is the latest element to be classified as essential for plant growth in both laboratory
and field conditions and an absolute requirement for nickel fertilizer under field conditions
in perennial species growing in the southeast of the United States has now been
established. Nickel clearly has a significant effect on the productivity of field-grown,
nitrogen-fixing plants, those in which ureides are a significant form of nitrogen and those
utilizing urea as a primary nitrogen source. Nickel is abundant in the crust of the Earth,
comprising about 3% of the composition of the earth. Nickel averages 50 mg Ni kg_1 in
soils and commonly varies from 5 to 500 mg Ni kg_1. Ubiquitous in plant tissues but
requirement is very low – 200ng to complete one life cycle. The basis for Nickel
requirement is not clear but may be related to mobilization of nitrogen during seed
germination. It is also known to be a component of two enzymes- Urease (catalyses the
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hydrolysis of urea into ammonia and carbon dioxide) and Hydrogenase (required in N
fixation).
Nickel deficient plants accumulate urea in their leaves and, consequently, show leaf tip
necrosis. Plants grown in soil seldom, if ever, show signs of nickel deficiency because the
amounts of nickel required are minuscule but nickel may indeed play a role in many, yet
undiscovered processes in plants.
Mechanism of Absorption of Elements:
Ion traffic into the root
Mineral nutrients absorbed from the root has to be carried to the xylem. This transport
follows two pathways namely apoplastic pathway and symplastic pathway. In apoplastic
pathway, mineral nutrients along with water moves from cell to cell through spaces
between cell wall by diffusion. The ions, which enter the cell wall of the epidermis move
across cell wall of cortex, cytoplasm of endodermis, cell walls of pericycle and finally
reach the xylem. In symplastic pathway, mineral nutrients entering the cytoplasm of the
epidermis move across the cytoplasm of the cortex, endodermis of pericycle through
plasmodesmata and finally reach the xylem.
The movement of ions is termed as flux. The
movement of ions into the cell is
called influx and the movement of ions from the cell to outside is known as efflux. Usually
the uptake of mineral ions by the plant cells or tissues involves two main phases. In the
first phase, tissues kept in mineral solution, show rapid uptake of ions into the free space or
outer space of the cells. The outer space includes the intercellular space and cell wall. The
entry of ions in to the outer space mostly does not require the expenditure of metabolic
energy. The ions exist in a freely exchangeable from in the outer space. In the second
phase, the uptake of ions by the tissues is slow and the ions move into the 'inner space' of
the cells. The inner space refers to the cytoplasm and the vacuole. Entry or exit of the ions
from the inner space usually requires the expenditure of metabolic energy. The ions in the
inners space are not in a freely exchangeable from.
P.R. Stout and Dr. Hoagland have proved that mineral nutrients absorbed by the roots are
translocated through the xylem vessels. Mineral salts dissolved in water moves up along
the xylem vessel to be transported to all the parts of the plant body. Translocation is aided,
by transpiration. As water is continuously lost by transpiration on the upper surfaces of the
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plant, it creates a transpirational pull, by which water along with mineral salts is pulled up
along the xylem vessel.
Theories of translocation of Solutes
Various theories have been proposed to explain the mechanism of mineral salt absorption
and can be placed under the following two categories: 1. Passive absorption. 2. Active
absorption.
Passive Absorption
It is the absorption of minerals without direct expenditure of metabolic energy. Passive
absorption of ions by root system was demonstrated by Briggs and Robertson (1957).
Theories of Passive Absorption
Mass Flow Theory
According to this theory ions are absorbed by the root along with mass flow of water under
the influence of transpiration. This theory failed to explain the salt accumulation against
osmotic gradient. An increase in transpiration pull increases the uptake of ions by the roots.
Thus, mass flow of ions through the root tissues occurs due to transpiration pull in the
absence of metabolic energy.
Ion Exchange Theory
Both cations and anions have a tendency to get adsorbed on the surfaces of the cell walls,
and exchange with ions present in the soil solution. This process of exchange between the
adsorbed ions and ions in solution is known as ion exchange. Mineral elements can be
absorbed in the form of molecules or as ions. When a positively charged ion as K+ is
absorbed by the cell, either a positively charged ion as H+ is displaced from the cell (ion
exchange) or a negatively charged ion must accompany it. Similarly anions can exchange
with free hydroxyl (OH-) ions. Likewise Cl- and Br- are also exchanged without disturbing
the electrical neutrality.
Donnan Equilibrium
This theory describes the effect of fixed or non-diffusible ions which mostly accumulate on
the inner surface of the outer membrane. Process is named after its discoverer F.G.Donnan.
If there is a negative non-diffusing charge on one side of a membrane, it will create a
potential gradient across the membrane from which ions will diffuse. The result will be an
electrochemical equilibrium. The concentration (chemical potential) of ions will not
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necessarily be the same inside and outside. Thus, as an electrical disequilibrium is
maintained because of diffusing charges. Concentration disequilibrium is established.
Therefore, according to Donnan, Donnan equilibrium is attained if the product of anions
and cations in the internal solution becomes equal to the product of anions and cations in
the external solution: Ci+ = Cations inside Ai- = Anions inside, Co+ = Cations outside, Ao= Anions outside i.e.;
[Ci+][Ai-] = [Co+][Ao-]
Theories of Active Absorption:
Movement of ions from the outer space of the cell to the inner space is generally against
the concentration gradient and hence requires energy. This energy is obtained through
metabolism either directly or indirectly. Various evidences indicate the active uptake of
ions by carrier mechanism.
1. Higher rate of respiration increases the salt accumulation inside the cell.
2. Respiratory inhibitors check the process of salt uptake.
3. By decreasing oxygen content in the medium, the salt absorption is also decreased.
These evidences indicate that salt absorption is directly connected with respiratory rate and
energy level in the plant body, as active absorption requires utilization of energy.
Carrier Mechanism
In carrier mechanism, activated ions combine with carrier proteins and form carrier ion
complex. This complex moves across the membrane and reaches the inner space by the
expenditure of energy. Within the cytoplasm, the complex breaks to release the ions. The
carrier moves out of the cytoplasm and is again ready to attach another ion to from a
complex.
Active Transport is Directly Coupled to Metabolic or Light Energy
To carry out active transport, a carrier must couple the uphill transport of the solute with
another, energy-releasing, event so that the overall free-energy change is negative.
Primary active transport is coupled directly to a source of energy other than ∆µ, such as
ATP hydrolysis, an oxidation– reduction reaction (the electron transport chain of
mitochondria and chloroplasts), or the absorption of light by the carrier protein (in
halobacteria, bacteriorhodopsin). The membrane proteins that carry out primary active
transport are called pumps. Most pumps transport ions, such as H+ or Ca2+. However,
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pumps belonging to the ―ATP binding cassette‖ family of transporters can carry large
organic molecules. Ion pumps can be further characterized as either electrogenic or
electroneutral. In general, electrogenic transport refers to ion transport involving the net
movement of charge across the membrane. In contrast, electroneutral transport, as the
name implies, involves no net movement of charge. For example, the Na+/K+-ATPase of
animal cells pumps three Na+ ions out for every two K+ ions in, resulting in a net outward
movement of one positive charge. The Na+/K+-ATPase is therefore an electrogenic ion
pump. In contrast, the H+/K+-ATPase of the animal gastric mucosa pumps one H+ out of
the cell for every one K+ in, so there is no net movement of charge across the membrane.
Therefore, the H+/K+-ATPase is an electroneutral pump.
Secondary Active Transport Uses the Energy Stored in Electrochemical-Potential
Gradients
Protons are extruded from the cytosol by electrogenic H+- ATPases operating in the
plasma membrane and at the vacuole membrane. Consequently, a membrane potential and
a pH gradient are created at the expense of ATP hydrolysis. This gradient of
electrochemical potential for H+,∆µH+, or the proton motive force (PMF), represents
stored free energy in the form of the H+ gradient. The proton motive force generated by
electrogenic H+ transport is used in secondary active transport to drive the transport of
many other substances against their gradient of electrochemical potentials.
Toxicity of Micronutrients:
As a group, the micronutrient elements are an excellent example of the dangers of excess.
Most have a rather narrow adequate range and become toxic at relatively low
concentrations. Critical toxicity, defined as the tissue concentration that gives a 10 percent
reduction in dry matter, varies widely between the several micronutrients as well as
between plant species. As noted earlier, critical concentrations for copper, boron, and zinc
are on the order of 20, 75, and 200 µg g−1 dry weight, respectively. On the other hand,
critical toxicity levels for manganese vary from 200 µg g−1 dry weights for corn, to 600 µg
g−1 for soybean, and 5300 µg g−1 for sunflower.
Toxicity symptoms are often difficult to decipher because an excess of one nutrient may
induce deficiencies of other nutrients. For example, the classic symptom of manganese
toxicity, which often occurs in waterlogged soils, is the appearance of brown spots due to
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deposition of MnO2 surrounded by chlorotic veins. But excess manganese may also induce
deficiencies of iron, magnesium, and calcium. Manganese competes with both iron and
magnesium for uptake and with magnesium for binding to enzymes. Manganese also
inhibits calcium translocation into the shoot apex, causing a disorder known as ‗‗crinkle
leaf.‘‘ Thus the dominant symptoms of manganese toxicity may actually be the symptoms
of iron, magnesium, and/or calcium deficiency.
Excess micronutrients typically inhibit root growth, not because the roots are more
sensitive than shoots but because roots are the first organ to accumulate the nutrient. This
is particularly true of both copper and zinc. Copper toxicity is of increasing concern in
vineyards and orchards due to long-term use of copper-containing fungicides as well as
urban and industrial pollution. Zinc toxicity can be a problem in acid soils or when sewage
sludge is used to fertilize crops.
In spite of the apparent toxicity of micronutrients, many plant species have developed the
capacity to tolerate extraordinarily high concentrations. For example, most plants are
severely injured by nickel concentrations in excess of 5 µg g−1 dry weight, but species of
the genus Alyssum can tolerate levels in excess of 10 000 µg g−1 dry weight.
Relatively high magnesium concentrations can elicit deficiency symptoms of other
essential cations like calcium or potassium.
Prior to the identification of copper as a micronutrient, it was regarded as a plant poison. In
1882, botanist Pierre-Marie-Alexis Millardet developed a copper-based formulation
Bordeaux mixture [CuSO4*5H2O + Ca(OH)2] that saved the disease-ravaged French wine
industry Copper may competitively inhibit magnesium accumulation in plants. The two
general symptoms of copper toxicity are stunted root growth and leaf chlorosis.
Most soils readily buffer phosphorus additions, and phosphorus is seldom present in the
soil solution at levels that cause direct toxicity. Perhaps the most common symptoms of
phosphorus excess are phosphate-induced micronutrient deficiencies, particularly Zn or Cu
deficiencies.
Boron toxicity symptoms consist of marginal and tip chlorosis, which is quickly followed
by necrosis. Boron does not accumulate uniformly in leaves, but typically concentrates in
leaf tips of monocotyledons and leaf margins of dicotyledons, where boron toxicity
symptoms first appear.
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Manganese, as a divalent cation, can compete with magnesium for binding sites on soil
particles as well as biological membranes within plants. However, manganese is required
in such small quantities that manganese toxicity usually occurs before quantities are high
enough to significantly inhibit magnesium uptake to physiologically deficient levels
Toxicity symptoms in plants under field conditions are very rare, whereas toxicity to
animals feeding on forages high in this element is well known. A narrow span exists
between nutritional deficiency for plants and toxicity to ruminants. Molybdenum
concentrations >10 mg/kg (dry mass) in forage crops can cause a nutritional disorder called
molybdenosis in grazing ruminants. This disorder is a molybdenum-induced copper
deficiency that occurs when the consumed molybdate (MoO42_) reacts in the rumen with
sulfur to form thiomolybdate complexes, which inhibit copper metabolism.
iii. Transport of Organic Substances
Phloem transport: flow from source to sink
Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a
sink. Usually the source is understood to be that part of the plant which synthesises the
food, i.e., the leaf, and sink, the part that needs or stores the food. But, the source and sink
may be reversed depending on the season, or the plant‘s needs. Sugar stored in roots may
be mobilised to become a source of food in the early spring when the buds of trees, act as
sink; they need energy for growth and development of the photosynthetic apparatus. Since
the source-sink relationship is variable, the direction of movement in the phloem can be
upwards or downwards, i.e., bi-directional. This contrasts with that of the xylem where the
movement is always unidirectional, i.e., upwards. Hence, unlike one-way flow of water in
transpiration, food in phloem sap can be transported in any required direction so long as
there is a source of sugar and a sink able to use, store or remove the sugar. Phloem sap is
mainly water and sucrose, but other sugars, hormones and amino acids are also transported
or translocated through phloem.
The Pressure Flow or Mass Flow Hypothesis
The pressure-flow model, first proposed by Ernst Munch in 1930, states that a flow of
solution in the sieve elements is driven by an osmotically generated pressure gradient
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between source and sink (∆ψр). The pressure gradient is established as a consequence of
phloem loading at the source and phloem unloading at the sink.
The accepted mechanism used for the translocation of sugars from source to sink is
called the pressure flow hypothesis. As glucose is prepared at the source (by
photosynthesis) it is converted to sucrose (a dissacharide). The sugar is then moved in the
form of sucrose into the companion cells and then into the living phloem sieve tube cells
by active transport. This process of loading at the source produces a hypertonic condition
in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. As osmotic
pressure builds up the phloem sap will move to areas of lower pressure. At the sink
osmotic pressure must be reduced. Again active transport is necessary to move the sucrose
out of the phloem sap and into the cells which will use the sugar – converting it into
energy, starch, or cellulose. As sugars are removed, the osmotic pressure decreases and
water moves out of the phloem.
Rates of movement in the phloem are quite rapid, well in excess of rates of diffusion.
Velocities average 1 m h–1, and mass transfer rates range from 1 to 15 g h–1 cm–2 of sieve
elements.
To summarise, the movement of sugars in the phloem begins at the source, where
sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a
water potential gradient that facilitates the mass movement in the phloem. Phloem tissue is
composed of sieve tube cells, which form long columns with holes in their end walls called
sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming
continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure
flow begins, and the sap moves through the phloem. Meanwhile, at the sink, incoming
sugars are actively transported out of the phloem and removed as complex carbohydrates.
The loss of solute produces a high water potential in the phloem, and water passes out,
returning eventually to xylem.
Early experiments
Sugar is Translocated in Phloem Sieve Elements
Early experiments on phloem transport date back to the nineteenth century, indicating the
importance of long-distance transport in plants. These classical experiments demonstrated
that removal of a ring of bark around the trunk of a tree, which removes the phloem,
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effectively stops sugar transport from the leaves to the roots without altering water
transport through the xylem.
A simple experiment, called girdling, was used to identify the tissues through which
food is transported. On the trunk of a tree a ring of bark up to a depth of the phloem layer,
can be carefully removed. In the absence of downward movement of food the portion of
the bark above the ring on the stem becomes swollen after a few weeks. This simple
experiment shows that phloem is the tissue responsible for translocation of food; and that
transport takes place in one direction, i.e., towards the roots. This experiment can be
performed by you easily.
When radioactive compounds became available, radiolabeled 14CO2 was used to show
that sugars made in the photosynthetic process are translocated through the phloem sieve
elements.
Phloem Sap Can be Collected and Analyzed
The collection of phloem sap has been experimentally challenging. A few species exude
phloem sap from wounds that sever sieve elements, making it possible to collect relatively
pure samples of phloem sap. Another approach is to use the stylet of an aphid as a ―natural
syringe.‖ Aphids are small insects that feed by inserting their mouthparts, consisting of
four tubular stylets, into a single sieve element of a leaf or stem. Sap can be collected from
aphid stylets cut from the body of the insect, usually with a laser, after the aphid has been
anesthetized with CO2. The high turgor pressure in the sieve element forces the cell
contents through the stylet to the cut end, where they can be collected. Exudate from
severed stylets provides a fairly accurate picture of the composition of phloem. Exudation
from severed stylets can continue for hours, suggesting that the aphid prevents the plant‘s
normal sealing mechanisms from operating.
Source-Sink Relationship:
Sap in the phloem is not translocated exclusively in either an upward or a downward
direction, and translocation in the phloem is not defined with respect to gravity. Rather, sap
is translocated from areas of supply, called sources, to areas of metabolism or storage,
called sinks.
Sources include any exporting organs, typically mature leaves that are capable of
producing photosynthate in excess of their own needs. The term photosynthate refers to
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products of photosynthesis. Another type of source is a storage organ during the exporting
phase of its development. For example, the storage root of the biennial wild beet (Beta
maritima) is a sink during the growing season of the first year, when it accumulates sugars
received from the source leaves. During the second growing season the same root becomes
a source; the sugars are remobilized and utilized to produce a new shoot, which ultimately
becomes reproductive. It is noteworthy that cultivated varieties of beets have been selected
for the capacity of their roots to act as sinks during all phases of development. Thus, roots
of the cultivated sugar beet (Beta vulgaris) can increase in dry mass during both the first
and the second growing seasons, so the leaves serve as sources during both flowering and
fruiting stages.
Sinks include any non-photosynthetic organs of the plant and organs that do not
produce enough photosynthetic products to support their own growth or storage needs.
Roots, tubers, developing fruits, and immature leaves, which must import carbohydrate for
normal development, are all examples of sink tissues. Both girdling and labeling studies
support the source-to-sink pattern of translocation in the phloem.
iv. Proteins
Classification of proteins based on structure and solubility:
Proteins are classified in several ways. Three major types of classifying proteins based on
their function, chemical nature and solubility properties are discussed here.
A. Functional Classification of Proteins:
Based on the functions they perform, proteins are classified into the following groups (with
examples)
1. Structural proteins: Keratin of hair and nails, collagen of bone.
2. Enzymes or catalytic proteins: Hexokinase, pepsin
3. Transport proteins: Hemoglobin, serum albumin.
4. Hormonal proteins: Insulin, growth hormone.
5. Contractile proteins: Actin, myosin.
6. Storage proteins: Ovalbumin, glutelin.
7. Genetic proteins: Nucleoproteins.
8. Defense proteins: Snake venoms, lmmunoglobulins.
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9. Receptor proteins for hormones, viruses.
B. Protein Classification based on Chemical Nature and Solubility
This is a more comprehensive and popular classification of proteins. lt is based on the
amino acid composition, structure, shape and solubility properties. Proteins are broadly
classified in to 3 major groups
1. Simple proteins: They are composed of only amino acid residues.
2. Conjugated proteins: Besides the amino acids, these proteins contain a non-protein
moiety known as prosthetic group or conjugating group.
3. Derived proteins: These are the denatured or degraded products of simple and
conjugated proteins.
The above three classes are further subdivided into different groups.
l. Simple proteins
(a) Globular proteins: These are spherical or oval in shape, soluble in water or other
solvents and digestible.
(i) Albumins: Soluble in water and dilute salt solutions by heat. e.g. serum albumin,
ovalbumin. lactalbumin(milk).
(ii) Globulins: Soluble in neutral and dilute salt solutions e.g. serum globulins vitelline
(egg yolk).
(iii) Glutelins: Soluble in dilute acids and alkalies and mostly found in plants e.g. glutelin
(wheat) oryzenin (rice).
(iv) Prolamines: Soluble in 70% alcohol e.g. gliadin (wheat), zein (maize).
(v) Histones: Strongly basic proteins, soluble in water and dilute acids but insoluble in
dilute ammonium hydroxide e.g. thymus histones, histones of codfish sperm.
(vi) Globins: These are generally considered a long with histones However; globins are not
basic proteins and are not precipitated by NH4OH.
(vii) Protamines: They are strongly basic and resemble histones but smaller in size and
soluble in NH4OH. Protamines are also found in association with nucleic acids e.g. sperm
proteins.
(b) Fibrous proteins: These are fiber like in shape, insoluble in water and resistant to
digestion, Albuminoids or scleroproteins constitute the most predominant group of fibrous
proteins.
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(i) Collagens are connective tissue proteins lacking tryptophan. Collagens, on boiling with
water or dilute acids yield gelatin which is soluble and digestible.
(ii) Elastins: These proteins are found in elastic tissues such as tendons and arteries.
(iii) Keratins: These are present in exoskeletal structures e.g. hair, nails, horns. Human
hair keratin contains as much as 14% cysteine.
2. Conjugated proteins
(a) Nucleoproteins: Nucleic acid (DNA or RNA) is the prosthetic group e.g.
nucleohistones, nucleoprotamnies.
(b) Glycoproteins: The prosthetic group is carbohydrate, which is less than 4% of protein;
the term mucoprotein is used if the carbohydrate content is more than 4%. e.g.mucin
(saliva) ovomucoid (egg white).
(c) Lipoproteins: Protein found in combination with lipids as the prosthetic group eg.
Serum lipoproteins, membrane lipoproteins.
(d) Phosphoproteins Phosphoric acid is the prosthetic group e.g. casein (milk), vitelline
(egg yolk).
(e) Chromoproteins The prosthetic group is coloured in nature e.g. hemoglobins,
cytochromes.
(f) Metalloproteins: These proteins contain metal ions such as Fe, Co, Zn, Cu, Mg etc., e.g.
ceruloplasmin (Cu),carbonic anhydrase (Zn).
3. Derived proteins
The derived proteins are of two types. The primary derived proteins are the denatured or
coagulated or first hydrolysed products of proteins. The secondary derived proteins are the
degraded (due to breakdown of peptide bonds) products of proteins.
(a) Primary derived proteins
(i) Coagulated proteins: These are the denatured proteins produced by agents such as heat,
acids, alkalies etc. e.g. cooked proteins, coagulated albumin (egg white).
(ii) Proteans: These are the earliest products of protein hydrolysis by enzymes, dilute acids,
alkalies etc. which are insoluble in water e.g. fibrin formed from fibrinogen.
(iii) Metaproteins: These are the second stage products of protein hydrolysis obtained by
treatment with slightly stronger acids and alkalies e.g. acid and alkali metaproteins.
(b) Secondary derived proteins:
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These are the progressive hydrolytic products of protein hydrolysis. These include
proteoses, peptones polypeptides and peptides.
v.
Basics of Enzymology
Enzymes are biomolecules that catalyze (i.e., increase the rates of) chemical reactions.
Nearly all known enzymes are proteins. However, certain RNA molecules can be effective
biocatalysts too. These RNA molecules have come to be known as ribozymes. In
enzymatic reactions, the molecules at the beginning of the process are called substrates,
and the enzyme converts them into different molecules, called the products. Almost all
processes in a biological cell need enzymes to occur at significant rates. Since enzymes are
selective for their substrates and speed up only a few reactions from among many
possibilities, the set of enzymes made in a cell determines which metabolic pathways occur
in that cell.
Like all catalysts, enzymes work by lowering the activation energy (Ea or ΔG‡) for a
reaction, thus dramatically increasing the rate of the reaction. Most enzyme reaction rates
are millions of times faster than those of comparable un-catalyzed reactions. As with all
catalysts, enzymes are not consumed by the reactions they catalyze, nor do they alter the
equilibrium of these reactions. However, enzymes do differ from most other catalysts by
being much more specific. Enzymes are known to catalyze about 4,000 biochemical
reactions. A few RNA molecules called ribozymes catalyze reactions, with an important
example being some parts of the ribosome. Synthetic molecules called artificial enzymes
also display enzyme-like catalysis.
Enzyme activity can be affected by other molecules. Inhibitors are molecules that decrease
enzyme activity; activators are molecules that increase activity. Many drugs and poisons
are enzyme inhibitors. Activity is also affected by temperature, chemical environment
(e.g., pH), and the concentration of substrate. Some enzymes are used commercially, for
example, in the synthesis of antibiotics. In addition, some household products use enzymes
to speed up biochemical reactions (e.g., enzymes in biological washing powders break
down protein or fat stains on clothes; enzymes in meat tenderizers break down proteins,
making the meat easier to chew).
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History / Discovery:
As early as the late 1700s and early 1800s, the digestion of meat by stomach secretions]
and the conversion of starch to sugars by plant extracts and saliva were known. However,
the mechanism by which this occurred had not been identified.
In the 19th century, when studying the fermentation of sugar to alcohol by yeast,
Louis Pasteur came to the conclusion that this fermentation was catalyzed by a vital force
contained within the yeast cells called "ferments", which were thought to function only
within living organisms. He wrote that "alcoholic fermentation is an act correlated with the
life and organization of the yeast cells, not with the death or putrefaction of the cells."
In 1878, German physiologist Wilhelm Kühne (1837–1900) first used the term
enzyme, which comes from Greek, "in leaven", to describe this process. The word enzyme
was used later to refer to nonliving substances such as pepsin, and the word ferment was
used to refer to chemical activity produced by living organisms.
In 1897, Eduard Buchner began to study the ability of yeast extracts that lacked any living
yeast cells to ferment sugar. In a series of experiments at the University of Berlin, he found
that the sugar was fermented even when there were no living yeast cells in the mixture. He
named the enzyme that brought about the fermentation of sucrose "zymase". In 1907, he
received the Nobel Prize in Chemistry "for his biochemical research and his discovery of
cell-free fermentation". Following Buchner's example, enzymes are usually named
according to the reaction they carry out. Typically, to generate the name of an enzyme, the
suffix -ase is added to the name of its substrate (e.g., lactase is the enzyme that cleaves
lactose) or the type of reaction (e.g., DNA polymerase forms DNA polymers).
Having shown that enzymes could function outside a living cell, the next step was to
determine their biochemical nature. Many early workers noted that enzymatic activity was
associated with proteins, but several scientists (such as Nobel laureate Richard Willstätter)
argued that proteins were merely carriers for the true enzymes and that proteins per se were
incapable of catalysis. However, in 1926, James B. Sumner showed that the enzyme urease
was a pure protein and crystallized it; Sumner did likewise for the enzyme catalase in
1937. The conclusion that pure proteins can be enzymes was definitively proved by
Northrop and Stanley, who worked on the digestive enzymes pepsin (1930), trypsin and
chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry.
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This discovery that enzymes could be crystallized eventually allowed their structures
to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found
in tears, saliva and egg whites that digests the coating of some bacteria; the structure was
solved by a group led by David Chilton Phillips and published in 1965. This highresolution structure of lysozyme marked the beginning of the field of structural biology and
the effort to understand how enzymes work at an atomic level of detail.
Nomenclature:
There is a long tradition of giving enzymes names ending in -ase. Urease,DNA Polymerase
etc. Exceptions are Pepsin, Trypsin, Renin etc. The names usually indicate the substrate
involved. Lactase catalyses hydrolysis of disaccharide Lactose into Glucose & Galactose It
should have been Lactosase. There is nothing in the name of this enzyme or many others to
indicate the type of the reaction being catalysed. See the case of Fumarase. Some names
such as Catalase indicates neither the substrate nor the reaction. Sometimes same enzyme
has 2 or more names or 2 different enzymes have same name. Because of such ambiguities
and ever increasing number of newly discovered enzymes there was felt a need for a
systematic way of naming and classifying enzymes. Nomenclature Committee of the
International Union of Biochemistry and Molecular Biology (NC-IUBMB) during the first
Enzyme Commission, (1961) devised a system of classification of enzymes that also serves
as a basis for assigning code numbers to them.
The commission places all enzymes in six major classes on the basis of the total reaction
catalyzed:
•
EC 1. Oxidoreductases
• EC 2. Transferases
• EC 3. Hydrolases
• EC 4. Lyases
• EC 5. Isomerases
• EC 6. Ligases
Oxido-Reductases: catalyze the transfer of hydrogen or oxygen atoms or electrons from
one substrate to another, also called oxidases, dehydrogenases, or reductases. Note that
since these are ‘redox’ reactions, an electron donor/acceptor is also required to complete
the reaction. Some examples are:
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Alcohol Dehydrogenase CH CH OH + NAD⁺ = CH CHO + NADH + H⁺
1.1.1.1 [Alcohol: NAD Oxidoreductase]
Lactate Dehydrogenase Lactate + NAD+ = Pyruvate + NADH + H
1.1.1.27
(Lactate: NAD+ oxidoreductase)
Transferases: catalyze group transfer reactions, excluding oxidoreductase.
Example: Hexokinase catalyses the transfer of Phosphate Group from ATP to Glucose
hence called ATP:D Hexose-6-Phosphotransferase with enzyme commission no as 2.7.1.1
other examples include 2.4.1.1 Phosphorylase, 2.7.7.7 DNA polymerase
Hydrolases: These break down large molecules into smaller ones by the introduction of
water (hydrolysis) and breaking of specific covalent bonds. Include lipases, esterases,
peptidases, proteases. Most Digestive enzymes belong to this group.
Example: Triacyl Glycerol Lipase {3.1.1.3} Triacyl Glycerol Acyl Hydrolase Glycerol
Ester Hydrolase, This pancreatic enzyme catalyses enzymatic hydrolysis of triacylglycerols
releasing fatty acids.
Lyases: catalyze non-hydrolytic removal of functional groups from substrates, often
creating / leaving a double bond in the product; or the reverse reaction, ie, addition of
functional groups across a double bond.
Include decarboxylases, aldolases etc in the removal direction, and synthases in the
addition direction.
RCOCOOH → RCHO + COO
Catalyze the cleavage of C-C, C-O, C-N, C-S and other bonds by other means than by
hydrolysis or oxidation, or conversely adding a group to a double bond.
Example: 1. The enzyme histidine ammonia-lyase (4.3.1.3) catalyzes the reaction shown
below which results in the formation of a double bond. Histidine ammonia-lyase is a
cytosolic enzyme catalyzing the first reaction in histidine catabolism,
2. Pyruvate decarboxylase (EC 4.1.1.1) catalyses the decarboxylation of pyruvic acid to
acetaldehyde and carbon dioxide in the cytoplasm Systematic name: 2-Oxo-acid CarboxyLyase
Isomerases: catalyzes isomerization reactions, including racemizations and cis-tran
isomerizations. These transfer groups within molecules to yield isomeric forms. For e.g,
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phosphohexose isomerase changes glucose 6 phosphate to fructose 6 phosphate. Example:
Alanine Racemase 5.1.1.1 L Alanine → D Alanine
D amino acids arise directly from their L isomers by the action of amino acid racemases
Ligases: catalyze covalent bonding of two substrates to form a large molecule. The energy
for the reaction is derived from the hydrolysis of ATP. These enzymes catalyse the
synthesis of new bonds coupled to the breakdown of ATP or other nucleoside
triphosphates.
Example: EC 6.3.1.2 glutamate—ammonia ligase Systematic name L-glutamate:
ammonia ligase
Characteristics of Enzymes:
Chemical Nature
All the enzymes are essentially proteins and possess properties characteristic to these.
Dixon and Webb (1964) have stressed the protein nature of an enzyme by defining it as ―a
protein with catalytic properties due to its power of specific activation‖.
Evidences Proving the Protein Nature of the Enzymes
(a) Elementary composition
In their elementary composition, the enzymes show the usual proportion of C, H, N and S,
as found in the proteins. Some crystalline enzymes, however,also contain minute quantities
of P or metal ions such as Cu2+, Mg2+, Zn2+ etc. On hydrolysis, the crystalline enzymes
yield the amino acids.
(b) Identical action of some enzymes over other enzymes and the proteins
Enzymes are subjected to the action of those enzymes which are specifically meant for the
breakdown of peptide bonds of proteins.
(c) Amphoteric nature
Like other proteins, the enzymes behave as ampholytes in an electric field. The isoelectric
point (pl) for various enzymes has also been determined.
(d) Denaturation
Enzymes, like other proteins, also undergo denaturation. If the crystalline proteinase
chymotrypsin is subjected to an unfavourable pH, some part of protein becomes denatured.
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This percentage of denatured protein is usually found to be equal to the per cent loss in
enzymic activity, thus proving a sort of correlation between the enzymes and the proteins..
(e) Formation of antibodies
Many purified enzymes, on injection into animal body, produce the specific antibodies.
Since many nonprotein materials have been shown to serve as antigens, this cannot be
treated as evidence in support of the protein nature of enzymes but simply a further support
to it. Chemically, the enzymes may be divided into 2 categories:
1. Simple-protein enzymes
These contain simple proteins only e.g., urease, amylase, papain etc.
2. Complex-protein enzymes
These contain conjugated proteins i.e., they have a protein part called apoenzyme (apo =
away from) and a nonprotein part called prosthetic group associated with the protein unit.
The two parts constitute what is called a holoenzyme, e.g., catalase, cytochrome c etc.
The activity of an enzyme depends on the fact that the non-proteinaceous prosthetic
group is intimately associated with the proteinaceous apoenzyme. But sometimes the
prosthetic group is loosely bound to the protein unit and can be separated by dialysis and
yet indispensable for the enzyme activity. In that case, this dialyzable prosthetic group is
called as a coenzyme or cofactor.
Thus Conjugated-protein enzyme = Protein part + Prosthetic group or Holoenzyme =
Apoenzyme + Coenzyme
Coenzymes are thermostable, dialyzable organic compounds. They may be either
attached to the protein molecules or may be present in the cytoplasm. The coenzyme
accounts for about 1% of the entire enzyme molecule. Sometimes, a distinction is made
between coenzymes and cofactors: the former includes the organic prosthetic groups and
the latter the metal ions (Fairley and Kilgour, 1966).
Characteristics: The enzymes possess many outstanding characteristics. These are
enumerated below:
1. Colloidal Nature
Enzyme molecules are of giant size. Their molecular weights range from 12,000 to over 1
million. They are, therefore, very large compared with the substrates or functional group
they act upon. It has been observed that the molecular weights of many enzymes prove to
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be approximately an n-fold multiple (where n is an integer) of 17,500 which is found to be
an unit in most proteins. On account of their large size, the enzyme molecules possess
extremely low rates of diffusion and form colloidal systems in water. Being colloidal in
nature, the enzymes are nondialyzable although some contain dialyzable or dissociable
component in the form of coenzyme.
2. Catalytic Nature or Effectiveness
An universal feature of all enzymatic reactions is the virtual absence of any side products.
Therefore, just as hemoglobin is precisely tailored to transport oxygen, an enzyme is
precisely adapted to catalyze a particular reaction. They act catalytically and accelerate the
rate of chemical reactions occurring in plant and animal tissues. They do not normally
participate in these reactions or if they do so, at the end of the reaction, they are recovered
as such without undergoing any qualitative or quantitative change. This is the reason why
they, in very small amounts, are capable of catalyzing the transformation of a large
quantity of substrate. Thus, the catalytic potency of enzymes is exceedingly great.
The catalytic power of an enzyme is measured by the ―turnover number‖ (a term
devised by Wechselzahl) or molecular activity (a term devised by Norman Arthur
Edwards and Kenneth Arnold Hassall, 1980) which is defined as the number of substrate
molecules converted into product per unit time, when the enzyme is fully saturated with
substrate. For example, a single molecule of catalase can convert 50,00,000 H2O2
molecules into H2O and O2 in a minute (Sumner and Somers, 1947). The value of turnover
number varies with different enzymes and depends upon the conditions in which the
reaction is taking place. However, for most enzymes, the turnover numbers fall between 1
to 104 per second. The turnover number of 600,000 sec-1 for carbonic anhydrase is one of
the largest known. Carbonic anhydrase catalyzes the hydration of carbon dioxide to
produce 3,60,00,000 molecules of carbonic acid per minute. This catalyzed reaction is 6
x107 times faster than the uncatalyzed one.
3. Specificity of Enzyme Action
With few exceptions, the enzymes are specific in their action. Their specificity lies in the
fact that they may act (a) on one specific type of substrate molecule or (b) on a group of
structurally-related compounds or (c) on only one of the two optical isomers of a
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compound or (d) on only one of the two geometrical isomers. Accordingly, four patterns of
enzyme specificity have been recognized:
a. Absolute specificity
Some enzyme are capable of acting on only one substrate. For example, urease acts only
on urea to produce ammonia and carbon dioxide. Similarly, carbonic anhydrase brings
about the union of carbon dioxide with water to form carbonic acid.
b. Group specificity
Some other enzymes are capable of catalyzing the reaction of a structurally related group
of compounds. For example, lactic dehydrogenase (LDH) catalyzes the interconversion of
pyruvic and lactic acids and also of a number of other structurally-related compounds.
c. Optical specificity
The most striking aspect of specificity of enzymes is that a particular enzyme will react
with only one of the two optical isomers. For example, arginase acts only on Larginine and
not on its D-isomer. Similarly, D-amino acid oxidase oxidizes the D-amino acids only to
the corresponding keto acids.
Although, the enzymes exhibit optical specificity, some enzymes, however,
interconvert the two optical isomers of a compound. For example, alanine racemase
catalyzes the interconversion between L- and D-alanine.
d. Geometrical specificity
Some enzymes exhibit specificity towards the cis and trans forms. As an example,
fumarase catalyzes the interconversion of fumaric and malic acids: It does not react with
maleic acid which is the cis isomer of fumaric acid or with D-malic acid.
4. Thermolability (= Heat sensitivity)
Being proteinaceous in nature, the enzymes are very sensitive to heat. The rate of an
enzyme action increases with rise in temperature; the rate being frequently increased 2 to 3
times for a rise in temperature of 10ºC, i.e., the value of temperature quotient or Q10 is 2 to
3. But at higher temperatures, the value of coefficient does not remain constant and
decreases rapidly. Above 60ºC, the enzymes coagulate and thus become inactivated,
because there occurs an irreversible change in their chemical structure. The enzymes of dry
tissues like seeds and spores, however, can endure still higher temperatures of about 100º
to 120ºC
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B. Sc. 3rd Year Single Paper Scheme 2016
4. Reversibility of a Reaction
The enzymes are capable of bringing about reversion in a chemical reaction. The digestive
enzymes catalyze the hydrolytic reactions which are reversible. For instance, lipase, which
catalyzes the synthesis of fat from glycerol and fatty acid, can also hydrolyze them into
their component units.
5. pH Sensitivity
The pH value or the H+ ion concentration of the medium controls the activity of an
enzyme to a great extent. This is mainly related to the degree of dissociation, to the electric
charge of the enzyme and, through this, to the formation of the enzyme substrate complex.
Each enzyme, thus, acts best in a certain pH value which is specific to it and its activity
slows down with any appreciable change (increase or decrease) in the H+ ion
concentration. In fact, the pH will affect the efficiency of an enzyme and usually there will
be a pH at which the activity is at a maximum. The activity will fall off on either side of
this value. A perusal of the pH values indicates that the approximate optimum pH value for
most enzymes lies near neutrality. This value depends on many factors such as: (a) the
nature of buffer system, (b) the presence of other colloids, activators or inhibitors, (c) the
age of the cell tissue, and (d) the nature of the substrate. Usually maximum enzyme activity
is obtained at or near the isoelectric point of the enzymes.
Mechanism of Enzyme Action:
Enzymes, as we have seen, cause rate enhancements that are orders of magnitude greater
than those of the best chemical catalysts. Yet they operate under mild conditions and are
highly specific as to the identities of both their substrates and their products. These
catalytic properties are so remarkable that many nineteenth century scientists concluded
that enzymes have characteristics that are not shared by substances of nonliving origin. To
this day, there are few enzymes for which we understand in more than cursory detail how
they achieve their enormous rate accelerations. Nevertheless, it is now abundantly clear
that the catalytic mechanisms employed by enzymes are identical to those used by
chemical catalysts. Enzymes are simply better designed.
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Theories of Enzyme Action
Fischer’s Lock and Key Model
Previously, the interaction of substrate and enzyme was visualized in terms of a lock and
key model (also known as template model), proposed by Emil Fischer in 1898. According
to this model, the union between the substrate and the enzyme takes place at the active site
more or less in a manner in which a key fits a lock and results in the formation of an
enzyme substrate complex. In fact, the enzyme-substrate union depends on a reciprocal fit
between the molecular structure of the enzyme and the substrate. And as the two molecules
(that of the substrate and the enzyme) are involved, this hypothesis is also known as the
concept of intermolecular fit. The enzyme-substrate complex is highly unstable and almost
immediately this complex decomposes to produce the end products of the reaction and to
regenerate the free enzyme. The enzyme-substrate union results in the release of energy. It
is this energy which, in fact, raises the energy level of the substrate molecule, thus
inducing the activated state. In this activated state, certain bonds of the substrate molecule
become more susceptible to cleavage.
Koshland’s Induced Fit Model
An important but unfortunate feature of Fischer‘s model is the rigidity of the active site.
The active site is presumed to be pre-shaped to fit the substrate. In order to explain the
enzyme properties more efficiently, Koshland, in 1958, modified the Fischer‘s model.
Koshland presumed that the enzyme molecule does not retain its original shape and
structure. But the contact of the substrate induces some configurational or geometrical
changes in the active site of the enzyme molecule. Consequently, the enzyme molecule is
made to fit completely the configuration and active centres of the substrate. At the same
time, other amino acid residues may become buried in the interior of the molecule.
Koshland‘s hypothesis has recently been confirmed by Lipscomb. The hydrophobic and
charged groups both are involved in substrate binding. In the absence of substrate, the
substrate binding and catalytic groups are far apart from each other. But the proximity of
the substrate induces a conformational change in the enzyme molecule aligning the groups
for both substrate binding and catalysis. Simultaneously, the spatial orientation of other
regions is also changed so that the other groups or other amino acids are now much closer.
On contact with the true substrate, all groups are brought into correct spatial orientation.
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As to the sequence of events during the conformational changes, 3 possibilities exist. The
enzyme may first undergo a conformational change, and then bind substrate. An alternative
pathway is that the substrate may first be bound and then a conformational change may
occur. Thirdly, both the processes may occur simultaneously with further isomerization to
the final conformation. Originally little more than an attractive hypothesis, Koshland‘s
model has now gained much experimental support. Conformational changes during
substrate binding and catalysis have been demonstrated for various enzymes such as
phosphoglucomutase, creatine kinase, carboxypeptidase etc.
Transition State Theory and Tight Binding
It was originally suggested by JBS Haldane, 1930 and Linus Pauling, 1946 that the
enzyme enhances reactivity in the substrate by putting the substrate under strain. The
suggestion was that tight binding might bring about a stretching or change in the angle of
some relevant bond in the substrate thereby lowering the reactions energy of activation.
The incredible catalytic rate enhancements caused by enzymes led Linus Pauling to
suggest that enzymes bind tightly to substrates distorted toward the transition state, thereby
concentrating them and enforcing catalysis. Wolfenden explained that chemically stable
analogues that resemble the transition state would be expected to bind more tightly than
substrate by factors resembling the rate enhancement imposed by enzymes.
vi. Biological Functions of Triacylglycerols
Biological lipids are chemically a diverse group of compounds, the common and defining
feature of which is their insolubility in water i.e; lipids are defined only by the common
physical property of solubility in non-polar organic solvents. Not all classes of lipids are
therefore, related to each other in chemical structure. Any, of a diverse group of organic
compounds that are grouped together because they do not interact appreciably with water.
Lipids are a class of nonpolar molecules that include the fatty acids, fats
(triacylglycerols), cholesterol, lipid-soluble vitamins (such as vitamins A, D, E and K),
waxes, soaps, glycerophospholipids, sphingolipids, steroid hormones, prostaglandins, and
derivatives of these compounds, as well. The biological functions of lipids are equally
diverse:
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The majority of lipids in biological systems function either as a source of stored
metabolic energy or as structural matrices and permeability barriers in biological
membranes. Very small amounts of special lipids act as both intracellular messengers and
extracellular messengers such as hormones and pheromones. One of the three large classes
of substances in foods and living cells, lipids contain more than twice as much energy
(calories) per unit of weight as the other two (proteins and carbohydrates). They include
the fats and edible oils (e.g., butter, olive oil, corn oil), which are primarily triglycerides;
phospholipids (e.g., lecithin), which are important in cell structure and metabolism; waxes
of animal or plant origin; and sphingolipids, complex substances found in various
membranes.
Other General Functions include:
1. Food material
Lipids provide food, highly rich in calorific value. One gram lipid produces 9.3
kilocalories of heat.
2. Food reserve
Lipids provide are insoluble in aqueous solutions and hence can be stored readily in the
body as a food reserve.
3. Structural component
Lipids are an important constituent of the cell membrane.
4. Heat insulation
The fats are characterized for their high insulating capacity. Great quantities of fat are
deposited in the subcutaneous layers in aquatic mammals such as whale and in animals
living in cold climates.
5. Fatty acid absorption
Phospholipids play an important role in the absorption and transportation of fatty acids.
6. Hormone synthesis
The sex hormones, adrenocorticoids, cholic acids and also vitamin D are all synthesized
from cholesterol, a steroidal lipid.
7. Vitamin carriers
Lipids act as carriers of natural fat-soluble vitamins such as vitamin A, D and E.
8. Blood cholesterol lowering
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Chocolates and beef, especially the latter one, were believed to cause many heart diseases
as they are rich in saturated fatty acids, which boost cholesterol levels in blood and clog the
arterial passage. But researches conducted at the University of Texas by Scott Grundy and
Andrea Bonanome (1988) suggest that at least one saturated fatty acid stearic acid, a major
component of cocoa butter and beef fat, does not raise blood cholesterol level at all. The
researchers placed 11 men on three cholesterol poor liquid diets for three weeks each in
random order. One formula was rich in palmitic acid, a known cholesterol booster; the
second in oleic acid; and the third in stearic acid. When compared with the diet rich in
palmitic acid, blood cholesterol levels were 14% lower in subjects put on the stearic acid
diet and 10% lower in those on the oleic acid diet.
9. Antibiotic agent
Squalamine, a steroid from the blood of sharks, has been shown to be an antibiotic and
antifungal agent of intense activity. This seems to explain why sharks rarely contract
infections and almost never get cancer.
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Unit: II
i. Nitrogen Metabolism:
General account
Nitrogen (N) compounds are found in all plant tissues, where they play key roles in growth,
reproduction, and photosynthesis. On a dry weight basis, nitrogen is the fourth most abundant
nutrient element in the plants. It is an essential constituent of proteins, nucleic acids,
hormones, chlorophyll and a variety of other important primary and secondary plant
constituents. Most plants obtain the bulk of their nitrogen from the soil in the form of either
nitrate (NO3 ‑) or ammonium (NH4 +), but the supply of nitrogen in the soil pool is limited
and plants must compete with a variety of soil microorganisms for available nitrogen. As a
result nitrogen is often a limiting nutrient for plants, in both natural and agricultural
ecosystems.
The bulk of atmosphere, 78% by volume consists of molecular nitrogen (N2 or dinitrogen),
an odourless and colourless gas. In spite of its abundance, higher plants are unable to convert
dinitrogen into a biologically useful form. The two nitrogen atoms in dinitrogen are joined by
an exceptionally stable bond (N≡N) and plants don’t have the enzyme that will reduce this
triple covalent bond. Only certain prokaryotic species are able to carry out this important
reaction. This situation presents plants with a unique problem with respect to the uptake and
assimilation of nitrogen. The plants thus depend on the prokaryotic organisms to convert
atmospheric dinitrogen into a usable form. The uptake and assimilation of nitrogen occurs
through phases like, nitrate uptake and ammonia uptake.
Nitrogen fixation
Symbiotic nitrogen fixers
In symbiotic association, the plant is identified as the host and the microbial partner is known
as the microsymbiont. The most common form of symbiotic association results in the
formation of enlarged and multicellular structures, called nodules, on the root (or
occasionally the stem) of the host plant.
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In the case of legumes, the microsymbiont is a bacterium of one of the three genera:
Rhizobium, Bradyrhizobium or Azorhizobium. Collectively, these organisms are referred to
as Rhizobia.Curiously, only one non-leguminous genus, Parasponia (of the family Ulmaceae)
is known to form root nodules with the Rhizobia symbiont.
Symbiotic nitrogen fixers, particularly legumes, contribute substantially more nitrogen to
the soil pool than do free living bacteria. Typically a hectare of legume-Rhizobium
association will fix twenty five to sixty kg of dinitrogen annually, while non symbiotic
organisms fix less than 5 kg per hectare.
Mechanism of legume-Rhizobium nitrogen fixation
The sequence of events beginning with bacterial infection of the root and ending with
formation of mature nitrogen fixing nodules has been studied extensively in legumes. Overall
the process involves a sequence of multiple interactions between the bacteria and the host
roots and can be discussed in three principle stages:
i). Colonisation and nodule initiation:
Rhizobia are free living saprophytic soil bacteria. The initial attraction of Rhizobia to host
roots appears to involve positive chemotaxis or movement towards a chemical. Rhizobia host
specificity is determined when the Rhizobia attach to the root hairs and must involve some
form of recognition between cells involves chemical linkages that form between unique
molecules on cell surfaces. In the case of Rhizobia host interactions, recognition appears to
involve two classes of molecules viz. Lectins and complex polysaccharides. Lectins are small,
non enzymatic proteins synthesised by the host and have the particular ability to recognise
and bind to specific carbohydrates. In addition rhicadhesin, a calcium binding protein thought
to be located on the surface of Rhizobium cell appears to be involved in recognition
mechanism.
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ii). Invasion of root hair and the formation of infection thread:
In this stage the bacterium penetrates the host cell wall in order to enter the space between the
wall and the plasma membrane. Once the Rhizobia reach the outer surface of the plasma
membrane, tip growth of the root hair ceases and the cell membrane begins to invaginate. The
result is a tubular intrusion into the cell called an infection thread, which contains the
invading Rhizobia. The infection thread elongates until it reaches the base of the root hair cell.
ii). Bacteria get released:
This is the final step in the infection process whereby the bacteria are released into the host
cells. The membrane of the infection thread buds off to form small vesicles each containing
one or more individual bacteria. Shortly after release, the bacteria cease dividing, enlarge and
differentiate into specialise nitrogen fixing cells called bacteroids. The bacteroids remain
surrounded by a membrane called the peribacteroid membrane. As the nodule enlarges and
matures, vascular connections are established which serve to import photosynthetic carbon
into the nodule and export fixed nitrogen from the nodule to the plant.
Biochemistry of nitrogen fixation :
Only prokaryotes are able to fix dinitrogen principally because only they have the gene
coding for enzyme dinitrogenase. The enzyme dinitrogenase has been purified from virtually
all known nitrogen fixing prokaryotes. It is a multimeric protein complex made up of two
proteins of different size. The smaller protein is a dimer consisting of two identical subunit
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polypeptides. The molecular mass of each subunit ranges from 24 to 36 kD, depending on the
bacterial species. It is called the Fe protein because the dimer contains a single cluster of 4
Iron ions bound to 4 sulphur groups (Fe4-S4). The larger protein in the dinitrogenase complex
is called the MoFe protein. It is a tetramer consisting of two pairs of identical subunits with a
total molecular mass of 220kD. Each MoFe protein contains 2 Mo ions in the form of a FeMo-S cofactor.
The overall reaction for reduction of dinitrogen to ammonia by dinitrogenase is as:
Note that the principle product of biological nitrogen fixation is ammonia, but that for every
dinitrogen molecule reduced, one molecule of hydrogen is generated.
Nitrate Assimilation:
Plants assimilate most of the nitrate absorbed by their roots into organic nitrogen compounds.
The first step of this process is the reduction of nitrate to nitrite in the cytosol. The enzyme
nitrate reductase catalyses this reaction:
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Where NAD(P)H indicates NADH or NADPH. The most common form of nitrate reductase
uses only NADH as an electron donor, another form of the enzyme that is found
predominantly in non-green tissues such as roots can use either NADH or NADPH.
The nitrate reductases of higher plants are homo-dimers; that is, they are composed of two
identical subunits, each with a molecular mass of 100kDa. Each subunit contains three
prosthetic groups viz., FAD (flavin adenine dinucleotide), heme and a molybdenum complex.
Molybdenum is bound to the enzyme via a complex with an organic molecule called a pterin
which acts as a chelator of the metal. Nitrate reductase is the principle molybdenum
containing protein in vegetative tissues, and one symptom of molybdenum deficiency is the
accumulation of nitrate that results from diminished nitrate reductase activity.
The genes for nitrate reductase from several higher plants have been cloned. Comparison
between the deduced aminoacid sequences and those of other well characterised proteins that
bind FAD, heme, or molybdenum has lead to the structural model for nitrate reductase. The
FAD binding domain accepts two electrons from NAD or NADPH. The electrons then pass
through the heme domain to the molybdenum complex, where they are transferred to nitrate.
Expression of the genes coding for nitrate reductase and the activity of the enzymes vary with
nitrate concentration and with light or carbohydrate levels. These factors induce the de novo
synthesis of the enzyme.
Nitrate reductase converts nitrate to ammonium:
Nitrite (NO2-) is a highly reactive and potentially toxic ion. Plant cells immediately transport
the nitrite generated during nitrate reduction from the cytosol into chloroplasts in leaves and
plastids in roots. In these organelles, the enzyme nitrite reductase reduces nitrite to ammonia.
Chloroplasts and root plastids contain different forms of enzyme but both forms transfer
electrons from ferredoxin to nitrite according to the reaction:
Reduced ferredoxin derives from photosynthetic electron transport in the chloroplasts and
from NADPH generated by the oxidative pentose phosphate pathway in non green tissues.
Nitrite reductase is encoded in the nucleus and synthesised as a precursor carrying a nitrogen
terminus transit peptide that targets it to the plastids. Nitrate and light induce the transcription
of nitrite reductase mRNA.
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Plants can assimilate nitrate in both roots and shoots:
In most plant species, both roots and shoots have the capacity to assimilate nitrate first as
nitrite and then as ammonium. The relative extent to which nitrate is reduced in the roots
or in the leaves depends on several factors, including the level of nitrate supplied to the
roots and the plant species. In many plants, when the roots receive small amount of nitrate,
nitrate is reduced primarily in the roots. As the supply of nitrate increases, a greater
proportion of the absorbed nitrate is translocated to the shoot and assimilated there.
Even under similar conditions of nitrate supply, the balance between root and shoot
nitrate metabolism measured by the proportion of nitrate reductase activity in each of the
two tissues or by the relative concentrations of nitrate and reduced nitrogen in the xylem
sap vary from species to species. In plants such as the cocklebur (Xanthium strumarium),
nitrate metabolism is restricted to the shoot; in other plants, such as white lupine (Lupinus
albus), most nitrate is metabolised in the roots. Generally, species native to temperate
regions rely more heavily on nitrate assimilation by the roots than do species of tropical or
subtropical origins.
Ammonium Assimilation:
Plant cells avoid ammonium toxicity by rapidly converting the ammonium generated from
nitrate assimilation or photorespiration into aminoacids. The primary pathway for this
conversion involves the sequential actions of glutamine synthetase and glutamate
synthase.
Multiple forms of the enzymes convert ammonium to aminoacids:
Glutamine synthetase (GS) combines ammonia with glutamate to form glutamine
This reaction requires the hydrolysis of one ATP and involves a divalent cation such as
Mg2+, Mn2+ and Co2+ as a cofactor. GS has a molecular mass of 350 kDa and is
composed of 8 identical subunits. Elevated levels of glutamine stimulate the activity of
glutamate synthase (also known as glutamine: 2-oxoglutarate aminotransferase or
GOGAT) this enzyme transfers the amide group of glutamine to 2-oxoglutarate, yielding
two molecules of glutamate. Plants contain two types of GOGAT: one accepts electrons
from NADH and other electrons from ferredoxin:
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Alternate pathway for ammonium assimilation
Glutamate dehydrogenase (GDH) catalyses a reversible reaction that synthesises or
deaminates glutamate.
An NADH dependent form of GDH is found in mitochondria and an NADPH
dependent form is localised in the chloroplasts of photosynthetic organs. Mitochondrial
form was thought to be involved in photorespiratory nitrogen metabolism because
photorespiration generates ammonium in the mitochondrion. However GS or Fd-GOGAT
are required for reassimilation of photorespiratory ammonia. Once assimilated into
glutamine and glutamate, nitrogen is incorporated into aminoacids via transamination
reactions. The enzyme that catalyses these reactions are known as aminotransferases.
Once assimilated into glutamine and glutamate, nitrogen is incorporated into other
aminoacids via transamination reactions. For example aspartate aminotransferase (ATT)
catalyses the reaction
Here the aminoacid glutamate is transferred to the carboxy atom of aspartate.
Aspartate is an aminoacid that participates in the malate-aspartate shuttle to transfer
reducing equivalents from mitochondria and chloroplast into the cytosol. All
transamination reactions require pyridoxal phosphospahte (vitamen B6) as a cofactor. The
enzymes that catalyse these reactions are known as aminotransferases. Aminotransferases
are found in the cytoplasm, chloroplast, mitochondria, glyoxysomes and peroxisomes. The
aminotransferases localised in the chloroplast have a significant role to play in aminoacid
synthesis.
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ii. Photosynthesis
Photosynthesis
The process in which electromagnetic radiations are converted into chemical energy by
the green plants is called photosynthesis. Or it can be defined as the process by which
green plants are able to synthesize energy rich sugar from the simple inorganic material
like CO2 and H2O in the presence of sunlight and chlorophyll pigment.
Light
12H2O + 6CO2
C6 H12O6 + 6O2 + 6H2O
Chlorophyll
Photosynthetic Pigments:
In
higher
consists
plants the chlorophyll
of
pigments
two closely related
Chlorophyll
a
and
chlorophyll b. Chlorophyll a is the
principal pigment and chrophyll b the
accessory pigment other accessory
pigments include carotenoids.
Structure of Chlorophyll
The chlorophylls have a porphyrin
like ring structure of a tetrapyrole
nucleus with a Mg atom coordinated
in the centre and a long hydrophobic
Molecular structure of Chlorophyll
hydrocarbon tail called phytol tail that anchors them in the photosynthetic membrane.
An isocyclic ring called cyclopentanon is attached to third pyrole ring. The empirical
formula of chlorophyll a is C55H72O5N4Mg. Chlorophyll a is blue green
microcrystalline solid. Chlorophyll b has empirical formula C55H70O6N4Mg. It is a
green black microcrystaline solid. It differs from the chlorophyll a in having an
aldehyde (CHO) group attached to carbon atom 3 instead of methyl (CH3) group.
Carotenoids
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These are a group of yellow, brown to reddish pigments which are associated with the
chlorophylls inside the chloroplast and alone inside chromoplasts. These are of two
types.
1. Carotenes
They are hydrocarbons with a general formula of C40H56 .The most common carotene
is B- carotene.
2. Xanthophyll (carotenols)
These are O2 containing derivatives of carotenes eg.C40H56O (Cryptoxanthin) C40H56 O2
(Lutein, Zeaxanthin).
The unique feature of both carotenoids and chlorophylls is the presence of a system
of alternating double bonds with resonating electrons which are rather easily excited by
photons of the visible light, especially at the blue and red ends.
Carotene
Xanthophyll
Absorption and Action Spectrum of Chlorophyll:
All the photosynthetic pigments do not absorb all the wavelengths of the visible
spectrum of light. If the amount of light absorbed by a pigment say Chl.a is
plotted against the different wavelengths of light, it will represent its absorption
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spectrum . If the actual rate of O2 evolved or CO2 consumed is plotted against the
different wavelengths of
light absorbed by the same pigment, it will represent
the action spectrum of photosynthesis. From absorption spectrum it becomes clear
that chlorophylls absorb more of
Absorption spectrum
blue and red light. If action and
absorption spectrum is compared,
It
is
observed
that
those
wavelengths of light which are
chiefly
absorbed
chlorophyll
by
pigments
the
also
stimulate the higher or maximum
rate
of
photosynthesis.
Experimental observations reveal
that the absorption and action
spectra of chlorophyll run almost
parallel to each other there by
indicate that it is the most efficient
photosynthetic pigment. Pigments
are often named after the wavelength which is absorbed to the maximum e.g Chl.a
700,
Chl.a 680.
Mechanism of Photosynthesis
Photosynthesis is completed in two phases; (1) Light Phase (2) Dark Phase.
1. Light Phase
It occurs in the thylakoid and intergranal membranes. This phase results in the
generation of energy rich molecules coenzyme NADPH and ATP called assimilatory
power which is utilized in the fixation of CO2 in the dark phase which occurs in the
stroma of the chloroplast. This phase is also celled Hill reaction after the name of its
discoverer. This phase can be discussed under the following headings
In chloroplasts light energy is changed into chemical energy by he help of two
functional units called photo systems. Light energy promotes the transfer of electrons
through a series of compounds that act as electron donors and electron acceptors. The
majority of electrons ultimately reduces NADP to NADPH and oxidizes H2O into O2.
Light is also used to generate a proton motive force across the thylakoid membrane
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which is used to synthesize ATP. Light phase can be discussed under the following
headings.
(a). Photosystems
A photosystem is the collection of different pigment molecules consisting of two
closely linked components one is the reaction centre where the principal reactions of
photosynthesis occurs. Other component is the antenna complex termed light harvesting
complexes (LHC S) which capture light energy and transmit it to reaction centre.
Both reaction centres and antennas contain tightly bound light absorbing
pigments molecules. Chla is the principal pigment involved in photosynthesis being
present in both reaction centre and antenna complex. . Antenna complex in addition
contains other pigments like Chl b and carotenoids. The presences of various antenna
pigments which absorb light at different wavelengths greatly extend the range of light
that can be absorbed and used for photosynthesis. The size of antenna system varies
considerably in different organisms .It is generally 200-300 chlorophylls per reaction
centre in higher plants.
In
bright
sunlight
a
chlorophyll molecule absorbs only a
few
photons
/
sec.
If
every
chlorophyll has a complete reaction
centre associated with the enzymes
that make up this system would be
idle for most of the time, only
occasionally being activated by photo
absorption.
However
if
many
pigments can sent energy into a
common reaction centre the system is
kept active a large fraction of the
time.
When chlorophyll a or any other pigment absorbs visible light, energy raises the
chlorophyll to a higher energy (exited). State. In the higher exited state chlorophyll is
extremely unstable, very rapidly gives some of its energy to the surroundings as heat
and enters the lowest exited state where it can be stable for maximum of several nano
seconds (10-9sec.)
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The exited chlorophylls have four alternative pathways for disposing of its
available energy.
1. It can remit photons (Phosphorescence).
2. It can convert its excitation energy into heat energy.
3. It can participate in energy transfer i.e. transfer its energy to another molecule.
4. The exited state causes chemical reaction to occur (Photochemistry).
The chemical reactions of photosynthesis are among the fastest known chemical
reactions. The physical phenomenon by which the excitation energy is conveyed from
the chlorophyll that absorbs the light to the reaction centre is thought to be resonance
(photosensitized resonance) transfer.
Emerson Enhancement Effect:
In late 1950s R. Emerson a biophysicist performed an experiment for measuring the
quantum yield of photosynthesis and revealed an effect known as red drop It was found
that any photon absorbed by chlorophyll or other pigments is as effective as any other
photon in driving photosynthesis. However the yield drops dramatically in the far red
region of chlorophyll absorption (greater than 680nm) called red drop. Thus light with
wavelength greater than 680 nm is much less efficient than light of shorter wavelength.
In another experiment Emerson measured the rate of photosynthesis separately with
light of two different wavelengths. When red and far red light were given together the
rate of photosynthesis was greater than the sum of their individual rates. This has been
called as Emerson enhancement effect. These observations led to the discovery that two
photochemical complexes now called Photosystem I and photosystem II (PSI and PSII )
operate in series to carry out the early energy storage reactions of photosynthe.
Types of Photosystems:
Photosystem I
It is driven by the light of wavelength 700nms. It is primarily present in the unstacked
thylakoids. It transfers electrons to the final electron acceptor NADP when it works in
coordination with PSII. It can work independently or play role in cyclic electron
transfer pathway but supports only ATP synthesis. It has reducing agent X, Fe-S protein
called ferredoxin, Plastoquinone, cytochrome complex and plastocyanin.
Photosystem II
It is driven by the light of wavelength 680nms. It is primarily present in the stacked
region (grana). It splits water to form oxygen. In combination with PSI it plays role in
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linear electron transfer and supports ATP synthesis, formation of NADPH for CO2
fixation. It contains Mn, Cl, Quencher Q, Plastoquinone cytochrome complex and
plastocyanin.
The relative quantiies of the two photosystems PSII and PSI in the chloroplasts
is about 1.5:1 respectively.
(b). Photolysis of water (Photocatalytic splitting of water):- Water is oxidized
according to following chemical reaction.
2H2O
4H+ + 4e- + 2O2
This equation indicates that four electrons are removed from two water
molecules generating an oxygen molecule and four hydrogen ions.
Water is very stable molecule. Oxidation of water to form molecular oxygen is
very difficult and the photosynthetic oxygen evolving complex is the only known
biochemical system that carries out this reaction.
The photochemically oxidized reaction centre chlorophyll of PSII. (P680+) is
the strongest biological oxidant known. The reduction potential of P680+ is more
positive than that of water and thus it can oxidize water to generate O2 + H+ ions.
The splitting of water which provides the electrons for reduction of P680+ in
PSII is catalyzed by a three protein complex The oxygen evolving complex located on
the laminar surface of the thylakoid membrane. The oxygen evolving complex contains
four Mn ions bound Cl and Ca ions.
The oxidation of two molecules of water to form O2 requires the removal of 4
electrons but absorption of each photon by PSII results in the transfer of just one
electron. PSII must loose an electron and then oxidize the O2 evolving complex four
times in a row for an O2 molecule to be formed. The electrons released from water are
transferred once at a time via the Mn ions and a nearby tyrosine side chain on the D1
subunit to the reaction centre P680+ where they regenerate the reduced chlorophyll
P680. The protons released from H2O remain in the thylakoid lumen and develop
proton motive force across the thylakoid membranes.
(c). Photoposphorylation:
It is the synthesis of ATP molecules in presence of light. It is of two types cyclic and
non cyclic
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B. Sc. 3rd Year Single Paper Scheme 2016
Non cyclic photophosphorylation (linear electron flow)
It involves PSII and PSI in an obligate series in which electrons are transferred from
water to NADP. This process begins with absorption of a photon by PSII causing an
electron to move from a P680 Chla to an acceptor plastoquinone (QB) on the stromal
surface. The resulting oxidized P680+strips one electron from the water after oxidizing it
to molecular oxygen protons and electrons. Protons which move in thylakoid lumen
contribute to proton motive force. After P680 absorbs a second photon of light the
semiquinone Q- accepts a second electron and picks up two protons from the stroma
generating QH2. After diffusing QH2 in the membrane binds to the Q0 site on the
cytochrome bf complex.
Cytochrome bf complex transfers electrons on, at a time to the CU2+ form of of
plastocyanin reducing it to Cu+ form. Reduced plastocyanin then diffuses in the
thylakoid lumen carrying electrons to P700+ in PSI, which has already got oxidized after
receiving the photons of light. The electrons excited in PSI can be transferred from
ferridoxin via the electron carrier FAD to NADP+ forming together with one proton
picked up from the stroma, the reduced molecule NADPH.
Protons are also transported into the lumen by the action of cytochrome bf complex
and contribute to the proton motive force. These protons must then diffuse to the ATP
synthetase enzyme where their diffusion down their electrochemical potential gradient
is used to synthesize ATP in the stroma.
Cyclic photophosphorylation
Reduced ferredoxin can donate two electrons to a Quinone (Plastoquinone) bound to a
site on stromal surface of PSI, the quinone then picks up two protons from the stroma to
form QH2. The QH2 then diffuses through the thylakoid membrane to Q0 binding site
on the luminal surface of the cytochrome bf complex. There it releases two electrons to
the cytochrome bf complex and two protons to the thylakoid lumen generating proton
motive force. As in linear electron flow these electrons return to PSI via plastocyanin.
A Q cycle operates in the cytochrome bf complex during cyclic electron flow, leading
to transport of two additional protons into lumen for each pair of electron transported
and a greater proton motive force.
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B. Sc. 3rd Year Single Paper Scheme 2016
The proton motive force generated during cyclic electron flow in chloroplasts
powers ATP synthesis by F0 to F1 complexes in the thylakoid membrane. This process
however generates no NADPH and no O2 is evolved.
The proton gradient developed will not allow continuing the electron transport
chain. According to law of thermodynamics the difference in charged particles between
two points is the source of energy. Thus this energy is known as proton motive force.
Overall gradient is known as electrochemical gradient. It is utilized to synthesize ATP
from ADP and Pi via a special H+ channel in thylakoid membrane known as ATP
synthetase. ATP synthetase is composed of two parts a hydrophobic membrane protein
CF0 and the protein that sticks out into the stroma called CF1. The CF0 contains proton
channel, when the protons are extruded from theCF0 component it rotates the CF1
component and during this step ATP is synthesized from ADP + Pi in the stroma
portion. The movement of charged particles from higher concentration to lower
concentration has been named as chemiosmosis by Robert Mitchell.
Linear electron flow in plants which requires both chloroplast photosystems PSI and
PSII
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B. Sc. 3rd Year Single Paper Scheme 2016
NonCyclic Photophosphorylation
Cyclic photophosphorylation
Dark phase or Dark reaction
It is the phase of CO2 fixation. It does not require light but requires assimilatory power
ATP and NADPH produced during photochemical phase for fixation and reduction of
CO2. The enzymes required for the process are present in the matrix or stroma of the
chloroplast. There are two main pathways for dark phase. 1. C3 or Calvin cycle 2. C4 or
Hatch and Slack cycle.
C3 cycle or Calvin cycle:
It is divided into three distinct phases. 1.Caboxylation 2.Glycolytic reversal 3.
Regeneration of RUBP. The various reactions occurring during the cycle are as under.
(1). To balance the overall reaction of the cycle let us start with 6 molecules of CO 2
combines with the 6 molecules of ribulose 1,5 biphosphate and 6 molecules of an
unstable intermediate compound 2 carboxy 3 keto ribitol 1, 5 biphosphate (B- keto
acid) is formed which splits into 12 molecules of 3 phosphoglyceric acid. This reaction
is catalysed by the enzyme rubisco (ribulose biphosphate carboxylase). This is the most
abundant protein on earth comprise about 16% of the chloroplast protein.
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B. Sc. 3rd Year Single Paper Scheme 2016
(2). 12 molecules of 3 phosphoglyceric acid are phosphorylated in presence of 12 molecules
of ATP and enzyme phosphoglycerokinase to form 12 molecules of 1, 3
diphosphoglyceric acid.
(3). 12 molecules of 1,3 diphosphoglyceric acid are now reduced in presence of 12
molecules of NADPH and enzyme triosephosphate dehydrogenase to form 12
molecules of 3 phosphoglyceraldehyde.
(4). 5 molecules of 3 phosphoglyceraldehyde are isomerised to 5 molecules of its isomer
dihydroxy acetone phosphate in presence of enzyme triosephosphate isomerase.
(5). 3 molecules of 3 phosphoglyceraldehyde undergo condensation with 3 molecules of
dihydroxy acetone phosphate to form 3 molecules of fructose 1, 6 diphosphate.
(6). 3 molecules of fructose 1, 6 diphosphate are dephosphorylated in presence of enzyme
phosphatase to form 3 molecules of fructose 6 phosphate.
(7). one of the molecules of fructose is converted to its isomer glucose 6 phosphate then
dephosporylated to glucoser the continuous operation of this cycle ADP + Pi and
NADP formed will be again utilized in the light reaction for the generation of ATP and
NADPH respectively. The CO2 needed in the cycle will be absorbed from the
atmosphere and Ribulose 1,5 biphosphate will be regenerate from the 4 molecules of 3
phosphoglyceraldehyde, 2 molecules of dihydroxy acetone phosphate and 2 molecules
of fructose 6 phosphate left behind in C3 cycle.
The various reactions involved in the regeneration of RUBP are as under.
(1). 2 molecules of 3 phosphoglyceraldehyde combine with the 2 molecules of fructose 6
phosphate in presence of enzyme transketolase to form 2 molecules of xylulose 5
phosphate and 2 molecules of erythrose 4 phosphate.
(2). 2 molecules of erythrose 4 phosphate combine with 2 molecules dihydroxy acetone
phosphate in presence of enzyme aldolase to form 2 molecules of sedoheptulose 1,7
diphosphate.
(3). 2 molecules of sedoheptulose 1,7 diphosphate are dephosphorylated to form 2
molecules of sedoheptulose 7 phosphate in presence of enzyme phosphatase.
(4). 2 molecules of sedoheptulose 7 phosphate condence with 2 molecules of 3
phosphoglyceraldehyde to form 2 molecules of xylulose 5 phosphate and 2 molecules
of ribose 5 phosphate.
(5). All the 4 molecules of xylulose 5 phosphate are isomerised to 4 molecules of ribulose 5
phosphate.
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B. Sc. 3rd Year Single Paper Scheme 2016
(6). 2 molecules of ribose 5 phosphate are isomerised to 2 molecules of ribulose 5
phosphate.
(7). All the 6 molecules of ribulose 5 phosphate are phosphorylated in presence of 6 ATP
molecules to get converted into 6 molecules of ribulose 1,5 biphosphate.
Overall reaction
6CO2 + 12 NADPH + 18 ATP + 11 H2O
Fructose 6 phosphate + 12 NADP + 12 ADP
This is called C3 cycle because first stable product formed is the carbon 3
compound 3 phosphoglyceric acid. It is called calvin cycle after name of its discoverer
Melvin calvin. Energy of 6,86,000 calories per molecule glucose is stored . This energy
is provided by a total of 18 ATP and 12 NDPH molecules, which represent 7,50,000
calories. The efficiency reached by the dark cycle is thus as high as 90 %.
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C3 cycle or Calvin cycle
Photorespiration and glycolic acid metabolism:
Photosynthesis is believed to have evolved in an atmosphere much richer in CO2 than it
is today and in relatively little O2 probably about 0.02% oxygen compared with 21 %
today.Since 1920 it has been known O2generally inhibits photosynthesis and the reason
for this was discovered in 1971. It was shown that the CO2 fixing enzyme rubisco will
accept not only CO2 but also O2 as a substrate. The two gases compete infact for the
same active site. If O2 is accepted by Rubisco the following reaction is catalysed.
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B. Sc. 3rd Year Single Paper Scheme 2016
RUBP
(1). O2 + RUBP
(5c)
Phospoglycolate + 3 phosphoglyceric acid
Oxygenase
(2c)
(3c)
If CO2 is accepted the following reaction is catalysed
RUBP
(2). CO2 +
RUBP
(5c)
3 phosphoglyceric acid
Carboxylase
(3c)
First reaction is called Oxygenation the same enzyme is therefore called RUPB
oxygenase. Second reaction is carboxylation and the enzyme is called RUBP
carboxylase. The enzyme is
always called ribulose biphosphate carboxylase –
oxygenase or RUBISCO. In reaction (1) one molecule of each 3 phosphoglyceric
acid and 2 phosphoglycolate are formed Instead of two GP molecules as in
reaction(2). Phosphoglycolatae (phophoglycolic acid) is converted immediately to
glycolate (glycolic acid) by removal of phosphate group in presence of enzyme
phosphatase.
The plants have the problem of what to do with the Glycolate and the
pathway which deals with it called photorespiration which is defined as a light
dependent uptake of O2 and giving out of CO2. The function of photorespiration is to
recover some of the carbon from the excess glycolate. It was discovered by Decker and
Tio in 1959. It is exhibited by plants like wheat, rice, legumes, sugar cane and maize.
Glycolate now leaves the chloroplast and moves into peroxisomes where it is
oxidized into glyoxylate in presence of enzyme glycolate oxidase and then aminated to
amino acid glycin in presence of enzyme aminotransferase.H2O2 (Hydrogen peroxide)
formed is converted back into water and O2 by enzyme catalase.
Two molecules of glycin interact inside the mitochondria to form a molecule of
serine, CO2 and ammonia is released in this process. The amino acid serine now enters
peroxisomes, where it is again deaminated to form glyceric acid which is again
converted to phosphoglyceric acid in chloroplast.
The pathway obviously requires close cooperation of biochemical activities among
three organelles, the chloroplast, the peroxisomes and the mitochondria. Remarkably
electron micrograph does show these three organelles very closely appressed to each
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B. Sc. 3rd Year Single Paper Scheme 2016
other indicating that there is indeed some important functional relationship among
them.
The pathway serves to recycle three carbon atoms (entering up as PGA) out of
the 4 carbon atoms i.e. 2 molecules of glycolate. There is loss of one of them as CO2. It
reduces the potential yield of C3 plants by 30% - 40%. The photorespiration occurs only
in the C3 plants.
However C4 plants have overcome the problem of photorespiration by performing
calvin cycle in the interior of leaves (bundle sheath cells) where both temperature & O2
are lower. They have further ensured high CO2 supply to cells performing Calvin cycle.
C4 Cycle or Hatch and Slack Cycle:
Kortschak
Harth
and
Burr
(1965)
14
CO2 that
demonstrated with the use of
in sugarcane leaves the chief labeled
synthesized products are C4 dicarboxylic
acids
like
mallate,
aspartate
Oxaloacetate. This observation was
confirmed by M.D Hatch and C. R.
Slack (1966) Later on these observations
have
been
confirmed
in
monocotyledonous plants like Zea mays,
Sorghum and a dicot Amaranthus etc. It
is also called B- Carboxylation pathway
and Cooperative photosynthesis. The
first stable compound of Hatch and
Slack cycle is 4 carbon oxaloacetic acid
Therefore it is called C4 cycle and the
plants are called C4 plants. Hatch and Slack cycle is completed in the chloroplast of
mesophyll cells and bundle sheath cells. Following reactions occur during this cycle.
In the mesophyll cells the CO2 acceptor is phosphoenol pyruvic acid (PEP)
instead of RUBP & the enzyme is PEP carboxylase instead of RUBP carboxylase. PEP
carboxylase has two advantages over RUBP carboxylase.
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B. Sc. 3rd Year Single Paper Scheme 2016
C4 cycle
1. It has much affinity for CO2.
2. It does not accept O2 & hence does not contribute to photorespiration.
Reactions occurring in the chloroplasts of mesophyll and bundle sheath cells are.
1. Phosphoenol pyruvic acid combines with CO2 in presence of PEP Carboxylase & forms
4 Carbon acid, oxaloacetic acid.
2. Oxalo acetic acid is quite unstable and is converted into mallic acid with the help of
NADPH and in presence of enzyme mallic dehydrogenase.
3. Mallic acid is now transported to bundle sheath cells it is decarboxylated to form
pyruvic acid and CO2. This reaction is aided by mallic enzyme. Here the conc. Of CO2
is increased so calvin cycle will start.
4. Pyruvic acid is then transported to mesophyll cells here it gets converted to PEP on the
expenditure of ATP.
Characters of C4 Plants
1. The leaves of C4 plants possess special anatomy called Kranz anatomy. The leaves of
C4 plant vascular bundles remain surrounded by bundle sheath containing chloroplasts
in abundance. The bundle sheath is surrounded by 1-3 layers of mosophyll cells which
posses very small intercellular spaces.
2. The chloroplasts of C4 plants are dimorphic. The chloroplasts of mesophyll cells of
normal type, but the chloroplasts of bundle sheath are comparatively quite larger in
size without grana and PSII.
3. C4 cycle is performed in mesophyll cells while C3 in the bundle sheath cells.
4. Two types of carboxylase:- PEP carboxylase in mesophyll cells and RUBISCO in
bundle sheath cells.
5. C4 plants are found in tropical and sub tropical regions.
6. They grow fast at high temperature and in more light intensities so called efficient
plants. 7. The optimum temperature required for their growth varies from 30-400C.
Sigificance of C4 cycle
1. In C4plants it increases the photosynthetic yield two to three times more than C3 plants.
2. In C4 plants, it performs a high rate of photosynthesis even when the stomata are nearly
closed.
3. It increases the adaptability of C4 plants to high temperature and high intensities.
4. It increases the rate of CO2 fixation at 25-300c as compared to C3plants
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B. Sc. 3rd Year Single Paper Scheme 2016
5. In C4 plants the O2 cannot have inhibitory effect. They lack photorespiration.
CAM Pathway ( Crassulacean Acid Metabolism):
Despite its name, CAM is not restricted to the family crassulaceae (Crassula,
Kalanchoe); it is found in numerous angiospermic families, Cacti and euphorbias are
CAM plants, as well as pineapple, Vanilla and agave.
The CAM mechanism enables plants to improve water use efficiency.
Typically, a CAM plant loses 50 to100 g of water for every gram of Co2 gained,
compared with values of 250 to 300 g and 400 to 500 g for C4 and C3 plants
respectively. Thus CAM plants have a competitive advantage in dry environment.
In CAM plants the stomata remain closed during the day and the CO2 is fixed
during the night which is captured by PEP carboxylase in the cytosol and is fixed in
organic acids Low temperature of 10-15 0C favours it. The acceptor of the CO2 is
phosphoenol pyruvate result is the formation of oxaloacetate which is reduced to
mallate. The malate accumulates and is stored in the large vacuoles that are a typical
but not obligatory, anatomical feature of the leaf cells of CAM plants. This is called as
the nocturnal acidification of leaf
With the onset of day the stomata close, preventing loss of water and
further uptake of CO2. The leaf cells deacidify as the reserves of vacuolar mallic acid
are consumed. Decarboxylation is usually achieved by the action of NADP malic
enzyme on mallate, because the stomata are closed , the internally released CO2 cannot
escape from the leaf and instead is fixed and converted to carbohydrate by the calvin
cycle.
The elevated internal concentration of CO2 effectively suppresses the
photorespiratory oxygenation of ribulose biphosphate and favours carboxylation.
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B. Sc. 3rd Year Single Paper Scheme 2016
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iii. Respiration
ATP –The biological energy currency
Adenosine triphosphate (ATP) is a nucleotide that consists of an adenosine and is
composed of an adenine ring a ribose sugar linked to three sequential phsphoryl groups
(-PO32-) via a phospodiester bond and two phospho anhydride bonds . The phosphate
groups are usually called as alpha, beta and gamma phosphates Gamma phosphate
group is the primary phosphate group on the ATP molecules that is hydrolyzed when
the energy is needed to drive anabolic reactions . ATP is the most abundant nucleotide
in the cell and the primary cellular energy currency in all life forms.
ATP is also known as adenosine 5/- triphosphate. It is formed from adenosine
diphosphate (ADP) and orthophosphate (PI). When fuel molecules are oxidized in
chemotrophs or when light is trapped by phototrophs.
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B. Sc. 3rd Year Single Paper Scheme 2016
ATP is very soluble in water and is quite stable solution that has a pH of 6.8-7.4, but is
rapidly hydrolyzed at extreme pH. Thus ATP is best stored as anhydrous salt
Although ATP is quite stable in solution . It is an unstable molecule in
unbuffered water. This is because once ATP gets in contact with unbuffered water it
hydrolyses to ADP and phosphate due to the strength of the bonds between the
phosphate group. ATP is commonly seen to be less than the strength of the hydrogen
bonds between its products (ADP + Phosphate and water.)
Generally ATP is connected to another reaction a process called coupling which means
the two reactions occur at the same time and at the same place, usually utilizing the
same enzyme complex. Release of phosphate from ATP is exothermic (a reaction that
gives off heat) and the reaction it is connected to is endothermic (requires energy input
in order to occur). The terminal phosphate group is then transferred by hydrolysis to
another compound, a process called phosphorylation, producing ADP, phosphate (Pi)
and energy.
The self-regulation system of ATP
The high energy bonds of ATP are actually rather unstable bonds. Because they are
unstable, the energy of ATP is readily released when ATP is hydrolyzed in cellular
reactions. Note that ATP is an energy-coupling agent and not a fuel. It is not a
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B. Sc. 3rd Year Single Paper Scheme 2016
storehouse of energy set aside for some future need. Rather it is produced by one set of
reactions and is almost immediately consumed by another. ATP is formed as it is
needed, primarily by oxidative processes in the mitochondria. Oxygen is not consumed
unless ADP and a phosphate molecule are available, and these do not become available
until ATP is hydrolyzed by some energy-consuming process ATP is not excessively
unstable, slow in the absence of a catalyst. This insures that its stored energy is
“released only in the presence of the appropriate enzyme”
Functions ofATP
The ATP is used for many cell functions including transport work moving substances
across cell membranes. It is also used for mechanical work, supplying the energy
needed for muscle contraction. It supplies energy not only to heart muscle (for blood
circulation) and skeletal muscle (such as for gross body movement), but also to the
chromosomes and flagella to enable them to carry out their many functions. A major
role of ATP is in chemical work, supplying the needed energy to synthesize the multithousands of types of macromolecules that the cell needs to exist.
ATP is also used as an on-off switch both to control chemical reactions and to
send messages. The shape of the protein chains that produce the building blocks and
other structures used in life is mostly determined by weak chemical bonds that are
easily broken and remade. These chains can shorten, lengthen, and change shape in
response to the input or withdrawal of energy. The changes in the chains alter the shape
of the protein and can also alter its function or cause it to become either active or
inactive.
The ATP molecule can bond to one part of a protein molecule, causing another
part of the same molecule to slide or move slightly which causes it to change its
conformation, inactivating the molecule. Subsequent removal of ATP causes the
protein to return to its original shape, and thus it is again functional. The cycle can be
repeated until the molecule is recycled, effectively serving as an on and off switch
(Hoagland and Dodson, 1995, p.104). Both adding a phosphorus (phosphorylation) and
removing a phosphorus from a protein (dephosphorylation) can serve as either an on or
an off switch.
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Respiration is the process by which cells convert food in energy. The food from which
energy is released is mainly glucose. There are two types of respiration aerobic and
anaerobic
Aerobic respiration
Aerobic respiration uses Oxygen to get the energy from food. The products of aerobic
respiration are CO2 and H2O. There are two stages of aerobic respiration.
1. Glycolysis
2. Krebs cycle
Glycolysis:
It is defined as the sequence of reaction converting glucose to pyruvate, with the
production of ATP. Glycolysis occurs in the cytoplasm of virtually all living cells, both
in the absence and presence of O2. Probably it was the first energy releasing process in
organisms when the life evolved.
The oxidative respiration in mitochondria of eukaryotic cells became possible
only after molecular oxygen had accumulated in the earth’s atmosphere as a result of
photosynthesis by cyanobacteria.
Reaction of glycolysis
1. Glucose is phosphorylated to glucose 6
phosphate by enzyme hexokinase. This is a
Mg+2 activated enzyme.
2.
Glucose 6 phosphate is isomerised by
enzyme glucose 6 phosphate isomerase into
fructose 6 phosphate.
3. Fructose 6 phosphate is phosphorylated by
enzyme phosphofructokinase to fructose 1, 6
diphosphate.
4. Fructose 1, 6 diphosphate is cleaved by
enzyme aldolase into two interconvertible
triosephosphates,
Glyceraldehyde
3
phosphate and Dihydroxy acetone phosphate.
5.
The enzyme triosephosphate isomerase
catalyses the reversible inter conversion of glyceraldehyde 3 phosphate and
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B. Sc. 3rd Year Single Paper Scheme 2016
dihydroxyacetone phosphate, thus two molecules of glyceraldehyde 3 phosphate are
obtained from one molecule of glucose.
6. Glyceraldehyde 3 phosphate is oxidized in presence of enzyme glyceraldehydes 3
phosphate dehydrogenase to 1, 3 biphosphoglycerate, it is simultaneously
phosphorylated by inorganic phosphate. Here 2NAD is changed to 2NADH.
7.
1,3,
biphosphoglycerate
is
dephosphorylated
in
presence
of
enzyme
phosphoglycerate kinase to 3 phosphoglycerate and ATP is changed to ADP.
8. Now the phosphate group changes its position from 3 to carbon 2 catalysed by
enzyme phosphoglyceromutase.
9. 2, phosphoglycerate is changed to phosphoenol pyruvate (PEP) by enzyme enolase
containing high energy enol phosphate.
10. Phosphoenol pyruvate is dephosphorylated to pyruvate in presence of enzyme
pyruvate kinase. Here ADP is changed to ATP.
In glycolysis there is a net formation of two ATP molecules and 2 NADH
molecules which in ETC oxidative phosphorylation release 6 molecules of ATP. So
total gain of ATP molecule is glycolysis is 8 i.e. one NADH molecule produces 3 ATP
molecules in ETC.
What happens to pyruvaye that is produced in glycolysis. The fate of pyruvate
varies from cell to cell and its environment. In cells having mitochondria and adequate
supply of oxygen, pyruvate is oxidized in TCA cycle. In the absence of oxygen
pyruvate is reduced by NADH to either lactate or alcohol. Such anaerobic routes are
called fermentation pathways.
Krebs Cycle (Oxidative Decarboxylation of Pyruvate)
Glycolysis occurs in the cytoplasm of the cell but pyruvate is metabolized
aerobically in the mitochondria. The pyruvate is transported to the mitochondria either
by simple diffusion or by the pyruvate – hydroxyl ion antiport system where pyruvate is
exchanged for a hydroxyl ion.
The pyruvate is oxidatively decarboxylated by the multienzyme complex called
the pyruvate dehydrogenase consisting of multiple copies of three enzymes ( E1. E2, E3)
each with specific binding site for the substrate and different cofactors. Since this
reaction links glycolysis with TCA it is also termed as link reaction. Pyruvate in this
reaction is changed to acetyle COA with the removal of CO2 and a pair of hydrogen
atoms. The hydrogen atoms released combine with NAD forms NADH.
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B. Sc. 3rd Year Single Paper Scheme 2016
Tricarboxylic Acid Cycle
Discovered by Hans Kreb therefore it is also called Krebs cycle. Krebs cycle is the
most important metabolic pathway for the energy supply to the body. About 65 – 75 %
of the ATP is synthesized in Krebs cycle. This cycle utilizes about two thirds of total
oxygen consumed by the body. The name TCA cycle is used, since at the onset of
cycle, tricarboxylic acids citrate and isocitrate participate
Acetyl CoA now enters the Krebs cycle. Each step is catalysed by a specific
enzyme. The various reactions occurring in the cycle are.
1. Acetyl CoA , condences with oxaloacetate (4–carbon compound )to form citric acid (
citrate) a 6-Carbon compound . CoA is liberated.
2. Citrate is converted into isocitrate by rearrangement of atom groups by the enzyme
aconitase.
3. Isocitrate gives of a pair of H atoms in presence of enzyme isocitrate dehydrogenase
to form oxalosuccinate, and NAD is changed to NADH.
4.
Oxalosuccinat in
presence of isocitrate
dehydrogenase loses a
molecule of CO2 and
forms ∞ Ketoglutarate
(5 C ). The pair of H
atoms passes to NAD
forming NADH. (the
enzyme
isocitrate
dehydrogenase
catalyses the reaction )
5.
∞ ketoglutarate is
transformed to succinyl
CoA
(4 C ) . This
reaction involves CoA
and NAD, the products
being CO2, NADH in
addition
CoA.
to
(With
succinyl
this
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B. Sc. 3rd Year Single Paper Scheme 2016
reaction all the C atoms that entered into citric acid cycle as pyruvic acid are released as
CO2 ).
6. Succinyl CoA is acted upon by enzyme (Succinyl thiokinase ) to form succinate.
This reaction releases sufficient energy to form ATP (in plants) or GTP (in animals).
7. Succinate undergoes dehydrogenation to form fumarate (by the enzyme succinate
dehydrogenase ). In this reaction FADH2 (reduced flavin adenine dinucleotode ) is
produced.
8. A molecule of water gets added to fumarate to form malate in presence of enzyme
Fumarase.
9. Malate is dehydrogenated (or oxidized) to produce oxaloacetate in presence of
enzyme Malate dehydrogenase. The pair of H atoms passes to NAD forming NADH2.
Oxaloacetate combines with another molecule of acetyl CoA to repeat the cycle.
TCA cycle – the Central Metabolic Pathway
The citric acid cycle is the final common oxidative pathway for carbohydrates fats
and amino acids. This cycle not only supplies energy but also provides many
intermediates required for the synthesis of amino acid, glucose, heame etc. Krebs cycle
is the most important central pathway connecting almost all the individual metabolic
pathways.
Energetics of Citric Acid Cycle
During the process of oxidation of acetyl CoA via citric acid cycle 4 reducing
equivalents (3 as NADH and one as FADH2) are produced. Oxidation of one NADH by
electron transport chain coupled with oxidative phosphorylation results in the synthesis
of 3ATP, where as FADH2 leads to the formation of 2 ATP.
Total ATP molecules produced during respiration:In Glycolysis
Direct
=
2
2 molecules of NADH
=
6
Total:
=
8
=
6
In Link reaction:
Pyruvic acid to Acetyle co-A
2 molecules of NADH
Citric acid cycle
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6NADH
=
18
2FADH2
=
4
Direct
=
2
Total
=
24
Grand total
=
38
Krebs cycle is both catabolic and anabolic in nature. Hence regard as amphibolic.
Anaerobic respiration:
In anaerobic respiration oxygen is not needed although it can take place in presence of
oxygen. It is also called as fermentation. In anaerobic respiration glycolysis takes place
and the pyruvate in absence of oxygen is converted into other products which are
alcohol or lactic acid called alcoholic fermentation and lactate fermentation
respectively
Alcoholic fermentation:- During this process at first glucose is converted into
pyruvate. Pyruvate in the presence of enzyme pyruvate decarboxylase is converted into
acetaldehyde one molecule of CO2 is liberated in this reaction.
Pyruvate decarboxylase
Pyruvate
Acetaldehyde + CO2
In the 2nd reaction acetaldehyde is reduced to ethyl alcohol in presence of enzyme
alcohol dehydrogenase. At this step one molecule of NADH is oxidized to NAD.
Alcoholdehydrogenase
Acetaldehyde
Ethyl alcohol
It can occur in any sugar solution. The fruit juices show alcoholic fermentation
when yeast powder is added or the juice is left as such open in air.
Lactic acid fermentation
It is carried out by lactic acid bacteria. Pyruvate is reduced in presence of enzyme
pyruvate dehydrogenase in presence of Zn2+ & FMN ( Flavin mononucleotide ) to
lactic acid. One glucose molecule gives two lacticacid molecules CO2 is not involved in
this reation.
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In muscle cells,Pyruvate is also changed into lactic acid which is carried to liver
through blood supply, where it is again converted into glucose and enters the blood.
Energy yield in fermentation
Fermentation yields only about 5% of the energy obtained by aerobic respiration. This
small amount of energy is sufficient to maintain the life of organisms. Such as yeasts,
many bacteria and other anaerobes.
Net gain of ATP in Glycolysis
=8
Loss of ATP in fermentation
=6
Balance ATP
=2
Importance of fermentation:
1. It supplements the energy provided by aerobic respiration during intense muscular
activity.
2. Brewing industry produces beer, wines by fermenting sugar solution with yeasts.
3.
Baking industry uses CO2 released by yeast cells in alcoholic fermentation in
raising the dough and making bread spongy.
4. Dairy industry produces yogurt, cheese and butter by fermenting milk sugar lactose
into lactic acid by strepto coccus lacti. Lactic acid coagulates the milk protein casein
and fuses droplets of milk fat.
5.
Tea and Tobacco leaves are cured (freed of bitterness and important pleasant
flavour) by fermentation with certain bacteria ( Bacillus megatherium ).
6.
Vinegar is produced by fermenting molasses with yeast to ethyl alcohol which is
oxidized to acetic acid by aerobic bacteria Acetobacter aceti.
7. Butyl alcohol and acetone are manufactured from molasses by fermentation with
bacteria Clostridium acetobutylicum
8. Fermentation is used for cleaning hides.
9. Retting of fibers by Pseudomonas.
10. Ensilage a nutrient fodder for cattle is prepared by fermentation with bacteria in air
tight chambers.
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Electron Transport Chain (Structural Organization of Respiratory Chain):
The inner mitochondrial membrane can be disrupted into five distinct respiratory or
enzyme complexes denoted as Complex I, II, III, IV, and V
The Complex I - IV are carriers of electrons while Complex V is responsible for
ATP synthesis.
Besides these enzyme complexes there are certain mobile electron carriers in the
respiratory chain. These include NADH, Coenzyme Q, Cytochrome C and Oxygen
The enzyme complexes ( I - V) and the mobile carriers are collectively involved
in the transport of electrons which ultimately combine with Oxygen to produce water.
Complex –I (NADH dehydrogenase)
Electrons from NDH generated in the mitochondrial matrix during the citric acid cycle
are oxidized by complex I. The electron carriers in the Complex I include a tightly
bound cofactor FMN and several Iron Sulphur proteins. Complex I then transfers these
electrons to ubiquinone. Four protons are pumped from the matrix to the
intermembrane space for every electron pair passing through the complex
Ubiquinone a small lipid soluble electron and proton carrier is located within the
inner membrane. It is not tightly associated with any protein and it can diffuse within
the hydrophobic core of the membrane bilayer
Complex II ( Succinate dehydrogenase): Oxidation of the succinate in the citric acid
cycle is catalyzed by this complex and the reducing equivalents are transferred via the
FADH2 and a group of iron sulphur protein into ubiquinone pool. This complex does
not pump protons.
Succinate + UQ → Fumarate + UQH2
Complex III (Cytochrome bc1 Complex)
This complex oxidizes reduced ubiquinone and transfers the electrons to cytochrome C
complex via an iron sulphur centre, two b type cytochromes (b565 and b560) and a
membrane bound cytochromr C1. Four protons per electron pair are pumped by
Complex III.
Cytochrome C is a small protein loosely attached to the outer surface of the inner
membrane and serves as mobile carrier to transfer electrons between complex III and
IV.
Complex IV( Cytochrome oxidase)
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This complex contains two copper centres (CUA and CUB) and cytochrome a and a3. It
is the terminal oxidase and brings about the reduction of O2 to two molecules of water.
Two protons are pumped per electron pair to intermembrane space of Mitochondriet of
electron et a.
In addition to set of electron carriers described above the plant mitochondria
contain two NAD(P)H dehydrogenase both Ca2+ dependent attached to outer surface of
the inner membrane facing the intermembrane space can oxidize NADPH. Electrons
from these external NADPH enter into main electron transport chain at the level of the
ubiquinone pool.
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Oxidative Phosphorylation: (Chemiosmotic Hypothesis):
Chemiosmotic hypothesis of ATP synthesis has been given by Peter Mitchell in 1961 is
now widely accepted concept of Oxidative phosphorylation. The concept is comparable
with energy stored in a battery separated by positive and negative charges.
Proton gradient
The inner mitochondria membrane as such is impermeable to protons (H+) and
hydroxyl ions (OH-).The transport of electrons through ETC is coupled with the
translocation of protons (H+) across the inner mitochondrial membrane (coupling
membrane) from the matrix to the intermembrane space. The pumping of protons
results in an electrochemical or proton gradient. This is due to the accumulation of
more H+ ions (low Ph) on the outerside of the inner mitochondrial membrane than the
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inner side. The proton gradient developed due to the electron flow in the respiratory
chain is sufficient to result in the synthesis of ATP.
The electron complexes I, II, III, IV. Acting as electron carriers also act as
proton pumps. They pump out protons across the inner membrane from matrix to
intermembrane space, because innermitochondrial membrane space is impermeable to
protons, a proton electro chemical gradient or a proton motive force is built up . The
intermembrane space becomes acidic and positively charged while the inner side of the
inner membrane i,e matrix becomes alkaline and negatively charged. A total of 10
protons are pumped out across the innermitochondrial membrane,
The free energy now stored in proton electrochemical gradient or proton motive
force can be used to carry chemical work i,e synthesis of ATP from ADP + P I . This is
accomplished through phosphorylating complex which are knob like structures situated
on Cristae in Mitochondria called F0 F1 – ATP synthetase
The F0 F1 – ATP synthetase complex (complex (V) consists of two major
components F1 and F0. The F is a peripheral membrane protein complex that is
composed of at least five different sub units three α three
one
one
and one
and
contains the catalytic site for converting the ADP and P I to ATP. This complex is
attached to the matrix side of the intermembrane. F0 is an integral membrane protein
complex that consists of at least three different polypeptides that form the channel
through which protons cross the innermembrane. For each ATP synthesized 3H+ pass
through the F0 from the intermembrane space to the matrix down the electrochemical
proton gradient.
A high resolution X ray structure shows that most of the FI complex of the
mammalian Mitochondria ATP synthesis supports a rotational mode for the catalytic
mechanism of ATP synthesis.
Rotational model
F0 F1 – ATP synthetase also called-ATPase is the worlds smallest rotary model
Paul Boyer (1997) at university of California Los Angels proposed the binding
change mechanism and supported rotational model for the catalytic mechanism of ATP
synthesis experimentally. The main points of the mechanism are
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1. The substrate is bound and products are released at three separate but interacting
catalytic sites, corresponding to the three catalytic sub units ( sub units ) each catalytic
site can exit in one of the three conformations tight, loose and open.
T state binds ADP + PI so tightly that they spontaneously from ATP and that binds ATP
very strongly.
L state binds ADP and PI more strongly.
O state that binds ATP poorly and ADP + PI
β. The binding changes are coupled to proton transport by rotation of
sub units . That
is the flow of proton down their electrochemical gradient through Fo complex causes
the sub unit to rotate.
γ. Rotation of the
sub unit that brings about the conformational changes in the
catalytic complex which does not rotate as is attached to the membrane. The
is located on the outside of the
sub unit
sub unit which anchors catalytic complex to the
membrane and prevents it from spinning.
A final rotation of returns the
Sub unit to O State, thereby releasing ATP and
beginning the cycle again.
Evidances in support of chemiosmotic hypothesis
1. The intact inner mitochondrial membrane is responsible for oxidative
phosphorylation.
2. The inner mitochondrial membrane is impermeable to proton (H+) and other ions
(OH-, K+, Cl-) etc
3. ATP can be generated by increasing the H+ ion concentrationon the outside of
mitochondria.
4. Certain compounds which increase the permeability of inner mitochondrial
membrane to protons inhibit ATP synthesis e.g. 2, 4, dinitrophenol.
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Binding Change Mechanism of ATP synthesis
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iv. Growth and development
Differentiation and Morphogenesis (elementary idea) phases of growth; concept of
phasic development; kinetics of growth
Growth, Differentiation and Morphogenesis:
Consider a typical annual weed. It may consist of billions of cells-some large, some
small, some highly specialized and others not, but all derived from a single fertilized
egg.
The growth is a permanent and irreversible change in the size of an organism or organ
or a cell usually accompanied by increase in dry weight. Development is the sum of all
the changes that progressively elaborate an organism‘s body. A plant‘s continuous
growth and development depend on processes that shape organs and generate specific
patterns of specialized cells and tissues within these organs. The development of body
form
and
organization,
including
recognizable
tissues
and
organs
is
called morphogenesis.
The specialization of cells with the same set of genetic instructions to produce a
diversity of cell types is called differentiation.
The diverse cell types of a plant, including guard cells, sieve-tube members, and xylem
vessel elements, all descend from a common cell, the zygote, and share the same DNA.
Cellular differentiation depends, to a large extent, on the control of gene expression.
Cells with the same genomes follow different developmental pathways because they
selectively express certain genes at specific times during differentiation.
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Each specialized cell type in an organism expresses a subset of all the genes that
constitute the genome of that species. Each cell type is defined by its particular pattern
of regulated gene expression. Cell differentiation is thus a transition of a cell from one
cell type to another and it involves a switch from one pattern of gene expression to
another. Cellular differentiation during development can be understood as the result of
a gene regulatory network. The major types of molecular processes that control cellular
differentiation involve cell signaling. Many of the signal molecules that convey
information from cell to cell during the control of cellular differentiation are
called growth factors. Growth in plants mainly occurs by the activity of meristematic
tissue. The new cells are continuously produced by cell division in a meristem. The
meristematic cells are located at the apex of every root and shoot known as root apex
and shoot apex respectively. Some plants like grasses, there are intercalary meristems at
the bases of internodes. The vascular cambium is another kind of meristem which by its
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activity causes increase in girth. The daughter cells derived from cell division in the
meristems, enlarge and bring about visible increase in size of the plant. The expanding
cells differentiate to form the different tissues and become part of the mature organ.
New cells form continuously at the apical meristems. Cells enlarge slowly in the apical
meristem and more rapidly in the subapical regions. The resulting increase in cell size
can range from several-fold to 100-fold, depending on the species and environmental
conditions. The increase in mass, or growth, that occurs during the life of the plant
results from both cell division and cell expansion, but what controls these processes?
Tissue growth is neither uniform nor random. The derivatives of the apical meristems
expand in predictable and cite-specific ways and the expansion patterns in these subapical regions largely determine the size and shape of the primary plant body.
The total growth of the plant can be thought of as the sum of the local patterns of
cell expansion, while morphogenesis (the development of body form and organization)
is the sum of patterns of both cell division and cell expansion
Plants are unique multicellular organisms that retain the capacity for unlimited
growth that is an overall increase in size, throughout their lives. They are able to do so
because they have meristems at certain locations in the body. Meristems are composed
of stem cells, which perpetuate themselves by cell divisions and also give rise to
derivative cells, which differentiate along new lines. As a result of meristematic
activity, fresh tissues and organs are
formed and the plant continues to
grow in height and in many cases
girth throughout its life. This form
of growth in plants is called as open
form of growth. The advantages of
this type of growth for organisms
that are rooted but subject to
predation are immediately obvious.
Meristematic system
Plant meristems are classified on
the basis of location. Based on their
location three different types of
meristems are:
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a) Apical meristem
Located at or near the tip of shoot and root and are called shoot and root apical
meristems; they are responsible for primary growth, including elongation of these
organs. Initiation of growth is by one or more cells situated at the tip of the organ which
maintain their individuality and position and are known as apical initials or apical cells.
These cells may be strictly terminal or subterminal.
b) Lateral meristem
Lateral meristems are located on the sides of roots or stems and add to the girth or
secondary growth of these organs. Vascular cambium and cork cambium are the
examples of lateral meristems. Lateral meristems are composed of initials that divide
chiefly in one plane (periclinally) and increase the diameter of an organ.
c) Intercalary meristem
Intercalary meristem typically occurs intercalated between mature regions, as at the
bases of grass leaves; however in a broader sense, they can be considered to occur at
the bases of leaf primordia. And above the nodes in stems of nearly all plants and add
to elongation growth of these organs.
Shoot and root apical meristems and vascular cambium remain active for the life of
the plant; hence, they are referred to as indetermediate meristems where as meristems
involved in leaf and flower development are active only for a short time and are used
up in the formation of those organs. They are called determinate meristems.
Phases of growth:
There are three phases of growth viz., Cell division, Cell enlargement and Cell
maturation.
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1) Cell division: A cell is metabolically highly active at the time of cell division. Its
cellular mass increases and a replication of genetic material (i.e., nucleic acid) take
place. Growth, as a result of division, is based on mitotic cell division. In the stages of
mitosis each chromosome splits lengthwise into two homologous chromatids, which
pass equally into daughter cells. Certain cell organelles, viz., chloroplast and mitochondria apparently duplicate themselves. As a result of division, each of the two daughter
cells is only half the size of the parent cell. These cells then proceed to
enlarge and synthesize their own
cytoplasm and cell wall material.
The
plane
(Direction)
and
symmetry of cell division are very
important
in
determining
plant
form. The plane in which a cell
Preprophase bands of
microtubules
divides is determined during late
interphase. The first sign of this
spatial
orientation
is
a
Nuclei
rearrangement of the cytoskeleton.
Microtubules
in
the
Cell plates
cytoplasm
become concentrated into a ring
called the preprophase band. The
band disappears before metaphase but it predicts the future planes of cell division. The
imprint consists of an ordered array of actin microfilaments that remain after the
microtubules disperse.
2) Cell enlargement
Cell division is followed by cell enlargement before the cell maturation takes place.
The cells increase in size mainly due to vacuolation, i.e., by absorption of water. A big
central vacuole appears which pushes cytoplasm to be limited to a thin boundary layer
against the cell wall. The new cell wall material is synthesised to cope with the
enlargement. It appears that cell enlargement due to vacuolation is governed by
osmosis. Since the diffusion pressure deficit (DPD) is equal to osmotic pressure (OP)
minus the wall or turgor pressure (TP), i.e., DPD= OP-TP, the water uptake may
involve either increased osmotic pressure or decreased turgor pressure or both.
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Considerable evidences suggest that wall pressure is reduced in vacuolating cells due to
increased plasticity of cell walls. It results absorption of water from neighbouring cells.
Not only the absorption of water, but the synthesis of new cytoplasm and cell wall
material also accompanies the cell enlargement.
3) Cell maturation
Cellular differentiation followed by cell division and enlargement leads to the
development of specialised mature tissues. For example, the meristematic cells of leaf
epidermis differentiate to form guard cells, trichomes, glands or epidermal cells. The
differentiation and maturation of various kinds of cells depend on the number of
internal, external, nutritional, genetic and hormonal factors. Thus, it is the final
differentiation processes in biological systems, such as the final ripening of a seed or
the attainment of full functional capacity by a cell, a tissue, or an organ.
Growth Kinetics:
Growth kinetics is actually the study of the growth and development of one part of an
organism in relation to another. Many growth phenomena in nature exhibit a
logarithmic or exponential increase. The size, mass or number increases by a constant,
similar to simple compound interest. The principal (current size, mass or number) times
the interest rate (growth rate) yields the interest (growth increase for that day). The
interest is added to the principal, to yield a new principal.
Absolute Growth Rate (AGR)
If you plot growth (size, mass or number) versus time, a constantly increasing growth
curve is obtained. If you calculate the slope between any two times, you get the
absolute growth rate, which is the change in actual growth over time. You get a
different slope, hence different AGR for each pair of times chosen to calculate the
slope.
Relative Growth Rate (RGR)
If you plot the logarithm of growth (size, mass or number) versus time, a linear line is
obtained. If you calculate the slope of the line, you get the relative growth rate, which is
the change in relative growth over time. Since the line is linear, you get the same RGR,
regardless of which time interval chosen to calculate the slope.
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Sigmoidal Growth Curve
Exponential growth can never be sustained indefinitely. Eventually, substrates are
depleted, the population exceeds the area available, tissues or individuals begin to die,
etc., which decreases the growth rate. Growth may still increase, but at a reduced rate
(ex. if crowding causes shading), it may reach a steady state (everything is in
equilibrium, for example in a population), or growth may begin to decrease (ex. due to
death or senescence of individuals or plant parts). If you plot long term growth versus
time you get the classical sigmoidal growth curve. If you plot the logarithm of the
sigmoidal growth curve, you get a linear line during the exponential phase, after which
the curve decreases over time.
Thus three main phases of growth are seen in the sigmoidal growth of plants viz., lag
phase, lag phase (exponential phase) and stationary phase.
(a) Lag phase
This is the longest of the all the growth phases. During this phase, there is little or no
cell growth. Instead of growing, the cells are busy replicating various proteins and
DNA in preparation for the next phase. The cells are not shut down during the lag
phase. They are very metabolically active, but are not getting any bigger.
(b) Log phase
The second phase of growth is called the ―log phase‖. In this phase the cells become
extremely active and begin the process of dividing. Every chemical in the cell is being
replicated in anticipation of the cell dividing. The log phase gets its name from
―logarithmic‖, which roughly means that there is massive growth. It is a period
characterized by cell doubling.
The number of new cells
appearing per unit time is
proportional to the present
population. If growth is not
limited, doubling will continue
at a constant rate so both the
number of cells and the rate of
population increase doubles
with each consecutive time
period.
For
this
type
of
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exponential growth, plotting the natural logarithm of cell number against time produces
a straight line. The slope of this line is the specific growth rate of the organism, which
is a measure of the number of divisions per cell per unit time. The actual rate of this
growth (i.e. the slope of the line in the figure) depends upon the growth conditions,
which affect the frequency of cell division events and the probability of both daughter
cells surviving.
(c) Stationary phase
The ―stationary phase‖ is due to a growth-limiting factor; this is mostly depletion of a
nutrient, and/or the formation of inhibitory products such as organic acids. An
widespread explanation is that the stationary phase results from a situation in which
growth rate and death rate have the same values (newly formed cells per time = dying
cells per time); Another not explanation of the stationary phase is that ―there isn‘t any
more enough space for the cells‖.
Changes in growth rates over time
If you calculate the absolute growth rate (AGR) over increments of time, then plot
AGR versus the time interval, you get a bell-shaped curve, i.e. the AGR changes
constantly with time. If you calculate the relative growth rate (RGR) over increments of
time, then plot RGR versus the time interval, you get a straight-line region during the
logarithmic phase followed by a decreasing RGR. The RGR is constant during the
logarithmic phase.
Differentiation and morphogenesis in relation to growth
Growth, as already said, is an irreversible increase in mass. Because mass is related to
cell volume and cell number, growth refers to an irreversible increase in cell size
(enlargement) or to an increase in cell size as well as cell number (cell division). Cell
division by itself is not sufficient to result in growth. Differentiation, in contrast, refers
to the acquisition of qualitative differences among cells of common ancestry, i.e., those
derived from a cell or a group of cells. It is by differentiation that cells in an organ or
tissue become different from each other or specialized for different functions, e.g., the
epidermis, mesophyll, xylem or phloem cells a leaf. From a functional viewpoint,
differentiation is equivalent to specialization. Morphogenesis is the acquisition of form,
how a plant or organ acquires its distinctive shape or form. Because plant cells
generally are fixed in relation to each other and because they are cemented by the cell
wall, morphogenesis in plants is essentially a function of planes of cell divisions and
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direction of cell growth. The control of these two processes, therefore, is central to a
study of plant morphogenesis.
Classically, plant growth has been analysed in terms of cell number or overall size (or
mass). Growth in plant cells is mainly through the cell expansion driven by turgor
pressure. During this process, cell increases in volume manifold and becomes highly
vacuolated. Growth in plants can be measured in terms of:
a) Increase in the length or girth
b) Increase in fresh or dry weight
c) Increase in area or volume
d) Increase in the number of cells produced
However, size is only one reliable criterion that may be used to measure growth.
For example, growth can be measured in terms of change in fresh mass (the weight of
living tissue) over a particular period of time. However the fresh weight of the plants
growing in soil fluctuates in response to changes in the water status. So this may be a
poor indicator of actual growth. In these cases, measurement of dry weight are often
more appropriate. Cell number is a common and convenient parameter to measure the
growth of unicellular organisms, but in multicellular plants, cells can divide in absence
of an increase in volume. For example, during the early stages of embryogenesis, the
zygote subdivides into progressively smaller cells with no net increase in the size of the
embryo. Only after it reaches the eight celled stage, does the increase in cell number
begin to mirror the increase in volume. Since the zygote is an especially larger cell, this
lack of correspondence between an increase in cell number and growth may be unusual,
but it points out the potential problem in equating an increase in cell number with
growth.
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v. Seed Dormancy and Germination:
The innate condition of a viable seed responsible for inhibition of their germination
under favorable environment is called as seed dormancy. In simple terms it is a failure
of a viable and mature seed to germinate immediately after harvest under favorable
environmental conditions. The seeds can remain in a dormant state for days, months or
even years. Due to dormancy majority of seeds fails to germinate immediately after
harvesting. In nature dormancy mechanism assures seed germination at the proper time
and allows various species to survive in a particular environment. Thus dormancy acts
as a device for optimizing the distribution of seed germination in time and space. It has
also played a significant role in adaptation and evolution of seed plants. However, seeds
of many plants i.e. maize, pea, beans etc undergo germination soon after their
maturation under favorable conditions.
Dormancy is not the only conditions which inhibit seed germination, sometimes seeds
may be quite capable of germination but fails to germinate due to non-availability of
sufficient moisture, aeration, temperature and requisite light. Such inability or imposed
dormancy is referred as quiescence.
Factors causing seed dormancy
Generally seed dormancy is caused by both morphological and physiological factors.
Broadly it can be classified in to primary and secondary dormancy under several
headings. The dormancy which develops when the seed is attached to the plant and
exists when first harvested is called as primary dormancy. The dormancy which
develops in a seed after its detachment from the plant when exposed to certain
unfavorable environmental conditions is termed as secondary dormancy.
Nikolaeva (1969 and 1977) classified dormancy in three broad classes‘ namely
exogenous dormancy, endogenous dormancy and combined dormancy.
Exogenous Dormancy
This dormancy is caused due to some feature located outside the embryo.
Dormancy due to seed coat
Each individual seed at the last stage of its development is protected by several layered
seed coat composed of impermeable material like polysaccharide, hemicellulose, fats,
waxes and proteins. During seed ripening due to dehydration the seed coat becomes
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hard and tough forming a protective covering around the embryo. The seed coat
impedes germination process due to following reasons:
Water impermeability
Seed coats of many plants have thick-walled layer of palisade like macrosclereid cells
coated with waxy substances (Rolston 1978). Being impermeable to water, it interferes
with the water uptake thus imposes seed dormancy (Ballard 1973). Disintegration of
such cells either due to mechanical injury or by soil microbial action promotes entry of
water and initiation of germination process (Brant 1971) e.g. seeds of family
Leguminaceae, Malvaceae, Solanaceae and Convolvulaceae.
Seed coats of few species have a small opening called strophiolar cleft (strophiole)
sealed with a cork-like plug which blocks the entry of water and oxygen. Fig. 15(i)
Under extreme moisture conditions the strophiolar plug (counter- palisade tissue)
swells up and blocks the entry of water (Dell 1980). However, under high temperatures
and extreme dry conditions the plug undergoes contraction forming deep fissure and
allows germination. e.g. Albizzia lophantha. In some species the plug is also dislodged
by a vigorous shaking of the seeds a treatment known as impactions (Hamly 1932) e.g.
Melilotus alba and Crotallaria.
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Gas impermeability
Seed coats of some seeds are impermeable to gases like oxygen and carbon dioxide
(Brown1940). The non-availability of O2 and excessive accumulation of CO2 inhibits
the respiratory activity and in turn represses the seed germination. Under oxygen debit
conditions growth inhibitors are produced which get deposited in the seed coat
(Edwards 1969). e.g. Graminaceae, fruit and forest trees.
Mechanical resistance
Some plant have seeds with hard and tough seed coats which provide mechanical
resistance to emerging embryo (Egley 1974). Though such seed coats are often
permeable to water and gases. e.g. Amaranthus, Capsella and Lepidium. Restriction in
embryo development due mechanical cause is considered as the primary factors for
dormancy. An embryo excised from such dormant seeds are able to grow therefore, it is
called as coat –imposed dormancy( Bewley et al 2013).The hard endocarp in peach
seeds slows water absorption and prevents leaching out of germination inhibitors (du
Toit et al 1979).
Dormancy due to inhibitors
A large number of biochemical substance like organic acids, phenolic, tannins and
alkaloids present in seed coat block the growth of embryo. Such substances are named
as germination inhibitors. (Torrey 1976). The molecular structure of few natural
germination inhibitors are given in Fig.15.
H
CH3
CH3 H5
3
C 4 C 2
CH
6'
C
1'
5'
1
H
HO
4'
COOH
2' OH
O
3'
CH3
H3C
COOH
C CH
CO
HO
O
OCH3
Ferulic acid
Coumarin
R-cis-Abscisic Acid
R
COOH
H
C
CH
C
H
C
H3C
HO
OH
Caffeic acid
O
O
O
Prasorbic acid
C
H
Phthalids
Fig.15. Chemical structure of some germination inhibitors.
They have been extracted from various plant parts and have been identified
(Evenari.1949). For example. Aflatoxin in Barley, Dichlobenil in Squash, Feruline and
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Caffeie acid in tomato and Coumarin in other species. It usually occurs in fleshy fruits,
hulls and capsules of many dry fruits.
2). Endogenous Dormancy
It is caused due to the factors present within the embryo. It is imposed either due to
morphological or physiological means. In many dormant embryos, the cotyledons are
responsible for inhibiting the growth of the axis. There are many finding which strongly
support this view point. Morever, amputation of one or both the cotyledons induce
germination in many species (Bulard 1963). Fig.15a.
Dormancy induced due to embryo
Seed dormancy is also related to the condition or state of embryo development. If the
embryo is immature or poorly developed (rudimentary) at the time of seed shedding
(Atwater 1980) the germination is delayed or postponed till the complete development
of embryo during the dormancy period. e.g. Ginkgo biloba, Caltha, Anemone etc. In
some cases embryo are fully developed and seeds are mature yet they are unable to
germinate promptly. Such seeds can be induced to germinate if stored in moist, well
aerated and low temperature conditions. This process is referred as stratification or after
ripening period. e.g. apple, pear and cherry etc.
Dormancy due to inhibitors
According to the ―Hormonal Theory‖ the embryo dormancy depends on the
interaction between the inhibitors like Abscisic acid and promoter hormones such as
Gibberellic acid. The ABA acts as inhibitor for the seed germination while as GA acts
as growth promoter. Inhibitors present within the endosperm leads to metabolic
blockade which represses germination. ABA has been isolated from dormant embryos
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of several species. A correlation has been found to exist between the depth of embryo
dormancy and the concentration of ABA. When GA levels are high and ABA levels are
low the seed is able to germinate thus break dormancy. The loss of embryo dormancy
and the ability of seed to germinate is thus related to a drop in the ratio of ABA and
GA. On the contrary if GA level is low and ABA level is high the seeds are unable to
germinate thus remain dormant. Fig.15b.
Germination can only commence when these inhibitors are leached out of the embryo
after soaking with water (Norton 1980) e.g. Xanthium, Fraximus etc. In nature,
inhibitors are leached out of the seeds by heavy soaking rains which in turn provide
ample soil moisture necessary for the survival of the seedling. Some germination
inhibitor or chemicals are leached out by some plants in its vicinity which inhibit the
seed germination of under growing plants a phenomenon known as allelopathy.
Molecular Mechanism of Seed Dormancy:
Molecular mechanism for understanding the hormonal control of seed dormancy was
unclear for many years. It was only after the identification of several rate limiting
hormone metabolism genes such as Nine Cis-Epoxy carotenoid deoxygenase (NCED4Wang et al 2013); NCED9 (Liu et al 2007); ABA deficient ABA1 (Bentsink et al 2006);
ABA insensitive AB13 (Zheng et al 2012); GA3 oxidase (GA3ox-Yano et al 2013) and
their regulatory mechanisms a broad picture of ABA and GA involvement in the seed
dormancy mechanism has been stipulated.
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The emerging mechanism of seed dormancy can be broadly discussed under following
headings:
1.
Role of dormancy related genes
2.
Repression of seed germination genes
Role of dormancy related genes
Certain natural variants of Arabidopsis have seed dormancy specific loci such as
DELAY OF GERMINATION (DOG) gene (Bentsink et al 2010). One such gene
namely DOG1 is expressed in seeds during the maturation stage and due to its
inactivation no dormancy is observed. The genetic role of DOG1 in seed dormancy and
the significance of its expression in environment sensing and adaptation has been
studied by Kronholm et al 2012, Footitt et al 2013,14. In ABA deficient mutant (aba-1)
the DOG1 is not able to impose seed dormancy indicating that DOG1 function is
dependent on ABA levels. Similarly in dog1 mutants no dormancy is imposed as ABA
levels are reduced while as GA levels are enhanced thereby indicating a possible link
between the DOG1 and hormone pathways in seed dormancy. Recently other dormancy
related genes have been discovered in rice namely Seed dormancy 4 (Sdr4)- Sugimoto
et al 2010.
There is an emerging evidence to suggest that regulation of transcriptional
elongation of dormancy gene (DOG1) may be a core mechanism of seed dormancy.
Transcriptional elongation and synthesis of RNA is determined by RNA Polymerase II
(Pol II) and assisted by transcriptional elongation factor S-II( TFIIS).(Kim et al 2010).
Transcriptional elongation may be a critical part of the dormancy mechanism as
mutation in TFIIS resulted in reduced seed dormancy (Grasser et al.2009) .TFIIS and
Pol-II interact with a Pol-II Associated Factor1 Complex (PAF1C). The PAF1C
complex in Arabidopsis consists of genes for proteins EARLY FLOWERING 7(ElF7),
EFF8,
VERNALIZATION
INDEPENDENCE
4
(VIP4),
VIP5
and
HOMOLOGOUS TO PARAFIBROMIN (PHP)-Yu & Michaels 2010.Fig.15.c.
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The seeds of the mutants like elf7, elf8, vip4, and vip5 all exhibit reduced dormancy (Liu
et al 2011) indicating the importance of PAF1C and transcriptional elongation for seed
dormancy. Further during transcriptional elongation the chromatin remodeling events
like histone ubiquitination and histone methylation are thought to be critical for
induction of seed dormancy. The mutants in these components show reduced seed
dormancy.
Repression of germination genes
The continuous repression of seed germination associated genes through Histone Deacetylation is considered important for maintenance of dormancy. In Arabidopsis
Histone De acetylase (HAD 19) interacts with SW1-INDEPENDENT 1(SNL1) or
SNL2 and Histone De-acetylation complex 1 (HDC1) and removes acetyl groups (AC)
from Histone 3 Lysine H3K9/18 and Lysine H3K14. Fig.15(d).
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The
de-acetylation
positively
Aminoacylopropane-1-Carboxylate
represses
Oxidase
germination
(ACOs);
genes
CYP707AS
such
plus
as
1-
ABA
deactivation gene. (Wang et al.2013). De acetylation occurs in both the promoter and
coding regions. Due to suppression of ABA deactivation genes there is an over
expression of ABA synthesizing gene which in turn suppresses germination thus
induces dormancy.
Dormancy due to light (Photo dormancy)
Light also plays an important role in seed germination (Crocker 1930) of certain plants
like Lettuce, Viscum album etc. Those seeds whose germination is affected by light are
said to be photoblastic. If light inhibits their germination they are called as –ively
photoblastic and if the germination is induced they are called as +ively photoblastic.
Light not only affect qualitatively but its quantitative requirement have also been
demonstrated. Quality of light influence rate of germination in Lettuce seeds. Red light
(660 nm) promotes germination while as Far-red (760nm) inhibits germination. This
shows the involvement of photo reversible pigment phytochrome in the process of
germination in many plants (Taylorson 1972). Phytochrome is a conjugated protein
containing two protein monomers and two chromophores and is believed to exist in two
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interchangeable forms designated as P735 or Pfr and P660 or Pr. Exposures of the imbibed
photoblastic seeds to red light (660 to 760 nm) changes phytochrome to Phytochrome
Pfr which induces germination. Red light promotes germination in positively
photoblastic seeds by synthesizing hormone cytokinin which antagonize inhibitors and
induce seed germination.
However, the exposure to far- red light (760 to 800nm)
changes Pfr to alternate Pr form which inhibits the germination. Fig.15.1
Germination
P730
Far-red
P650
No germination
Red
Fig.15.1. Showing effect of
red and far-red light on the seed
germination
Low temperature requirement
Some seeds remain dormant during winter as they have a low temperature requirement
for germination. After passing through winter months they germinate in the following
spring. e.g. apple and peach seeds.
Combinational dormancy
More than one dormancy is prevalent among many temperate seeds. Therefore such
dormancy is produced by combination of two or more factors which act in
complementary fashion. They thus require separate after-ripening requirements for
development of epicotyl, radicle and hypocotyl. Some seeds germinate in autumn
develop a root system and require 1-3 months chilling treatment to release epicotyl
from dormancy. e.g. Lilium, Viburnum and Peony. However, certain seeds require
chilling treatment twice to break double dormancy, one for breaking radicle dormancy
and second for epicotyl dormancy .e.g. Trillium
Advantages of Seed Dormancy

Dormancy assures seed germination at the proper time.

Due to dormancy seeds can pass through adverse conditions thus promotes their
chances of survival.

It acts as a strategy to avoid competition within their siblings.

It enables the successful establishment of seedlings.

It minimizes the chances of extinction of all individuals under disastrous
conditions.
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
Dormancy can prolong the storage of seeds thus ensures agricultural security.

It serves as an efficient strategy to ensure successful germination.

Dormancy optimizes the spatial and temporal distribution of seed germination.

Dormancy prevents the in-situ germination i.e.. Vivipary in majority of species.
Disadvantages

Seed dormancy prevents uniform germination.

Dormancy interferes in seed testing procedures.
Methods of Breaking Dormancy
To induce germination process several methods of breaking seed dormancy are
available. They are effectively used either in isolation or in combination. They are
discussed under two main headings:
1. Natural methods
Several adequate natural factors such as moisture, aeration and temperature etc are
helpful in breaking dormancy in many species. The impermeable seed coat can become
permeable to water and gases due to the softening of seed coat by the action of natural
agents like microorganisms, abrasion due to wind or digestive activity of birds and
animals. The softening of seed coats by microorganisms like fungi and bacteria is most
effective at 100C under sterile medium conditions.
2. Artificial methods
To overcome seed dormancy various treatments are available which can be classified
into three groups:
a. Seed Coat treatments
It involves both physical or chemical treatments all aimed at making hard seed coat
permeable to water or gases. It is achieved by
Scarification
A method employed in softening or cracking the seed coat to make them permeable to
water and gases is known as scarification. Though time consuming this process also
occurs under natural conditions involving soil microorganisms. However, various
mechanical and chemical methods are usually preferred.
Acid scarification
Dry seeds with thick seed coat are often treated with concentrated acids like Sulphuric
acid, Hydrochloric acid and Nitric acid having specific gravity 1.84 in a ratio of 1:2,
one part seeds and two parts acid. e.g. Rosaceous seeds. Seeds are stirred cautiously at
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intervals and minimum agitation is preferred to avoid uncontrollable heat which could
otherwise damage the embryo. Depending upon the species the treatment time may
vary between 10 minutes to 6 hours. Since treatment time vary with different seed
types, a preliminary test on a small lot is recommended for better results (Heit 1967).
Finally seeds are removed from acid and rinsed with running water or with a water
having sodium bicarbonate to neutralize traces of acids. Such acid treated seeds can
either be sown immediately when wet or dried and stored.
Hot water scarification
Seeds are treated with hot water (77 to 100oC) and soaked in a gradually cooling water
for about 12 to 24 hours. For better germination percentage seeds are usually sown
immediately after this treatment. Hot water and aerated steam treatments are also used
as a disinfectant procedures against seed borne diseases of vegetables and cereals such
as Alternaria blight in broccoli and onion, loose smut of wheat and barley.
High thermal scarification
Seeds are treated with high temperature during storage. Hard seed coats are modified
by high temperature. In some species incubation at 40-5000C for few hours to 1-5 days
may be effective in overcoming dormancy. Hard seed coats of many closed cone
species of pines are modified by high temperature. During forest fire the resins that seal
the cone are melted and seeds are released for germination.
Mechanical scarification
To promote and facilitate germination seed coat is damaged using mechanical means
viz. cutting or chipping hard seed coats by rubbing with sandpaper or using mechanical
scarifiers (i.e. Disk scarifier), removing entire seed coat or making small incisions. The
seed coat should not be deeply pitted or cracked to prevent the exposure of embryo.
Some seeds are with a cork plug within their small opening in the seed coat. Such seeds
are compressed together shaken vigorously to remove the corky plug. Such a
specialized mechanical treatment is called as impaction. e.g. Crotolaria and Trigonilla
arabica.
Embryo treatment
Some seeds have dormancy due to the condition of embryo. Such dormancy can be
overcome by various methods:
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Stratification or moist chilling
Low temperature incubation under moist and well aerated medium is known as
stratification. OR it is a method of handling dormant seeds by subjecting imbibed seeds
to a period of chilling to after ripen the embryo. The term stratification originated as
formers could place seeds in stratified layers interspersed with moist soil or sand in
outdoor pits during winter. Under refrigerated stratification dry seeds are exposed to
low temperature( O to 100C) for few weeks to several months under well aerated moist
medium like washed sand, peat moss, vermiculite or composted saw dust. e.g.
Rosaceous plants. It is believed that embryo of seeds which require stratification are
immature and need certain chemical changes which promote seed germination. The
seeds are usually planted without drying to avoid injury or reversion to secondary
dormancy. Some of the chemical changes that occur during stratification:

Leads to changes in the concentration of various growth regulators.

Helps in decomposition of cyanogenic glycosides in rosaceous seeds.

Alters the concentration of nitrogen and phosphorous in different parts of the seed.

Shifting of constituent amino acids, organic acids and enzymes.
Chemical treatment
To facilitate germination seeds are alternatively treated with hormones and other
chemicals (Weaver 1972) To induce germination commonly used growth regulators
include- Gibberellic acid (GA3) seeds are soaked with GA3 500-1000ppm for 12 hours;
Kinetin (6-furfurylamino purine) (10-15 ppm) a commercial preparation of Cytokinins
applied to seeds for 3 minutes.; ethephon commercial ethylene generating chemical is
used in seeds of Cocklebur. In addition to growth regulators many chemicals are also
used to stimulate seed germination. Thiourea (CS (NH2)2 at 0.5 to 3 % solution is used
to stimulate germination in seeds which fail to germinate under complete darkness or at
high temperature similarly Sodium hypochlorite (1 gallon to 100 gallon water)
promotes seed germination of rice by overcoming water soluble inhibition in the hull (
Mikkelson & Sinah 1961).
Miscellaneous approaches
Light exposure
The seeds which respond to exposure of light for germination are known as
photoblastic seeds. To initiate germination in viability testing, light influence is quite
important. There are three categories of such seeds.
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A single exposure of light promotes seed germination. The imbibed seeds are provided
with a light by cool-white fluorescent lamps at an intensity of (800 t0 1345 lux) for at
least eight hours daily. For example Lettuce seeds germinate when exposed to red light
for few seconds to several minutes while as far-red light inhibit their germination thus
confirming the involvement of photo reversible pigment phytochrome in the process of
seed germination. Other examples incudes grasses, conifers and many other vegetables.
Light inhibits seed germination i.e. seeds require complete darkness for germination.
They are –ively photoblastic.
Seeds germinate either in light or in darkness thus are non-photoblastic in nature.
Seed priming
The various procedures used to initiate germination in freshly harvested seeds prior to
sowing is called as seed priming. It can be achieved by osmoconditioning, infusion and
fluid drilling. The osmoconditioning is an integrated method in which seeds are
intermingled with polyethylene glycol (PEG) solution, chemicals and hormones and
incubated at 15 to 20oC for 7 to 21 days. Such seeds are washed with distilled water, air
dried and stored until used (Khan et al. 1978). The infusion is a method of
incorporating chemicals such as growth regulators, fungicides, insecticides, antibiotics
etc into seeds by means of organic solvents.(Khan.1978) The fluid drilling involves the
treatment and pre-germination of seeds under conditions of aeration, light, optimum
temperature suspended in a special gel i.e. sodium alginate, synthetic clay and others (
Gray 1981).
Seed structure and germination
Seed is a miniature of a plant body or a fertilized ovule containing an embryonic plant
enclosed by a protective seed coat. Some seeds are provided with reserved food
material in the form of endosperm tissue surrounding the embryo, they are called as
endospermic seeds e.g. Maize grains, while as in other food is stored in cotyledons they
are called as non-endospermic seeds. e.g. beans. An embryo is comprised of radicle
and plumule, the radicle elongates giving rise to primary root whereas the plumule
elongates to produce aerial shoot. On the basis of number of cotyledons seeds can be
monocotyledonous or dicotyledonous containing one or two cotyledons respectively.
Fig.15.2.
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Seed Germination:
The process of reactivation of metabolic machinery of the seed and the emergence of
radicle & plumule leading to formation of seedling is called as seed germination. In
physiological terms seed germination begins with initiation of biochemical reactivation
and ends with emergence of radicle (Jann and Amen 1977). Germination can be
hypogeal or epigeal. Fig.15.3.
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In the former one regions of elongation is epicotyl which becomes curved brings
plumule above the soil and lengthening of hypocotyl does not raise the cotyledons
above the ground. e.g. Maize and Pea. In the latter one hypocotyl elongates and raises
the cotyledons above the soil. e.g. Beans. A specialized type of germination known as
precocious germination or vivipary is noticed in number of mangrove plants like
Rhizophora, Sonneratia Coconut etc. In these plants seeds cannot germinate under
marshy habitat due to lack of oxygen and presence of saline conditions. Therefore seeds
germinate insitue within the fruit while still attached to the mother plant. Radicle
appears first and hypocotyl elongates more than 70 cm and becomes heavy. Soon it
breaks its connections with the fruit and falls down in a parabolic fashion fixing radicle
in the mud and keeps plumule away from saline water (Stephens 1969). The radicle
forms a root system and establishes a seedling as a new plant.Fig.15.4.
Physiology of Seed Germination:
Germination is the resumption of metabolic activity and growth of seed tissue. It occurs
in several stages.
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Stage I. Activation
The process of seed germination begins with the absorption of moisture involving
imbibition or osmosis through seed coat or its aperture. Entry of water leads to
hydration of protoplasm, swelling of endosperm, softening of seed coat. The seed coat
is consequently ruptured due to imbibition pressure and due to more swelling in the
living cells of seed. Under hydrated condition protoplasm resumes its metabolic activity
and synthesizes various enzymes (Bewley & Black 1978). The synthesis of enzymes
during germination involves activation of unique germination controlling mRNA which
are transcribed during seed development but their translation is delayed until imbibition
of the seed. The emergence of radicle due to cell elongation marks the beginning of
germination and the end of stage first (Haber & Luippold 1960).
Stage II. Digestion and translocation
Hydration which leads to activation of metabolic activity is followed by diverse
enzymatic activities. Reserved food material namely lipids, proteins and carbohydrates
located in endosperm of monocots and cotyledons of dicot seeds are hydrolyzed by
hydrolytic enzymes namely amylase which converts starch into soluble sugars,
proteases breaks down proteins into amino acids, lipases acts on fats and splits into
glycerol and fatty acids, nuclease converts nucleic acid into nucleotides and maltase
converts malt into sugars. Soon after their degradation the soluble forms of food
material is mobilized and translocated to embryonal axis by diffusion from cell to cell.
In some seeds such as cereals, scutellum absorbs glucose from endosperm and converts
it to sucrose which is translocated through phloem.
The mobilization of reserved food material in germinating seeds is under hormonal
control. The hormones like GA3 are usually synthesized in the embryonic axis which in
turn causes de- novo synthesis of many hydrolytic enzymes. In barley and rice
endosperm embryo axis produces GA3 which is translocated to aleurone tissue layer to
secrete hydrolytic enzymes into endosperm for digestion of reserved food material.
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Fig.15.5.
Role of hormones in the de- novo synthesis of enzymes vary from species to species.
e.g. GA induces α- amylase in barley but fails in case of Pea seeds. Similarly in
Cucurbita maxima cytokinin promotes the synthesis of protolytic enzymes where GA3
is quit ineffective. Most probably the hormone induces the synthesis of RNA in storage
tissue which initiates enzyme synthesis. Growth hormones regulate both mobilization
of food reserves and type of germination. High auxin and low GA3 content in Phaseolus
mongo promotes hypogeal type of germination, while low auxin and high GA3 induces
epigeal type of germination.
Stage 3. Seedling Growth
It is marked with the continued cell division in the growing points of embryo axis
leading to expansion of the seedling structures. Once growth begins from embryo axis,
both fresh and dry weight of the seedling shows a marked increase and subsequent
decline in total weight of storage tissue. As the germination proceeds, the structure of
the seedling becomes evident. The embryo axis bearing one or two cotyledons has a
plumule or growing point of shoot at the upper end above the cotyledons and a radicle
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or growing point of the root at the lower end. Further seedling stem divided into
hypocotyl a section below the cotyledons and an epicotyl above the cotyledons.
Factors effecting seed germination
For viable seeds, the availability of following factors is essential for successful
germination.
Water
Water plays a significant role in seed germination. No germination takes place in seeds
having below 40 to 60 percent of water on fresh weight basis. Dry seeds show a
variation in their water absorption, with initial rapid uptake due to imbibition, a slow
period followed by rapid increase corresponding to seedling development. Water plays
a pivotal role in softening of seed coat, swelling of embryo & endosperm, facilitates
entry of oxygen, hydration of protoplasm and solubility of food material.
Temperature
Temperature is the most important environmental factor that regulates germination
affecting both germination percentage and germination rate. Though germination
occurs over a wide range of temperatures (5-400C) and optimum is usually about 2530oC. For every kind of seed a minimum and maximum temperature are required
below and above which no germination takes place. Many seeds usually fail to
germinate under (0-50C) temperature and between (45-480C). The effect of temperature
is primarily on the membrane structure and their function. Extremely low temperatures
freezes membrane lipids and high temperatures can also effect protein structure causing
their denaturation thereby effects many physiological processes going on within the
seed during germination.
Oxygen
Oxygen is essential for oxidation of metabolites, provide energy required by
germinating seeds. For rapid and uniform germination a good gaseous exchange is
required between the germinating medium and the embryo. To maintain good aeration
it is the reason that soil is ploughed before sowing. Soon after imbibition the rate of
oxygen uptake by seeds is increased and is proportional to the amount of metabolic
activity. Entry of water due to imbibition is responsible for resumption of metabolic
activity which is anaerobic at the beginning but soon becomes aerobic due to the
availability of oxygen. Rate of oxygen uptake is an indicator of germination progress
and is considered as a measure of seed vigor.
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Light
Light also effects seed germination. It can either inhibit germination or can promote
seed germination. It has been experimentally seen that light interferes in the seed
germination of some species like Phacelia, Nigella, Allium, Amaranthus and Phlox,
while as it favours germination of other species like Oenthera, Rumex, and Viscum etc.
Quality of light also influence the rate of germination. Red light (650nm) promotes
seed germination while as Far-red (700-820 nm) inhibits germination. It has been
established that there is an involvement of photo-reversible pigment phytochrome in the
process of germination.
vii.
Physiology of Flowering:
The main turning point in the life of flowering plant is the transition to flowering, on
which they embark a different times ranging from several days after the outset of
germination in miniature ephemerals to many decades in giants of the plant kingdom.
The transition to flowering is a result of functional activity and interaction of all
vegetative organs and is realized in the plant as an integral organism. The transition
from vegetative growth to reproductive growth (flower formation) is caused by internal
factors.
Photoperiodism:
Plants constantly monitor changes in their light environment in order to maximize
photosynthetic rate (e.g. by phototropism and shade avoidance) and to appropriately
regulate key developmental transitions (e.g. seed germination and the induction of
flowering). Plants sense light quality, direction, quantity, and periodicity through
phytochromes (which possess red and far–red absorbance maxima), cryptochromes and
phototropins (which absorb blue–light) and UV–B photoreceptors (of unknown
molecular nature).
Fluctuations in the length of day and night affect developmental processes and
behavior of many organisms. The ability of an organism to detect day length is termed
as photoperiodism. These phenomena allow detection of seasonal changes and
anticipation of environmental conditions such as low temperature and desiccation.
Circadian rhythms and photoperiodism have the common property of responding to
cycles of light and darkness.Plant responses controlled by day length are the initiation
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of flowering, asexual reproductionand the formation of storage organs (like tubers of
potato) and the onset of dormancy.
Photoperiodism was first described in detail by Garner and Allard in 1920 through
the demonstration that many plants flower in response to change in day length.
Subsequently they also showed that some plant species flower when day length falls
below a critical day length, whereas other plants accelerate flowering in response to day
length longer than a critical day length. These plants are called short (SD) and long day
(LD) plants, respectively. Plant species that flower under any photoperiodic conditions
are referred to as day–neutral plants (DNP). The discovery of photoperiodism played a
prominent role in development of knowledge about the physiological nature of
flowering.
Selection development and breeding of photoperiod–insensitive varieties has been
major event in modern crop husbandry that led to green–revolution increased
productivities of Wheat and Rice in Asia. Many desert plants evolved to germinate,
grow, and flower quickly whenever sufficient water is available. These are also DNPs
under natural conditions; plants monitor day length by measuring the length of the
night. Primarily the duration of darkness determines flowering of SDPs and LDPs. The
minimum dark period required for flowering, is made ineffective by interruption with a
short exposure to light, called a night break (NB). However, interrupting a long day
with a brief dark period does not cancel the effect of the long day. Flowering is
effectively inhibited in many SDPs by night–break treatments of only a few minutes. In
contrast longer exposures (night–break) are required to promote flowering in LDPs.
The effect of night–break varies according to the time when it is given. It is most
effective for SDPs and LDPs when given middle of a dark period of 16 hours. The
discovery of the night–break effect established the central role of the dark period. This
also provided valuable clue for studying, photoperiodic timekeeping. The discovery of
this phenomenon has also led to the development of commercial methods for regulating
the time of flowering in horticultural plant species.
The phenomenon of night–break (NB) has been extensively used as a tool to study
the photoperiodic control of flowering. In rice (Oryza sativa), 10 minutes of light
exposure in the middle of a 14–h night caused a clear delay in flowering. Erwin
Bunning first proposed that the photoperiodic time–keeping mechanism is associated
with circadian clock (Bunning, 1936) –– an autonomous mechanism that generates
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biological rhythms with a period of approximately 24 hours. The control of flowering
according to this model by photoperiodism is achieved by an oscillation of phases with
different sensitivities to light. The light promotes or inhibits flowering depending upon
on the phase of circadian rhythm in which light is given. When a light signal is
administered during the light sensitive phase of the rhythm, the effect is either to
promote flowering in LDPs or to prevent flowering in SDPs. This model is called the
external coincidence model, has been supported by a number of physiological studies
for the control of flowering time, indicating that the basis of day length measurement is
the interaction of an external light signal with circadian rhythm. In contrast, another
model called the internal coincidence model, proposes that the floral response occurs
under conditions in which two differentially entrained (synchronized) rhythms are
brought into same phase under day lengths that promote flowering, but that under other
day lengths these two rhythms are out of phase. Studies of photoperiodism in insects
support this model but details of analyses have not been carried out to test in plants.
Plants exhibit several adaptations for avoiding the ambiguity of day length signals.
The photoperiodic responses are coupled with temperature requirements. Winter wheat
does not respond to photoperiod until after a cold period (vernalization or over–
wintering).
Vernalization:
As the plants are sessile organisms, they have to readily alter their development and
growth responses to survive an ever–changing environment. In all facets of
development, from germination to flowering, plants use correct amalgamation of
multiple external signals including light and temperature. Temperature as an
environmental factor profoundly influences developmental programs of plants.
Temperature plays a major role in controlling the degree of seed dormancy/seed
germination and vegetative growth. In some plant species, long cool winter periods, are
required to enable flowering. This inductive process, called vernalization, is a strategy
that ensures flowering only occurs in the more desirable spring or summer climate. This
vernalization is the process by which flowering is induced/promoted by a cold
treatment given to a hydrated seed or to growing plant. Day seed do not respond to the
cold treatment. Without the cold treatment, plants that require vernalization show
th
delayed flowering or remain vegetative. Over the 20 century, vernalization has been
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studied extensively at the physiological level. Gassner (1918) reported that a wide
range of plant species require cold treatment/exposure to flower. In fact, the term
vernalization comes from studies of flowering in cereals. The infamous Russian
geneticist Trofim Lysenko, who studied the effect of cold on flowering, coined the term
jarovization to describe what we now vernalization. Spring cereals are called jarovoe in
Russian (derived from Jar, the god of spring) and cold exposure causes a winter cereal
to behave like a jarovoe (i.e. flower rapidly). Jarovization was translated from Russian
into vernalization; vernal is derived from latin word for spring, vernum.
A useful definition of vernalization is ―the acquisition or acceleration of the ability
to flower by a chilling treatment‖. As noted in this definition, cold exposure does not
necessarily cause flowering but rather renders the plant competent to so. Vernalization
should be followed by inductive photoperiod to flower. If vernalized plants are grown
non–inductive photoperiods, they continue to grow vegetative. However, if such plants
are later shifted to inductive photoperiods, they still flower. This shows that the
vernalized plant remember their prior vernalization; that is, they had acquired
competence to flower but did not actually do so until the photoperiod requirement was
met. Thus a cellular memory is established by exposure to cold treatment that is stable
through mitosis, but, importantly not through meiosis. The length of this memory
winter varies among plant species.
Two types of experiments demonstrate that this acquisition or acceleration of
flowering after chilling treatment occurs at the shoot apex. One is to locally chill only
certain parts of the plant. Another is to graft short tips: In most species, if vernalized
shoot tip as grafted to non–vernalized stock, it will flower, but a non vernalized shoot
tip grafted to vernalized stock will not flower.
The effective temperature range for vernalization is just below freezing to about
10°C, with broad optimum usually between about 1 and 7°C. The effect of cold
increases with duration (4 to 12 weeks) of the cold treatment until the response is
saturated. Vernalization is a quantitative response with increasing periods of low
temperature causing progressively the earlier flowering until a saturation point is
reached.
Vernalization can be lost as a result of exposure to devernalizing, conditions, such
as high temperature, but the longer the exposure to low temperature, the more
permanent the vernalization effect.
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Mechanism of Vernalization
There are two main hypothetical theroies as discussed bellow:
(A) Phasic development:
The main points of this theory were given by Lysenko (1934) are as follows:
The growth(increase in size) and development (the progressive change in the
characteristics of the new organ) are two distinct phenomena
1.
The process of the development of an annual seed plant consists of a series of phases
which must occur in some predetermined sequences.
2.
Commencement of any of these phases will take place only when the preceding phase
has been completed.
3.
The phases require different external conditions for completion such as light and
temperature.
4.
Vernalization accelerates the thermophasei,e that phase of development which is
dependent upon temperature.
(B). Hormonal Theories.
As it known that vernalization probably involves the formation of floral hormones called
as Venalin. Based on this fact, many hypothetical schemes have been proposed by
different workers. The first hormonal theory proposed by Lang and Melchers (1947) is
schematically shown below.
D
Higher temp.
Cold
A
Normal Temp.
B
C
Flowering
Thermolabile
According to this scheme, the precursor A is converted into a thermolabile compound B
during cold treatment. Under normal conditions B changes into C which ultimately causes
flowering. But at higher temperatures B is converted into D and flowering does not take
place which is called Devernalization.
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viii. Plant Movements:
Although, the power of movement is generally perceived as an animal trait not
normally associated with plants. Yet, to surprise of many, movement pervades the life
of green plant as well. The change in the state of either complete body or any specific
plantb part due to internal conditions or external or environmental stimulus is called
movement. Plant movement is mostly slow and is a key factor in determining the
orientation of a plant in space.
Plant movements may take place either due to sensitivity of protoplasm or due to
external factors such as light, gravity, temperature, touch, water, air etc, which is called
stimulus and the consequent orientation or change is called response. The stimulus can
be perceived by a specific plant part or region called perceptive part or region and the
plant part or region that exhibits the response is called responsive part or region.
Movements can be broadly classified into two types:
(A) Physical or mechanical movements such as seed dispersal.
(B) Vital movements, which are exhibited by living cells, tissues and organs due to
the sensitivity of protoplasm. These movements are further divided into two types.
(I) Movement of locomotion
In which acell or group of cells or zoospores move spontaneously or autonomous
(without involving external stimulus e.g. movements in Chlamydomonas, Volvox and
cyclosis in Tradescantia) or in response to external stimulus called Induced
movements of locomotion. The external response can be light (Phototactic movement),
Temperature (thermotactic movement), Chemical (Chemotactic movement), water
current (Rheotactic movement) etc.
(II) Movement of curvature
It is the movement or formation of curvature or bending of attached organs of fixed
plants. Curvature movements may also take place spontaneously or autonomously
i,e.Without involving external stimulus. These movements include Nutation or
circumnutation (regular and rotary movement of plant organs, most typically stem apex
e.g, tendrils) Epinasty and hyponasty (due to unequal growth on upper and lower
surface of petal; faster growth on upper surface and slower growth on lower surface
shows curvature on lower, side thus flower opens and is called epinasty, the vice versa
results in the formation on upper side, thus flower closes and is called hyponasty).
Curvature movements may also take place in response to external stimulus, which are
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called induced or paratonic (growth) movements. These growth or curvature
movements are further classified into two types i.e. Directive ortropic movements and
Non directive or nastic movements.
Tropism or Tropic Movements:
The movements occurring in response to unidirectional stimulus are called tropic
movements or tropism. These movements are named and classified in the basis of
external stimulus into following types:
Phototropism
The tropic movement in response to light is called phototropism. Some parts of plant
such as stem, branches, leaves, pedicle, flowers etc. Move towards the light and are
called positive phototropic. On the otherhhand the plant parts such as roots, rhizoid
etc move away from the light and are called negative phototropic.
Phototropism has played an important role in the early history of plant physiology.
Cell elongation in phototropically stimulated grass coleoptiles led to Went‘sdiscovery
of auxin, which is an important milestone in plant physiology. Same hormone was
found to be involved in phototropism. Since then phototropism has been studied several
etiolated grass coleoptilesincluding oats, wheat, and maize and epicotyls and
hypocotyls of sunflower and pea seedlings.
Mechanism of phototropism
The phototropic response in not necessarily a simple phenomenon. It involves a
complex interaction betweenlight and optical properties of tissue. Since phototropism is
an exogenously stimulated phenomenon, thus requires a specific preceptor. The
mechanism is accomplished in three possible steps
1.
Signal perception
2.
Signal transduction
3.
Response
The use of monochromatic light of different wavelengths has helped in the
identification of the possible preceptors. Most experiments have pointed out the
involvement of b-carotene and a UV absorbing biochemical flavinoid, riboflavin as the
key players in the signal perception and transduction in phototropism. Moreover, the
use of some chemicals in the experiments has further strengthened the evidences in
support of the involvement of these two important compounds in phototropism. For
example, a chemical Norflurazon, that interferes in the carotenoid biosynthesis.
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Similarly, photosensitivity is lost in the seedlings treated with potassium iodide or
phenyl acetic acid. These chemicals quench the excitation by flavins by light but do not
affect carotenes. However, the evidence strongly favoursriboflavin as a candidate for
the perception of light response. Another important evidence is the use of mutants of
fungus Phycomyces, which have no measurable carotene, yet exhibitnormal phototropic
responses.
The photoreceptors are predominantlylocated either at the tip of epicotyls in grasses
and hypocotyls of dicots. In coleoptiles of grass 1-5mmis highly photosensitive. The
terminal 0.2mm of coleoptiles is about 6000 times more sensitive than 3mm below the
tip.
Role of auxin in phototropism
The phototropic response is due to the unequal rate of growth on the two sides of an
organ caused by the asymmetric distribution of auxin. This resultsinto the accumulation
of more auxin on the darkened side than on illuminated side. Hence, the darkened side
grows more rapidly and curves towards light. Plant physiologists have propoed the
following few reasons for the unequal distribution of auxin:
1.
The rate of auxin production may be reduced on the illuminated side.
2.
The auxins may be destroyed by the action of light.
3.
Transverse or latera ltransport of auxin from illuminated to the darkened side.
4.
Decreased translocation of auxins out of tip.
5.
Decreased sensitivity to the auxin.
Out of the above, second and third are considered to be the most important because
they are also supported by some experimental evidences.
Diaphototropism: When plant parts move towards the direction perpendicular to the
incident light, it is called diaphototropism e.g. movement of leaf blades.
Geotropism
The tropic movements in response to the gravity are called geotropism. Some plant
parts e.g. stem moves away from the gravity are called negative geotropic. However
some plants parts move in the direction of the gravity are called positive geotropic e.g.
roots and rhizoids. Geotropism is of the following types:
a)
Positively orthogeotropic: When organ grows in the center of earth e.g roots
b) Negatively orthogeotropic: When the organ grows away from the earth e.g Stem
and branches
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c) Diageotropic: When the axis of organ grows perpendicular to the direction of
gravitational force e.g rhizome
d) Plageogeotropic: When axis mof the organ makes an angle other than right angle
e.g. Side roots of first and second order.
Mechanism of geotropism
Since the stimulus in geotropism is external, therefore, the phenomenon is
accomplished in the following steps
a)
Perception (physical action of gravity and its reception by the plant)
b) Transformation of information or signal transduction (involves the metabolic
changesand subsequent synthesis and transport of auxin.
c)
Response (bendingof the plant part towards or away from the gravity).
Role of auxin in geotropism
Like phototropism, geotropismis also due to the asymmetric or unequal distribution of
auxin in the plant.There are sufficient experimental evidences which suggest that auxin
diffuses out to the lower half of the tip of horizontally placed plants. The shoot turns
upward because the growth is acceleratedon the lower surfaceand the root turns
downwards, because auxins are known to inhibit root growth or elongation.An
additional advantage of the unequal distribution of auxin is that it also facilitates the
flowering in plants. Flowering is initiated due tom auxin accumulation on the lower
side of the tip.
Role of calcium in geotropism
Experiments with root geotropism have provided promising evidences of a role for Ca
by involving Ca2+-Calmodulin protein complex. The accumulation of Ca exhibits
opposite tendencyin gravi-stimulated coleoptiles and roots.In a horizontally oriented
root, Ca moves on the lower side of the capbut it moves towards the upper side of the
coleoptiles.
How accumulation of Ca in the lower side of the root is translated into a differential
growth response has beenhas been explained by two possible pathways:
(a) Calcium acts as a sink for auxin, thereby attracting auxin on the lower side of the
rootwhere the excessively high concentration of auxin inhibits root elongation and
the upper side elongatesnormally, thus causing positive geotropism.
(b) Calcium sensitises the tissue to auxin. Thus its accumulation results in the
inhibition of root growth causing the bending towards gravitation.
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Thigmotropism
The tropic movement in response to the touch stimulus is called thigmotropiosm. The
stem tendrils of the family Cucurbitaceae when comes in contact with a solid support,
they grow fast and encircle around the support and help the plant to climb. The part
which is not in contact with the supportgrows faster and the part which touches the
support shows slower growth. Thus the growth on the outer surface of tendril is always
more in comparison to that on the inner surface. Thus the tendril coils around the
support. Similarly the leaves of Dionia become closed due to touch.
The thigmotropic response in tendril is is perceived by the tip. The sensitivity of the
tendril decreases from tip to the back. These responses may be due to the changes in
ATP and inorganic phosphate upon touch stimulus in pea seedlings and or may also be
due to the changes in membrane permeability or movement of metabolites or excessive
water uptake.
Hydrotropism
The movement in response to the water stimulus is called hydrotropism.It is of two
types:
(a) Positive hydrotropism: when the tropic movement is towards water it is called
Positive hydrotropism e.g. roots.
(b) Negative hydrotropism: When the movement of the plant is away from the water
stimulus, it is called negative hydrotropism e.g. stem.
Hydrotropism can be easily demonstrated by a simple experiment.
Take a
regulartube and after filling it with water slightly tilt it. Now place a perforated plate on
itand cover the plate with wet wooden powder.Place the seeds on the powder and allow
the seeds to germinate.. When the seeds germinate, their radiclesmove towards
gravitational stimiulus through the pore, then turn upwards towards water. It proves that
root shows positive hydrotropism, which is stronger than the geotropism.
Thermotropism
The tropic movement in response to the temperature change in one side of a plant organ
is called thermotropism.It has been reported in several plants e.g. Tulipa sylvestris
where peduncles turn towards sun throughout the day. Thermotropism may be regarded
as a special type of phototropism. In thermotropism, the stimulus is due to longer
wavelengths of infra-redradiation, whereas in phototropism stimulus is due to shorter
wavesof visible light.
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The bending of shoot is due tothe negative thermotropism whereas bending of root is
due to positive thermotropism.. If the temperature is blow optimum, plants will curve in
the direction of warmerside and if above the optimum, it will curve towards colder side.
Rheotropism
The tropic movement in response to the water-current is called rheotropisme.g. bending
of root tip. Generally, roots of seedlings are moresensitive to the stimulus of running
water than those of more adult plants. The roots of Crucifereae and Gramineae the most
sensitive whereas the roots of some plants are insensitive. It has been found that the
hyphae of Mucor and Phycomyces are negative rheotropic while those of Botrytris
cinera are positive rheotropic. Rheotropism is not a clearly understood phenomenon
and the following views are put forward to explain it:
(i) It is the reaction to the stimulus of water pressure.
(ii) It is a type of hydrotropism
(iii) It is a special case of chemotropism.
Chemotropism
The tropic movement in response to the chemical stimulus is called chemotropism. e.g.
the growth of pollen tube on the stigma into the style is always due tochemotropism.
Aerotropism
The tropic movement in response to the changes in the concentration of gas particularly
oxygen is called aerotropism.
Traumatotropism
The tropic movement in response to the wounding of apex is called traumatotropism.
Galvanotropism
The tropic movement in response to the electric current is called galvanotropism.
Osmotropism
The tropic movement in response to osmoregulation in the system is called
osmotropism.
Nastic Movements:
In addition to the directed movements of tropism, many plants and plant parts,
especially leaves, exhibit nastic movements, in which the direction of movement is not
related to any vectorial component of stimulus.Nastic responses may involve
differential growth, in which case the movement is permanent. Alternatively, the
movement may be reversible, caused byturgor changesin a specialised motor organ.
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Epinast and thermonasty are the examples of nastic responsesinvolving differential
growth. Epinasty is the bending of an organ, commonly petiole and leaveswhose tips
are inclined towardsthe ground.It is not a response to gravity, however,but appears to
depend on an unequalflow of auxin through the upper and lower sides of petiole.
Epinsty is also a common response to ethylene or excessive amount of auxin. The
reverse response called hyponasty is less commonbut can be induced by gibberellins.
A typical exemple of thermonasty is the repeated opening and closing of some flower
petals, such as tulip ans Crocus. In spite oftheir repeatednature, however, thermonatic
movements are permanent and result from alternating differential growth on two
surfaces of the petal.
The most dramatic nastic movements are all turgor movements, which may broadly
separated into three catagories.
(1) The leisurly rhythemic sleep leaf movements calleds nyctinasty,
(2) Very rapid sismonastic movements in response to touch e.g'.Leaves of Mimosa
pudica.On touching the plant the leaflets of the plant become closed and leaves
hang down. Since these plants respond to touch, they are sometimes considered
thigmonastic. However, seismonastic plants respond to a wider variety of stimuli
including shaking by wind, falling rain drops, wounding by cutting and intense
heat and burning
(3) The thigmonastic curling ofthread like appendagesin climbing plants and vines.
Nyctinastic and seismonastic responses depend on differential turgor movementsin
specilaized motor organs called pulvinus (plural pulvini). The pulvinus is a
bulbous structure most often found in plant families characterized by the presence
of compound leaves such as Leguminoseae and Oxalidaceae. It occurs at the base
of the petiole (primary pulvinus), the pinna (secondry pulvinus). The pulvinus
contains large, thin walled motor cells, which alter the position of the leaf by
undergoing reversible changes in turgor.
ix.
Plant Hormones:
Plant hormones or the phytohormones, as they are often called, are the chemical
substances that regulate the growth and development in the plant body right from the
dormancy and germination of seeds, buds or other propagules, to the formation of
various tissues and organs, elongation of roots and shoots, flowering, abscission (of
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leaves, flowers and fruits) and even senescence. It is due to this reason that these
substances are variously named as growth-substances, growth-factors, growthregulators or plant growth hormones as well.
Characteristics of phytohormones:
1.
A plant hormone is a chemical substance that is synthesized in one part of the plant
and is then translocated to some other part(s), where it causes a physiological
response and, thus, brings about the specific effect on the growth and development.
2.
A plant hormone is synthesized and required in very small concentrations.
3.
A particular plant hormone may have a positive effect on a process of growth and,
thus, promote it; or may have a negative effect on it and inhibit it.
4.
A particular plant hormone may promote certain processes, inhibit some others and
not affect others.
There are, so far, recognized five main groups of phytohormones. These are:
1.
Auxins
2.
Gibberellins
3.
Cytokinins
4.
Ethylene, &
5.
Abscisic acid (ABA)
Besides these, the various other less important growth regulators that have been isolated
from some plants include polyamines (like putrescine, cadaverine, spermidine,
spermine etc.), Triacontanol, Brassins, and some growth inhibitors like Lunularic acid,
Batasins, Jasmonic acid etc.
1.
Auxins:
Auxins constitute one of the most important groups of plant hormones, because of their
manifold roles in plants. The principal naturally occurring auxin (Indole-3-acetic acid,
IAA) was the first growth hormone to be discovered in plants.
Discovery
During the latter part of the nineteenth century, Charles Darwin and his son Francis
studied plant growth phenomena involving tropisms. One of their interests was the
bending of plants towards light. This phenomenon, which is caused by differential
growth, is called phototropism. In some experiments the Darwins used seedlings of
canary grass (Phalaris canariensis). The coleoptiles of canary grass, and many other
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grasses, are very sensitive to light, especially to blue light. If illuminated on one side
with a short pulse of dim blue light, they bend (grow) towards the source of light pulse
within an hour. The Darwins found that the tip of the coleoptile perceived the light, for
when they covered the tip with foil, the coleoptile would not bend. But the actual
growth zone is several millimeters below the tip. So, they concluded that some sort of
signal is produced in the tip, travels to the growth zone, and causes the shaded side to
growth faster than illuminated side.
Many investigators followed their experiments to find out the nature of the growth
stimulus in coleoptiles. Boysen Jensen (1910-1913) found that the response to the light
stimulus was lost by decapitation of the coleoptile that was recovered if the tip was
placed back on the stump.
Frits Went (1926), discovered that some unidentified compound probably caused
curvature of oats (Avena sativa) coleoptiles towards light (phototropism). He carried
out a series of experiments on Avena coleoptiles and demonstrated the presence of a
substance in the tips that is carried to the growth zone and causes the curvature. The
chemical which was called ‗auxin‘ (from Greek ‗auxen‘ meaning to grow) by Kogl,
Hagensmit and Went was later on isolated from a variety of materials. The first
crystalline auxin, chemically Indole-3-acetic acid was obtained from human urine by
Kogl and Kostermann in 1934. Auxin was also obtained from malt and corn meal by
Hagensmit et al. in 1942. Now we know that auxins are present in the tissues of all
higher plants, although in small quantities. These auxins are of fundamental importance
in the physiology of growth and differentiation.
Indole acetic acid is believed to be the principal naturally occurring auxin. Other
naturally occurring auxins include indole-3-acetal dehyde, indole-3-pyruvic acid,
indole-3-ethanol, 4-chloro-indole acetic acid, indole aceto nitrile and phenyl acetic acid.
Some synthetic auxins include indole-butyric acid, naphthalene acetic acid, phenyl
acetic acid etc.
Physiological Effects of Auxins:
Cell elongation and longitudinal growth: Auxins are well known to promote
elongation of stem and coleoptile. However, this effect is not observed when the
treatment of exogenous auxins is given to the intact plants. Probably, it is because the
required amount of auxin is already present in plants. It has been remarkably observed
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that exogenous application of auxins promotes growth if the apex of the shoot is
removed before auxin treatment.
1.
Cell elongation in the cambium: Auxins are responsible for the initiation and
promotion of cell division in cambium. This effect of auxin is particularly
important in secondary growth of stem and differentiation of xylem and phloem
tissues.
2.
Cell division and tissue culture: Initiation of cell division by auxins is very useful
in tissue culture experiments and formation of callus (i.e., undifferentiated mass of
cells). Healthy growth of callus depends on the auxin added in the culture.
3.
Root growth and root initiation: Ausins promote growth only at extreme low
concentrations (i.e., 107 to 10-13 M concentrations). At higher concentrations they
always inhibit root growth. In fact the concentration of auxin, which is most
suitable for shoot elongation, is always inhibitory to root growth. Here, it is worth
mentioning that the concentration of auxin which is inhibitory to root growth
causes initiation of adventitious roots from the nodes or basal regions of stem. For
this reason the auxins, particularly Indole butyric acid (IBA) and Napthalene acetic
acid (NAA) are used by the plant gowers to induce root formation in stem cuttings
and cut leaves.
4.
Apical dominance: It is common observation in vascular plants, especially in
taller ones, that if the apical bud is intact and growing the lateral buds remain
suppressed. Removal of apical bud causes fast of lateral buds. The influence of
apical bud in suppressing the growth of lateral buds is termed apical dominance.
According to Thimann and Skoog (1934) and Thimann (1937), the auxin
synthesized in apical meristem is translocated downwards and inhibits the growth
of lateral buds.
5.
Prevention of abscission layer: Shedding of mature leaves from the stem or ripe
fruits from the stem is called abscission. Generally a layer of tissue is formed at the
base of the organ. This layer of tissue is called abscission zone. It has been shown
that abscission zone does not occurs when the concentration of auxin is high,
particularly when the gradient of auxin is steep; i.e., more auxin on the distal side
and less auxin on the proximal side. The abscission zone formation occurs rapidly
when the auxin gradient becomes slight or neutral. Moreover, the plant hormone
ethylene is found to promotre ther abscission. Thus, a high concentration of auxin
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prevents the formation of abscission layer and the phenomenon is controlled by the
concentrations of auxin and ethylene.
6.
Parthenocarpy: AUxin is well known to induce parthenocarpy (i.e., the formation
of seedless fruits withour fertilization). In a number of plants. External application
of auxin on flowers causes development of seedless fruits in tomatoes, apples,
cucumbers, etc.
7.
Role
of
auxins
as
herbicides:
Many
synthetic
auxins
lik
e
2,4-
dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2methyl-4-cholorophenoxyacetic acid (MCPA), etc. have been used as selective
herbicides (i.e., eradicants of weeds). They are used as potent defoliants due to
their destructive properties.
8.
Enzymatic activity of auxins: Auxins have been found to regulate the activity of
many enzymes either by de-novo synthesis of enzymes or by enhancing the activity
of some already existing enzymes, like peroxidase activity in tobacco cells and in
mungbean cotyledons.; and wall-bound cellulose and glucose synthetase activity
in pea, bean and maize seedlings.
9.
Flower initiation: Auxins generally inhibit the flowering in pineapple (Ananas
sativus), spraying of certain auxins initiates uniform flowering in the whole crop.
The auxins help in delaying the flower formation in lettuce.
10. Plant growth movements: Auxins regulate some of the important growth
movements including tropisms like phototropism and gravitropism.
11. Respiration: Many auxins induce synthesis of many enzymes that result in an
increase in respiration. Further, auxins may stimulate respiration probably by
increasing the availability of reparatory substrates
12. Other effects:
i)
Stimulation of respiration probably by increasing the availability of
repiratory substrates;
ii)
Increase in the storage of solutes inside the cells. induction of feminizing
effect on some plants..
2.
iii)
Increase the synthesis of ethylene.
iv)
Healing injury by enhancing the cell division around the injured area.
Gibberellins:
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Gibberellins constitute a large group related compounds that, unlike auxins, are defined
by their chemical structure rather than by their biological activity. Gibberellins are most
often associated with the promotion of stem growth. However, they play important
roles in a variety of physiological processes.
Discovery
Gibberellins were first studied in Japan in studies with the diseased rice plants that
grew excessively tall. These plants could not support themselves and eventually died
from combined weakness and parasitic damage. This disease was known by the
Japanese as the ‗bakanae disease‘ (‗foolish seedling disease; Japanese word ‗bakanae‘
means ‗foolish‘). It is caused by a fungus then known as Gibberella fujikuroi, which is
actually the sexual or the perfect stage of the fungus Fusarium monoliforme. Japanese
plant pathologists under the supervision of E. Kurosawa in 1926 demonstrated that
some chemical substance secreted by the fungus was responsible for causing the
disease. T. Yabuta and T. Hayashi (1935) isolated an active compound from the fungus
and named it ‗gibberellin‘. This was the first gibberellin of its kind to be discovered and
isolated. Cross et al. (1961) isolated six different gibberellins from the fungus.
MacMillan et al. (1961) isolated three different gibberellins from bean seeds.
More than 125 different gibberellins have now been discovered from various fungi
and other plants including more than 60 from seed plants, although no single species
contains more than 15 and most species have only a few. All gibberellins have either 19
or 20 carbon atoms grouped in a total of either four or five ring systems, and all have
one or more carboxyl groups. They are known as gibberellic acids and abbreviated as
GA with a subscript, such as GA1, GA2, GA3 and so on, so as to distinguish them.
Physiological Effects of Gibberellins:
1. Promotion of plant growth
Gibberellins stimulate extensive growth in intact plants. They enhance elongation of
intact stems much more than that of excised stem segments. In this respect, their effects
are opposite to those of auxins. Because of this prop[erty gibberellins are able to
overcome genetic dwarfness in some species, such as maize, bean and pea. In maize,
dwarf varieties are about one fifth in height of the normal varieties. It is believed that
dwarfness in these species is due to a gne mutation which blocks gibberellin synthesis.
Lack of gibberellin causes shortening of internodes and reduced height on the plant.
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Exogenous application of gibberellins fulfills the requirement and the plant achieves
normal height. Gibberellin have little effect ehen applied to plants of normal height.
2. Promotion of seed germination and bud growth
Giberellins promote seed germination in several species, which otherwise fail to
germinate unless subjected to low temperature, long days or red light. Gibberellins
seem to act a substitute for all these treatments. They enhance cell elongation so the
radicle can push through the endosperm, seed coat or fruit that restricts its growth.
Subsequently, the hormone also stimulates hydrolysis and transport of stored food
materials from endosperm and cotyledons to the growing root-shoot axis, especially in
the cereals. Gibberellins also help overcome dormancy in buds in evergreen and
deciduous trees and shrubs.
3. Induction of flowering
Anton Lang and his associates (1957) demonstrated that flowering in many herbaceous
plants was correlated with rise in endogenous gibberellin levels. Exogenous application
of gibberellins induces flowering in some photoperiod sensitive and cold requiring
species. Gibberellin replaces the requirements both for long day in long day plants and
for low temperature in cold requiring species. Applied gibberellins generally do not
induce flowering in short day plants except for a few species.
Gibberellins change sex expression of plants, as well. In most species gibberellins
induce the formation of male flowers; in a few they induce the formation of female
flowers. Gibberellins also promote fruiting and fruit development also in some plants.
4. Prevention of senescence
It has been shown that the exogenous application of gibberellins can prevent
senescence of leaves. In leaves of many plants in which the principal anti-senescence
hormone (kinetin) has a very little effect, the gibberellins act powerfully. Furthermore,
gibberellins have also been found to delay senescence of fruits besides that of leaves.
Gibberellins are often sprayed on the fruits and leaves or oranges to prevent rind
disorders that appear during storage. The hornmone delays senescence and maintains
former rinds.
5. Mobilization of food and minerals in seeds
During germination of seeds, the organic food and minerals stored in the form of
macromolecules in endosperm and cotyledons are released and transported to the
growing root and shoot axis, until the seedling is established in the soil. Thereafter, the
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seedling starts abstracting nourishment from the soil and atmosphere. Gibberellins are
known to stimulate the hydrolysis of stored macromolecules and their transport to the
embryonic axis.
3.
Cytokinins:
Cytokinins are a very important group of plant hormones, which are primarily involved
in inducing cell division in parenchymatous cells. Although Cytokinins regulate many
cellular processes, the control of cell division is central in plant growth and
development and is considered diagnostic for this class of plant growth regulators.
Discovery:
Gottleib Haberlandt in 1913 demonstrated that an unknown compound present in
vascular tissues of various plants stimulated cell division that caused the cork cambium
formation and wound-healing in cut potato tubers. This compound was late identified
and known as ‗cytokinin‘, as it stimulated cytokinesis. Johannes von Overbeek (1940‘s)
found that the milky endosperm from the immature coconuts is also rich in compounds
that promote cytokinesis. Jablonski and Skoog (1954) reported that vascular tissue cells
contain a substance that stimulated cell division on pith cells. Miller et al. (1956) found
the coconut milk to be effective in promoting continuous cell division of tobacco stem
pith cells in culture. They also observed that a combination of yeast extract and IAA
was also effective in promoting the division of cells in cultures. Their subsequent work
revealed the active material in coconut milk and yeast extract that promoted the growth
in tissue cultures. They finally succeeded in isolating and purifying the active
substance, which they named ‗kinetin‘ (which is chemically 6-furfuryl amino-purine).
Miller and Letham (1965) also isolated a substance from the immature maize kernels,
which they named ‗zeatin‘. The biological properties of ‗kinetin‘ and ‗zeatin‘ are
similar, though in some instances zeatin is more active. All such compounds were, then,
placed under the same group of plant hormones— the cytokinesis.
Physiological effects of cytokinins
1. Promotion of cell division and organ formation
A major function of naturally occurring Cytokinins is to promote cell division. in the
presence of auxins, nearly all Cytokinins stimulate cell division and subsequent callus
growth in the parenchymatous cells from several plants. By manipulating the ratio of
cytokinin to auxin, plantlets can be developed from the tissue in culture.
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Cytokinins stimulate bud formation on intact tissues such as leaves, roots,
cotyledons or stem pieces as well. Auxins inhibit this sytokinin action and herefore a
balance between auxin and cytokinin is required to achieve differenciation and growth
buds in plants.
2. Promotion of seed germination
Application of Cytokinins can promote germination even break dormancy in some
seeds. The seed germination in photoblastic seeds, which germinate only in light and
fail to germinate in dark, seems to be phytochrome mediated.. however, if these seeds
are soaked ina solution of Cytokinins, their germination is greatly enhanced in dark, as
compared to those seeds which are not soaked in Cytokinins.
3. Expansion of cotyledons and leaves
Cytokinins induce expansion of excised cotyledons in several dicots. Most of these
species contain fats as a major food reserves in cotyledons. Cytokinins cause cotyledon
expansion by increasing cell size, although increased cell division may also account for
some expansions.
4. Delaying of senescence
Senescence of leaves is characterized by the breakdown of proteins, chlorophylls,
nucleic acids and lipids and outflow of the products to other younger tissues. The
senescence is often accompanied by yellowing is accelerated if the leaves are excised
from the plant and kept in dark. A.C. Chibnall first demonstrated in 1954 that the
formation of roots in many detached dicot leaves retarded the onset senescence of the
leaves. He proposed that roots produced some hormones that delayed senescence in
such leaves. Subsequently, it was shown by A.E. Richmond and A. Lang (1957) that
cytokinin treatment extended the life span of detached leaves by delaying cholorophyll
and protein degradation. This property of Cytokinins to delay senescence is, as such,
known as ‗Richmond & Lang Effect.‖.
5. Promotion of chloroplast development
Exogenous application of cytokinins promote cholorplast development in callus tissues
and excised cotyledons. Application of cytokinins in dark promotes lamellar
development and if light is also applied simultaneously (with cytokinins) grana and
chlorophyll also appear.
6. Anthocyanin synthesis
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Anthocyanins are flavonoid pigments, which are responsible for the red, pink, purple
and blue colours in plants. Cytokonin treatment increases anthocyanin content in many
cultures cells and tissues in parts of intact plants.
4.
Eyhylene:
Ethylene is a unique growth regulator in that it is the only growth regulator which is
gaseous in nature. Furthermore, this growth regulator is both a growth regulator in
certain respects and in other respects it is a growth inhibitor.
Discovery:
The ability of certain gases to stimulate fruit ripening has been known for many
years. Even the ancient chinese knew that their picked fruits would ripen more quickly
in a room with burning incense. Nel Jubow (1901) showed that the illuminating gas
which induces the ripening of unripe fruits contains ethylene. Gane (1934)
demonstrated that the ripening of fruits was due to ethylene. With the availability of gas
chromatography, it was found that the ethylene is present in and synthesized by the
plant tissues.
Ethylene is a volatile gas (CH2=CH2) present in the atmosphere as a component of
smoke and other industrial gases and formed by the incomplete combustion of carbonrich compounds such as coal, petroleum etc. Being a gas at ordinary physiological
temperatures ethylene synthesized by and within plants is transported through the plant
tissues by gaseous diffusion, and, thus, there is lack of directional control. It is because
this reason that ethylene is not regarded as a hormone by the plant physiologists and is
simply known as a plant growth regulator.
Physiological effects of ethylene:
1. Seed germination
Ethylene is known to break dormanyand induce germaination in a number of plants. It
also causes the increased extension growth of the seedling in some other plants. In a
few plants like maize, ethykene has been recorded to inhibit germination.
2. Growth inhibition and morphogenetic effects
In most cases exogenous application of ethylene inhibits plant growth. In most dicots
the elongation growth of stems, roots and leaves is inhibited. But the hormone ehnaces
radial growth, as a result both stem and root swell in response to ethylene.
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3. Epinastic responses
Exposure to ethylene causes epinastic movement in petioles, as a result of which the
leaves bend down. This is because of more growth on upper side than on lower side of
the petiole. Stem epinasty is also caused due to ethylene.
4. Flowering inhibition and sex expression
Ethylene inhibits flowering in most plants, although it is known to promote flowering
in mango and pine apple. Inhibition of flowering by ethylene is controlled by the
photoperiod. Ethylene changes sex expression also in unisexual plants. It increases
female flowers in several members of cucurbiatceae. It also induces sterility in
cucurbits and wheat.
5. Ripening of fruits
Acceleration of fruit ripening was the first discovered effect of ethylene. The hormone
is now known to accelerate ripening of mature fruits in most cases including banana,
apple, tomato, avocado etc. Ethylene appears to play an important role in natural
ripening of fruits.
6. Acceleration of senescence and abscission
Ethylene is an important hormone governing the senescence and abscission of plant
parts, both natural and induced. It accelerates senescence of leaves, flowers and fruits.
Abscission is the most widely demonstrated response of ethylene. Ethylene induces
abscission of leaves, fruits, petals and flowers.
5.
Abscisic Acid (ABA):
Abscisic acid, alongwith some other related compounds like phaseic acid, 4/dihydroxyphaseic acid, xanthoxin etc., constitute an important group of plant
hormones, which is a growth inhibitor group.
Discovery:
Robinson et al., (1963) and Robinson and Warburg (1964) isolated a compound from
the dormant buds of Acer pseudopalatinues as a ‗dormancy inducing factor‘. The called
it ‗dormin‘. Okuhama et al., (1963, 1965) and Addicot et al. (19..) isolated the same
substance from the cotton fruits as an ‗abscission accelerating factor‘ and they called it
‗abscisin-II‘. Later it was found that the ‗dormin‘ and ‗abscisin-II‘ are identical and
they were given the common name abscisic-acid. Later various other compounds with
similar properties were also identified (names mentioned above).
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Physiological effects of ABA:
1. Inhibition of seed germination
Exogenous application of ABA inhibits seed germination of most non-dormant seeds.
For causing inhibition, ABA must be continuously present. As soon as it is removed by
washing, germination can take place. Endogenous ABA also inhibits germination.
Inhibition is because of inhibition of enzymes involved in germination process.
2. Inhibition of seedling growth
ABA inhibits the growth of the seedlings in some cases. It has been suggested that
ABA inhibits seedling growth by decreasing the water potential.
3. Inhibition of bud growth
Exogenous ABA induces bud dormancy in woody plants. As in dormant seeds, ABA
content in dormant buds is high and it decreases by treatments which lead to breaking
the dormancy. The hormone inhibits lateral growth of the bud, as has been reported in
tomato.
4. Stomatal closing
The most significant and best known effect of ABA is its control of stomatal closing.
Exogenous application of ABA to epidermal strips causes stomatal closure. In fact,
factors such as water stress, chilling etc. which induce stomatal closure also increase
ABA content of guard cells. The response of ABA on stomatal closing is very fast and
occurs within a few minutes of ABA application.
5. Senescence and abscission
Numerous studies indicate that ABA is an endogenous factor involved in senescence
and abscission of leaves and other plant organs. Exogenous application of ABA induces
primary yellowing in leaf tissues in a variety of species ranging from deciduous trees to
herbaceous plants. It also accelerates abscission in leaves in a number of species.
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Unit: III
i. Plants & Environment:
Atmosphere – Gaseous Composition and Layering of Atmosphere:
Gaseous Composition:
The atmosphere is a mixture of several gases-Nitrogen (N2) 780,840 ppmv (78.084%)
Oxygen (O2) 209,460 ppmv (20.946%) Argon (Ar) 9,340 ppmv (0.9340%) Carbon
dioxide (CO2) 383 ppmv (0.0383%) Neon (Ne) 18.18 ppmv (0.001818%) Helium (He)
5.24 ppmv (0.000524%) Methane (CH4) 1.745 ppmv (0.0001745%) Krypton (Kr) 1.14
ppmv (0.000114%) Hydrogen (H2) 0.55 ppmv (0.000055%) Nitrous oxide (N2O) 0.3
ppmv (0.00003%) Xenon (Xe) 0.09 ppmv (9x10-6%) Ozone (O3) 0.0 to 0.07 ppmv
(0%-7x10-6%) Nitrogen dioxide (NO2) 0.02 ppmv (2x10-6%) Iodine (I) 0.01 ppmv
(1x10-6%) Carbon monoxide (CO) trace Ammonia (NH3) trace.
Though 0.033% by volume, carbon dioxide is most impotant meteorologically as it
is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.
Varying quantities of water vapour, dust particles, smoke, and salts (coming from
oceans) etc is present in the atmosphere. There is sufficient movement of air upto about
100 km which keeps it well mixed and homogeneous. The general circulation of air (the
mixture of gases in the lower atmosphere) is responsible for the specific climatic
conditions and is responsible for water cycle. Atmosphere is an essential part of the life
support system as it supplies oxygen, carbon dioxide and nitrogen. Air is the medium
for locomotion of flying animals and aids in dispersal of pollen, spores, seeds and
fruits. The strong moving current of the air is the wind- an ecological factor of great
significance. Dispersal of microorganisms, pollen, seed, fruit, maintenance of plant
size, anatomy, formation of deserts, transpiration, rainfall etc is governed by moving
air. The different elements of atmospheric air, the pressure exerted by it on the earth,
plants and animals, the horizontal movement of air – the wind, air moisture (its
humidity) all play a role in the life and distribution of plants and animals.
Layering of Atmosphere:
Atmosphere may be defined as a transparent gaseous envelope surrounding the earth
(the vast expanse of air which envelops the earth is known as atmosphere). The
atmosphere consisting of a mixture of various gases extends to many miles and is held
around the earth by the gravitational force. The vertical profile of the atmosphere shows
several concentric layers. These layers vary in density, temperature composition and
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properties. It is thickest near the surface and thins out with height until it eventually
merges with space. The main layers from the surface of the earth upwards / outwards
are:
Troposphere (tropos = turning, mixing) Lowest, densest extends upto 16-17 km on the
equator and about 7-8 kilometers at the poles. It contains about 80 - 90% of the gases,
the average temperature decreases with increasing height uptotropopause {15-20ºC to
−57ºC}. All sorts of disturbances are limited to this layer. Rain storms, clouds etc are
restricted to this layer. Weather occurs in this layer. The upper limit of the troposphere
is called tropopause, the boundary between the stratosphere and troposphere generally
lies at about 17km. In the troposphere, the air cools gradually as it gets further from the
earth.
Stratosphere: From the Latin word "stratus" meaning spreading out. The stratosphere
extends from the troposphere's 7–17 km range to about 51 km. Temperature increases
with height. At the top of the stratosphere temperature is about 0-3ºC; (around 32ºF).
This is a calm zone with no disturbances, cooler and clearer. Many jet aircrafts fly in
the stratosphere because it is very stable. The stratosphere contains the ozone layer, the
part of the earth's atmosphere which contains relatively high concentrations of ozone. It
is mainly located in the lower portion of the stratosphere from approximately 15–35 km
above earth's surface. About 90% of the ozone in our atmosphere is contained in the
stratosphere. Ozone concentrations are greatest between about 20 and 40 km, where
they range from about 2 to 8 parts per million.
If the entire ozone content is
compressed at normal pressure and temperature, it will constitute a layer only 3mm
thick. And yet on all counts it has played a vital role in the evolution of life. The life
sustaining role comes from the fact that radiations below 300nm i.e., UV are
biologically harmful. Any large scale depletion can have catastrophic influence on all
biological systems including the plants. If all the UV rays are allowed, all bacteria
would be destroyed, animal tissues burnt etc. The upper limit of the stratosphere is
called stratopause.
Mesosphere: meaning middle (middle shell of gases in the atmosphere between
stratosphere and thermosphere). The mesosphere extends from about 50 km to the
range of 80–85 km. Temperature decreases with height, reaching −100 °C in the upper
mesosphere. This is also where most meteors burn up when entering the atmosphere.
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Thermosphere: From 80–85 km to over 640 km, temperature increasing with height.
Although the temperature can rise to 1,500 °C, a person would not feel warm because
of the extremely low pressure. It is also where the space shuttle orbits. Ionosphere:
The part of the atmosphere (considered a part of thermosphere) that is ionized by solar
radiation stretches from 50-80 to 1,000 km and typically overlaps both the exosphere
and the thermosphere. It plays an important part in atmospheric electricity and forms
the inner edge of the magnetosphere. Because of its charged particles, it has practical
importance because it influences, for example, radio propagation on the earth.
Exosphere: The atmosphere merges into space in the extremely thin exosphere. This is
the upper limit of our atmosphere.(Highly rarified) From 500–1,000 km
up to
10,000 km contain free-moving particles that may migrate into and out of the
magnetosphere or the solar wind. He & H may be lost into space.
Water – Properties, Reservoir and Water Cycle:
Water is essential for life. The types of plants and animals distributed on the earth and
in water bodies greatly depend upon the availability and nature of water, fresh or sea
water. The organisms have to face different problems of water scarcity or abundance. In
aquatic ecosystem, organisms have to face the problem of excess water. These maintain
water balance in different ways. Water problem in terrestrial ecosystem is a major
limiting factor. Land organisms having to face the problem of deficiency of water,
develop various devices to minimize the loss of water.
Importance of Water
i) Required for solubilization of chemicals and several biochemical reactions like
hydrolytic digestion of polymeric nutrients, photosynthesis, etc.
ii) Important for the working of macromolecules, as a good ionizer, transport of
materials, etc.
iii) Acts as a habitat for hydrophytic and aquatic animals and as an agent of geological
change.
iv) Acts as an agent of energy transfer and use.
v) Water is a tremendous factor in neutralising heat radiations of sunlight, so also acts
as a 'temperature buffer'
Properties of Water:
The physical arrangement of its component molecules makes water a unique substance.
A molecule of water consists of two atoms of hydrogen (H) joined to one atom of
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oxygen (O), represented by the chemical symbol H2O. The H atoms are bonded to the
O atoms asymmetrically, such that the two H atoms are at one end of the molecule and
the O atom is at the other. The bonding between the two hydrogen atoms and the
oxygen atom is via shared electrons (called a covalent bond), so that each H atom
shares a single electron with the oxygen. The shared hydrogen atoms are closer to the
oxygen atom than they are to each other. As a result, the side of the water molecule
where the H atoms are located has a positive charge, and the opposite side where the
oxygen atom is located has a negative charge, thus polarizing the water molecule
(termed a polar covalent bond Because of its polarity, each water molecule becomes
weakly bonded with its neighboring molecules. The positive (hydrogen) end of one
molecule attracts the negative (oxygen) end of the other. The angle between the
hydrogen atoms encourages an open, tetrahedral-like arrangement of water molecules.
This situation, when hydrogen atoms act as connecting links between water molecules,
is hydrogen bonding. The simultaneous bonding of a hydrogen atom to the oxygen
atoms of two different water molecules gives rise to a lattice arrangement of molecules.
These bonds, however, are weak in comparison to the bond between the hydrogen and
oxygen atoms. As a result, they are easily broken and reformed.Water has some unique
properties related to its hydrogen bonds.
One property is high specific heat —the number of calories necessary to raise 1
gram of water 1 degree Celsius. The specific heat of water is defined as a value of 1,
and other substances are given a value relative to water. Water can store tremendous
quantities of heat energy with a small rise in temperature. As a result, great quantities of
heat must be absorbed before the temperature of natural waters, such as ponds, lakes,
and seas, rises just 1°C. These waters warm up slowly in spring and cool off just as
slowly in the fall. This process prevents the wide seasonal fluctuations in the
temperature of aquatic habitats so characteristic of air temperatures and moderates the
temperatures of local and worldwide environments. The high specific heat of water also
is important in the thermal regulation of organisms. Because 75–95 percent of the
weight of all living cells is water, temperature variation is also moderated relative to
changes in ambient temperature. Due to the high specific heat of water, large quantities
of heat energy are required to change its state between solid (ice), liquid, and gaseous
(water vapor) phases. Collectively, the energy released or absorbed in transforming
water from one state to another is called latent heat. Removing only 1 calorie (4.184
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joules) of heat energy will lower the temperature of a gram of water from 2°C to 1°C,
but approximately 80 times as much heat energy (80 calories per gram) must be
removed to convert that same quantity of water at 1°C to ice (freezing point of 0°C).
Likewise, it takes 536 calories to overcome the attraction between molecules and
convert 1 g of water at 100°C into vapor, the same amount of heat needed to raise 536 g
of water 1°C.
The lattice arrangement of molecules gives water a peculiar density–temperature
relationship. Most liquids become denser as they are cooled. If cooled to their freezing
temperature, they become solid, and the solid phase is denser than the liquid. This
description is not true for water. Pure water becomes denser as it is cooled until it
reaches 4°C. Coolingbelow this temperature, results in a decrease in density. When 0°C
is reached, freezing occurs and the lattice structure is complete— each oxygen atom is
connected to four other oxygen atoms by means of hydrogen atoms. The result is a
lattice with large, open spaces and therefore decreased density. Water molecules when
frozen occupy more space than they would in liquid form. Because of its reduced
density, ice is lighter than water and floats on it. This property is crucial to life in
aquatic environments. The ice on the surface of water bodies insulates the waters
below, helping to keep larger bodies of water from freezing solid during the winter
months.
Due to hydrogen bonding, water molecules tend to stick firmly to each other,
resisting external forces that would break these bonds. This property is called cohesion
and a very important property for raising water to tips of tall trees.
In a body of water, these forces of attraction are the same on all sides. At the
water‘s surface, however, conditions are different. Below the surface, molecules of
water are strongly attracted to one another. Above the surface is the much weaker
attraction between water molecules and air. Therefore, molecules on the surface are
drawn downward, resulting in a surface that is taut like an inflated balloon. This
condition, called surface tension, is important in the lives of aquatic organisms. For
example, the surface of water is able to support small objects and animals, such as the
water striders ( Gerridaespp.) and water spiders ( Dolomedesspp.) that run across a
pond‘s surface. To other small organisms, surface tension is a barrier, whether they
wish to penetrate the water below or escape into the air above. For some, the surface
tension is too great to break; for others, it is a trap to avoid while skimming the surface
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to feed or to lay eggs. If caught in the surface tension, a small insect may flounder on
the surface. The nymphs of mayflies (Ephemeropteraspp) and caddis flies
(Trichopteraspp.) that live in the water and transform into winged adults are hampered
by surface tension when trying to emerge from the water. While slowed down at the
surface, these insects become easy prey for fish. Cohesion is also responsible for the
viscosity of water.
Viscosity is the property of a material that measures the force necessary to separate
the molecules and allow an object to pass through the liquid. Viscosity is the source of
frictional resistance to objects moving through water. This frictional resistance of water
is 100 times greater than that of air. The streamlined body shape of many aquatic
organisms, for example most fish and marine mammals, helps to reduce frictional
resistance. Replacement of water in the space left behind by the moving animal
increases drag on the body. An animal streamlined in reverse, with a short, rounded
front and a rapidly tapering body, meets the least water resistance. The perfect example
of such streamlining is the sperm whale (Physeter catodon).
Water‘s high viscosity relative to air is due largely to its greater density. The
density of water is about 860 times greater than that of air (pure water has a density of
1000 kg/m3.
Although the resulting viscosity of water limits the mobility of aquatic organisms,
it also benefits them. If a body submerged in water weighs less than the water it
displaces, it is subjected to an upward force called buoyancy. Because most aquatic
organisms (plants and animals) are close to neutral buoyancy (their density is similar to
that of water), they do not require structural material such as skeletons or cellulose to
hold them erect against the force of gravity. Similarly, when moving on land, terrestrial
animals must raise their mass against the force of gravity for each step they take. Such
movement requires significantly more energy than swimming movements do for
aquatic organisms. But water‘s greater density can profoundly affect the metabolism of
marine organisms inhabiting the deeper waters of the ocean.
Because of its greater density, water also experiences greater changes in pressure
with depth than does air. At sea level, the weight of the vertical column of air from the
top of the atmosphere to the sea surface is 1 kg/cm 2 or 1 atmosphere (atm). In contrast,
pressure increases 1 atm for each 10 m in depth. Because the deep ocean varies in depth
from a few hundred meters down to the deep trenches at more than 10,000 m, the range
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of pressure at the ocean bottom is from 20 atm to more than 1000 atm. Recent research
has shown that both proteins and biological membranes are strongly affected by
pressure and must be modified to work in animals living in the deep ocean.
Global Reservoirs of Water:
Water is the major constituent of hydrosphere and covers 4/5 of the earth‘s surface. In
fact water is all around us, every cubic mm of air even over dry deserts has water
vapour. Water is present in the soil beneath our feet. The total volume of water in the
hydrosphere is 1.4 billion cubic kilometre. About 97% is oceanic, only 03% is fresh –
77.2% stored in ice caps & glaciers, 22.4% is ground water, the rest 0.36% is
distributed in lakes, swamps, rivers and streams. Repeating in other words - the total
volume of water on Earth is approximately 1.4 billion cubic kilometers (km3) of which
more than 97 percent resides in the oceans. Another 2 percent of the total is found in
the polar ice caps and glaciers, and the third largest active reservoir is groundwater (0.3
percent). Over the oceans, evaporation exceeds precipitation by some 40,000 km3. A
significant proportion of the water evaporated from the oceans is transported by winds
over the land surface in the form of water vapor, where it is deposited as precipitation.
Water Cycle:
All marine and freshwater aquatic environments are linked,either directly or indirectly,
as components of the water cycle(also referred to as thehydrologic cycle; —the
processby which water travels in a sequence from the air to Earthand returns to the
atmosphere.Solar radiation, which heats Earth‘s atmosphere and providesenergy for the
evaporation of water, is the driving forcebehind the water cycle.Precipitation sets
thewater cycle in motion. Water vapor, circulating in the atmosphere,eventually falls in
some form of precipitation. Some ofthe water falls directly on the soil and bodies of
water. Some isintercepted by vegetation, dead organic matter on the ground,and urban
structures and streets (interception).Because of interception, which can be
considerable, various amounts of water never infiltrate the ground but evaporatedirectly
back to the atmosphere. Precipitation that reaches thesoil moves into the ground
byinfiltration. The rate of infiltrationdepends on the type of soil, slope, vegetation, and
intensityof the precipitation. During heavy rainswhen the soil is saturated, excess water
flows across the surfaceof the ground assurface runoff or overland flow. At places,
itconcentrates into depressions and gullies, and the flow changes from sheet to
channelized flow—a process that can be observedon city streets as water moves across
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the pavement into gutters. Because of low infiltration, runoff from urban areas might
beas much as 85 percent of the precipitation.Some water entering the soil seeps down to
an imperviouslayer of clay or rock to collect as groundwater. From there, water finds its
way into springs and streams. Streams coalesceinto rivers as they follow the topography
of the landscape. In basins and floodplains, lakes and wetlands form. Rivers eventually
flow to the coast, forming the transition from freshwater tomarine environments.
Water remaining on the surface of the ground, in the upper layers of the soil, and
collected on the surface of vegetation—aswell as water in the surface layers of streams,
lakes, and oceans— returns to the atmosphere by evaporation. The rate of evaporation
is governed by how much water vapor is in the air relative to the saturation vapor
pressure Plants cause additional water loss from the soil. Through their roots, they take
in water from the soil and lose it through the leaves and other organs in a process called
transpiration.
Of the 111,000 km3 of water that falls as precipitation on the land surface, only
some 71,000 km3 is returned to the atmosphere as evapotranspiration. The remaining
40,000 km3 is carried as runoff by rivers and eventually returns to the oceans. This
amount balances the net loss of water from the oceans to the atmosphere through
evaporation that is eventually deposited on the continents (land surface) as
precipitation. The relatively small size of the atmospheric reservoir (only 13 km3) does
not reflect its importance in the global water cycle. The importance of the atmosphere
in the global water cycle is better reflected by the turnover time of this reservoir. The
turnover time is calculated by dividing the size of the reservoir by the rate of output
(flux out). For example, the turnover time for the ocean is the size of the reservoir (1.37
x106km3) divided by the rate of evaporation (425 km3 per year) or more than 3000
years. In contrast, the turnover time of the atmospheric reservoir is approximately 0.024
year. That is to say, the entire water content of the atmosphere is replaced on average
every nine days.
Soil Development:
Soil is the medium for plant growth; the principal factor controlling the fate of water in
terrestrial environments; nature‘s recycling system, which breaks down the waste
products of plants and animals and transforms them into their basic elements; and
habitat to a diversity of animal life, from small mammals to countless forms of
microbial life . As familiar as it is, soil is hard to define. One definition says that soil is
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a natural product formed and synthesized by the weathering of rocks and the action of
living organisms. Another, states that soil is a collection of natural bodies of earth,
composed of mineral and organic matter and capable of supporting plant growth. Soil is
not just an abiotic environment for plants. It is teeming with life—billions of minute
and not so minute animals, bacteria, and fungi. The interaction between the biotic and
the abiotic makes the soil a living system. Soil scientists recognize soil as a threedimensional unit, or body, having length, width, and depth.
Soil formation begins with the weathering of rocks and theirminerals. Weathering
includes the mechanical destruction ofrock materials into smaller particles as well as
their chemical modification.Mechanical weathering results from the interactionof
several forces. When exposed to the combined actionof water, wind, and temperature,
rock surfaces flake and peelaway. Water seeps into crevices, freezes, expands, and
cracksthe rock into smaller pieces. Wind-borne particles, such as dust and sand, wear
away at the rock surface. Growing roots of trees split rock apart. Without appreciably
influencing their composition, physical weathering breaks down rock and minerals into
smaller particles. Simultaneously, these particles are chemically altered and broken
down throughchemical weathering. The presence of water, oxygen, and acids resulting
from the activities of soil organisms and the continual addition of organic matter (dead
plant and animal tissues) enhance the chemical weathering process. Rainwater falling
on and filtering through this organic matter and mineral soil sets up a chain of chemical
reactions that transform the composition of the original rocks and minerals.
Soil Formation Involves Five Interrelated Factors:
Five interdependent factors are important in soil formation: parent material, climate,
biotic factors, topography, and time.Parent material is the material from which soil
develops. The original parent material could originate from the underlyingbedrock;
from glacial deposits (till); from sand and silt carried by the wind (eolian); from gravity
moving material down aslope (colluvium); and from sediments carried by flowing
water (fluvial), including water in floodplains. The physical character and chemical
composition of the parent material are important in determining soil properties,
especially during the early stages of development.
Biotic factors—plants, animals, bacteria, and fungi—all contribute to soil
formation. Plant roots can function to break up parent material, enhancing the process
of weathering, as well as to stabilize the soil surface and thus reduce erosion. Plant
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roots pump nutrients up from soil depths and add them to the surface. In doing so,
plants recapture minerals carried deep into the soil by weathering processes. Through
photosynthesis, plants capture the Sun‘s energy and transfer some of this energy to the
soil in the form of organic carbon. On the soil surface, microorganisms break down the
remains of dead plants and animals that eventually become organic matter incorporated
into the soil.
Climate influences soil development both directly and indirectly. Temperature,
precipitation, and winds directly influence the physical and chemical reactions
responsible for breaking down parent material and the subsequent leaching (movement
of solutes through the soil) and movement of weathered materials. Water is essential for
the process of chemical weathering, and the greater the depth of water percolation, the
greater the depth of weathering and soil development. Temperature controls the rates of
biochemical reactions, affecting the balance between the accumulation and breakdown
of organic materials. Consequently, under conditions of warm temperatures and
abundant water, the processes of weathering, leaching, and plant growth (input of
organic matter) are maximized.
Topography, the contour of the land, can affect how climate influences the
weathering process. More water runs off and less enters the soil on steep slopes than on
level land, whereas water draining from slopes enters the soil on low and flat land.
Steep slopes are also subject to soil erosion and soil creep—the downslope movement
of soil material that accumulates on lower slopes and lowlands.
Time is a crucial element in soil formation: all of the factors just listed assert
themselves through time. The weathering of rock material; the accumulation,
decomposition, and mineralization of organic material; the loss of minerals from the
upper surface; and the downward movement of materials through the soil all require
considerable time. Forming well-developed soils may require 2000 to 20,000 years.
Physico Chemical Properties of Soil:
Soils are distinguished by differences in their physical and chemical properties.
Physical properties include color, texture, structure, moisture, and depth, pH etc. All
may be highly variable from one soil to another.
Color is one of the most easily defined and useful characteristics of soil. It has
little direct influence on the function of a soil but can be used to relate chemical and
physical properties.Organic matter (particularly humus) makes soil dark or black. Other
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colors can indicate the chemical composition of the rocksand minerals from which the
soil was formed. Oxides of iron give a color to the soil ranging from yellowish-brown
to red, whereas manganese oxides give the soil a purplish to black color. Quartz, kaolin,
gypsum, and carbonates of calcium and magnesium give whitish and grayish colors to
the soil. Blotches of various shades of yellowish-brown and gray indicate poorly
drained soils or soils saturated by water. Soils are classified by color using standardized
color charts.
Soil texture is the proportion of different-sized soil particles. Texture is partly
inherited from parent material and partly a result of the soil-forming process. Particles
are classified on the basis of size into gravel, sand, silt, and clay. Gravel consists of
particles larger than 2.0 mm. They are not part of the fine fraction of soil. Soils are
classified based on texture by defining the proportion of sand, silt, and clay. Sand
ranges from 0.05 to 2.0 mm, is easy to see, and feels gritty. Silt consists of particles
from 0.002 to 0.05 mm in diameter that can scarcely be seen by the naked eye; it feels
and looks like flour. Clay particles are less than 0.002 mm, too small to be seen under
an ordinary microscope. Clay controls the most important properties of soils, including
its water-holding capacity and the exchange of ions between soil particles and soil
solution. A soil‘s texture is the percentage (by weight) of sand, silt, and clay. Based on
proportions of these components, soils are divided into texture classes. Soil texture
affects pore space in the soil, which plays a major role in the movement of air and
water in the soil and the penetration by roots. In an ideal soil, particles make up 50
percent of the soil‘s total volume; the other 50 percent is pore space. Pore space
includes spaces within and between soil particles, as well as old root channels and
animal burrows. Coarse-textured soils have large pore spaces that favor rapid water
infiltration, percolation, and drainage. The finer the texture the smaller the pores, and
the greater the availability of active surface for water adhesion and chemical activity.
Very fine- textured or heavy soils, such as clays, easily become compacted if plowed,
stirred, or walked on. They are poorly aerated and difficult for roots to penetrate.
Soil depth varies across the landscape, depending on slope, weathering, parent
materials, and vegetation. In grasslands, much of the organic matter added to the soil is
from the deep, fibrous root systems of the grass plants. By contrast, tree leaves falling
on the forest floor are the principal source of organic matter in forests. As a result, soils
developed under native grassland tend to be several meters deep, and soils developed
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under forests are shallow. On level ground at the bottom ofslopes and on alluvial plains,
soils tend to be deep. Soils on ridge tops and steep slopes tend to be shallow, with
bedrock close to the surface.
The pH of soil in terrestrial environments or of water in aquatic ones is a condition
that can exert a powerful influence on the distributionand abundance of organisms. The
protoplasm of the root cells of most vascular plants is damaged as a direct result of
toxic concentrations of H+ or OH− ions in soils below pH 3 or above pH 9,
respectively. Further, indirect effects occur because soil pH influences the availability
of nutrients and/or the concentration of toxins . Increased acidity (low pH) may act in
three ways:
(i) Directly, by upsetting osmoregulation, enzyme activity or gaseous exchange across
respiratory surfaces;
(ii) Indirectly, by increasing the concentration of toxic heavy metals, particularly
aluminum (Al3+) but also manganese (Mn2+) and iron (Fe3+), which are essential
plant nutrients at higher pHs; and
(iii) Indirectly, by reducing the quality and range of food sources available to animals
(e.g. fungal growth is reduced at low pH in streams and the aquatic flora is often
absent or less diverse).
Tolerance limits for pH vary amongst plant species, but only a minority is able to
grow and reproduce at a pH below about 4.5. In alkaline soils, iron (Fe3+) and
phosphate (PO4 3+), and certain trace elements such as manganese (Mn2+), are fixed
in relatively insoluble compounds, and plants may then suffer because there is too little
rather than too much of them. For example, calcifuge plants (those characteristic of acid
soils) commonly show symptoms of iron deficiency when they are transplanted to more
alkaline soils. In general, however, soils and waters with a pH above 7 tend to be
hospitable to many more species than those that are more acid. Chalk and limestone
grasslands carry a much richer flora (and associated fauna) than acid grasslands and the
situation is similar for animals inhabiting streams, ponds and lakes.
Soil Profile:
Initially, soil develops from undifferentiated parent material. Over time, changes occur
from the surface down, through the accumulation of organic matter near the surface and
the downward movement of material. These changes result in the formation of
horizontal layers that are differentiated by physical, chemical, and biological
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characteristics. Collectively, a sequence of horizontal layers constitutes a soil profile.
This pattern of horizontal layering, or horizons, is easily visible where a recent cut has
been made along a road bank or during excavation for a building site. The simplest
general representation of a soil profile consists of four horizons: O, A, B, and C.
The surface layer is the O horizon, or organic layer. This horizon is dominated by
organic material, consisting of partially decomposed plant materials such as leaves,
needles, twigs, mosses, and lichens. This horizon is often subdivided into a surface
layer composed of undecomposed leaves and twigs (Oi), a middle layer composed of
partially decomposed plant tissues (Oe), and a bottom layer consisting of dark brown to
black, homogeneous organic material—the humus layer (Oa). This pattern of layering
is easily seen by carefully scraping away the surface organic material on the forest
floor. In temperate regions, the organic layer is thickest in the fall, when new leaf litter
accumulates on the surface. It is thinnest in the summer after decomposition has taken
place.
Below the organic layer is the A horizon, often referred to as the topsoil. This is
the first of the layers that are largely composed of mineral soil derived from the parent
materials. In this horizon, organic matter (humus) leached from above accumulates in
the mineral soil. The accumulation of organic matter typically gives this horizon a
darker color, distinguishing it from lower soil layers. Downward movement of water
through this layer also results in the loss of minerals and finer soil particles, such as
clay, to lower portions of the profile— sometimes giving rise to an E horizon, a zone
or layer of maximum leaching, or eluviation (from Latin ex or e, ―out,‖ and lavere , ―to
wash‖) of minerals and finer soil particles to lower portions of the profile. Such E
horizons are quite common in soils developed under forests, but because of lower
precipitation they rarely occur in soils developed under grasslands.
Below the A (or E) horizon is the B horizon, also called the subsoil. Containing
less organic matter than the A horizon, the B horizon shows accumulations of mineral
particles such as clay and salts due to leaching from the topsoil. This process is called
illuviation (from the Latin il, ―in,‖ and lavere , ―to wash‖). The B horizon usually has a
denser structure than the A horizon, making it more difficult for plants to extend their
roots downward. B horizons are distinguished on the basis of color, structure, and the
kind of material that has accumulated as a result of leaching from the horizons above.
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The C horizon is the unconsolidated material that lies under the subsoil and is
generally made of original material from which the soil developed. Because it is below
the zones of greatest biological activity and weathering and has not been sufficiently
altered by the soil-forming processes, it typically retains much of the characteristics of
the parent materials from which it was formed. Below the C horizon lies the bedrock.
Basic Concepts of Climate Change and its Impact on Plants:
Is Earth‘s climate changing? According to the Intergovernmental Panel on Climate
Change (IPCC), the answer is unequivocallyyes. This conclusion is drawn from a suite
of observations that allow scientists to track changes in the global climate over the past
century. Widespread direct measurements of surface temperature began around the
middle of the 19th century.These direct measures from instruments such as
thermometers are referred to as the instrumental record. Observations of othersurface
weather variables, such as precipitation and winds, havebeen made for about 100
years.Besides measurements made at the land surface, observationsof sea surface
temperatures have been made from ships since themid-19th century. Measurements of
theupper atmosphere have been made systematically since the late1940s, but since the
late 1970s, Earth-observing satellites have been providing a continuous record of global
observations for awide variety of climate variables.So what do these climate records
reveal? The global averagesurface temperature has increased by 0.74°C (+ - 0.2°C)
since theearly 20th century. The five warmest years in the instrumentalrecord since
1850 are, in descending order, 2005, 2010, 1998,2003, and 2002. New analyses of daily
maximum and minimumland-surface temperatures for 1950 to 2000 show that
thediurnal temperature range is decreasing. On average, minimumtemperatures are
increasing at about twice the rate of maximumtemperatures (0.2°C versus
0.1°C/decade). In other words, nighttimetemperatures (minimum) have increased more
than daytimetemperatures (maximum) over this period.New analyses also indicate that
global ocean heat content has increased significantly since the late 1950s. More than
halfof the increase in heat content has occurred in the upper 300m ofthe ocean; in this
layer the temperature has increased at a rate ofabout 0.04°C/decade.
Climate reconstructionsof the more recent past (1000 years ago to the present),
however, suggest that the warming trend observed overthe past century is consistent
with that expected from the risingatmospheric concentrations of greenhouse gases. A
consensushas begun to emerge, with the most recent IPCC report (2007)stating that
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most of the observed increase in global averagetemperature since the mid-20th century
is ―very likely‖ due toobserved changes in the atmospheric concentrations of
greenhousegases.
Climate Change influences almost every aspect of the ecosystem: the physiological and
behavioral response of organisms; the birth, death, and growth rates of populations; the
relative competitive abilities of species; community structure; productivity; and cycling
of nutrients.
Plant structures: leaves and roots
Plant structure and physiology are markedly altered due to increased levels of CO2; this
includes increased leaf expansion and cell wall extensibility and often cell turgor
pressure, leading to increased leaf and root growth. However, various studies have
reported that this initial increase would not be sustained leading to an overall decline in
growth and productivity.
Seasonal growth
Contrasting seasonal growth responses to elevated CO2 and temperature in certain
species suggests that pasture management may change in the future.The grazing season
may be prolonged, but whole-season productivity may become more variable than
today. This is shown by studies of perennial ryegrass where, in spring, increased leaf
extension occurred in elevated CO2 whilst in summer it was reduced. In high
temperature it was reduced in both seasons. Water reserves gained during the winter
may, in some cases, be depleted earlier.
Climate change and agriculture
Climate change will affect agriculture through effects on crops and weeds, soils, insects
and disease. In terms of crops, the main climatic variables that are important are
temperature, solar radiation, water and atmospheric CO2 concentration. However,
whilst plant development is generally increased by temperature, CO2 enrichment can
accelerate it even further in some cases, whilst in other cases it may have no effect or
retarding effects.Plant growth and crop yields depend on temperature and temperature
extremes. The optimum range for C3 crops is 15–20°C and for C4 crops it is 25–30°C.
The variation in temperature requirements and temperature extremes of different
cultivars of the same species, and among species, is quite wide for most crops. C3
plants are sensitive to higher CO2 and typically respond with an increase in
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photosynthesis and growth, whilst C4 plants don't respond so dramatically. Water stress
has often been observed to be ameliorated by increasing concentrations of CO2.
The effects on yield and phenology
Through global warming, an anticipated increase in temperature can potentially have
various effects, e.g. spikelet sterility in rice, reversal of vernalisation in wheat, reduced
formation of tubers in potatoes, loss of pollen viability in maize. Yields can be severely
affected if temperatures exceed critical limits for periods as short as 1 h during anthesis
(flowering). Flowering is a very important event in crop development, as it is a phase
which is particularly vulnerable to environmental stresses (Roberts et al.,
1993).Increased temperature can speed phenological development, reducing the grainfilling period for crops and lowering yield. Crop yields were greater under elevated
CO2, but warmer temperatures reduced the duration of crop growth and, hence, the
yield of crops such as winter wheat and onion.
ii. Ecological Adaptations:
The special characteristics of plants and animals that enable them to be successful in
certain environmental conditions are called adaptations.
Hydrophytic Adaptations:
As the aquatic environment is uniform throughout, the hydrophytes develop very few
adaptive features.
A. Morphological
(i) Roots
Root systems in hydrophytes are poorly developed which may or may not be branched
in submerged hydrophytes. Roots are meaningless as the plant body is in direct contact
with water and acts as absorptive surface and absorbs water and minerals. This may
probably be the reason why roots in hydrophytes are reduced or absent. Roots of
floating hydrophytes show very poor development of root hairs.
Roots in floating plants do not possess true root caps but very often they develop
root pockets or root sheaths which protect their tips from injuries. Exact functions of
these root pockets, however, are not fully understood. Some rooted hydrophytes like
Hydrilla, Vallisneria, Elodea canadensis, though they derive their nourishments from
water by their body surfaces, are partly dependent on their roots for minerals from the
soil.
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Roots are totally absent in some plants, e g., Ceratophyllum, Salvinia, Azolla,
Utricularia, etc. In Jussiaea repens two types of roots develop when the plants grow on
the surface of water, some of them are floating roots which are negatively geotropic
having spongy structures. The floating roots keep the plants afloat.
(ii) Stem
In aquatic plants, stem is very delicate and green or yellow in colour. In some cases it
may be modified into rhizome orrunner, etc.
(iii) Leaves
(a) In floating plants leaves are generally peltate, long, circular, light or dark green in
colour, thin and very smooth. Their upper surfaces are exposed in the air but lower
Leaves are generally in touch with water. In lotus plant petioles of leaves show
indefinite power of growth and they keep the laminae of leaves always on the surface of
water.
(iv) Heterophylly
Some aquatic plants develop two different types of leaves in them. This phenomenon is
termed as heterophylly. Examples are Sagittaria sagitifolia, Ranunculus aquatilis,
Limnophila heterophylla, Salvinia, Azolla etc. In this phenomenon, generally the
submerged leaves are linear ribbon shaped or highly dissected and the leaves that are
found floating on or above the surface of water are broad circular or slightly lobed. The
occurrence of heterophylly is associated probably with the following characteristic
physiological behaviours of these aquatic plants:
1. Quantitative reduction in transpiration.
2. The broad leaves on the surface overshadow the submerged dissected leaves of the
same plant and thus they reduce the intensity of light falling on the submerged
leaves. The submerged leaves require light of very low intensity.
3. Plants show very little response to drought because the necessity of excess water
during drought period is compensated by submerged leaves which act as water
absorbing organs.
4. Variation in the life-forms and habitats.
5. Broad leaves found on the surface of water transpire actively and regulate the
hydrostatic pressure in the plant body.
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(v) Leaves of free floating hydrophytes are smooth, shining and frequently coated with
wax. The wax coating protects the leaves from chemical and physical injuries and
also prevents the water clogging of stomata.
(vi) In floating plants of water hyacinth, Trapa etc., the petioles become
characteristically swollen and develop sponginess which provides buoyancy to
these plants.
(vii) Leaves in submerged hydrophytes are generally small and narrow. In some case,
e.g., Myriophyllum, Utricularia, Ceratophyllum, etc., they may be finely dissected.
The small slender and terete segments of dissected leaves offer little resistance
against the water currents. In this way plants are subjected to little mechanical
stress and strain of water.
(viii) In the Amphibious plants, the leaves that are exposed to air show typical
mesophytic features. They are more tough than the leaves of other groups of
hydrophytes.
(ix) Pollination and dispersal of fruits and seeds are accomplished by the agency of
water. Seeds and fruits are light in weight and thus they can easily float on the
surface of water.
(x) Vegetative reproduction is common method of propagation in hydrophytes. It is
accomplished either through fragmentation of ordinary shoots or by winter buds.
B. Anatomical Modifications
The anatomical modifications in hydrophytes aim mainly at:
1. Reduction in protecting structures,
2. Increase in the aeration,
3. Reduction of supporting or mechanical tissues, and
4. Reduction of vascular tissues.
Various anatomical adaptations of hydrophytes are listed below:
1. Reduction in protecting structures:
(a) Cuticle is totally absent in the submerged parts of the plants. It may be present in the
form of very fine film on the surfaces of parts which exposed to atmosphere.
(b) Epidermis in hydrophytes is not a protecting layer but it absorbs water, minerals and
gases directly from the aquatic environment. Extremely thin cellulose walls of
epidermal cells facilitate the absorption process.
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(c) Epidermal cells contain chloroplasts, thus they can function as photosynthetic tissue,
especially where the leaves and stems are very thin, e.g. Hydrilla (Fig. 8.8).
(d) Hypodermis in hydrophytes is poorly developed. Its cells are extremely thin walled.
2. Increase in the aeration
(a) Stomata are totally absent in submerged parts of the plants. In some exceptional
cases, vestigial and functionless stomata have been noticed. In these cases exchange of
gases takes place directly through cell walls. In the floating leaves, stomata develop in
very limited number and are confined only to the upper surface. In amphibious plants
stomata may be scattered on all the aerial parts and they develop comparatively in
larger number per unit area than those on the floating leaves
(b) Air chambers:
Aerenchyma in submerged leaves and stem is very much developed. Air chambers are
filled with respiratory gases and moisture. These cavities are separated from one
another by one or two cells thick chlorenchymatous partitions. CO2 present in the air
chambers is used in the photosynthesis and the O2 produced in the process of
photosynthesis and also that already present in the air chambers is used in respiration.
The air chambers also develop finely perforated cross septa which are called
diaphragms. The diaphragms afford better aeration and perhaps check floating. The
Aerenchyma provides buoyancy and mechanical support to aquatic plants. Air
chambers are abundantly found in the fruits of hydrophytes rendering them buoyant and
thus facilitating their dispersal by water. Development of air chambers in the plants is
governed by habitat. This point is clear from the anatomy of Jussiaea sp. In this case,
air chambers develop normally if plants are growing in water but they seldom develop
if the plants are growing on the land.
3. Reduction of supporting or mechanical tissues:
(a) Mechanical tissues are absent or poorly developed in the floating and submerged
parts of plants because buoyant nature of water saves them from physical injuries.
The thick walled sclerenchymatous tissue is totally absent m submerged and
floating hydrophytes. They may, however, develop in the cortex of amphibious
plants particularly in the aerial or terrestrial parts. Generally elongated and loosely
arranged spongy cells are found in the plant body. These thin-walled cells, when
turgid, provide mechanical support to the plants.
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(b) The reduction of absorbing tissue (roots act chiefly as anchors and root hairs are
lacking).
(c) In water lily and some other plants, special type of star shaped lignified cells, called
asterosclereids, develop which give mechanical support to the plants.
4. Reduction of vascular tissues
Conducting tissue is very poorly developed. As the absorption of water and nutrients
takes place through the entire surface of submerged parts, there is little need of vascular
tissues in these plants. In the vascular tissues, xylem shows greatest reduction. In some
cases, it consists of only a few tracheids while in some, xylem elements are not at all
developed. Some aquatic plants, however, show a lacuna in the centre in the place of
xylem. Such spaces resemble typical air chambers.
Phloem tissue is also poorly defined in most of the aquatic plants but in some cases
it may develop fairly well. Sieve tubes of aquatic plants are smaller than those of
mesophytes. Phloem parenchyma is extensively developed. Endodermis may or may
not be clearly defined. The Vascular bundles are generally aggregated towards the
centre. Secondary growth in thickness does not take place in the aquatic stem and roots.
C. Physiological adaptations in hydrophytes
The aquatic plants exhibit a low compensation point and low osmotic concentration of
cell sap. Osmotic concentration of cell sap is equal or slightly higher than that of water.
Nutrients are absorbed by the submerged plants through the general plant surface. The
gases are exchange from the water through the surface cells.
The gases produced during photosynthesis and respiration are partly retained in the
air chambers of aerenchyma to be utilized as and when required. There is no
transpiration from the submerged hydrophytes. However emergent plants and free
floating hydrophytes have excessive rate of transpiration. Mucilage cells and mucilage
canals secrete mucilage to protect the plant body from decay under water.
Xerophytic Adaptations
Plants which grow in dry habitats or xeric conditions are called xerophytes. Places
where available water is not present adequate quantity are termed xeric habitats.Xeric
habitats may be of following types:
1. Habitats physically dry (where water retaining capacity of the soil is very low and
the climate is dry, e.g., desert, rock surface, waste land, etc.).
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2. Habitats physiologically dry (places where water is present in excess amount but it is
not such as can be absorbed by the plants easily. Such habitats may be too salty or
too acidic, too hot or too cold.
3. Habitats dry physically as well as physiologically, e.g., slope of mountains.
Xerophytes are characteristic plants of desert and semi-desert regions, yet they can
grow in mesophytic conditions where available water is in sufficient quantity. These
plants
can
withstand
extreme
dry conditions,
low
humidity and
high
temperature.When growing under un-favourable conditions, these plants develop
special structural and physiological characteristics which aim mainly at the
following objectives:
(i) To absorb as much water as they can get from the surroundings
(ii) To retain water in their organs for very long time
(iii) To reduce the transpiration rate to minimum; and
(iv) To check high consumption of water
Xerophytes are categorized into several groups according to their drought resisting
power. These groups are as follows:
1. Drought escaping plants
These xerophytes are short-lived. During critical dry periods they survive in the form of
seeds and fruits which have hard and resistant seed-coats and pericarps respectively. At
the advent of favourable conditions (which are of very short duration), the seeds
germinate into new small sized plants which complete their life cycles within a few
weeks‘ time. The seeds become mature before the dry condition approaches.In this
way, plants remain unaffected by extreme conditions. These are called ephemerals or
drought evaders or drought escapers. These plants are very common in the semiarid
zones where rainy season is of short duration. Examples—(Papilionaceae), some
inconspicuous compositae (e.g., Artemesia) and members of families Zygophyllaceae,
Boraginaceae, some grasses etc.
2. Drought enduring plants
These are small sized plants which have capacity to endure or tolerate drought.
3. Drought resistant plants:
These plants develop certain adaptive features in them through which they can resist
extreme droughts. Xerophytes grow on a variety of habitats. Some grow on rocky soils
(Lithophytes) some in deserts, some on the sand and gravels (Psammophytes) and some
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may grow on the waste lands (Eremophytes). Some plants of xeric habitat have water
storing fishy organs, while some do not develop such structures.
On this ground xerophytes can be divided into two groups which are as follows:
(1) Succulent xerophytes.
(2) Non-succulents, also called true xerophytes.
Succulent xerophytes are those plants in which some organs become swollen and fleshy
due to active accumulation of water in them or in other words, the bulk of the plant
body is composed of water storing tissues. Water stored in these tissues is consumed
during the period of extreme drought when the soil becomes depleted of available
water.
1. Morphological Adaptations of Xerophytes
(A) Roots
Xerophytes have well developed root systems which may be profusely branched. It is
extensive and more elaborate than shoot system. Many desert plants develop superficial
root system where the supply of water is restricted to surface layer of the earth. The
roots of perennial xerophytes grow very deep in the earth and reach the layers where
water is available in plenty. Root hairs are densely developed near the growing tips of
the rootlets. These enable the roots to absorb sufficient quantity of water.
(B) Stem
(i) Stems of some xerophytes become very hard and woody. It may be either aerial or
subterranean.
(ii) They are covered with thick coating of wax and silica as in Equisetum. Some may
be covered with dense hairs as is Calotropis.
(iii) In some xerophytes, stems may be modified into thorns, e.g., Duranta, Ulex, etc.
(iv) In stem succulents, main stem itself becomes bulbous and fleshy and it seems as if
leaves in these plants are arising directly from the top of the roots, Kleinia articulata.
(v) Stems in some extreme xerophytes are modified into leaf-like flattened, green and
fleshy structures which are termed as phylloclades. Many cacti and cocoloba
(Muehlenbeckia) are familiar examples for this. In Ruscus plants, the branches
developing in the axils of scaly leaves become metamorphosed into leaf-like structures,
the phylloclades or cladophylls.
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In Asparagus plant also a number of axillary branches become modified into small
needle-like green structures which look exactly like leaves. They are called cladodes. A
number of species of Euphorbia also develop succulence and become green. In these
plants, leaves are greatly reduced, so the main function of leaves, the photosynthesis, is
taken up by these green phylloclades or cladodes which are modified stems.
(C) Leaves
(i) In some xerophytes the leaves, if present, are greatly caducous, i.e., they fall early in
the season, but in the majority of the plants leaves are generally reduced to scales, as in
Casuarina, Ruscus, Asparagus, etc.
(ii) Some evergreen xerophytes have needle-shaped leaves, e.g., Pinus
(iii) In leaf succulents, the leaves swell remarkably and become very fleshy owing to
storage of excess amount of water and latex in them. Plants with succulent leaves
generally develop very reduced stems. Examples of leaf succulents are Sedum , Aloe
spinossissima, Mesembryanthemum, Kleinia ficoides and several members of family
Chaenopodiaceae.
(iv) In majority of xerophytes, leaves are generally much reduced and are provided with
thick cuticle and dense coating of wax or silica. Sometimes they may be reduced to
spines, as for example, in Ulex, Opuntia, Euphorbia splendens, Capparis and Acacia.
(v) Generally, the leaves of xerophytic species possess reduced leaf blades or pinnae
and have very dense network of veins. In Australian species of Acacia (Babool) the
pinnae are shed from the rachis and the green petiole swells and becomes flattened
taking the shape of leaf. This modified petiole is termed as phyllode. The phyllode
greatly reduces the water loss, stores excess amount of water and performs
photosynthesis.
(vi) Trichophylly: In some xerophytes especially those growing well exposed to strong
wind, the under surfaces of the leaves are covered with thick hairs which protect the
stomatal guard cells and also check the transpiration. Those xerophytes which have
hairy covering on the leaves and stems are known as trichophyllous plants. Zizyphus,
Nerium, Calotropis procera, are important examples.
(vii) Rolling of leaves: Leaves in some extreme xerophytic grasses have capacity for
rolling or folding. In these cases stomata are scattered only on the upper or ventral
surface and as the leaves roll upwardly, stomata are effectively shut away from the
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outside atmosphere. This is effective modification in these plants for reducing the water
loss. Sun-dune grass is an important example for this.
(D) Flowers, fruits and seeds. Flowers usually develop in the favourable conditions.
Fruits and seeds are protected by very hard shells or coatings.
2. Anatomical Modifications in the Xerophytes
A number of modifications develop internally in the xeric plants and all aim principally
at water economy.The following are the anatomical peculiarities met within xerophytes:
(i) Heavy cutinisation, lignification‘s and wax deposition on the surface of epidermis
and even in the hypodermis are very common in xerophytes. Some plants secrete wax
in small quantity but some are regular source of commercial wax. Shining smooth
surface of cuticle reflects the rays of light and does not allow them to go deep into the
plant tissues. Thus, it checks the heavy loss of water.
(ii) Epidermis
Cells are small and compact. It is single layered, but multiple epidermes is not
uncommon. In Nerium leaf, epidermis is two or three layered. In stems, the epidermal
cells are radially elongated. Wax, tannins, resin, cellulose, etc., deposited on the surface
of epidermis form screen against high intensity of light. This further reduces the
evaporation of water from the surface of plant body. Certain grasses with rolling leaves
have specialized epidermis. In these, some of the epidermal cells that are found in the
depressions become more enlarged than those found in the ridges. These enlarged cells
are thin walled and are called bulliform cells or motor cells or hinge cells. These are
found usually on the upper surface of leaves between two parallel running vascular
bundles.The highly specialized motor cells facilitate the rolling of leaves by becoming
flaccid during dry periods. In moist conditions these cells regain their normal turgidity
which causes unrolling of the leaf margins. Bulliform cells are of common occurrence
in the leaf epidermis of sugarcane, bamboo, Typha and a number of other grasses.
(iii) Hairs
Hairs are epidermal in origin. They may be simple or compound, uni- or multicellular.
Compound hairs are branched at the nodes. These hairs protect the stomata and prevent
excessive water loss. In some plants, surfaces of stems and leaves develop
characteristic ridges and furrows or pits. The furrows and pits in these plants are the
common sites of stomata. Hairs found in these depressions protect the stomata from the
direct strokes of strong wind.
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(iv) Stomata
In xerophytes, reduction of transpiration is of utmost importance. It is possible only if
the stomatal number per unit area is reduced or if the stomata are elaborately modified
in their structures. In xerophytes, number of stomata per unit area of leaf is greater than
in mesophytes. They are generally of sunken type. In some cases, they may be found in
the furrows or pits.Subsidiary cells of sunken stoma may be of such shapes and
arrangement that they form an outer chamber that is connected by narrow opening or
the stoma. Such type of specialized stomata are very common in conifers, Cycas,
Equisetum, etc. Walls of the guard cells and subsidiary cells are heavily cutinized and
lignified in many xeric plants.
These devices have little value in directly reducing transpiration when stomata are
open. When the plants are wilting and stomata are closed then only lignified or
cuticularized walls of guard cells have protecting properties and under such
circumstances only cuticular transpiration is possible which is of little significance.
In dorsiventral leaves stomata are generally found on the lower surface, but m rolling
leaves they are scattered mostly on the upper surface. In the rolled leaves, stomata are
protected from the direct contact of outside wind. This is very important rather secured
device for lowering the rate of transpiration in xerophytic grasses.
(iv) Hypodermis
In xerophytes, just below the epidermis, one or several layers of thick walled
compactly grouped cells may develop that form the hypodermis. The cells may be
much like those of epidermis and may either be derived from epidermis or from the
cortex (in case of stem) or from the mesophyll (in case of leaf). The hypodermal cells
may sometimes be filled with tannin and mucilage.
(vi) Ground tissue
(a) In the stem, a great part of body is formed of sclerenchyma. In those cases, where
the leaves are either greatly reduced or they fall in the early season, the photosynthetic
activity is taken up by outer chlorenchymatous cortex. The chlorenchymatous tissue is
connected with the outside atmosphere through stomata. The gaseous exchange takes
place in regular manner in the green part of stem.
(b) In succulent stems and leaves, ground tissues are filled with thin walled
parenchymatous cells which store excess quantity of water, mucilage, latex, etc. This
makes the stems swollen and fleshy (Figs. 8.33, 8.34).
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(c) In the leaves, mesophyll is very compact and the intercellular spaces are greatly
reduced. Palisade tissue develops in several layers. There are some xerophytes in which
mesophyll is surrounded by thick hypodermal sheath of sclerenchyma from all the sides
except from below. This sheath forms a diaphragm against intense light. Such
xerophytes in which sclerenchyma is extensively developed are called sclerophyllous
plants. In succulent leaves, spongy parenchyma develops extensively which stores
water (Figs. 8.33, 8.34). In Pinus, the spongy cells of mesophylls are star shaped (Fig.
8.36).
(d) Intercellular spaces are greatly reduced. Cells in the body are generally very small,
thick walled and compactly grouped. They may be spherical, rounded or cuboid in
shape. Such cells are very common in xerophytes.
(vii) Conducting tissues: Conducting tissues, i.e., xylem and phloem develop very well
in the xerophytic body.
3. Physiological Adaptations in Xerophytes
It was long assumed that the structural adaptations in the body of xerophytes were
useful in reducing the transpiration but now a number of experiments related with the
physiology of these plants reveal some facts which are contrary to the early
assumptions. Works of Maximov support that except succulents, true xerophytes show
very high rate of transpiration. Under similar conditions, the rate of transpiration per
unit area in xerophytes is much higher than that in mesophyte. Stomatal frequency per
unit area of leaf surface in xerophytes is also greater than that in the mesophytic leaf.
(1) Succulents are well known to contain polysaccharides, pentosans and a number of
acids by virtue of which they are able to resist drought. The structural modifications in
these succulent xerophytes are directly governed by their physiology. How does the
succulence develop? Metabolic reaction which induces development of succulence is
the conversion of polysaccharides into pentosans. Pentosans have water binding
property. These pentosans together with nitrogenous compounds of the cytoplasm cause
accumulation of excess amount of water in the cells and consequently the succulence
develops.
(2) Another experimented fact in the physiology of succulent plants is that their stomata
open during night hours and remain closed during the day. This unusual feature is
associated with metabolic activity of these plants. In dark, these plants respire and
produce acids. The heavy accumulation of acids in the guard cells increases osmotic
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concentration which, in turn, causes inward flow of water in the guard cells. When
guard cells become turgid the stomata open. In the sunlight, acids dissociate to produce
carbon dioxide which is used up in the photosynthesis and as a result of this osmotic
concentration of cell sap decreases which ultimately causes closure of stomata.
(3) In xerophytes, the chemical compounds of cell sap are actively converted into wall
forming compounds that are finally incorporated into the cell walls. Conversions of
polysaccharides into anhydrous forms as cellulose, formation of suberin, etc., are some
examples.
(4) Some enzymes, such as catalases, peroxidases, are more active in xerophytes than in
mesophytes. In xerophytes, amylase enzyme hydrolyses the starch very actively.
(5) The capacity of xerophytes to survive during period of drought lies not only in the
structural features but also in the resistance of the hardened protoplasm to heat and
desiccation.
(6) Regulation of transpiration. Presence of the cuticle, polished surface, compact cells
and sunken stomata protected by stomatal hairs regulate the transpiration.
(7) High osmotic pressure of cell sap. The xerophytes have very high osmotic pressure
which increases the turgidity. The turgidity of cell sap exerts tension force on the cell
walls. In this way, wilting of cell is prevented. High osmotic pressure of cell sap also
affects the absorption of water.
Ecological Adaptation to Light:
Light is one of the most important ecological factors, which influence a wide range of
plant aspects. Different environments offer varied light conditions and plants undergo
modifications to survive under such light regimes, phenomenon called as adaptation.
The two light environments, which represent extremes of irradiance, are bright sunny
and light limited shady environments. Plants adapted to these irradiance extremes are
called as heliophytes and sciophytes respectively. The plant modifications, which
ensure their survival under these extremities of light regimes or light conditions, are
morphological, anatomical and physiological in their nature. The morphological
adaptations of plants to low light regimes (sciophytes) are increased leaf size, thin and
smooth leaves, horizontal leaf orientation, distichous phyllotaxy; All these features are
aimed at maximizing energy absorption in the light limited or shady environments.
Further, sciophytes have a less extensive root system, low stomatal density, single
layered palisade, high content of chlorophyll, high PSII/PSI ratio, high chl b/chl a ratio,
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a low rate of transpiration, low rate of photosynthesis and respiration as adaptations to
light limited or shady environments which collectively ensure their survival in such
light limited environment. Heliophytes on the other hand have a small leaf size, thick
leaf, high leaf area index, high stomatal density, small chloroplasts, multilayered
palisade, high leaf density, vertical leaf orientation, spiral phyllotaxy, high Rubisco and
soluble protein content, extensive root system, well developed vascular system, high
rate of transpiration, high rate of photosynthesis and respiration, a higher light
compensation point as adaptations to bright sun environments.
Adaptation to Chilling:
Tolerance to freezing is the most common strategy for plants to survive freezing stress.
The ability to increase freezing tolerance in response to low nonfreezing temperatures,
i.e. cold acclimation, is essential for plants to overwinter in temperate habitats. Cold
acclimation is thought to be governed by a complex of genes, inducible by low
temperatures, and involves a number of changes that could protect plant cells against
freezing stress. Synthesis of the products of cold-regulated genes leads to the final
increase in freezing resistance. Because of the complexity of the changes required to
achieve maximal cold hardiness, many genes may be involved in the overall process of
cold acclimation.
In the Polar and Alpine regions plant species have to adapt to some of the most
extreme conditions on Earth. In these conditions, not only vascular plants (such as
trees, bushes, grasses) but mosses, fungi, lichens and liverworts have adapted to survive
the extremes. Unlike animals, the majority of plant species do not have a way by which
they can move when their environment dramatically changes. Arguably this means that
the adaptations that develop as a response to changes in an organism‘s habitat are vital
for survival.
Plants grow close to the ground to reduce damage caused by wind and ice particles.
Have small leaves to conserve water which can be lost through the leaf surface. They
possess shallow root systems that allow the plant to grow in the active layer and avoid
the permafrost. Grow in close proximity to one another so that each plant acts as a
barrier for others for the wind and ice particles. Stem, buds and leaves are covered in
small hairs which creates a layer of insulation for protection against cold temperatures.
Can photosynthesise in extremely cold weather conditions that allow the plant to store
energy despite lack of sunlight for large parts of the year. Develop and produce seeds in
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a relatively short time that allows for germination to be possible in a small favourable
climatic window of time. Ability to survive on bare rock (Lichen) showing survival of a
species where soil does not exist
iii. Forest Type of India:
Champion and Seth (1968)
Tropical Wet Evergreen Forest
These are tall, dense and multi-layered forests generally found in regions having
rainfall in excess of 2500 mm. The total area under these forests is about 4.1 million ha,
distributed mainly in the Western Ghats, Upper Assam, Arunachal Pradesh, Andaman
and Nicobar Islands. The flora in these forests has Malayan affinities. Bamboos and
canes occur in specific locations. Ferns and epiphytes are also common.
Tropical Semi-evergreen Forest
These forests occur in areas adjoining tropical wet evergreen, and form a transition
between evergreen and moist deciduous forests. They are found locally in the Western
Ghats, Assam, and Arunachal Pradesh, parts of Orissa and Andaman and Nicobar
Islands. The total area under this forest type is 2.4 million ha. The growth of this forest
is poorer when compared with that of wet evergreen forest. The canopies are not
continuous and species richness is lower. Bamboos, canes, ferns, and epiphytes are
abundant.
Tropical Moist Deciduous Forest
These forests occur over an area of 22.4 million ha, distributed mainly in the Western
Ghats, Assam, Arunachal Pradesh, Mizoram, Bihar, West Bengal, Orissa, and
Uttaranchal. This forest type occurs in a strip along the foothills of Himalaya, another
strip along the east side of Western Ghats and in a large area in Chhota Nagpur and
north-east hills. These forests are common in areas where rainfall is 1500-2000 mm
with a dry season of 4 to 6 months. The most important forest communities are those
consisting of sal (Shorea robusta) and teak (Tectona grandis). The teak forests are
characteristics of southern form, whereas sal forests form the greater proportion in the
northern form. Bamboos are quite common. Bambusa arundinacea and Dendrocalamus
hamiltonii are the most common bamboo. These forests are usually 2 to 3 layered with
a much lower number of species as compared with the earlier type-groups.
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Littoral and Swamp Forests
These forests consist of evergreen species of varying densities and height, usually
associated with mesic habitats. These forests occupy an area of 0.7 million ha along the
coast. These forests are mostly in their developmental stage and are seral in nature; they
occur throughout the country, wherever wet and waterlogged conditions prevail. The
littoral and tidal forests occur along the coast, the latter being especially associated with
deltas of larger rivers. Swamp forests occur in north-east India along major river
systems. Mangrove forests are generally dominated by trees of the genera –
Rhizophora, Avicennia, Sonneratia, BruguieraKandelia and Ceriops. Some genera like
Heriteira and Xylocarpus could be locally important, as in Sundarbans.The tidal and
swamp forests are dominated by several evergreen and semi-evergreen species, while
species like Baringtoniaspp, Syzygiumcumini, and Dilleniaspp occur in seasonal
swamps. Fresh water swamps contain species such as Terminaliaarjuna, Lagerstroemia
speciosa,Trewianudiflora, and Myristica spp.
Tropical Dry Deciduous Forest
These forests occur from Kanyakumari to the foothills of the Himalaya in irregular
wide strips in areas having rainfall between 750 mm and 1250 mm. These forests are
concentrated in Rajasthan, Madhya Pradesh, Maharashtra, Orissa, Uttar Pradesh,
Karnataka, Andhra Pradesh and Tamil Nadu. The total area under these forests is
approximately 29.7 million ha. These forests consist of trees less than 25m high, with a
light canopy consisting of deciduous trees. Dry teak and dry sal communities
predominate in the southern and northern regions respectively. In some areas both these
species are absent and a mixture of trees like Anogeissuspendula, Boswelliaserrata,
Hardwickiabinata,Acacianilotica,Madhucaindica, and Buteamonosperma occupies the
area.Acacia catechu and Dalbergiasissoo are conspicuously present on newly formed
soils.
Tropical Thorn Forest
These forests occupy a large strip in southern Punjab, Haryana, northern Gujarat and
almost entire Rajasthan, where rainfall is about 250mm and 750 mm. Such forests are
also found over a large area in the upper Gangetic plains and Deccan plateau. The total
area under this forest type is about 5.2 million ha. These forests are open, consisting of
short trees, generally belonging to thorny leguminous species. The characteristic
species include Prosopis cineraria, Acacia leucophloea, Acacia nilotica, Ziziphusspp,
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and Salvadora spp. Acacia tortilis and Prosopis chilensis have been widely planted in
this region. In south India, important species are Acacia chundra and Acacia planifrons.
These forests are highly degraded due to severe biotic pressure and occur in the form of
scrub forests in most of the areas.
Tropical Dry Evergreen Forest
These forests are found in a relatively small area (0.1 million ha) on the Carnatic coast,
which receives little or no summer rainfall. The forests are low but often dense with
hard-leaved evergreen trees in which thorny species predominate. The characteristic
species are Memecylonedule, and Mababuxifolia.
Sub-tropical Broad-leaved Hill Forest
These forests occur in the lower slopes of the Himalaya in Bengal and Assam and on
other hill ranges such as Khasi, Nilgiri, Mahabaleshwar, Pachmarhi, Amarkantak and
Parasnath, occupying an area of about 0.3 million ha. Important species in the southern
hills are Syzygiumcumini, Ficusspp, and some species of Lauraceae. The northern form
consists of species like Quercus and Castanopsis.
Sub-tropical Pine Forest
Sub-tropical chir pine (Pinus roxburghii) forest occurs throughout the central and
western Himalaya, and Khasi pine forest occurs in Khasihills.These forests are almost
pure throughout their zone of distribution. The understorey is also not pronounced.The
total area of these forests is approximately 3.7 million ha, distributed in several
Himalayan states.
Sub-tropical Dry Evergreen Forest
These forests occur in areas with low rainfall and consist of xerophytic, thorny and
small-leaved evergreen species. Such forests are localized in the northwest corner of the
country in an area of approximately 0.2 million ha.The typical species are
Oleacuspidata and Acacia modesta in the top canopy and Dodonea shrub in the
degraded forests.
Montane Wet Temperate Forest
These forests are a characteristic feature of the eastern Himalaya and are found between
1800 m and 3000 m elevation in high rainfall areas. These forests occupy about 1.6
million ha. Some of the tops of southern hills, e.g. Nilgiris, are also occupied by these
forests. In northern form of these forests, characteristic genera are Quercus,
Castanopsis, Machilus,and Rhododendron. In the southern hills, important species
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belong to Syzygium and Ternostroemia. Rhododendron nilagiricum is an important
component in Nilgiri hills. The forests are luxuriant with dense undergrowth.
Himalayan Moist Temperate Forest
These are commercially important forests and are found between 1500m and 3000m
elevations in the Himalaya. These are concentrated in the central and western
Himalaya, except in areas where rainfall is below 1000 mm.The total area under these
forests is reported to be about 2.7 million ha. These forests are classified into two
forms; the lower form consists of Quercusleucotrichophora, Quercus. floribunda, Pinus
wallichiana and Cedrus deodara.As the altitude increases, the upper form consisting of
Abiespindrow, Piceasmithiana, and Quercussemecarpifolia becomes dominant. The
east Himalayan hills are occupied by Quercus. lineata, Quercuslamellosa,
Quercuspachyphylla, Tsugadumosa, Piceaspinulosa and Abiesdensa. Cupressustorulosa
is a conspicuous species found on limestone. Alders (Alnusnepalensis) and blue pine
(Pine wallichiana) colonise the new sites.
Himalayan Dry Temperate Forest
These are open evergreen forests with open scrub undergrowth. These forests occur in
the upper ranges of the Himalaya in low rainfall areas and cover about 0.2 million ha.
These forests consist of both coniferous and broad-leaved species. In the western
Himalaya, the characteristic species are Pinus gerardiana, Cedrus deodara and Quercus
ilex. At higher elevation, Juniperus macropoda communities are also found. In the
eastern Himalaya, the common species are from Abies and Picea. In higher hills,
Juniperus wallichiana is common. Locally, between 2500 and 4000 m elevation, a few
other species like Larixgriffithiana, Populuseupheretica, Salix spp., Hippophoe spp. and
Myricaria spp. also occur.
Sub-alpine Forest
These forests occur throughout the Himalaya above 3000 m elevation up to the tree
limit. Some of the characteristic species in the western Himalaya are Abies spectabilis
and Betulautilis while those in the eastern Himalaya are Abiesdensa and Betula spp.
Highlevel blue pine (Pinus wallichiana) forests occur on exposed sites. Rhododentron
forms the understorey.
Moist Alpine and Dry Alpine Scrub
The moist alpine scrub occurs above the tree line up to about 5500 m elevations in
Himalaya. The shrubs are rarely more than 1 m in height. Common species are
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Rhododentron, Juniperus spp and Betula spp. The dry scrub forests consist of open
xerophytic dwarf shrub in areas where rainfall is below 400 mm. The major genera
represented are Juniperus, Carangana, Eurotia, Salix, and Myricaria.
iv. PopulationEcology
The individuals of a species living within a habitat constitute a population. In ecology,
a population is a group of individuals of same species inhabiting the same area and
functioning as a unit of biotic community. Individuals inhabiting the same area form
local populations. A set of local populations connected by dispersing individuals is
called a metapopulation. They all experience similar ecological processess at a
particular stage of the life cycle. Similar populations of a species occupying different
geographical areas are called sister populations. Population is a dynamic unit. Number
of individuals may increase or decrease due to many factors due to birth rate, death rate,
migration etc.
Characteristics of Population:
Populations have a number of attributes:
a) Density
It is the total number of individuals of a species per unit area or volume at a given time
such as number of animals per square kilometre or number of trees per hectare etc.
Population density reflects the success of a species in any given area. Species density
varies from time to time and from one area to another. E.g, rainy season / dry season
Population density (P.D) can be calculated as, density = N/S where N = number of
individuals of a species at a specific time &S = unit area in a region /space. The space is
described in two dimensions (such as km2, m2, cm2) for terrestrial organisms and in
three dimensions (such as m3, cm3, ml3) for aquatic organisms and organisms
suspended in air
The size and density of a population are affected by various factors. Some of the
important ones are:
b) Birth rate or Natality Rate
Natality rate is the rate at which new individuals are added to a particular population by
reproduction (birth of young ones or hatching of eggs or germination of seeds/spores).
It is generally expressed as number of births per 1,000 individuals of a population per
year. Higher realized natality rate increases the population size and population density.
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c) Death or Mortality Rate
Mortality rate is the rate at which the individuals die or get killed. It is the opposite of
natality rate. Mortality rate is generally expressed as number of deaths per 1,000
individuals of a population per year. Lowest death rate for a given species in most
favourable conditions is called potential mortality while the actual death rate being
observed in existing conditions is called realized mortality. Realized mortality
decreases the population size and population density. The percentage ratio of natality
over mortality expressed in percentage is called vital index. Vital index determines the
normal rate of growth of a population.
d) Age Distribution (Age Composition)
The relative abundance of the organisms of various age groups in the population is
called age distribution of population. With regard to age distribution, there are three
kinds of populations. i) Rapidly growing population is a population, which has high
birth rate and low death rate, so there are more number of young individuals in the
population. ii) Stationary population is a population, which has equal birth and death
rates, so population shows zero population growth. iii) Declining population is a
population, which has higher death rate than birth rate, so the population has more
numbers of older individuals. Various age groups in a population determine its
reproductive status. The three ages referred to above be called ecological ages – pre
reproductive, reproductive & post reproductive
e) Dispersal
The majority of the organisms disperse at one time or the other during their life. These
individuals move into or move out of the population and affect the size of the
population. i) Immigration: Immigration is the permanent entry of new individuals of
same species into a population from outside. It increases the size of local population. ii)
Emmigration: Emmigration is the permanent movement/departure of individuals of
same species out of the local population due to several reasons such as lack of food,
scarcity of space (over crowding), etc. Emmigration decreases the size of local
population, but the species spread to new areas. If more individuals are added than lost,
then the population will show positive growth. If more individuals are lost than added,
then the population will show negative growth. But if the two rates are equal, then the
population will become stationary and is called zero growth. Popula
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e) Biotic Potential and Environmental Resistance
The inherent maximum capacity of an organism to reproduce or increase in number is
termed biotic potential (designated by the symbol ―r‖). Biotic potential is realized only
when the environmental conditions are nonlimiting so that the natality rate is maximum
and mortality rate is minimum. Under these conditions the population size increases,
however nature keeps a check on the expression of biotic potential. This environmental
check on population size or its biotic potential is called environmental resistance.
f) Carrying Capacity
Population density of an area is largely determined by available resources like food,
water and space in the region. The maximum number of individuals of a population,
which can be provided with necessary resources for healthy living, is called carrying
capacity of the habitat. Size of a population increases till it reaches the carrying
capacity of the habitat. When the resources become short of requirement, the
population size begins to decrease through different determinants like natality,
mortality etc.
g) Population Growth
The growth of population is measured as an increase in its size over a period of time.
Let us assume that a population having the initial size No, increases to size Nt after
time interval t, then the change in population size is given by the expression: Nt = No +
B + I – D – E where No = size of population at the beginning of change. B = natality
rate, I = rate of immigration, D = mortality rate and E = rate of emmigration.
Population Growth Forms
Populations have characteristic patterns of growth with time. These patterns are known
as Population Growth Forms. There are two basic population growth forms designated
as i) J - shaped population growth form & ii) S - shaped population growth form.
In J-shaped population growth form, the population grows exponentially and after
attaining the peak value, the population may abruptly crash. The exponential growth
cannot be sustained infinitely because not only environment is ever changing, food and
space are limited. It can be represented by the following exponential equation: dN / dt =
rN where dN / dt is the rate of change in population size, r is the biotic potential and N
is the population size.
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In S - shaped or sigmoid growth the population shows an initial gradual increase in
size, followed by an exponential increase and then a gradual decline to near constant
level.
This slow down following the exponential phase occurs due to increasing
environmental resistance. Generally the population size stabilizes with time with minor
fluctuations around this upper limit. The maximum number of individuals of a
population that can be sustained indefinitely in a given habitat represents its carrying
capacity designated by ―K‖. The S shaped sigmoid growth form is represented by the
following equation which includes an expression for environmental resistance. dN / dt
= rN (K - N /K) = rN (1- N/K) where K - N /K or 1- N/K stands for environmental
resistance.
Population Interactions:
The interactions between populations of species in a community are broadly
categorised into positive (beneficial) and negative (inhibition) interactions, depending
on the nature of effect on the interacting organism.
Positive Interactions amongst different species in a community
1) Proto Cooperation
It is a positive inter specific interaction in which both the partners are mutually
benefited and increase the chance of their survival. However, the interaction is not
obligatory for their survival as both can live without this interaction. For e.g., a)
Crocodile bird (Pluvianusaegyptius) enters the mouth of the crocodile and feed on
parasitic leeches. By this the bird gets food and the crocodile gets rid of blood sucking
parasites. b) Hermit crab (Eupagurusprideauxi) lives inside an empty gastropod shell
and fixes a sea- anemone (Adamsiapallicata) on the shell. The sea- anemone provides
camouflage (protective colouration) and defence to the crab, while crab helps in
dispersal of the sea anemone and provides new feeding grounds. c) Beautifully
coloured antennae of the shrimp on the coral reefs attract the fishes. When the fishes
approach the shrimp it feeds on the parasites on the fish‘s body. Fishes get a cleaning
service done and shrimp obtains its food.
2) Mutualism
It is a positive interspecific interaction in which members of two different species
favour the growth and survival of each other and their association is obligatory. Both
the partners are benefitted by this interaction and it is a short or life long association as
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a means of obtaining food by one or both the partners. Protocooperation between sea
anemone and hermit crab Rhizobium bacteria in the nodules
Mutualism is also referred as ' symbiosis' (sym-together; bios-life) or 'symbiotic
interaction' and the partners are referred as 'symbionts'. a) Mutualism between animal
and animal species. - Termites (white ants) are not capable of digesting wood, which
they ingest as food. A multi flagellate protozoan Trichonympha campanula, which lives
in the intestine of white ant secretes 'cellulase' enzyme to digest the cellulose of wood.
In return, the ant provides food and shelter to the protozoan. b) Mutualism between
bacteria and plant species - Symbiotic nitrogen fixation by Rhizobium leguminosarum
(bacteria) occurs in the nodules on the secondary roots of leguminous plants like pea,
gram etc. The bacteria obtain carbohydrates and shelter from leguminous plants while
in return they fix nitrogen as nitrites and nitrates for the plant which is required for their
growth.
c) Mutualism between plant and animal species: Green alga Zoochlorella is a symbiont
in the parenchyma of flatworm, Brown alga Zooxanthella lives as a symbiont with the
mollusc –Tridacna, Pollination of flowers by insects is a mutual interaction.
3) Commensalism
It is a positive interspecific interaction in which a smaller member called 'commensal',
is benefited, while the larger member called 'host', is neither benefitted nor harmed.
Commensalism represents a beneficial relation. For e.g., a) Epiphytes are small green
plants found growing on other plants for space only. They absorb water and minerals
from the atmosphere by their hygroscopic roots and prepare their own food. The plants
are not harmed in any way. Examples: Orchids, lianas and Vanda hanging mosses are
common epiphytes found on the tree of tropical rain forests. b) Epizoans are animals,
which grow on other plants or animals exhibiting Commensalism. Examples: 1) Several
species of Barnacles (molluscs) grow on the hard shell of Limulus polyphemus (horse
shoe crab). 2) Sucker fish (Echeneis) gets attached to the under surface of sharks by its
sucker. This provides easy transport for new feeding grounds and also food pieces
falling from the sharks prey to Echeneis. 3) Entamoeba coli (protozoan) lives as a
commensal in the intestine of man.
3) Scavenging
Is a direct food related interspecific interaction in which one partner called scavenger or
saprobiont eats the dead bodies of other animals, which have died either naturally or
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killed by some other animals. Scavengers clean the environment and the available food
is ultimately disposed off by which a major part of nutrients again enter for recycling.
Animals such as foxes, hyenas, vultures etc. are the animals, which are natural
scavengers. Dogs, crows, ants are occasionally seen to do the work of scavengers.
Negative interactions in a community are classified into 1) Predation 2) Parasitism 3)
Competition 4) Amensalism 5) Mimicry.
1. Predation
It is a negative, direct food related interspecific interaction between two species of
animals in which larger species called predator attacks, kills and feeds on the smaller
species called prey. Predator population adversely affect the growth and survival of
smaller prey population and therefore predation is considered an antagonistic
interaction.Examples: i) There are certain carnivorous plants also referred, as
insectivorous plants that act as predators in nature. Plant like Nepenthes (pitcher plant),
Drosera (sundew), Dionoeae (Venus fly trap) etc. feed on insects to fulfill their nitrogen
requirement. ii) All carnivorous animals and scavengers are predators.Some predators
(such as frog) act as prey for others (snake) which inturn are prey to a higher carnivores
(eagle). iii) Herbivorous animals, eating plants or seeds, are also predators as they feed
on individuals or future individuals.
2. Parasitism
It is a type of antagonistic interspecific interaction in which smaller partner, called
parasite, derives food and shelter from in or on the body of larger partner, called host,
which inhibits the survival of the host. The host can survive without the parasite, but
the parasite cannot survive without the host. Parasitism can be rightly explained as
weaker attacking the stronger.Parasitic interaction is generally found among
protozoans, flat forms, nematodes and arthropods besides many plants.
Types of Parasitism
Facultative and obligate parasitesFacultative parasites are those which become a
parasite only when they are in access to a host (or in association with a host), otherwise
they lead a free, independent life.Examples: Prawn, Oyster etc. Obligate parasites are
those, which cannot live independently and necessarily need a host for food and shelter.
Obligate parasites are usually host specific. Examples: Taenia, Trypanosoma,
Entamoeba, Cuscuta etc.
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Holoparasites and HemiparasitesHoloparasites are those organisms, which are
dependent upon their host for their entire nutritional requirement. Example: Cuscuta a
total parasite of Acacia. On the other hand, hemiparasites (or semi parasites) derive
only a part of nourishment (usually water and minerals) from their host and synthesize
their own food by photosynthesis. Example: Viscum (mistletoe) and Loranthus are
partial stem parasites.
Santalum is a partial root parasite. Parasitic plants grow
haustorial roots into the host plant tissues, which make connections with the vascular
tissue of the host plant and suck in the required nutrients.
3. Competition
When Darwin spoke of the struggle for existence and survival of the fittest in nature; he
was convinced that interspecific competition is a potent force in organic evolution. It is
generally believed that competition occurs when closely related species compete for the
same resources that are limiting, but this is not entirely true. Firstly, totally unrelated
species could also compete for the same resource. For instance, in some shallow South
American lakes visiting flamingoes and resident fishes compete for their common food,
the zooplankton in the lake. Secondly, resources need not be limiting for competition to
occur; in interference competition, the feeding efficiency of one species might be
reduced due to the interfering and inhibitory presence of the other species, even if
resources (food and space) are abundant. Therefore, competition is best defined as a
process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic
rate of increase) is significantly lower in the presence of another species. According
to Gause when resources are limited the competitively superior species will eventually
eliminate the other species, but evidence for such competitive exclusion occurring in
nature is not always conclusive. Strong and persuasive circumstantial evidence does
exist however in some cases. For e.g, The Abingdon tortoise in Galapagos Islands
became extinct within a decade after goats were introduced on the island, apparently
due to the greater browsing efficiency of the goats. Another evidence for the occurrence
of competition in nature comes from what is called ‗competitive release’. A species,
whose distribution is restricted to a small geographical area because of the presence of
a competitively superior species, is found to expand its distributional range
dramatically when the competing species is experimentally removed. In general,
herbivores and plants appear to be more adversely affected by competition than
carnivores. Gause‘s ‗C
p
v
xclus
P
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species competing for the same resources cannot co-exist indefinitely and the
competitively inferior one will be eliminated eventually. This may be true if resources
are limiting, but not otherwise. More recent studies do not support such gross
generalisations about competition. While they do not rule out the occurrence of
interspecific competition in nature, they point out that species facing competition might
evolve mechanisms that promote co-existence rather than exclusion. One such
mechanism is ‘resource partitioning’. If two species compete for the same resource,
they could avoid competition by choosing, for instance, different times for feeding or
different foraging patterns. MacArthur showed that five closely related species of
warblers living on the same tree were able to avoid competition and co-exist due to
behavioural differences in their foraging activities.
It is an antagonistic interaction in which two or more members of same species
(intraspecific), or two or more members of different species (interspecific) of same
trophic level compete for common resource like light, moisture, space, and nutrients
etc. which are in short supply in relation to the member of individuals. On the basis of
nature of struggle, competition is of two types.
a) Direct interference type Where members of two different populations are mutually
and actively inhibitory to each other. In such a case competition benefits the organism,
which is more suitably adapted while the other is at a disadvantage. The size of
population of the latter decreases, finally leading to its elimination.
b) Competition resource use type In this each population inhibits the other indirectly for
the resource in short supply. Example: Trees, shrubs and herbs in a forest, struggle for
sunlight, water and nutrients. Similarly, grasshoppers, rats, rabbits, deer (herbivores)
compete for food especially during draught and less food availability.
Significance of competition: Competition is extremely important in determining the
distribution of closely related species, essential in determining habitat range and
speciation.
4. Amensalism
It is an antagonistic interspecific interaction in which one species is inhibited while
other species is neither benefitted nor harmed. In simple words, in amensalism, one
organism does not allow other organism to grow or live near it. It is also called
antibiosis and the affected species is called amensal and the affecting species is called
inhibitor. Such inhibition is achieved through the secretion of certain chemicals called
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allochemics or allelopathic substances. Examples: Most common phenomenon of
antibiosis is formation of antibiotics that are antagonistic to the microbes. 1)
Penicilliumnotatum releases the antibiotic substance Penicillin, which inhibits the
growth of variety of bacteria. 2) Streptomyces griseus produce antibiotic Streptomycin,
which again inhibits the growth of many bacteria. 3) Roots of certain plants produce
allochemic substances which check the growth of other plants to conserve resources,
such as, Convolvulus arvensis; a weed inhibits the germination and growth of wheat. 4)
Older plants of Grevillea robusta inhibit the growth of its own seedlings.
5. Mimicry
It is a phenomenon in which a living organism modifies its form, appearance, structure
or behaviour and looks like another living organism or some inanimate (non- living)
object so as to defend from its predators, or to increase the chances of capturing the
prey. Individual which shows mimicry is called mimic while the animate or inanimate
object with which it resembles is called model. Mimicry increases the survival value of
the organism. Henry Bates, an English naturalist, first explained the concept of mimicry
and so the phenomenon is referred as Batesian mimicry also.
v. Community Ecology:
Biotic Community is an association of a number of interrelated and independent
populations belonging to different species, in a common environment which can
survive in nature. Biotic community comprises a number of populations of different
species interacting between them and the abiotic environment. Large number of biotic
communities is found in nature due to existence of diverse habitats with characteristic
environmental conditions. Each stable and self-sufficient biotic community possesses
ecological characteristics which differentiate it from other communities. These are
described below:
a) Habitat
Biotic community occupies particular area with specific physical environment, which
acts as a limiting factor, regulating the population size of various species within the
community.
b) Self-sufficiency
Nutritionally, each community comprises autotrophic plants and heterotrophic animals,
perfectly balanced. The remains and dead bodies of these producers and consumers are
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decomposed by the decomposers (bacteria and fungi) and help in the recycling of
materials.
c) Species composition
The kinds of plants and other organisms present in a community indicate its species
composition. It differs from one community to another. Seasonal variations also occur.
d) Species diversity
Community is formed of a number of different populations. The number of species and
population abundance in a community also vary greatly. Species diversity depends
upon size of the area, diversity of habitats in that area, soil type, altitude etc. Some
communities, such as tropical rain forest and coral reef community show high species
diversity with many different kinds of species living at each trophic level. In others like
a desert there may be relatively few species in the entire community.
e) Species Dominance
Nature of community is determined by only a few species called dominants which exert
a major controlling influence – tall trees in a forest, tall grasses in a grass land.
Sometimes communities are named after dominant species or after a particular
environment, e.g, Pine forest, Desert community
f) Physiognomy & Stratification
A community is first noticed by its physiognomy. It refers to the external appearance or
look of the community. The external appearance is the total effect created by the
combination of vertical structure and architecture of dominant species of vegetation,
e.g, high physiognomy of a forest, low physiognomy in a grass land. Stratification of a
community depicts vertical layering of the vegetation. A stable community comprises
of various strata, each comprising the population of particular kind of species. Their
growth forms determine the structure of a community and on their arrangement;
community shows either horizontal layering called zonation or vertical layering called
stratification - Canopy, under storey tree layer, shrubs, and herbs. In aquatic
ecosystems, stratification from surface to bottom is determined by light penetration,
temperature and oxygen profile – Littoral, Limnetic, Profundal, and Benthic.
Stratification tends to increase the number of species and efficient use of resources.
g) Key stone and link species
The species having much greater influence on community characteristics relative to
their low abundance or biomass are called key stone species. Fig trees in a tropical rain
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forest (produce fruit in abundance) these species play a vital role in controlling the
relative abundance of other species. Their removal causes serious disruption in
community functioning. Only a few species work as key stone species and several
others work as critical link species. Mycorrhizal fungi, (pollinators, dispersal agents)
in soil are critical link species as they establish essential links in the absorption of
nutrients from the soil.
h) Ectones, Edge Effect & edge species
The transition zone between two communities is known as ecotone (also called tension
zone), it contains few species from both the communities. The total number of species
is often greater in the ecotone than in the adjoining communities. The tendency of
increased variety and density of some organisms at the community border is known as
edge effect. The organisms which occur primarily or spend the greatest amount of their
time in junctions between communities are called edge species.
i) Succession: Communities are dynamic and develop as a result of a directional
change called succession over a period of time.
Ecological Succession
"The occurrence of relatively definite sequence of communities over a long period of
time in the same area resulting in establishment of stable or climax community is
known as ecological or biotic succession". The first community which inhabits the area
will be referred as 'pioneer community' and the last and stable community formed in the
area will be referred as 'climax community'. The intermediate communities are called
'transitional or seral communities'. The whole series of changes in community
characteristics from pioneer stage to climax stage constitute a 'sere' and the intermediate
stages are the 'seral stages'. Usually the initial stages of succession are comprised of
lower forms.
Community development reasonably directional and predictable, Succession is
community controlled (modifies its own environment) and culminates in a stabilized
ecosystem.
Causes
Interactions among the organisms in a community influence the structure, composition
and function of a community. In succession, during period of time a community makes
the area less favourable for itself and more favourable for the next serial community.
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Succession can also be caused by physical and chemical factors of the environment
such as fire, landslides, erosion, catastrophic factors, etc. Migration Competition etc
Types :1) Primary succession
Also referred as 'perisere' is a type of biotic succession that occurs on a substratum
devoid of life earlier like bare rock, sand dunes, new island exposed out of the sea etc.
where there was previously not any sort of life. Primary succession takes a very long
time (more than thousands of years in case of climax forest on bare rock).2) Secondary
succession It is the biotic succession that occurs in an area, which had an existing biotic
community and have become bare due to destruction by fire, landslide, earthquake etc.
The sequence of successional stages is called 'subsere', and time required for the
completion of sere is much shorter than primary succession. a) autogenic b) allogenic ?
General process (Mechanism of succession)
Process of primary succession occurs through a number of sequential steps, which
follow one another.
a) Nudationis the development of a bare area without any form of life. Causes of
nudation may be topographic (soil erosion, landslide, earthquake etc.) or climatic
(glaciers, hailstorm, fire etc.) or biotic (human activities, epidemics etc.)
b) Invasion - is the successful establishment of a species in a bare area, which happens
in three steps.
i) Migration (dispersal) - reaching of seed or spores in a bare area through various
agencies (wind, water etc.)
ii) Ecesis - also called 'establishment' involves the adjustment of the migrated species
with the prevailing conditions of the area. iii) Aggregation - once established, the
organisms increase in number through the process of reproduction.
c) Competition and co-action Involves the development of intraspecific and
interspecific competitions among the members (as there is natural resistance like
limited food and space). Affect each others life.
d) Reaction involves the modification of the environment, influenced by living
organisms, and existing community are replaced by next seral community and the
process is repeated.
e) Stabilization is the stage where the final terminal community (climax communityslow growing & long lived) becomes stabilized and can maintain itself in equilibrium
with climate of the area..
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Xerarch Succession - Xerosere:
It is a type of xerosere and involves the ecological succession on bare rock surfaces.
Rocky surface is characterized by- i) Deficiency of water ii) Absence of organic matter
iii) Surface temperatures are very high. Various stages of succession are:
a) Lichen stage-Lichen species like Graphis, and Lecanora forms the pioneer
community in a lithosere, as they can tolerate desiccation. Organic acids produced by
lichens corrode the rock surface and release minerals for proper growth of lichens.
Wind borne soil particles and organic matter collect in the depressions, which initiates
soil formation, leading to the growth of foliose lichens like Parmelia, etc. Foliose
lichen retains more water and accumulates more soil particles, helping in the
development of a fine layer of soil on the rock surface.
b) Moss stage - Accumulation of soil and humus leads to the growth of mosses such as
Polytrichum. Over a period of time, collection of more soil and organic matter favour
the growth of moisture loving mosses like Hypnum, etc.
c) Herb stage - Death and decay of mosses produce a mat of organic moss on partially
fragmented rock help the germination to seeds of hardy grasses like Eleusine, etc.
Further decomposition of these annual grasses lead to replacement by perennial grasses
like Cymbopogon, etc. Some small animals also invade the habitat.
d) Shrub /Scrub stage - Further weathering on rocks and death of herbs make the
habitat more suitable for the growth of shrubs like Rhus, and Zizyphus etc. Since the
shrubs are larger in size and their roots penetrate more deeply in the rocky substratum
causes more weathering and soil formation.
e) Forest stages - Many light demanding, stunted and hardy trees invade the area.
Vegetation finally becomes mesophytic. A steady state is reached between the
environment and the biotic community. Type of climax community depends upon the
climate. For e.g., a rain forest is formed in a moist tropical area, coniferous forest in
temperate area, a grassland in areas with less rainfall etc.
Hydrarch Succession - Hydrosere:
Hydrosere, also called hydrarch involves the ecological succession in the newly formed
pond or lake.
a) Plankton stage Germination of encysted spores in the newly formed water body
forms the pioneer community; Spores could have reached the water body through wind
or animals. Planktonic stage includes minute autotrophic diatoms, pyhtoflagellates,
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cyanobacteria etc. Population of phytoplankton is regulated by zooplanktons. Their
dead and decomposed organic matter mixes with silt and forms soft mud at the bottom
of the pond.
b) Rooted submerged stage Rooted submerged hydrophytes like Hydrilla, etc. grow
on the soft mud. Due to death and decay of these plants and deposition of sand and silt,
leads to a slow rise in the bottom level (soil layer) of the pond. Buried older plants form
good humus for next seral stage.
c) Rooted floating stage Area is now invaded by species of floating, leaved, anchored
plants like Nymphaea,etc.which help the water become rich in mineral and organic
matter. Later free floating species like Azolla, Lemna, Pistia, Eicchornia, etc. appear.
This rapid growth of plants builds up the pond bottom and makes the water shallower.
d) Reed swamp stage Also called amphibious stage and plants like Typha, etc., replace
the floating plants. These plants produce abundant amount of organic wastes and lose
huge amounts of water by transpiration. Addition of organic matter raises the
substratum of the pond and becomes unsuitable for growth of amphibious plants.
e) Sedge meadow stage Also called marsh meadow stage where the area is now made
up of plant species like Carex(Sedge), and herbs like Caltha, etc. They form a mat like
vegetation with their much branched rhizomatous system. Finally the marshy
vegetation disappears due to the development of mesic conditions.
f) Wood land stage First the peripheral part of the area is invaded by some shrubby
plants, which can tolerate bright sunlight and water logged conditions. Plants that grow
are Cornus (Bogwood), Cephlanthus (Button brush), etc. The next to invade trees are
Populus (Cottonwood), Alnus (Alder), etc. Further fall in the water table, along with
mineralization and soil buildup favours the arrival of plants for next seral community.
g) Forest stage It is the formation of climax community, which depends upon the
climatic conditions. For e.g., tropical deciduous or monsoon forests are formed in
regions of moderate rainfall, tropical rain forests in areas with heavy rainfall, mixed
forests in temperate regions.
Importance of Ecological SuccessionEcological succession is of great importance as
i)
It provides information, which help to have control on the growth rate of one or
more species in a given geographical area.
ii) ii) It helps in reforestation and forest management programmes.
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iii) iii) Ecological succession is an orderly and evolutionary process of community
development in a habitat.
vi. Ecosystem:
The living components interact among themselves as well as with their physical
environment like soil, air and water. The branch of biology, which deals with the
relationship of organisms with their environment, is known as 'ecology'. The term
ecology is derived from the Greek root "oikos" - meaning "house", combined with
"logy" - meaning "the science of‖ or "the study of". Sir Arthur. G. Tansley, a British
ecologist, coined the term 'Ecosystem' in the year 1935, which refers to a whole
community of organisms and its environment as one unit.
An ecosystem may be natural (like forest, lake, ocean etc) or man-made (such as an
aquarium, a crop field etc), temporary (like a rainfed pond) or permanent (like a lake,
forest, etc), aquatic (such as pond, ocean etc) or terrestrial (like grassland, forest, etc).
An ecosystem comprises of two basic components i) Abiotic components and ii) Biotic
components.
Abiotic Components:
These include the non-living, physico - chemical factors such as air, water, soil and the
basic elements and compounds of the environment. Abiotic factors are broadly
classified under three categories. Climatic factors which include the climatic regime
and physical factors of the environment like light, humidity, atmospheric temperature,
wind, etc. Edaphic factors which are related to the structure and composition of soil
including its physical and chemical properties, like soil and its types, soil profile,
minerals, organic matter, soil water, soil organisms. Inorganic substances like water,
carbon, sulphur, nitrogen, phosphorus and so on. Organic substances like proteins,
lipids, carbohydrates, humic substances etc.
Biotic Components:
It comprises the living part of the environment, which includes the association of a
number of interrelated populations belonging to different species in a common
environment. The populations are that of animal community, plant community and
microbial community. Biotic community is distinguished into autotrophs, heterotrophs
and saprotrophs.
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Autotrophs (Gr: auto - self, trophos - feeder) are also called producers, converters or
transducers. These are photosynthetic plants, generally chlorophyll bearing, which
synthesize high-energy complex organic compounds (food) from inorganic raw
materials with the help of sunlight, and the process is referred as photosynthesis.
Autotrophs form the basis of any biotic system. In terrestrial ecosystems, the autotrophs
are mainly the rooted plants. In aquatic ecosystems, floating plants called
phytoplankton and shallow water rooted plants called macrophytes are the dominant
producers.
Heterotrophs (Gr: heteros - other; trophs - feeder) are called consumers, which are
generally animals feeding on other organisms. Consumer's also referred as Phagotrophs
(phago - to ingest or swallow) or macro consumers are mainly herbivores and
carnivores. Herbivores are referred as First order consumers or primary consumers, as
they feed directly on plants. For e.g., Terrestrial ecosystem consumers like cattle, deer,
rabbit, grass hopper, etc. Aquatic ecosystem consumers like protozoans, crustaceans,
etc. Carnivores are animals, which feed or prey upon other animals. Primary carnivores
or Second order consumers include the animals which feed on the herbivorous animals.
For e.g., fox, frog, predatory birds, smaller fishes, snakes, etc. Secondary carnivores or
Third order consumers include the animals, which feed on the primary carnivores. For
e.g., wolf, peacock, owl, etc. Secondary carnivores are preyed upon by some larger
carnivores. Tertiary carnivores or Quaternary consumers include the animals, which
feed on the secondary carnivores. For e.g. lion, tiger, etc. These are not eaten by any
other animals. The larger carnivores, which cannot be preyed upon further are called
top carnivores.
Saprotrophs (Gr: sapros - rotten; trophos - feeder) are also called decomposers or
reducers. They break down the complex organic compounds of dead matter (of plants
and animals). Decomposers do not ingest their food. Instead they secrete digestive
enzymes into the dead and decaying plant and animal remains to digest the organic
material. Enzymes act upon the complex organic compounds of the dead matter.
Decomposers absorb a part of the decomposition products for their own nourishment.
The remaining substances are added as minerals to the substratum (mineralization).
Released minerals are reused (utilised) as nutrients by the plants (producers).
The characteristic structure of an ecosystem is obtained by the systematic physical
organisation of the abiotic and biotic components of that particular ecosystem. The two
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main structural features of any ecosystem are its 'species composition' and
'stratification'. An ecosystem can be represented by depicting the producers consumer‘s relationship in the given ecosystem.
This is also called the 'Trophic structure' of an ecosystem, wherein each animal
population forms the various trophic levels. The producers (green plants) always form
the first trophic level. Herbivores, which feed on producers, are at the second trophic
level followed by secondary consumers, tertiary consumers and so on.
Few important functional aspects of an ecosystem are:
Biological diversity and maintenance of stability -A system with high species
diversity and low dominance is less productive but stable. On the other hand, a system
with a community with low species diversity and high dominance is more productive
but unstable.
'The amount of organic matter or biomass produced by an individual organism,
population, community or ecosystem during a given period of time is called
productivity'. Primary production refers to all or any part of the energy fixed by plants
possessing chlorophyll. The total amount of solar energy converted (fixed) into
chemical energy by green plants (by the process of photosynthesis) is called 'Gross
Primary Production' (GPP). The rate at which, organic matter is synthesized by
producers per unit time and area is called 'Gross Primary Production' (GPP).
A certain portion of gross primary production is utilised by plants for maintenance
(largely respiratory energy loss) and the remainder is called 'Net Primary Production
(NPP)' which appears as new plant biomass. Or 'The rate of organic matter build up or
stored by producers in their bodies per unit time and area is called net primary
production (NPP)'. GPP - Energy lost by respiration and maintenance = NPP
Thus, productivity by heterotrophic organisms in the ecosystem is called
secondary productivity. Or the rate of increase in the biomass of heterotrophs per unit
time and area is called secondary productivity. Secondary productivity serves as an
index of significance of the population in terms of food resources available to the
heterotrophic populations, including man, in the food chain.
Lindemann (1942) put forth Ten percent law for the transfer of energy from one
trophic level to the next. According to the law, during the transfer of organic food from
one trophic level to the next, only about ten percent of the organic matter is stored as
flesh. The remaining is lost during transfer or broken down in respiration. Plants utilise
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sun energy for primary production and can store only 10% of the utilised energy as net
production available for the herbivores. When the plants are consumed by animal,
about 10% of the energy in the food is fixed into animal flesh which is available for
next trophic level (carnivores). When a carnivore consumes that animal, only about
10% of energy is fixed in its flesh for the higher level. So at each transfer 80 - 90% of
potential energy is dissipated as heat (second law of thermodynamics) where only 10 20% of energy is available to the next trophic level.
The energy, in the biological world, flows from the sun to plants and then to all
heterotrophic organisms such as microorganisms, animals and man.
Energy Flow:
Is the key function in an ecosystem and it is unidirectional. 'The study of energy
transfer at different trophic level is known as 'Bioenergetics'. The storage and
expenditure of energy in an ecosystem is in accordance with the laws of
thermodynamics (basic laws of thermodynamics).
The first law of thermodynamics is the law of conservation of energy, which says that
'energy can neither be created nor destroyed but can be transformed from one form into
another'. In biological system, solar energy is converted into chemical energy and is
stored in food materials as internal energy. Second law of thermodynamics states that
processes of energy transformation will not occur spontaneously unless there is
degradation of energy from a non - random to a random form.
Only the photosynthetically active radiation (PAR) is the energy available to
autotrophs. A major portion (90 - 95%) of this energy is lost in the form of heat of
evaporation and sensible heat. Around 1 to 5% is used for photosynthesis (primary
production). Thus, at each transfer, heat energy (random form) dissipates. Hence the
energy transfer is not 100% efficient and there is degradation of energy from a nonrandom to a random form.
Thus in an ecosystem there is,
a) Constant flow or transfer of energy from sunlight through plants (producers) to
animals (consumers) in the form of food. b) A decrease in useful energy during each
transformation or transfer at each successive trophic level.
c) Return of entire solar energy trapped by green plants back to the environment as
heat.
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Food Chain:
Is a series of groups of organisms called trophic levels, in which, there is repeated
eating and eaten by so as to transfer food energy". Or "The series of populations or
organisms of an ecosystem through which food and energy contained in it passes with
each member becoming the food of the later is called a food chain". For e.g., a) Food
chain observed in a river:b) Food chain observed in a pasture:The transfer of food
energy from plant sources through a series of organisms forms a 'food chain'. The base
of the food chain is always formed by a plant (producer / autotroph), which is grazed on
by a herbivore, which is predated over by a carnivore, which may be eaten by another
carnivore. A food chain, can therefore, be represented as grass – Deer – Lion where
each link represents a trophic level.
In a food chain,
a) There is repeated eating in which each group eats the smaller one and is eaten by the
larger one. Thus, it involves a nutritive interaction between the biotic components of an
ecosystem.
b) The plants and animals which depend successively on one another form the limbs of
a food chain.
c) There is unidirectional flow of energy from sun to producers and then to a series of
consumers of various types. Thus, a food chain is always straight and proceeds in a
progressing straight line.
d) Usually 80 to 90% of potential energy is lost as heat at each transfer on the basis of
second law of thermodynamics.
e) Usually there are 4 or 5 trophic levels. Shorter food chains provide greater available
energy and vice - versa.
f) Omnivores occupy more than one trophic level and, some organisms occupy different
trophic positions in different food chains.
There are mainly two types of food chains operating in nature. a) Grazing food
chain b) Detritus food chain.
Grazing food chain is generally seen in ecosystems such as grassland, pond or lake
where a substantial part of the net primary production is grazed on by herbivores (cattle
and rodents). Usually upto 50% of the NPP is grazed on by these animals in their
respective ecosystems and the remaining 50% goes to the decomposer organisms as
dead organic matter. Thus, in these ecosystems, the food chain is herbivore based.
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a) These are directly dependent upon solar radiations as the primary source of energy
and the producers (green plants) synthesize their plant biomass by the process of
photosynthesis. Producers form the first trophic level.
b) Herbivores or primary consumers eat upon the producers and form the second
trophic level.
c) Herbivores are in-turn eaten by different categories of carnivores forming the higher
trophic levels.
d) Grazing food chains are longer food chains and they always end at decomposer level.
Detritus food chain, as seen in a forest ecosystem, wherein the dominant primary
consumers (herbivores) are the insects, which usually consume less than ten percent of
the net primary production. The major remaining portion of more than 90% is
consumed later as dead plant material by small detritus feeding organisms such as
micro arthropods, oligochaetes and microorganisms. The detritus feeding organisms
process the detritus in their gut by reducing it into small pieces, digesting it partially or
completely, thus making organic material available for bacterial or fungal attack. These
microorganisms also act as food for many soil animals. This food chain is decomposer
organism based and is called 'detritus or decomposer food chain'.
a) Primary source of energy is dead organic matter called 'detritus' which are fallen
leaves, plant parts or dead animal bodies. b) Primary consumers are 'detritivores'
including protozoans, bacteria, fungi, etc which feed upon the detritus saprophytically.
c) Detritivores are inturn eaten by secondary consumers such as insect larvae,
nematodes, etc.
d) Detritus food chains are generally shorter than grazing food chains.
e) In nature, detritus food chains are indispensable as the dead organic matter of
grazing food chain is acted upon by the detritivores to recycle the inorganic elements
into the ecosystem.
Food Web:
Food web can be defined as, "a network of food chains which are interconnected at
various trophic levels, so as to form a number of feeding connections amongst different
organisms of a biotic community". Food webs are indispensable in ecosystems as they
allow an organism to obtain its food from more than one type of organism of the lower
trophic level.
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Generally, a food web operates according to taste and food preferences of the organism,
yet availability of food source and other compulsions are equally important. For
example tigers normally do not eat fish or crabs, but in Sundarbans they are forced to
feed on them in the absence of their natural prey.
Ecological Pyramids:
Are also called 'Eltonian pyramids' after C. Elton. Charles Elton (1927) developed the
concept of ecological pyramids who noted that "…the animals at the base of a food
chain are relatively abundant while those at the end are relatively few in number…"
Ecological pyramids can be defined as, "a graphical representation of an ecological
parameter like number of individuals or amount of biomass or amount of energy
present in various trophic levels of a food chain with producer forming the base and top
carnivores at the tip". Graphical representation of the trophic structure is done by
drawing ecological pyramids, where the basal, mid and top tiers show the parameter
values for producers, herbivores and carnivores in the ecosystem.
An ecological pyramid may be upright (tapering towards the tip), or inverted (widens
towards the tip) or spindle shaped (broader in the middle and narrow above and below).
On the basis of the parameters used, ecological pyramids are of three types: Pyramid of
Numbers, Pyramid of Biomass, and Pyramid of Energy
"Pyramid of numbers is the graphic representation of number of individuals per unit
area of various trophic levels stepwise with producers forming the base and top
carnivores the tip".
The shape of the pyramid of numbers vary from ecosystem to ecosystem. In
aquatic ecosystems and herbaceous communities, autotrophs are present in large
numbers per unit area. They support a lesser number of herbivores, which inturn
support fewer carnivores. So, the producers are smallest sized but maximum in number
while, top carnivores are larger in size but lesser in number, so these cannot be used as
prey by another. Hence the pyramid of numbers is upright.
In a parasitic food chain, for e.g., an oak tree, the large tree provides food to
several herbivorous birds. The birds support still larger population of ectoparasites
leading to the formation of an inverted pyramid. When a large tree support larger
number of herbivorous birds which inturn are eaten by carnivorous birds like falcon and
eagle, which are smaller in number, it forms a spindle shaped pyramid.
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"Pyramid of biomass is the graphic representation of biomass present per unit
area of different trophic levels, with producers at the base and top carnivores at the tip".
The total amount of living or organic matter in an ecosystem at any time is called
'Biomass'. In a terrestrial ecosystem, the maximum biomass occurs in producers, and
there is progressive decrease in biomass from lower to higher trophic levels. Thus, the
pyramid of biomass in a terrestrial ecosystem is upright.
In an aquatic habitat the pyramid of biomass is inverted or spindle shaped where the
biomass of trophic level depends upon the reproductive potential and longevity of the
member.
"Pyramid of energy is a graphic representation of the amount of energy trapped
per unit time and area in different trophic level of a food chain with producers forming
the base and the top carnivores at the tip". Pyramid of energy is always upright. It is so
because at each transfer about 80 - 90% of the energy available at lower trophic level is
used up to overcome its entropy and to perform metabolic activities. Only 10% of the
energy is available to next trophic level.
Biogeochemical Cycles:
Earth is the source of matter for all living organisms, as they require several (about 40)
elements for their growth and life processes. As these elements are provided by the
earth and are used by the organisms for their body building and metabolism, they are
called 'Biogenetic nutrients' or 'Biogeochemicals'. These materials have been used again
and again in the formation of new generations of the organism. In other words, the
matter is continuously recycled. By definition, "The movement or circulation of
biogenetic nutrients through the living and non-living components of the biosphere or
of any ecosystem is called biogeochemical cycling".
Biogeochemical cycles are the cyclic pathways through which chemical elements
move from environment to organism and back to the environment". Since such
movements of elements and inorganic compounds is essential for maintenance of life,
they are also called 'nutrient cycles'. Biogeochemical cycles are basically of two types:
a) Gaseous cycles like carbon (as carbon dioxide), oxygen, nitrogen, etc. b)
Sedimentary cycles like sulphur, phosphorus, etc.
.
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Nitrogen Cycle:
Nitrogen is an important structural component of many necessary compounds,
particularly proteins. Atmosphere is the reservoir of free gaseous nitrogen and nitrogen
compounds are found in bodies of organisms and in the soil. Living organisms cannot
pickup elemental gaseous nitrogen directly from the atmosphere (except for nitrogen
fixing bacteria). It has to be converted into nitrates to be utilised by plants.
Nitrogen Fixation
Conversion of nitrogen into ammonia & or nitrates is called nitrogen fixation. It occurs
in 3 ways. 1) Atmospheric nitrogen fixation 2) Industrial nitrogen fixation.3) Biological
nitrogen fixation
Atmospheric Nitrogen Fixation
Thunderstorms and lightning are the common photochemical and electrochemical
reactions in nature, which convert atmospheric gaseous nitrogen to oxides of nitrogen.
They get dissolved in water forming nitrous acid and nitric acid, which inturn combine
with other salts to produce 'nitrates'. An average amount of 7.6 x 106 metric tonnes per
year of nitrogen is estimated to be produced in nature.
Industrial Nitrogen Fixation
Ammonia is produced in industry by combining nitrogen and hydrogen under high
pressure of 200 atmospheres and extreme high temperature of 400oC (Haber's process).
Biological Nitrogen Fixation
The process is the transformation of gaseous nitrogen into nitrates by living organisms.
Biological nitrogen fixation occurs by:
a) Symbiotic nitrogen fixation, which is brought about by certain bacteria such as: i)
Rhizobium species in the root nodules of legumes (pea family) ii) Nostoc and
Anabaena (cyanobacteria) in the coralloid roots of Cycas. iii) Actinomycetes in the root
nodules of Alnus, Casuarina, etc. An average of about 54 x 106 metric tonnes / year of
nitrates is estimated to be produced by biological fixation.
b) Free living (asymbiotic) nitrogen fixing organisms, are primitive nitrogen fixers,
which operate under poor aeration conditions by reductional process. These include - i)
obligatory aerobes such as Azotobacter. ii) facultative aerobes such as Escherichia,
Bacillus, etc. iii) anaerobic bacteria like Clostridium. iv) photosynthetic bacteria like
Rhodospirillum (purple bacteria) etc.
Nitrogen cycling involves several stages
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Ammonification involves the decomposition of protein of dead plants and animals, and
nitrogenous wastes like urea, uric acid, etc to ammonia, in the presence of ammonifying
bacteria or putrefying bacteria. Common ammonifying bacteria are Bacillus ramosus,
Bacillus vulgaris and Bacillus mycoides. Part of the plant proteins are broken down
into nitrogenous animal proteins. In animal body, the plant proteins are consumed and
are broken down into nitrogenous wastes like urea, uric acid and ammonia. Nitrogenous
wastes are excreted out which are acted upon by decomposing microorganisms such as
Actinomycetes and fungi. Proteins
Amino acids
Ammonia
Nitrification involves the oxidation of ammonia to nitrates through nitrites in the
presence of nitrifying bacteria, which are also chemosynthetic autotrophs.
First, ammonia is converted into nitrites by Nitrosomonas and Nitrococcus bacteria.
The nitrites are then converted into nitrates by Nitrobacter and Nitrocystis, which are
now available for plant absorption.
Denitrification
It is a biological process where in the ammonium compounds, nitrates and nitrites are
reduced to molecular nitrogen in the presence of denitrifying bacteria such as Bacillus
subtilis, Micrococcus denitrificans, Pseudomonas stutzeri, Pseudomonas aeruginosa,
etc. Denitrification reduces soil fertility and is stimulated by water logging, poor
drainage, lack of aeration and accumulation of organic matter in the soil.
Carbon Cycle:
When you study the composition of living organisms, carbon constitutes 49 per cent of
dry weight of organisms and is next only to water. If we look at the total quantity of
global carbon, we find that 71 per cent carbon is found dissolved in oceans. This
oceanic reservoir regulates the amount of carbon dioxide in the atmosphere (Figure
14.6). Do you know that the atmosphere only contains about 1per cent of total global
carbon? Fossil fuels also represent a reservoir of carbon. Carbon cycling occurs through
atmosphere, ocean and through living and dead organisms. According to one estimate 4
× 1013 kg of carbon is fixed in the biosphere through photosynthesis annually. A
considerable amount of carbon returns to the atmosphere as CO2 through respiratory
activities of the producers and consumers. Decomposers also contribute substantially to
CO2 pool by their processing of waste materials and dead organic matter of land or
oceans. Some amount of the fixed carbon is lost to sediments and removed from
circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel,
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and volcanic activity are additional sources for releasing CO2 in the atmosphere.
Human activities have significantly influenced the carbon cycle. Rapid deforestation
and massive burning of fossil fuel for energy and transport have significantly increased
the rate of release of carbon dioxide into the atmosphere.
Sulphur Cycle:
The global Sulphur cycle is a good example to illustrate linkage between the air, water,
and soil.Like the nitrogen cycle, it also illustrates the key role played by
microorganisms.
There are at least four major inputs of sulphur into the atmosphere from land—
volcanic activity, soil dust, industrial activity and activity of sulphur bacteria which
releases H2S into the atmosphere. Sulphur also enters the atmosphere from the oceans
in three ways— deep sea hydrothermal vents, biogenic gas (dimethy1 sulfide) and sea
water whipped by winds forms aerosols containing SO4.
Plants and animals require a continuous supply of sulphur and its compounds in order
to synthesize some amino acids such as cysteine and methionine, and proteins. Only a
few organisms gain their sulphur requirements in such organic forms as amino acids
and cysteine. Inorganic sulphate (SO4) is the major source of biologically significant
sulphur.
Most of the biologically incorporated sulphur is mineralized by bacteria and fungi
in ordinary decomposition by species of Aspergillus and Neurospora, among others.
Under anaerobic condition, however, some may be reduced directly to sulphides,
including hydrogen sulphide, by bacteria such as Escherichia and Proteus. Some
organic sulphur enters the atmosphere as sulphur dioxide (SO2) through incomplete
combustion of fossil fuels, especially coal. This is one of the major sources of air
pollution today. According to Schindler (1988), anthropogenic inputs of sulphur in
industrial areas may contribute 90% of the sulphur input into the air.Most of the input is
in the form of SO2 which in the presence of water, may be converted to HSO3 or SO4
by the following reactions:
SO2——-> SO3 —–> H2SO4⥢ H+ + SO42–,SO2+H+——–> HSO3
The deposition of SO24-, HSO3 and H+ ions lowers the pH of precipitation, causing
acid rain problem. Inorganic sulphur as sulphate may precipitate out, but since it is
relatively soluble, it serves as a source of elemental sulphur in many ecosystems.
Sulphate is also reduced under anaerobic conditions to elemental sulphur or to
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sulphides including hydrogen sulphide, by such heterotrophic bacteria as species of
Desulfovibrio and Desulfomonas (SO2-4 ——> S2). Colourless sulphur bacteria such as
species of Beggiatoa (chemosynthetic bacteria) often abundant in sulphur springs
oxidize hydrogen sulphide to elemental sulphur, and species of Thiobacillus oxidize it
to sulphate.
Other species of Thiobacillus oxidize sulphide to sulphur, and still others oxidise
sulphur to sulphate as shown in the following reactions:
H2S —->S——->SO4, H2S—->SO4
The sedimentary aspect of the sulphur cycle involves the precipitation of sulphur in
the presence of iron under anaerobic conditions. Ferrous sulphide is insoluble in neutral
or alkaline water, and consequently sulphur has the potential for being bound up under
these conditions to the limits of the amount of iron present.
Because of the thermodynamics of this ferrous sulphide (FeS) system, other nutrients
important to biological systems, can also get trapped for varying periods of time.
Among these are copper, cadmium, zinc and cobalt. When iron sulphides are formed in
the sediments, phosphorus is converted from insoluble to soluble form and thus
becomes available to organisms. This is an excellent example of how one cycle
regulates another.
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UNIT: IV
i.
Archaeobotany and Ethnobotany
Plants Use Through Ages:
Ever since ancient times, in search for rescue for their disease, the people looked for
drugs in nature. The beginnings of the medicinal plants‘ use were instinctive, as is the
case with animals. In view of the fact that at the time there was not sufficient
information either concerning the reasons for the illnesses or concerning which plant
and how it could be utilized as a cure, everything was based on experience. In time, the
reasons for the usage of specific medicinal plants for treatment of certain diseases were
being discovered; thus, the medicinal plants‘ usage gradually abandoned the empiric
framework and became founded on explicatory facts. Until the advent of iatrochemistry
in 16th century, plants had been the source of treatment and prophylaxis. Nonetheless,
the decreasing efficacy of synthetic drugs and the increasing contraindications of their
usage make the usage of natural drugs topical again.
Historical sources relevant for study of medicinal plants’ use
The oldest written evidence of medicinal plants‘ usage for preparation of drugs has
been found on a Sumerian clay slab from Nagpur, approximately 5000 years old. It
comprised 12 recipes for drug preparation referring to over 250 various plants, some of
them alkaloid such as poppy, henbane, and mandrake.
The Chinese book on roots and grasses ―Pen T‘Sao,‖ written by Emperor Shen
Nung circa 2500 BC, treats 365 drugs (dried parts of medicinal plants), many of which
are used even nowadays such as the following: Rhei rhisoma, camphor, Theae folium,
Podophyllum, the great yellow gentian, ginseng, jimson weed, cinnamon bark, and
ephedra.
The Indian holy books Vedas mention treatment with plants, which are abundant in
that country. Numerous spice plants used even today originate from India: nutmeg,
pepper, clove, etc.
The Ebers Papyrus, written circa 1550 BC, represents a collection of 800
proscriptions referring to 700 plant species and drugs used for therapy such as
pomegranate, castor oil plant, aloe, senna, garlic, onion, fig, willow, coriander, juniper,
common centaury, etc.
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According to data from the Bible and the holy Jewish book the Talmud, during
various rituals accompanying a treatment, aromatic plants were utilized such as myrtle
and incense.
In Homer's epics The Iliad and The Odysseys, created circa 800 BC, 63 plant
species from the Minoan, Mycenaean, and Egyptian Assyrian pharmacotherapy were
referred to. Some of them were given the names after mythological characters from
these epics; for instance, Elecampane (Inula helenium L. Asteraceae) was named in
honor of Elena, who was the centre of the Trojan War. As regards the plants from the
genus Artemisia, which were believed to restore strength and protect health, their name
was derived from the Greek word artemis, meaning ―healthy.‖ Herodotus (500 BC)
referred to castor oil plant, Orpheus to the fragrant hellebore and garlic, and Pythagoras
to the sea onion (Scilla maritima), mustard, and cabbage. The works of Hippocrates
(459–370 BC) contain 300 medicinal plants classified by physiological action:
Wormwood and common centaury (Centaurium umbellatum Gilib) were applied
against fever; garlic against intestine parasites; opium, henbane, deadly nightshade, and
mandrake were used as narcotics; fragrant hellebore and haselwort as emetics; sea
onion, celery, parsley, asparagus, and garlic as diuretics; oak and pomegranate as
adstringents.
Theophrast (371-287 BC) founded botanical science with his books ―De Causis
Plantarium‖— Plant Etiology and ―De Historia Plantarium‖—Plant History. In the
books, he generated a classification of more than 500 medicinal plants known at the
time. Among others, he referred to cinnamon, iris rhizome, false hellebore, mint,
pomegranate, cardamom, fragrant hellebore, monkshood, and so forth. In the
description of the plant toxic action, Theophrast underscored the important feature for
humans to become accustomed to them by a gradual increase of the doses. Owing to his
consideration of the said topics, he gained the epithet of ―the father of botany,‖ given
that he has great merits for the classification and description of medicinal plants.
In his work ―De re medica‖ the renowned medical writer Celsus (25 BC–50 AD)
quoted approximately 250 medicinal plants such as aloe, henbane, flax, poppy, pepper,
cinnamon, the star gentian, cardamom, false hellebore, etc.
In ancient history, the most prominent writer on plant drugs was Dioscorides, ―the
father of pharmacognosy,‖ who, as a military physician and pharmacognosist of Nero's
Army, studied medicinal plants wherever he travelled with the Roman Army. Circa 77
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AD he wrote the work ―De Materia Medica.‖ This classical work of ancient history,
translated many times, offers plenty of data on the medicinal plants constituting the
basic materia medica until the late Middle Ages and the Renaissance. Of the total of
944 drugs described, 657 are of plant origin, with descriptions of the outward
appearance, locality, mode of collection, making of the medicinal preparations, and
their therapeutic effect. In addition to the plant description, the names in other
languages coupled with the localities where they occur or are grown are provided. The
plants having mild effect are dominant, but there are also references to those containing
alkaloid or other matter with strong effect (fragrant hellebore, false hellebore, poppy,
buttercup, jimson weed, henbane, deadly nightshade). Dioscorides‘ most appreciated
domestic plants are as follows: willow, camomile, garlic, onion, marsh mallow, ivy,
nettle, sage, common centaury, coriander, parsley, sea onion, and false hellebore).
Camomile (Matricaria recucita L.), known under the name Chamaemelon, is used as
an antiphlogistic to cure wounds, stings, burns, and ulcers, then for cleansing and
rinsing the eyes, ears, nose, and mouth. Owing to its mild carminative action, it is
particularly appropriate for usage with children. Dioscorides deemed that it had
abortive action, on which he wrote, ―The flower, root, and the entire plant accelerate
menstruation, the release of the embryo, and the discharge of urine and stone, provided
that they are used in the form of an infusion and baths.‖ This untrue belief was later
embraced by both the Romans and the Arabs; hence the Latin name Matricaria, derived
from two words: mater denoting ―mother,‖ i.e. matrix, denoting ‗uterus‘. Dioscorides
differentiated between a number of species from the genus Mentha, which were grown
and used to relieve headache and stomach ache. The bulbs of sea onion and parsley
were utilized as diuretics, oak bark was used for gynaecological purposes, while white
willow was used as an antipyretic. As maintained by Dioscorides, Scillae bulbuswas
also applied as an expectorant, cardiac stimulant, and antihydrotic. It is worth
underscoring that Dioscorides pointed to the possibility of forgery of drugs, both the
domestic ones such as opium forged by a yellow poppy (Glaucium flavum) milk sap
and poppy, and the more expensive oriental drugs, transported by the Arab merchants
from the Far East, such as iris, calamus, caradmomum, incense, etc.
Pliny the Elder (23 AD-79), a contemporary of Dioscorides, who travelled
throughout Germany and Spain, wrote about approximately 1000 medicinal plants in
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his book ―Historia naturalis.‖ Pliny's and Dioscorides‘ works incorporated all
knowledge of medicinal plants at the time.
The most distinguished Roman physician (concurrently a pharmacist), Galen (131
AD–200), compiled the first list of drugs with similar or identical action (parallel
drugs), which are interchangeable—―De succedanus.‖ From today's point of view,
some of the proposed substitutes do not correspond in a pharmacological context and
are absolutely unacceptable. Galen also introduced several new plant drugs in therapy
that Dioscorides had not described, for instance, Uvae ursi folium, used as an
uroantiseptic and a mild diuretic even in this day and age.
In the seventh century AD the Slavic people used Rosmarinus officinalis, Ocimum
basilicum, Iris germanica, and Mentha viridis in cosmetics, Alium sativum as a remedy
and Veratrum album, Cucumis sativus, Urtica dioica, Achilea millefolium, Artemisia
maritime L., Lavandula officinalis, Sambuci flos against several injurios insects, i.e.
louses, fleas, moths, mosquitos, and spiders and Aconitum napellus as a poison in
hunting.
In the Middle Ages, the skills of healing, cultivation of medicinal plants, and
preparation of drugs moved to monasteries. Therapy was based on 16 medicinal plants,
which the physicians-monks commonly grew within the monasteries as follows: sage,
anise, mint, Greek seed, savory, tansy, etc.
Charles the Great (742 AD–814), the founder of the reputed medical school in
Salerno, in his ―Capitularies‖ ordered which medicinal plants were to be grown on the
state-owned lands. Around 100 different plants were quoted, which have been used till
present days such as sage, sea onion, iris, mint, common centaury, poppy, marsh
mallow, etc. The great emperor especially appreciated the sage (Salvia officinalis L.).
The Latin name of sage originates from the old Latins, who called it a salvation plant
(salvare meaning ―save, cure‖). Even today sage is a mandatory plant in all Catholic
monasteries.
The Arabs introduced numerous new plants in pharmacotherapy, mostly from India,
a country they used to have trade relations with, whereas the majority of the plants were
with real medicinal value, and they have persisted in all pharmacopoeias in the world
till today. The Arabs used aloe, deadly nightshade, henbane, coffee, ginger, strychnos,
saffron, curcuma, pepper, cinnamon, rheum, senna, and so forth. Certain drugs with
strong action were replaced by drugs with mild action, for instance, Sennae folium was
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used
as
a
mild
laxative,
compared
to
the
purgatives Heleborus
odorus and Euphorbium used until then.
Throughout the Middle Ages European physicians consulted the Arab works ―De
Re Medica‖ by John Mesue (850 AD), ―Canon Medicinae‖ by Avicenna (980-1037),
and ―Liber Magnae Collectionis Simplicum Alimentorum Et Medicamentorum‖ by Ibn
Baitar (1197-1248), in which over 1000 medicinal plants were described.
For Macedonia, St Clement and St Naum of Ohrid's work are of particular
significance. They referred to the Nikeian pharmacological codex dating from year 850,
and transferred his extensive knowledge on medicinal plants to his disciples and via
them to the masses.
Marco Polo's journeys (1254-1324) in tropical Asia, China, and Persia, the
discovery of America (1492), and Vasco De Gama's journeys to India (1498), resulted
in many medicinal plants being brought into Europe. Botanical gardens emerged all
over Europe, and attempts were made for cultivation of domestic medicinal plants and
of the ones imported from the old and the new world. With the discovery of America,
materia medica was enriched with a large number of new medicinal plants: Cinchona,
Ipecacuanha, Cacao, Ratanhia, Lobelia, Jalapa, Podophylum, Senega, Vanilla, Mate,
tobacco, red pepper, etc. In 17th century, Cortex Chinae, yielded from quinine
bark Cinchona succirubra Pavon, under the name countess‘ powder, since the Countess
of Chinchon was the first one who used it, was introduced to European medicine.
Quinine bark rapidly overwhelmed England, France, and Germany despite the fact that
there was many an opponent to its use among distinguished physicians—members of a
range of academies.
Paracelsus (1493-1541) was one of the proponents of chemically prepared drugs out
of raw plants and mineral substances; nonetheless, he was a firm believer that the
collection of those substances ought to be astrologically determined. He continuously
emphasized his belief in observation, and simultaneously supported the ―Signatura
doctrinae‖—the signature doctrine. According to this belief, God designated his own
sign on the healing substances, which indicated their application for certain diseases.
For example, the haselwort is reminiscent of the liver; thus, it must be beneficial for
liver diseases; St John's wort Hypericum perforatum L. would be beneficial for
treatment of wounds and stings given that the plant leaves appear as if they had been
stung.
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While the old peoples used medicinal plants primarily as simple pharmaceutical
forms—infusions, decoctions and macerations—in the Middle Ages, and in particular
between 16th and 18th centuries, the demand for compound drugs was increasing. The
compound drugs comprised medicinal plants along with drugs of animal and plant
origin. If the drug the theriac was produced from a number of medicinal plants, rare
animals, and minerals, it was highly valued and sold expensively.
In 18th century, in his work Species Plantarium (1753), Linnaeus (1707-1788)
provided a brief description and classification of the species described until then. The
species were described and named without taking into consideration whether some of
them had previously been described somewhere. For the naming, a polynomial system
was employed where the first word denoted the genus while the remaining polynomial
phrase explained other features of the plant (e.g. the willow Clusius was named Salix
pumila angustifolia antera). Linnaeus altered the naming system into a binominal one.
The name of each species consisted of the genus name, with an initial capital letter, and
the species name, with an initial small letter.
Early 19th century was a turning point in the knowledge and use of medicinal
plants. The discovery, substantiation, and isolation of alkaloids from poppy (1806),
ipecacuanha (1817), strychnos (1817), quinine (1820), pomegranate (1878), and other
plants, then the isolation of glycosides, marked the beginning of scientific pharmacy.
With the upgrading of the chemical methods, other active substances from medicinal
plants were also discovered such as tannins, saponosides, etheric oils, vitamins,
hormones, etc.
In late 19th and early 20th centuries, there was a great danger of elimination of
medicinal plants from therapy. Many authors wrote that drugs obtained from them had
many shortcomings due to the destructive action of enzymes, which cause fundamental
changes during the process of medicinal plants drying, i.e. medicinal plants‘ healing
action depends on the mode of drying. In 19th century, therapeutics, alkaloids, and
glycosides isolated in pure form were increasingly supplanting the drugs from which
they had been isolated. Nevertheless, it was soon ascertained that although the action of
pure alkaloids was faster, the action of alkaloid drugs was full and long-lasting. In early
20th century, stabilization methods for fresh medicinal plants were proposed, especially
the ones with labile medicinal components. Besides, much effort was invested in study
of the conditions of manufacturing and cultivation of medicinal plants.
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On account of chemical, physiological, and clinical studies, numerous forgotten
plants and drugs obtained thereof were restored to pharmacy: Aconitum, Punica
granatum, Hyosciamus, Stramonium, Secale cornutum, Filix mas, Opium, Styrax,
Colchicum, Ricinus, and so forth. The active components of medicinal plants are a
product of the natural, most seamless laboratory. The human organism accepts the drug
obtained from them best in view of the fact that man is an integral part of nature.[29]
There are scores of examples of this kind; perhaps they will instigate serious research
into the old manuscripts on medicinal plants, which would not be observed out of
curiosity about history but as potential sources of contemporary pharmacotherapy.
In present days, almost all pharmacopoeias in the world—Ph Eur 6, USP XXXI, BP
2007-proscribe plant drugs of real medicinal value. There are countries (the United
Kingdom,[32] Russia, Germany that have separate herbal pharmacopoeias. Yet, in
practice, a much higher number of unofficial drugs are always used. Their application is
grounded on the experiences of popular medicine (traditional or popular medicine) or
on the new scientific research and experimental results (conventional medicine). Many
medicinal plants are applied through self-medication or at the recommendation of a
physician or pharmacist. They are used independently or in combination with synthetic
drugs (complementary medicine). For the sake of adequate and successfully applied
therapy, knowledge of the precise diagnosis of the illness as well as of medicinal plants,
i.e. the pharmacological effect of their components is essential. Plant drugs and
phytopreparations, most commonly with defined active components, verified action
and, sometimes, therapeutic efficiency, are applied as therapeutic means. In the major
European producer and consumer of herbal preparations—Germany, rational
phytotherapy is employed, based on applications of preparations whose efficiency
depends on the applied dose and identified active components, and their efficiency has
been corroborated by experimental and clinical tests. Those preparations have been
manufactured from standardized plant drug extracts, and they adhere to all
requirements for pharmaceutical quality of drugs.
With the new Law on Drugs and Medical Devices dated September 2007 and
enacted in the Republic of Macedonia, dry or sometimes fresh parts of medicinal plants
(herbal substances) may be used for preparation of herbal drugs, herbal processed
products, and traditional herbal drugs. Herbal substances may also be utilized for
manufacture of homeopathic drugs, which are stipulated in the current law, too. In the
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Republic of Macedonia herbal preparations are dispensed without a medical
prescription, as ―over the counter‖ (OTC) preparations.
Ethnobotany and its Scope:
Ethnobotany (from ethnology, study of culture, and botany, study of plants) is
the scientific study of the relationships that exist betweenpeople and plants.
Ethnobotanists aim to document, describe and explain complex relationships
between cultures and (uses of) plants, focusing primarily on how plants are used,
managed and perceived across human societies. This includes use for food, clothing,
currency, ritual, medicine, dye, construction, cosmetics and a lot more. Richard Evans
Schultes, called the "father of ethnobotany", explained the discipline in this way:
Ethnobotany simply means investigating plants used by primitive societies in
various parts of the world.
As can be seen in the dictionary definition, as well as overview studies, the field of
ethnobotany has expanded to include not only primitive or native peoples, and not only
a utilitarian relationship, but also the relationship between people and plants in the
modern setting, incorporating cognitive, ecological, and symbolic aspects.
Intellectual property rights and benefit-sharing arrangements are important issues
in ethnobotany
Though the term "ethnobotany" was not coined until 1895 by the US botanist John
William Harshberger, the history of the field begins long before that.Pythagoreanism,
which originated in 500 BC included a refusal to eat beans, perhaps because of the
human relationship of the beans through matter. While possibly related to the condition
known as favism, it is more likely because of magical thinking, and a contemporary
belief that beans and humans originated from the same matter, or because of the beans
shape bearing similarity to detain human organs in shape. Theophrastus the father of
botany wrote of plants and people's usage of them in his works. In A.D. 77, the Greek
surgeon Pedanius Dioscoridespublished De Materia Medica, which was a catalog of
about 600 plants in the Mediterranean. It also included information on how the plants
were
used,
especially
for
medicinal
purposes.
This illustrated
herbal
publication contained information on how and when each plant was gathered, whether
or not it was poisonous, its actual use, and whether or not it was edible (it even
provided recipes). Dioscorides stressed the economic potential of plants. For
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generations, scholars learned from this herbal publication, but did not commonly
venture into the field until after the Middle Ages due to the Inquisition
The first individual to study the emic perspective of the plant world was a German
physician working in Sarajevo at the end of 19th century: Leopold Glueck. His
published work on traditional medical uses of plants done by rural people in Bosnia
(1896) has to be considered the first modern ethnobotanical work. The term
"ethnobotany" was first used by a botanist named John W. Harshberger in 1895 while
he was teaching at the University of Pennsylvania.
Other scholars analyzed uses of plants under an indigenous/local perspective in the
20th century: Matilda Coxe Stevenson, Zuni plants (1915); Frank Cushing, Zuni foods
(1920); Keewaydinoquay Peschel, Anishinaabe fungi (1998), and the team approach of
Wilfred Robbins, John Peabody Harrington, and Barbara Freire-Marreco, Tewa pueblo
plants (1916).
In the beginning, ethonobotanical specimens and studies were not very reliable and
sometimes not helpful. This is because the botanists and the anthropologists did not
always collaborate in their work. The botanists focused on identifying species and how
the plants were used instead of concentrating upon how plants fit into people's lives. On
the other hand, anthropologists were interested in the cultural role of plants and treated
other scientific aspects superficially. In the early 20th century, botanists and
anthropologists better collaborated and the collection of reliable, detailed crossdisciplinary data began.
Beginning in the 20th century, the field of ethnobotany experienced a shift from the raw
compilation of data to a greater methodological and conceptual reorientation. This is
also the beginning of academic ethnobotany. The so-called "father" of this discipline
is Richard Evans Schultes, even though he did not actually coin the term
"ethnobotany". Today the field of ethnobotany requires a variety of skills: botanical
training for the identification and preservation of plant specimens; anthropological
training to understand the cultural concepts around the perception of plants; linguistic
training, at least enough to transcribe local terms and understand native morphology,
syntax, and semantics.
Mark Plotkin, who studied at Harvard University, the Yale School of
Forestry and Tufts University, has contributed a number of books on ethnobotany. He
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completed a handbook for the Tirio people of Suriname detailing their medicinal
plants; Tales of a Shaman's Apprentice (1994); The Shaman's Apprentice, a children's
book with Lynne Cherry (1998); and Medicine Quest: In Search of Nature's Healing
Secrets (2000).
Plotkin was interviewed in 1998 by South American Explorer magazine, just after
the release of Tales of a Shaman's Apprentice and the IMAX movie Amazonia. In the
book, he stated that he saw wisdom in both traditional and Western forms of medicine:
No medical system has all the answers—no shaman that I've worked with has the
equivalent of a polio vaccine and no dermatologist that I've been to could cure a fungal
infection as effectively (and inexpensively) as some of my Amazonian mentors. It
shouldn't be the doctor versus the witch doctor. It should be the best aspects of all
medical systems (ayurvedic, herbalism, homeopathic, and so on) combined in a way
which makes health care more effective and more affordable for all.
A great deal of information about the traditional uses of plants is still intact with
tribal peoples.[10] But the native healers are often reluctant to accurately share their
knowledge to outsiders. Schultes actually apprenticed himself to an Amazonian
shaman, which involves a long-term commitment and genuine relationship. In Wind in
the Blood: Mayan Healing & Chinese Medicine by Garcia et al. the visiting
acupuncturists were able to access levels of Mayan medicine that anthropologists could
not because they had something to share in exchange. Cherokee medicine priest David
Winston describes how his uncle would invent nonsense to satisfy visiting
anthropologists.
Another scholar, James W. Herrick, who studied under ethnologist William N.
Fenton, in his work Iroquois Medical Ethnobotany (1995) with Dean R. Snow (editor),
professor of Anthropology at Penn State, explain that understanding herbal medicines
in traditional Iroquois cultures is rooted in a strong and ancient cosmological belief
system. Their work provides perceptions and conceptions of illness and imbalances
which can manifest in physical forms from benign maladies to serious diseases. It also
includes a large compilation of Herrick‘s field work from numerous Iroquois authorities
of over 450 names, uses, and preparations of plants for various ailments. Traditional
Iroquois practitioners had (and have) a sophisticated perspective on the plant world that
contrast strikingly with that of modern medical science.
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Scope of Ethnobotany
ETHNOBOTANY BASICS
What is ethnobotany? Ethno + Botany = Ethnobotany people + study of plants =
the study of the interactions of people with plants
Ethnobotany is an interdisciplinary science, which includes aspects of both
sciences and humanities. Ethnobotany can serve as a gateway to many different
disciplines like the ones listed. Define and analyze how the disciplines below
are relevant to the study of ethnobotany.
Discipline
Relevance to Ethnobotany
1. Agriculture
How humans have domesticated
and managed plants, especially in
traditional agriculture systems
2. Agroforestry
How humans have managed the
land
for
the
simultaneous
production of food, crops, and trees
3. Anthropology
How different cultures use plants
4. Archeology
Paleoethnobotany – how ancient
cultures used plants
5. Botany
The study of the structure and
composition of plants
6. Chemistry
The study of the composition of
substances and active chemicals in
plants, especially medicinal plants
7. Ecology
How human interactions with plants
and ecosystems affect plant ecology
Economic botany – the economic
8. Economics
uses of plants
9. Forestry
The human management of forests
and forest trees
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10.Horticulture
The management of useful plants
(fruits, vegetables, ornamentals) in
home gardens or orchards
11.Linguistics
The terminology for plants and
plant parts by people of different
language groups
12.Medicine
How
humans
use
plants
for
medicinal purposes
13.Religious Studies
Ritual uses of plants by different
cultures and religions
14.Sociology
How humans use plants in various
societies
15.Systematics
Folk-taxonomy,
how
different
people classify plants
\
ii.
Food Plants:
Rice
Botanical name
Oryza sativa
Vern:
Dhan Chawal
Family:
Gramineae
Rice is the Principal Food 60 percent of mankind. Approximately 40 million hectares of
land in our country are under rice cultivation. The rice product annually is about 654
million toones nut paddy ( rice enclosed within husks) production is about 80-90
million tonnes. Half of the Indian population is dependent upon rice as food.
Botanical description
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The rice plant like wheat and maize belongs to the family Gramineae. The plant is a
variable annual which grows to a height of 2-6ft. there are usually 7-12 tillers
developed from the lower nodes of the plant. It produces several stems or culms in a
plant. A stem may usually have 7-10 nodes but may sometimes have as many twenty
nodes. The hollow internodes of the stem are fully or partially covered by the lead
sheaths. Internodes are hollow internodes in tall varieties but may be solid in dwarf
plants. The lamina of the uppermost leaf below the panicle (flag or boot leaf) is wider
and shorter than the others. Tillers develop from lower nodes of the stem. The
inflorescence is a loose terminal panicle. The spikelet‘s are one flowered and are
laterally compressed. It is a self-pollinated plant. The lemma and palea together are
known as hull or husk and the rice grain with the hull is known as :paddy‘ or Dhan.
Rice contains sic functional stamens which distinguishes it from other cereals. The
lemma and the palae may or may not possess awns. The presence of awn is considered
and advantages since it is less subject to bird attacks. Mature rice grain is a caryopsis.
The grain whose husk or hull has been removed is called as ―husked‖ or Brown or
cleaned rice. The brain of the rice consists of the pericarp, nuscellus, alerone layer and
the embryo (germ). The brain of the rice grain gets removed in the millin process after
hich the rice is called ―milled‘ or ; polished‘ rice. Brain is quite rich in oils, proteins,
minera; salts and vitamins. Hence polished rice is devoid of all these nutrients. The
plant is distinguished from other cereals by the presence of six stamens.
The colour of the grain helps in determining the quality and variety of rice. The grain
with red colour is supposed to be interior to others,
Cultivation
The rice grows best on damp soils underlaid with a semi-impervious subsoil in places
where it can be flooded.
Inn tropical countries like India three crops of rice are produced according to
requirement of different agro-climatic regions. These are (a) Aus (b) Aman (c) Boro.
Aus, also known as Upland rice, is sown in May-June and harvested in September October, Aman, also called lowland rice is sown in June-July to be harvested in
November - December. Boro is sown in December-January and harvested in MarchApril. During this period growing varieties are produced. Boro rice is grown in semidry areas.
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Before planting the filed are ploughed thoroughly. The seeds are grown in seed beds.
The seedling are transplanted to the filed when 20-25 cm tail. Extensive weeding
exercise and careful application of fertilizers is necessary for successful cultivation.
Addition of fertilizers just before panicle formation greatly increase the yield. The role
of blue green alagae in replenishing the fertility of rice fields is now being greatly
emphasized. In Kashmir Valley green manuring of rice crop with Lens esculentus has
been found to be very effective.
A consideration interest has been aroused in India during last two decades in the
Japanese method of rice cultivation. This combines improved cultural practices with
proper manuring and plant protection measures. The increased acre yields obtained by
this method have naturally led to its popularity. Low land rice is grown on soils
submerged under water for 60-90 days during growing season. Following seasonal
categories of rice are recognized in India:
1.
Summer rice
2.
Winter rice.
3.
Deep water paddy
4.
Jhum paddy
Milling. The mature plants turn yellow and are harvested by cutting their stalks.
Threshing is usually carried out within two or three days of cutting crop and is
accomplished either by beating the cut crop against a wooden log or by pedal and
power threshers. The husk is removed either by hand pounding or power driven
machines. Hand pounding id done with pestle and mortar. Modern machine milling
removes the husk, brain, germ, and part of the endopserm. It is called milled or
‗polished rice;. In polishing the embryo which contains proteins vitamins and oils is
knocked out due to lack of strong link between embryo and endosperm.
Uses
Clean Dirty Vases and Other Unusually-Shaped Bottles. What‘s that, you don‘t
have the world‘s smallest hands? Cleaning vases and bottles with slender and unusually
shaped necks is a total pain — until now. Drop in a couple tablespoons of rice, pour in
some warm water, and let it soak for about 10 minutes. Then shake, shake, shake, pour
it out, rinse, and marvel at your newly-cleaned vase.
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2. Rescue Wet Electronics. As the ultimate klutz, I‘ve had to use this trick more than
once. When you drop your electronics in water, remove the battery if you can, and bury
the phone in rice. Trust me, it works!
3. Make a Pet Toy. Make a small satchel, fill it with a combination of rice and catnip,
sew it up, and watch your cat go nuts!
4. Clean Coffee and Spice Grinders. As much as we love them, keeping coffee and
spice grinders clean can be a pain. Remedy that with rice: remove as much debris as
you can, cover the area with rice, and run it through the grinder. The rice will magically
absorb a lot of the buildup. Some of the country‘s most prestigious roasters use this
trick. It works especially well with instant rice!
5. Check if Oil is Hot Enough. Want to check the temperature of the oil you‘re using
to deep fry? All you need to do is drop a grain of rice into it. If the rice pops up to the
surface of the oil and begins cooking, the oil is ready for frying. In my experience, it‘s
much more reliable than a deep fry thermometer.
6. Bake a Perfect Pie. Fancy stores sell $15 jars of pie weights, the little beads placed
on unfilled crusts when you blind bake them. What a racket — rice works just as well.
You wouldn‘t want to cook with the rice afterward, but you can use them again as pie
weights.
7. Keep Salt Separated. A classic restaurant trick: store a few grains of rice in your
salt shaker to prevent that annoying clumping.
8. Ripen Fruit Faster. Just can‘t wait for that fruit to ripe? Store fruit in a container of
rice to speed up the ripening time. Just make sure to check on it twice a day so it
doesn‘t get too ripe. And yes, you can cook with the rice!
10. Make a Heating Pad. Sew a little pouch with a natural fabric like cotton or wool.
Fill the pouch up with some rice, sew it shut, and you‘ve got yourself a heating pad. If
you‘re not feeling that ambitious, you can just fill an old sock with rice and tie the end
close. When you‘re ready to use the heating pad, heat it up in the microwave. It‘ll stay
warm for up to an hour.
11. Get Glowing Skin. If you‘re like me, you just never get the amount of water right
when you‘re cooking rice. It always seems like there‘s not enough water, and then you
overcompensate and the rice is drowning. Don‘t stress — save that leftover rice water
and put it to use! Let it cool, refrigerate, and use to it wash your skin. For best results
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use brown rice water and use within 3 days. Apply it with a washcloth and rinse. Brown
rice is high in vitamin E, and will give your skin that radiant glow it deserves.
Potato
Botanical name:
Solanim tuberosum
Vern
Aalu
Family
Solanaceae
The potato is one of the cheapest sources of starchy food. Potato now occupies a
prominent position in the world‘s food economy. Certainly it was the potato that helped
to keep Germany alive during the two world wars as the potato safe underground could
not destroyed by burning standing crops of other food plants.
Botanical description
The potato plant is an erect, branching, spreading annual attaining the height of 3-4 ft.
potato belongs to family Solanaceae. The leaves are pinnately compound with large or
primary leaflets and small or secondary leaflets. The plant may or may not produce
flowers. When flowers are present, they are white, yellow or purple in colour which
after fertilization produce berries (fruits). Each berry may contain as many as 200
seeds. These berries are inedible.
The part used for consumption are the potato tubers which are the swollen tips of
the underground branches. The tubers bear groups of buds borne in the axil of aborted
leaves. These buds are called‘ eyes‘. Potatoes are propagated vegetatively by means of
tuber places. Having atleast one eye on them.
The tuber has large number od starch bearing cells. The starch grains swell up on
cooking an break up the thin cell walls to give the typical mealiness of a cooked potato.
Cultivation
Potato is essentially a crop of cool of cool moist regions and grows best in climate
where cool rights alternate with warm days during the period of tuber formation.
Disease free potato tubers are planted whole or into section (eye) is careful prepared
seed beds. Large seed potatoes are cut into many sections, each having at least one bud
or ‗eye‘. Tubers should be cut at right angles to the main axis to eliminate apical
dominance. Potatoes are planned in rows which are spaced 30-50cm apart. In each row
they are planted about 10 cm deep and at intervals of 20cm, by hand or by using a
potato planter.
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Potato tubers are ready for harvest in 3-4 months after planting. Crop may be harvested
by hand digging of tubers or by tractor drawn mechanical diggers. The plant tops are
cut off or defoliated by chemicals like maleic hydrazide. Defoliants are sprayed about 2
weeks before digging of tubers and also to prevent pre mature sprouting. The harvested
tubers, after washing out the sticking soil are either sent to the market for sale or stored
in a cool environment at about 4c.
Uses
Removal of Dark Circles
Potatoes are a natural under eye brightener and so, applying potato juice or potatoes
directly on the affected area can banish dark eye rings.
2. Treatment of Wrinkles
Potato is considered as an effective anti-aging beauty agent, particularly in warding off
wrinkles. Regular application of potato juice imparts a healthy glow to your skin. It also
helps to soften your skin as well as keep wrinkles at bay.
3. Treatment of Dark Spots
Potatoes are a natural way of fading away those dark spots.
4. Removal of Facial Blemishes
Facial blemishes adversely affect your appearance and potato juice is a great natural
way to get rid of them.
5. Treatment Of Sunburns
Potatoes are wonderful for treating sun burnt skin.
6. Skin Lightening
Potatoes are a natural skin lightening agent.
7. Potato For Dry Skin
If you have dry skin, potatoes will help give you that smooth skin feeling in minutes.
8. Lifting Of Dead Skin Cells
Potatoes can also remove dead skin cells on the face.
9. Natural Cleanser
Potato can be used as a natural facial cleanser.
10. Treatment Of Rashes, Itching Sensation And Insect/Pest Bite
11. Removal of Eye Puffiness:
To remove puffiness around the eyes:
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12. Promotes Healthy Hair
Potatoes are a great natural ingredient to help achieve healthy hair.
13. Treatment Of Grey Hair
Potato peelings can be used to restore Grey hair.
14. Potato For Hair Loss
To combat hair loss, you can make a hair mask comprising of potato and honey.
iii.
Fibres
The fibre yielding plants fulfil the second requirement of human beings i.e., clothing.
Such plants are known to mankind since time immemorial. The utilization of fibre is
directly related to the advancement of civilization. Till now more than two thousand
species of plants have been worked out which yield fibres. However, the commercial
fibre yielding plants are few in number.
Botanically, they are sclerenchymatous fibres occurring in different parts of the plants.
The longest fibre of Boehmeria nivea measures 55 cms, in length. Chemically, they are
made up of cellulose or hemicelluloses with deposition of lignin.
Jute
Botanical Name:
Corchorus capsularis
Vernacular Name: Patsan
Family:
Tiliaceae
Jute is the most important bast fibre and among natural fibres it is only second to
cotton. The commercial fibre jute is obtained from the stems of two species of
Corchorus, C. capsularis and C. olitorius. In India, it is mainly grown in West Bengal,
Assam, Bihar and Orrisa.
Morphology
The plants are 2.5 to 3 meter tall, somewhat shrubby annuals. The leaves are alternate,
stipulate, serrate, simple and ovate. Two curved bristles (auricles) are present at the
base of the leaf. The flowers are solitary or arranged in 3-4 flowered cymes. They are
bracteate, pedicellate, hermaphrodite and yellow. The fruit is loculicidal capsule which
may be round (C. capsularis) or long (C. olitorius). The fibre is obtained from
secondary phloem. The fibres are very long, stiff, lignified and with yellow lustre.
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Fig. A Jute Plant
Cultivation of Jute
The plant requires rich loamy, alluvial soil and warm humid climate. The land is first
ploughed and cleaned several times. Seeds are sown by broadcasting or by dibbling,
usually in the month of April/May. After emergence of seedlings thinning is done in
two instalments, one when they are about 10 cm in height and again when they are
about 15 cm in height, so as to have a spacing of 10 cm between seedlings. During the
first four or five weeks of the crop weeding is necessary two or three times.
Manuring
Farmyard manure is applied at the rate of 4-7 tons per hectare during land preparations.
Nitrogen is applied in two instalments, 40-80 kg per hectare. Phosphate and Potash are
applied as basal doses, the quantity of former being half of nitrogen and that of latter
equal to nitrogen.
Harvesting of the crop is done at the flowering stage. If harvested early good quality
fibres are obtained. The plants are cut close to the ground with a sickle. The plants are
then allowed to dry and shed off the leaves. The cut stems are now tied in bundles 6-9
inches in diameter.
Extraction
Fibres are separated from the stems by the process of retting in pools of stagnant water.
Retting is a microbiological process which helps the softer gummy tissues to rot out
and as a result the bast fibres get separated. The bundles are laid flat in water at least
0.6-0.9 m deep. The bundles are made to sink in the water by placing on the top of them
stones and logs. The process of retting requires 10-30 days. It depends upon the nature
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of water, the kind of fibre, depth of the pond and its temperature. Retting is complete
when the bark separates out from the wood. The stems are usually whipped on the
surface of water. The retted and loosened fibres are pulled apart from the sticks by
hand. By the other method namely the beat, break, jerk method, 10-12 stems are taken
together, their root ends are beaten with a mallet until the fibres become loose. The
separated fibres are then rinsed, cleaned, wrung and stacked on clean ground. The dried
fibres are then tied together in small bundles. Now they are ready for marketing.
Economic importance: Jute is next to cotton in importance and has got diverse uses as
given below:
1.
Jute is an important bagging material and is extensively used for gunny bags.
2.
Jute is extensively used in the manufacture of carpets, curtains, shirtings, blankets
and is also mixed with silk.
3.
The short fibres, known as jute butts are used in paper making.
4.
The fibre is used in the manufacture of coarse cloth, twine, ropes, etc.
5.
Oil from jute seeds is used for cooking purposes and in the manufacture of soaps.
6.
An infusion of leaves is tonic and febrifuge and is also used as a demulcent in
cystitis and dysuria.
iv.
Vegetable oils
Vegetable oils are obtained from plant (vegetable) sources. Oils are the complex
chemical compounds which have triesters of glycerol with the long chain of organic
acids chiefly palmitic acid, stearic acid and oleic acid. Vegetable oils are distinguished
into volatile or essential and fixed or fatty oils depending on their behaviour on
heating.
Essential Oils: The essential or volatile oils evaporate on coming in contact with the
air emitting pleasant fragrance. Chemically, they are combination of several organic
substances, such as benzene derivatives, terpenes and various other hydrocarbons and
straight-chain compounds. They are distributed in about 60 families of plant kingdom.
The important families which provide essential oils are Lamiaceae, Rutaceae, Apiaceae,
Asteraceae, Lauraceae, Fabaceae and Poaceae.
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Essential oils can be extracted by distillation in a still, or by expression i.e., by applying
pressure or by using solvents. The later is done in two ways: by enfleurage and by
maceration. Essential oils are used in the manufacture of perfumes, soaps and other
toilet preparations. Several essential oils are used in medicines.
Fatty Oils: The fatty oils are liquid at room temperature and usually contain oleic acid.
They are known as fixed oils because of their non-volatile nature. Chemically, the fatty
oils consist of glycerine in combination with a fatty acid. They lack strong odour in
pure state, become rancid after long exposure to air. They are insoluble in water but
soluble in various organic solvents.
Fatty oils are stored up in seeds as a reserve food, but also found in fruits, tubers, stems
and other plant organs. The oils may be removed by solvents or by hydraulic pressure.
Several fatty oils are edible and are used as cooking media.
Mustard
Botanical name:
Brassica compestris
Vernacular Name: Sarsoon
Family:
Brassicaceae
The oil obtained from various species of Brassica is known as rape, colza or mustard
oil. Some important species of Brassica which are extensively used for oil extraction
are: B. compestris var. sarson (yellow sarson), B. compestris var. dichotoma (Brown
sarson), B. compestris var. toria (Lahi) B. juncea (Indian mustard) and B. napus (Kali
sarson).
Three varieties of B. compestris – sarson, dichotoma and toria are collectively known as
rape, whereas B. juncea is called mustard.
Morphology
Mustard plants are erect, slender, branched annual herbs about 50-150 cm tall. The
leaves are generally lyrate (pinnatipartite). The flowers are small, yellow and are
arranged in corymbose racemes, each having a typical cruciferous plan, i.e. four free
sepals, four free petals which are clawed, tetradynamous stamens and a bicarpellary
syncarpous ovary, initially unilocular but later becoming bilocular due to formation of
false septum (replum). The fruit is siliqua.
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Cultivation
Fig. A Mustard Plant
Twig
Mustard is mainly grown as a rabi crop, either as a pure crop or mixed with wheat or
barley. It prefers medium or heavy loam soils. A fine tilth is required for these crops. In
irrigated areas 4-6 ploughings and in rain fed areas 1-2 ploughings may be given. In the
case of mixed cropping, the seed rate depends on the proportion of rape to the main
crop. When grown pure, 5 kg of seed is considered proper for 1 hectare of land. The
seeds are broadcasted on prepared fields during Oct.-Nov. In mixed cropping, mustard
is sown in rows 1.8-2.4 m apart, across the main crop and in a pure crop, sowing is
done in rows 30 cm apart. Thinning is done after three weeks in order to maintain a
plant to plant distance of 10-15 cm.
Manuring and Fertilization
The optimum dosage of nitrogen for mustard in rain fed areas is 40 kg per hectare,
whereas in irrigated areas 40-60 kg per hectare is required.
Irrigation
Only two irrigations, one at the time of flowering and the other at the time of pod
formation are required. The crop produces flowers in April-May. The crop matures in
110-160 days. The crop is usually harvested by means of hand sickles. Threshing is
conveniently done by beating the fruits on the plants taken in bundled with a wooden
stick. Trampling them under the feet of blocks is also quite common. Seeds are
separated from the husk by winnowing. The seed is finally dried and stored
Extraction of the Oil: Oils and fats are usually contained as insoluble droplets within
the cells of seeds. Seeds contain 30-48% of oil depending upon the variety and the
climatic conditions. The oil is extracted by mechanical expression or solvent extraction.
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I.
Mechanical Expression: Expression is the most important method of oil
extraction. It involves application of pressure to the oil bearing tissues to squeeze out
the fat. It is usually done by hydraulic pressing or screw pressing. Expression may be of
hot or cold type. Hot expression involves free cooking of the kernel, usually with
steam, to facilitate oil flow, whereas cold expression involves mere pressing of the
kernel without steam. The oil is then passed through filter presses to refine it.
II. Solvent Extraction: By this process even every low proportion of oil can be
extracted. A number of solvents like gasoline, benzene, carbon disulphide, petroleum
ether and chlorinated hydrocarbons are used for the purpose. Fatty oils are freed from
extracting solvents by distillation. The oil thus obtained is subjected to refining process
to remove impurities. Solvent extraction is often practiced on the press cake left after
expression to remove the small percentage of oil left.
Economic Importance
1.
Mustard oil is one of the major edible oils and is the chief cooking medium in
India.
2.
Different soaps and rubber substituents are prepared were oil is used as an
important ingredient.
3.
Mustard oil is used as body massage and hair oil.
4.
In machines, the mustard oil is used as a lubricant to avoid friction.
5.
The oil is also used as a preservative for pickles and for flavouring curries and
vegetables.
6.
v.
The oil cake known as ‗Khali‘ is used as cattle feed.
Wood
Wood is defined as a complex secondary tissue produced in the stems of gymnosperms
and dicotyledonous angiosperms by the actively dividing cambium cells.
The most conspicuous part of wood is the secondary xylem with concentric rings. It is
made up of four kinds of cells i.e. vessels, tracheid‘s, wood fibers and wood
Parenchyma.
Classification of Wood
(i) Porous wood and Non-porous wood: In gymnosperms the wood lacks vessel
elements since cavities of vessels in cross section are called pores. Therefore the
gymnospermous wood lacks pores and as such called non-porous wood. Such woods
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also lack wood fibers which impart hardness and toughness to wood. Thus non-porous
wood is also called soft wood because of its texture. The common examples of nonporous wood are pine, spruce, fir, deodar, red wood and other gymnosperms.
In angiosperms especially the dicots the vessels form the chief component of wood and
there are abundant pores in the cross section of such woods. It is therefore, called
porous wood. Since this type of wood also contains abundant wood fibers making it
hard and tough, porous wood is also called as hard wood e.g. kikar, shisham, oak,
walnut etc.
(ii) Sap wood and heart wood:The wood in several years old stem consists of
secondary xylem. In a cross section, two distinct parts can be recognized:
(a) Sap wood or Alburnum: It is the outer light coloured part consisting of younger
annual rings, the cells of which are less lignified and physiologically active. They
conduct and store food materials. These outer rings of wood consist of living
parenchyma and wood ray cells in addition to vessels, tracheid‘s and fibers.
(b) Heart wood or Duramen: The inner layers of wood consisting of older annual
rings are dark coloured due to the presence of tannins, pigments and accumulated
products in its conducting cells. These layers form the heart wood or duramen. It
consists of dead cells and appears darker and drier than the sap wood. The
parenchymatous cells grow through the pits in the walls of neighboring vessels and
tracheid‘s and enlarges in their lumen, thus blocking them. These protrusions are called
tyloses.
The heart wood is very strong and purely mechanical in function. It discourages
the visits of parasitic organisms such as fungi, insects and other wood destroying
organisms due to presence of various chemicals such as resins, gums, tannins, oils and
bitter substances in it. These substances also impart durability, strength, hardness and
weight to heart wood. This type of wood is therefore more valuable for outdoor
construction.
(iii) Split wood or coopers wood
All types of wood used in making barrels, casks and similar objects for holding wet or
dry goods like liquors, honey, grams, flour etc. are called coopers wood. Such wood has
the following properties:
(i) It should be strong and durable.
(ii) It should be water tight and retain liquids without leakage.
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(iii) It should be light and without knots.
(iv) It should not give undesirable taste, odour or colour to liquids and other stored
materials.
(v) It should have straight grams.
The common examples of split wood in India are Grewia tiliafolia, Alnus nepalensis,
Grewia vestita, Ougenia dalbergioids etc.
Seasoning of Wood: The process of drawing out moisture from wood without causing
uneven shrinkage, wrapping, shakes and other defects is called as seasoning of wood. It
is done in two ways:
(i) Open air seasoning: It is done under natural heat of the sun by placing logs of
wood horizontally one above the other in open protected shade. The process is
continued till the final moisture content is 12-30%. This is a useful technique from
commercial point of view, though being a slow process.
(ii) Kiln seasoning: It is done by drying the wood artificially using hot air at desired
temperature and humidity. Usually steam heated chambers are used where hot air is
circulated by fans and kept humid by introducing steam. The process is continued till
moisture content drops to about 10 %. This process is more efficient than open air
seasoning.
Uses of wood: The important and chief uses of wood are:
1.
Fuel: The dry, hard wood forms an excellent fuel at home and in industry. The
wood of mulberry, sheesham, kikar, walnut, willow etc. is commonly used as fuel in
rural and hilly areas.
2.
Timber: The wood produced in conifers and some dicotyledons is used to make
doors, windows, beams and other objects for construction of buildings, bridges etc.
3.
Boxes and Cratis: The soft wood of Chil, Kail, Almus, and Poplar has a good nail
holding property. It is light weight. Thus, this type of wood is used to make boxes and
crates for vegetables, fruits etc.
4.
Furniture: The seasoned wood of teak, Shisham, Chinar, Kikar etc. is durable,
resistant to decay and polishable. It is therefore used to make furniture.
5.
Vehicles: The hard wood of Toon,Kikar, Shisham, etc. is used to make the bodies
of trucks, wagons, carts, tongas and other carriages.
6.
Telegraph, electric and fencing poles: The round or split wood of various trees is
used for this purpose after treatment with preservatives.
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7.
Boat and ship Building: The wood of Shorea robusta, Tectona grandis,
Tamarindus indicus, Cedrus deodaraetc. after special treatment is used in making boats
and ships.
8.
The wood obtained from various trees is used in making musical instruments,
agricultural implements, picture frames, wood pipes, toys, handles for knives,
playground equipment‘s etc.
9.
The joinery andcabinet making as well as flooring, wall paneling depend upon
wood obtained from Cedrella tuna, Cedrus deodara, Chloroxylon swietiana etc.
10. Perfumes and oils are also obtained from various types of woods e.g. Sandalwood,
Deodar (Cedar oil).
11. Large number of household and other articles e.g. match boxes, combs, walking
sticks, spinning and weaving machines, tobacco pipes etc. are made from wood.
12. Wood after special treatment obtained from Pterocarpus macrocarpus, Dalbergia
sissi, D. latifolia etc. is used for making butts of guns and gun carriages as well as
railway tracks.
Important sources of firewood and timber:
1.
Tectona grandis: It is commonly called teak or sggwan and found in Assam,
Madras, M.P and South eastern Asian countries. It yields timber of high quality and is
used in making furniture, cabinet works, shipbuilding etc. it is greasy to touch, hard and
very durable.
2.
Dalbergia sisso: It is called Tahli or Shesham. It is a tall, deciduous, leguminous
tree found all over India. Its heart wood is of excellent quality and mainly used for
making furniture of high quality. It is also used for making doors, frames, ploughs and
outdoor construction material. Its wood is brown to dark in colour, strong, durable,
seasonal, highly polishable and ornamental.
3.
Shorea robusta: It is a tall tree belonging to family Dipterocarpaceae. It yields
strong and highly durable wood used for making furniture, cart wheels, doors, windows
and railway sleepers. The tree grows all over India and its wood is moisture decay
resistant.
4.
Cedrus deodara: Commonly called Deodar, it is a tall, evergreen conifer found in
the western Himalayas at a height of 4,000 to 10,000 feet. Its wood is soft, insect
repellent and used for telegraph and electric poles, boat making, railway sleepers, floor
and wall paneling as well as for making doors, windows and cabinets.
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5.
Pinus roxburghii: It is a tall, evergreen conifer growing in the Himalayas up to an
altitude of 5,000 feet. Its wood is used as timber for electric poles, boxes and crates and
more commonly for making doors and windows. It is also a source of turpentine oil.
6.
Cedrela toona: Also called toon, it belongs to family Meliaceae and is found
throughout India. Its wood is brown, light and easily seasonable. It is used for making
furniture, cigar boxes and house building.
7.
Albizia lebbek: It is a large, deciduous tree belonging to family leguminosae, sub-
family Mimosoidae. Its heart wood is strong, tough and durable. It is used for making
agricultural implements, wheels and kohloos.
8.
Acacia modesta: It is a moderate sized, spiny tree of family Leguminosae and
yields hard and durable heart wood used as fuel wood as well as timber for cart wheels,
wooden crushers etc.
9.
Morus sps.: These are medium sized trees of the family Moraceae. They are
commonly cultivated for their edible composite fruits and for rearing silk worms.
Commonly called mulberry, the wood of trees of these species i.e. M. indica, M. alba
and M. nigrais used as high energy fuel as well as for making sports goods.
10. Poplar: It is a tall, deciduous tree belonging to family Moraceaeand common in
India sub-continent. Its wood is used as fuel as well as in buildingconstruction as
beams.
11. Ulmus lacvigata: It is a long, woody tree belonging to family Ulmaceae and called
―Bren‖ in Kashmiri. Its wood is very strong and hard. After seasoning it is used in
making beams of buildings and for roofing. Its twigs are used as fuel wood.
12. Plane tree: Botanically called Platinus orientalis, is very large tree, belonging to
family Caprifoliaceae. Its wood is used as fuel as well as for making furniture and wall
paneling.
vi. Spices and Condiments:
Spices and Condiments (With Special Reference to Kashmir)
International Organization for Standardization (ISO) has defined spices and condiments
as “such natural plant or vegetable products or mixtures thereof, in whole or ground
form, as are used for imparting flavor, aroma and piquancy to and for seasoning the
foods”.
Benefits of using spices: The important beneficial uses of spices and condiments are:225
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1.
They are well known appetizers and form an essential part of the culinary art.
2.
They had tang and flavor to other insipid foods.
3.
They intensify salivary flow and secretion of amylase, hexosamines and
neuraminic acid.
4.
They have anti-microbial and anti-biotic properties.
5.
They increase the rate of perspiration and have a cooling effect on the body.
6.
Some of them have pharmaceutical importance and help in overcoming various
ailments.
Important spices and Condiments used in Kashmir:
(1) TURMERIC:
Botanical name ………………………. Curcuma domestica, Syn. Curcuma longa
Family ………………………………… Zingiberaceae
Common name ………………………… Haldi
The plant is a perennial herb, 60-90 cm high, with a short thickened rhizome bearing a
tuft of large, broad, lanceolate leaves. It is grown in warm and moist regions, thriving
best on well drained loamy or alluvial fertile soils.
Part Used: The spice turmeric is the dried, cleaned and polished rhizome of
Curcumadomestica.
Chemical Components: Cured, finished and dried turmeric rhizome contains moisture
(5.8 %), proteins (8.6 %), fats (8.9 %), carbohydrates (63.0 %), fiber (6.9 %), minerals
(6.8 %), calcium (0.2 %), phosphorous (0.26 %), iron (0.05 %), sodium (0.01 %)and
potassium (2.5 %), Vit. A(175 mg/100 g) and Vit. B(0.09 mg/100 g). the calorific value
of turmeric is 390 calories per 100 grams.
Steam distillation of turmeric yields an orange yellow volatile oil consisting of d-α
phellandrene, d-sabiene, cincol, borneol, zingiberene and sesquiterpenes. The colour of
the spice is due to colouring substance called ―curcumin‖.
Uses:
1.
It is used as a spice and colouring agent.
2.
It is stimulant, tonic, stomachic and depurative.
3.
It is used to colour and add flavor to butter, cheese, ghee, pickles and other food
stuffs.
4.
It acts as a blood purifier and vermicide.
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5.
Boiled with warm milk and taken internally, it relieves sore throat and common
cold.
6.
In burnt and powdered form it is used to relieve dental troubles.
7.
It relieves gall stones.
8.
Smearing turmeric paste on the face and limbs clears the skin from pimples and
beatifies it.
9.
It is also used to dye silk, leather, fiber, paper and many other articles of use.
10. It is an anti-oxidant because of its phenolic composition.
11. It is also indicated in case of diabetes, leprosy and healing up of internal injuries.
(2) GINGER:
Botanical name ………………………. Zingiber officinale
Family ………………………………… Zingiberaceae
Common name ………………………… Adhrak (Moist form) & Sounth (dried,
powdered form)
The plant is a perennial herb cultivated in several parts of the world including India. It
has a robust branched rhizome covered with small scale leaves and fibrous roots. It
requires a warm, humid climate, bright sunshine, good rainfall and sandy loam soil.
Part Used: Rhizomes of the plant are harvested and used as spice in two forms:
(a) Preserved green Ginger and
(b) Dried or cured Ginger.
For green ginger rhizomes are harvested before being fully mature. They are dried,
cleaned, boiled, scraped and finally boiled with sugar.
Dried ginger is prepared by drying the rhizome after cleaning dirt and fibrous roots,
washing in water and scraping the scale leaves and fibrous roots.
Chemical Composition: The chemical composition of ginger depends on agro-climatic
conditions, methods of curing, drying, packaging and storage. On an average it contains
moisture (6.9 %), protein (8.6 %), fat (6.4 %), fiber (5.9 %), carbohydrates (66.5 %),
calcium, (0.01 %), phosphorous (0.15 %), iron (0.011 %), sodium (0.03 %), potassium
(1.4 %), other minerals (5.7 %) and vitamins. Its calorific value is 380 calories/100
grams.
Uses:
1.
Ginger is highly esteemed as a spice due to its characteristic odour and pungent
taste.
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2.
Dried ginger (sounth) is widely used for flavouring foods, for extraction of oleoresins and for the production of an essential oil of ginger.
3.
Green ginger is used in preparations of pickles and other culinary preparations.
4.
It is extensively used in flavoring baked foods, soft drinks, liquors and processed
meat.
5.
It is stimulant, digestive and extensively used in pharmaceutical preparations.
6.
It has aphrodisiac properties and therefore used in preparation of tonics.
7.
Its active principles dilate blood vessels, thus producing warmth and increasing
perspiration.
8.
It has anti-oxidant properties and as such used in edible oils and protects to protect
them against oxidative rancidity.
9.
It is also used in distilleries for the preparation of ginger beer, ginger wine etc.
10. It cures dyspepsia and flatulent colic as well as alcoholic gastritis.
(3) CORIANDER:
Botanical name ………………………. Coriandrum sativum
Family ………………………………… Apiaceae
Common name ………………………… Dhania
The plant is a small annual herb of 0.3-1.0 meter height.
Part Used: Dried ripe fruits of the plant are used as spice in whole as well as dried and
powdered form.
Uses:
1.
The entire plant when young and green is used as flavoring agent in chutneys and
sauces.
2.
The seeds are extensively used as condiments in the preparation of pickling spice,
sausages etc.
3.
The seeds are also used for flavouring pastry, buns, cookies, cakes and tobacco
products.
4.
The seeds are considered to be carminative, diuretic, stomachic, anti-bilious and
aphrodisiac.
5.
The seeds are chewed to correct foul breath.
6.
An infusion of seeds with cardamom and caraway seeds is useful in flatulence,
indigestion, vomiting and intestinal disorders.
7.
Oil of coriander is used in perfumes.
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8.
The oleoresin extracted from seeds is used for flavouring beverages, pickles,
sweets etc.
(4) CHILLI (RED PEPPER):
Botanical name ………………………. Capsicum annum
Family ………………………………… Solanaceae
Common name ………………………… Lal Mirch
The plant is an annual herb with a somewhat bushy habit growing to a height of 3-4
feet.
Part Used: The many seeded berries varying in shape and size are used as spice in
unripe and green as well as ripe and red stage.
Chemical Composition: The shape, size, pungency and chemical composition of
chilies varies from one variety to another. On an average, chilies contain moisture (10
%), proteins (15.9 %), fats (6.2 %), carbohydrates (31.6 %), fibers (30.2 %), minerals
(6.1 %) and vitamins. The pungency is due to a crystalline phenolic compound called
capraicin and red colour due to caprorubin, zeaxanthin, capsanuhin, cryptoxanthin,
lurein, α-carotene and β-carotene.
Uses:
1.
As food flavourant: Chilies are widely used as spice in all curried dishes, seasoning
of eggs, fisg, meat, sauce, pickles etc. they are also used as colourants.
2.
Medicinal properties: chilies are used as counter irritants in neuralgia, lumbago and
rheumatic disorders. They also act as tonic and aphrodisiac. As per some reports,
chilies are believed to retard cancer.
3.
Chilies are source of vitamins A & C when green and fresh.
4.
They also act as appetizers as they stimulate taste buds and salivary glands.
5.
They are useful in atonic dyspepsia.
(5) BLACK PEPPER:
Botanical name ………………………. Piper nigrum
Family ………………………………… Piperaceae
Common name ………………………… Marsch or Kali Mirchi
The plant is a weak, trailing shrub growing to about 9-10 meters with adventitious
roots, leaves and axillary buds at its nodes. The mature fruit is bright red and later turns
black and wrinkled on drying. The plant requires warm, humid, tropical climate, good
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rainfall of 250 cm or moreand temperature ranging from 20-35 0C. The soil must be
humus rich, well drained and loamy.
Part Used: Called as king of spices. The spice is obtained from dried unripe berries of
Piper nigrum. In India, it is widely grown in Kerala, Karnataka, Tamil Nadu and
Pondicherry.
Uses:
1.
It is used as a spice in the form of black or white pepper.
2.
It is useful in dyspepsia, malaria, tremors, delirium and hemorrhoids.
3.
It is also used to relieve sore throat, piles and skin troubles.
4.
It is widely used in preparation of sauces, soups, curries and pickles.
5.
It retards rancidity in oils, fats and meat.
6.
It is also used as flavourant, insecticide and preservative.
(6) CARDAMOM:
Botanical name ………………………. Elletaria cardamomum
Family ………………………………… Zingiberaceae
Common name ………………………… Chhoti Elaichi
The plant is a tall, herbaceous perennial growing to a height of 2-4 meters. It has a
branched, underground rhizome which sends up several erect leafy shoots. The plant is
essentially tropical and grows best under the natural canopy of evergreen forests
between 600 and 1500 meters. The plant likes warm, humid climate, good rainfall and
well drained humus rich loamy soils.
Chemical Composition: On an average, cardamom contains 7-10 % moisture, 5.5-10.5
% volatile oil, 3.8-6.9 % ash, 6.7-12.8 % fiber, 7.0-14.0 % proteins, 39.0-40.9 % starch,
Vitamin B1 0.18 g/100g and vitamin B2 0.12 g/100g. The characteristic aroma is due
to a volatile oil in its seeds. The chief components of this oil are terpineol, borneol,
turpinene, limonene and sabinene.
Uses:
1.
It is used for flavouring sweets, pastries and other fast foods.
2.
It is chewed as a masticatory.
3.
It is used in pickles, puddings, curries etc.
4.
The oil extracted from its hulls and seeds are used in making perfumes and for
flavouring bitters and liquors.
5.
It is diuretic, carminative, digestive, stomachic and with a cooling effect.
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6.
It is used to check nausea, indigestion, vomiting and bad breath.
7.
In Kashmir it is used in making Kahwa, a sugary hot aqueous extract of tea leaves,
cinnamon and ginger.
(7) FENNEL:
Botanical name ………………………. Foeniculum vulgare
Family ………………………………… Apiaceae (Umbelliferae)
Common name ………………………… Saunf (Badiyan)
The plant is a stout, aromatic, perennial herb with decompound leaves and small yellow
flowers borne in compound, terminal umbels. The spice, by the name of fennel, is the
dried, ripe fruits of the plant. These fruits are small, oblong, ellipsoidal or cylindrical,
straight or slightly curved cremocarps with greenish-yellow colour. It requires a fairly
mild climate.
Chemical Composition: The fennel seeds contain 6.3 % moisture, fat (10 %),
carbohydrates (42.3 %), minerals (13.4 %), fibre (18.5 %) and vitamins B1, B2, C, A
and niacin.
The volatile oil obtained by steam distillation of crushed fennel seeds is a colourless,
pale yellow liquid with a characteristic taste and odour. It contains 70 % anethole and 6
% fenchone and possesses a sweet taste.
Uses:
1.
The plant is aromatic and grown as a pot herb.
2.
Its leaves are used in sauce and leaf stocks are used as vegetable.
3.
Its leaves are diuretic and roots used as purgative.
4.
The seeds are used for flavouring soaps, dishes, sauces, bread rolls, pastries,
pickles and liquors.
5.
It has paramedical properties and used in disorders of chest, spleen and kidneys.
6.
The fennel oil is used in flatulence, infant colic and as vermicide against hook
worms.
7.
The seed husk, after extraction of oil from seeds, is used as feed for cattle due to
good protein and fat content.
(8) SAFFRON:
Botanical name ………………………. Crocus sativus
Family ………………………………… Iridiaceae
Common name ………………………… Kung
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The plant is a native to southern Europe and is now grown in Spain, Austria, France,
England etc. In India, it is grown in theKarewa of Pampore (Kashmir), Kishtwar
(Jammu) and some other places etc.
The plant is a low growing perennial bulbous herb with linear, radical leaves. The
violet or bluish flowers are born singly on a scape. The plant is successfully grown in
cold, sub-tropical, low rainfall, hilly regions. A warm spring and long autumn are
conducive to flowering.
The spice, called saffron, is obtained by drying the stigmas and tops of styles of the
flowers of the plant after hand picking them at the end of autumn. The finest quality of
saffron called ―ShahiZaffron‖ is obtained from the red tips of stigmas.
Chemical Composition: Saffron contains 15.6 % moisture, 13.35 % carbohydrates, 0.6
% essential oil, 5.6 % fixed oil, 43.64 % total N-free extract, 4.48 % crude fiber, 4.27 %
ash. The ash is rich in K & P. the characteristic colour is due to a glycoside ―Crocin‖
and its bitterness is due to another glycoside called ―Picrocrocin‖.
Uses:
1.
It is most commonly used as flavouring and a colouring agent.
2.
It acts as stomachic, stimulant, sedative, emmenagogul and abortifacient.
3.
It is given in measles and exanthematous diseases to promote eruptions.
4.
It is used in fevers, melancholia and enlargement of the liver and spleen.
5.
It is an important ingredient of Ayurveda and Unani medicines.
6.
In Kashmir, it is most commonly used in wazwan preparations and for preparation
of ―Kehwa‖, an invigorating liquid beverage.
(9) CLOVES:
Botanical name ………………………. Syzigium aromaticum
Family ………………………………… Myrtaceae
Common name ………………………… Loung or Roung
The plant is a small evergreen tree, 12-15 meter in height with as large number of
branches spreading from near the base in an upward direction. The leaves are opposite,
glabrous and dotted with oil glands. The crimson flowers are borne in terminal
paniculate cymes. In cultivated state, flowers do not open.
Clove of commerce is the dried, unopened floral buds of the plant. In India, it is mostly
grown in Nilgiris, Tanbasi Hills, Kanyakumari (Tamil Nadu), Kottayam and Quillion
districts (Kerala). The plant likes tropical, maritime climate with warm, humid
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conditions. Deep, volcanic loamy soil, annual rainfall of 150-250 cm and temperature
of 15-40 0C is conducive to its growth.
Chemical Composition: On an average, clove contains 5.4 % moisture, 6.3 % volatile
oil, 15.5 % fat, 11.0 % crude fibre, 57.7 % carbohydrates, 5.0 % mineral matter and
vitamins (A, B1, B2, C and Niacin).
The essential oil obtained on steam distillation contains free eugenol (70-90 %),
eugenol acetate and caryophylline.
Uses:
1.
Being strongly aromatic it is used as a flavouring spice in food preparations.
2.
Cloves have a warming effect on the body.
3.
They are used as aromatic, stimulant and carminative as well as anti-spasmodic.
4.
They are useful in dyspepsia and gastric irritations.
5.
They are used with betel nut for chewing and refreshing the mouth.
6.
Clove oil is extensively used for flavouring food products and fermented
beverages.
7.
It is also used in chewing gums, gargles and for scanting soaps.
8.
The clove oil is used as a cleaning agent in laboratories for histological work.
9.
It (clove oil) is also used for dental troubled and bleeding gums.
(10) CINNAMON:
Botanical name ……………………….
Cinnamomum verum (syn.
Cinnamomum
zeylanicum)
Family ………………………………… Lauraceae
Common name ………………………… Dalchini
The plant is a small, evergreen tree of 9-15 meters height with leathery, dark glossy
green and highly aromatic leaves. It prefers hot, moist climate and gro9ws best at low
altitudes on poor white sands. It prefers an average temperature of 25- 30 0C and 200250 cm of rainfall.
Part Used: The spice is obtained from the dried, inner bark of the tree. In India, it is
widely cultivated in Kerala and Western Ghats.
Chemical Composition: On an average cinnamon contains 10 % moisture, 5 %
protein, 2 % fat, 20 % fibre, 60 % carbohydrates, 4 % ash (Ca, P Fe, Na and K) and
vitamins. The principle aromatic substance in this spice is ―Cinnamic aldehyde‖.
Uses:
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1.
It is extensively used as a spice in food preparations.
2.
It acts as a astringent, stimulant and carminative.
3.
Powdered cinnamon is used in chocolates, incenses, perfumes etc.
4.
It is helpful in checking nausea and vomiting.
5.
Cinnamon bark oil is used for flavouring confectionary, liquors, pharmaceuticals
and soaps.
6.
Medicinally it is used to cure flatulent colic, gastric debility and mouth refresher.
7.
It is also used in rheumatism.
(11) CUMIN:
Botanical name ………………………. Cuminum cyminum
Family ………………………………… Apiaceae
Common name ………………………… Zera
The plant is a low growing annual herb with a much branched angular stem and
decompound leaves. The flowers are small, white and borne on terminal compound
umbels. The fruit is an ovoid, elongated, laterally compressed and greyish or yellowishbrown schizocarp.
The plant requires a fairly mild climate and grows as a cold weather crop. In India, it is
mostly grown in U.P, Punjab, Rajasthan, Gujarat and Tamil Nadu.
Part Used: The spice is obtained from the dried, ripe fruits of the plant.
Chemical Composition: The cumin seeds are found to contain about 6 % moisture, 18
% protein, 24 % fat, 36 % carbohydrates, 8 % minerals matter, 9 % fiber and vitamins.
On steam distillation the seeds yield a volatile oil which is colourless or pale yellow but
darkens on keeping. The oil contains 20-20 % cumin aldehyde.
Uses:
1.
Cumin seeds are used as a condiment in curry powders and for flavouring dishes.
2.
Cumin is carminative, stomachic, astringent and useful in dyspepsia and diarrhea.
3.
The cumin oil is used in liquors, cordials and perfumery.
4.
The cumin seeds are also used in veterinary medicines.
5.
Cumin is widely used in sausages, pickles, cheese and for seasoning cakes and
breads.
(12) MINT:
Botanical name ………………………. Mentha aarvensis
Family ………………………………… Lamiaceae (Labiatae)
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Common name ………………………… Pudina
The plant is a perennial herb with running root stocks and a rigid branching stem with
frequent nodes bearing adventitious roots and aerial shoots. It grows wild in pastures
and waste land as well as on the borders of crop fields. It likes to grow in humid, warm
conditions and prefers a nitrogen rich soil.
In India, about 8 species of mint are known to grow. The world demand of pepper
mint oil and menthol, obtained after steam distillation of the plant, is met from 3
species of mint i.e. M. arvensis, M. piperitaand M. spicata.
Uses:
1.
Mixed with chilies (green) and coriander, it forms a very good sauce or chutney.
2.
It is also used for flavouring meat, soaps, teas, vinegars etc.
3.
The fresh leaf chops are used in beverages, soups, jellies, salads etc.
4.
It acts as a carminative, stomachic and anti-flatulent.
5.
It is also used in ointments, cough drops, inhalants and tonic for kidneys.
(13) AJOWAN:
Botanical name ………………………. Trachys permumammi
Family ………………………………… Apiaceae (Umbelliferae)
Common name ………………………… Jaiyan
It is an annual herb growing in wild and also cultivated. In India it is grown mainly
in U.P, M.P, Bihar, Punjab, Bengal, Tamil Nadu, etc. the crop is sown in October and
November and is harvested in May-June. The spice is obtained from the schizocarps of
the plant. These schizocarp fruits also yield an essential oil on steam distillation.
Uses:
1.
The dried seeds are used as spice and condiment throughout India.
2.
They are used as flavourants, anti-oxidants and preservatives and as drugs.
3.
The ajowan oil is used for obtaining thymol crystals which are used in medicine,
perfumery, cosmetics etc.
4.
Ajowan is an anti-flatulent, anti-acid, tonic and produces a cooling effect.
(14) MUSTARDS:
Botanical name ………………………. Brassica sps.
Family ………………………………… Brassicaceae (Cruciferae)
Common name ………………………… Sarsoon
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The plants of the species of mustard are annual herbs, commonly grown as rabbi
crop in India, especially Jammu and Kashmir. Different species of this spice, i.e.
Brassica juncea, B. nigra and B. rugosa have been used as condiment. The seeds of
these species also yield edible oil, commonly used to fry vegetables, meat, cheese etc.
In Kashmir, the mustard seeds are generally used in the preparation of pickles.
vii.
Medicinal And Aromatic Plants (MAPs):
Morphology of Podophyllum
Podophyllum (family Berberidaceae)
commonly
called
Mayapple is
a herbaceous woodland plant, typically growing in colonies derived from a single root.
The stem grows 30–40 cm tall, with palmately lobed umbrella-like leaves up to 20–
40 cm diameter with 3–9 shallowly to deeply cut lobes. The plant produces several
stems from a creeping underground rhizome; some stems bear a single leaf and do not
produce any flower or fruit, while flowering stems produce a pair or more leaves with
1–8 flowers in the axil between the apical leaves.
The flowers are white, yellow or red, 2–6 cm diameter with 6–9 petals, and mature
into a green, yellow or red fleshy fruit 2–5 cm long.
The common name is mayapple as the flower appears in early May, fruits are produced
in early summer and ripens later in summer.
Many species of plants have mycorrhizae to assist with nutrient uptake in infertile
conditions. Mayapple plants are considered obligately dependent upon such
mycorrhizae, although it may also be facultatively dependent upon rhizome age and soil
nutrient levels. Plants are commonly found infected by the rust Allodus podophylli,
appearing as honeycomb-patterned orange colonies under the leaves, and yellowish
lesions on the upper surface.
Uses of Podophyllum
 All parts of the plant are poisonous, including the green fruit, but once the fruit has
turned yellow, it can be safely eaten with the seeds removed. They contain
podophyllotoxin or podophyllin that is used as a purgative and as a cytostatic
 Parts of Podophyllum are used as an emetic, cathartic, and antihelmintic agents
by American Indians, the poisonous root are boiled and the extract is used to cure
stomach aches.
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 The rhizome of the mayapple has been used for a variety of medicinal purposes,
originally by indigenous inhabitants and later by other settlers.The roots of the
podophyllum plant have a long history among native North American tribes as a
laxative or antiparasite agent.
 Podophyllotoxin obtained from Podophyllum is used to make etoposide (an anticancer
agent) and teniposide (an agent that treats leukemia).
 Extracts of the podophyllum plant are often applied to the skin to treat genital warts,
mouth and tongue sores related to HIV, and some skin cancers.
 Posalfilin is a drug containing podophyllin and salicylic acid that is used to treat
the plantar wart.
 CPH 82, a form of podophyllum that is taken by mouth, is useful in treating rheumatoid
arthritis. However, when taken by mouth, Podophyllum can be lethal and should be
avoided.
Lavandula
Lavandula (common name lavender) is a genus of 39 known species of flowering
plants in the mint family, Lamiaceae. It is native to the Old World and is found from
Cape Verde and the Canary Islands, Europe across to northern and eastern Africa, the
Mediterranean, southwest
Asia to southeast India.
Many members of the genus are cultivated extensively in temperate climates as
ornamental plants for garden and landscape use, for use as culinary herbs, and also
commercially for the extraction of essential oils. The most widely cultivated species,
Lavandula angustifolia, is often referred to as lavender, and there is a color named for
the shade of the flowers of this species.
Scientific classification
Kingdom: Plantae
Order: Lamiales
Family: Lamiaceae
Subfamily: Nepetoideae
Tribe: Lavanduleae
Genus: Lavandula
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Morphology of Lavandula
The genus includes annual or short-lived herbaceous perennial plants, and shrub like
perennials, subshrubs or small shrubs. Leaf shape is diverse across the genus. They are
simple in some commonly cultivated species; in others they are pinnately toothed, or
pinnate, sometimes multiple pinnate and dissected. In most species the leaves are
covered in fine hairs or indumentum, which normally contain the essential oils. Flowers
are borne in whorls, held on spikes rising above the foliage, the spikes being branched
in some species. Some species produce coloured bracts at the apices. The flowers may
be blue, violet or lilac in the wild species, occasionally blackish purple or yellowish.
The calyx is tubular. The corolla is also tubular, usually with five lobes (the upper lip
often cleft, and the lower lip has two clefts).
Nomenclature and taxonomy
Linnaeus recognised five species in Species Plantarum (1753), L. multifida and L.
dentata (Spain) and L. stoechas and L. spica from Southern Europe.
Upson and Andrews, classified Lavandula into three subgenera.
 Subgenus Lavandula is mainly of woody shrubs with entire leaves. It contains the
principal species grown as ornamental plants and for oils. They are found across the
Mediterranean region to northeast Africa and western Arabia.
 Subgenus Fabricia consists of shrubs and herbs, and it has a wide distribution from
the Atlantic to India. It contains some ornamental plants.
 Subgenus Sabaudia constitutes two species in the southwest Arabian peninsula
and Eritrea, which are rather distinct from the other species, and are sometimes placed
in their own genus Sabaudia. In addition, there are numerous hybrids and cultivars in
commercial and horticultural usage.
Etymology
The English word lavender is derived from the Latin lavare (to wash), referring to the
use of infusions of the plants. The botanic name Lavandula as used by Linnaeus is
considered to be derived from this and other European vernacular names for the plants.
However it is suggested that this explanation may be apocryphal, and that the name
may actually be derived from Latin livere, "blueish". The names widely used for some
of the species, "English lavender", "French lavender" and "Spanish lavender" are all
imprecisely applied. "English lavender" is commonly used for L. angustifolia, though
some references say the proper term is "Old English Lavender". The name "French
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lavender" may be used to refer to either L.stoechas or to L. dentata. "Spanish lavender"
may be used to refer to L. stoechas, L. lanata or L. dentata.
Cultivation
The most common form in cultivation is the common or English lavender Lavandula
angustifolia (formerly named L. officinalis). A wide range of cultivars can be found.
Other commonly grown ornamental species are L. stoechas, L. dentata, and L. multifida
(Egyptian lavender). Because the cultivated forms are planted in gardens worldwide,
they are occasionally found growing wild as garden escapes, well beyond their natural
range. Commonly such adventitious establishment is apparently harmless at best, but in
some cases Lavandula species have become invasive. For example, in Australia,
Lavandula stoechas has become a cause for concern; it occurs widely throughout the
continent, and has been declared a noxious weed in Victoria since 1920. It also is
regarded as a weed in parts of Spain. Lavenders flourish best in dry, well drained,
sandy or gravelly soils in full sun. All types need little or no fertilizer and good air
circulation. In areas of high humidity, root rot due to fungus infection can be a problem.
Organic mulches can trap moisture around the plants' bases, encouraging root rot.
Gravelly materials such as crushed rocks give better results.
Uses of Lavendula
1.
Commercially, the plant is grown mainly for the production of essential oil of
lavender, having antiseptic and anti-inflammatory properties.
2.
Lavendula extracts are also used as fragrances for bath products.
3.
English lavender (Lavandula angustifolia) yields an essential oil with sweet
overtones, and can be used in balms, salves, perfumes, cosmetics, and topical
applications.
4. Lavandula intermedia (also known as Dutch lavender), yields a similar essential oil,
but with higher levels of terpenes including camphor, which add a sharper overtone
to the fragrance.
5. The lavandins (Lavandula intermedia) are a class of hybrids of L. angustifolia and L.
latifolia. Lavandins are widely cultivated for commercial use, since their flowers
tend to be bigger than those of English lavender and the plants tend to be easier to
harvest, but lavandin oil is regarded by some to be of a lower quality than that of
English lavender, with a perfume less sweet.
6. It is grown as a condiment and used in salads and dressings.
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7. Flowers yield abundant nectar from which bees make a high quality honey.
8. Flowers can be candied and are sometimes used as cake decorations.
9. Lavender flavours baked goods and desserts (it pairs especially well with chocolate),
and is also used to make "lavender sugar".
10. In the United States, both lavender syrup and dried lavender buds are used to make
lavender scones and marshmallows.
11. The essential oil was used in hospitals during World War I. Lavender is used
extensively with herbs such as chamomile in aromatherapy.
12. Lavendula is having anxiolytic effects and influence on sleep quality. Lavender
oil is approved for use as an anxiolytic in Germany under the name Lasea.
13. Lavender oil may be an effective medicament in treatment of several neurological
disorders."
14. Lavender honey (created from bees feeding on lavender plants), instead of lavender
essential oil has the best effects of uninfected wounds.
15. Lavender is traditionally regarded as a safe oil and, although it was recently
reported that lavender oil, and its major constituent linalyl acetate, are toxic to
human skin cells in vitro, contact dermatitis to lavender oil appears to occur at only
a very low frequency.
viii. Beverages:
Beverages are liquids specifically prepared for human consumption. In addition to basic
needs, beverages form part of the culture of human society. Despite the fact that most
beverages, including juice, soft drinks, and carbonated drinks, have some form
of water in them; water itself is often not classified as a beverage, and the
word beverage has been recurrently defined as not referring to water. Beverages are
categorised into two broad categories
1.
Alcoholic Beverages
2.
Non Alcoholic Beverages
Alcoholic Beverages
An alcoholic beverage is a drink containing ethanol, commonly known as alcohol,
although in chemistry the definition of an alcohol includes many other compounds.
Alcoholic beverages, such as wine, beer, and liquor have been part of human culture
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and development for 8,000 years. Many brands of alcoholic beverages produced
worldwide include.
a)
Beer – is produced by the saccharification of starch and fermentation of the
resulting sugar. The starch and saccharification enzymes are often derived from malted
cereal grains, most commonly malted barley and malted wheat. Most beer is also
flavoured with hops, which add bitterness and act as a natural preservative, though
other flavourings such as herbs or fruit may occasionally be included. The preparation
of beer is called brewing.
b) Cider – is a fermented alcoholic beverage made from apple juice. It is also called
"apple wine".
c)
Distilled beverages – (also known as liquor and spirits) it is
produced
by distillation of a mixture produced from alcoholic fermentation, such as wine. This
process purifies it and removes diluting components like water, for the purpose of
increasing its proportion of alcohol content (commonly known as alcohol by
volume, ABV). As distilled beverages contain more alcohol they are considered
"harder".
d) Cocktails – a cocktail refers to any kind of alcoholic mixed drink that contains two
or more ingredients. As generally understood today, a cocktail requires at least one
alcoholic
component—typically
a distilled
spirit,
although beer and wine are
permissible—and one sweet component; it may also contain a souring or bittering
ingredient.
e)
Wine – an alcoholic beverage made from fermented grapes or other fruits. The
natural chemical balance of grapes lets them ferment without the addition
of sugars, acids, enzymes, water, or other nutrients. Yeast consumes the sugars in the
grapes and converts them into alcohol and carbon dioxide. Different varieties of grapes
and strains of yeasts produce different types of wine.
Non-alcoholic beverages
Non alcoholic beverages signify drinks that would normally contain alcohol, such
as beer and wine but are made with less than 0.5 percent alcohol by volume. The
category includes drinks that have undergone an alcohol removal process such as nonalcoholic beers and de-alcoholized wines.
a)
Caffeinated
beverages –
a caffeinated
drink or
caffeinated
beverage
contains caffeine, a stimulant which is legal and popular in most developed countries.
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b)
beverages – chocolate is
Chocolate
a
processed,
typically
sweetened food produced from the seed of the tropical Theobroma cacao tree. Its
earliest documented use is by the Olmecs of south central Mexico around 1100 BC. The
majority
of Mesoamerican people
made
chocolate
beverages,
including
the Mayans and Aztecs, who made it into a beverage known as xocolātl meaning
"bitter water".
c)
Soft drinks – a soft drink is a beverage that typically contains water (often, but not
always, carbonated water), usually a sweetener and a flavoring agent. The sweetener
may be sugar, high-fructose corn syrup, fruit juice, sugar substitutes (in the case of diet
drinks) or some combination of these. Soft drinks may also contain caffeine, colorings,
preservatives and other ingredients.
Tea
Botanical Name: Camellia sinensis (Syn. Thea sinensis)
Family
Theaceae
Morphology of tea
Tea plant is low growing ever green shrubby bush, it grows to a height of 3 to 4 ft.
Stem is profusely branched, bearing elliptical leathery; leaves with seareated margins.
The characteristic fragrance and aroma of the leaves is due to the presence of numerous
oil glands. The leaves are alternately arranged and in their axils are produced, either
singly or in groups of 2-4 whitish or pink coloured flowers. The flowers produce at
maturity 3 celled woody capsule each compartment of which contains a brown seed
about 1.25cm in diametyer. The shrub of tea is constantly pruned resulting in the
development of new tender shoots called flushes which are commercially important.
Fruit is a capsule 1-4 lobed smooth with 1-6 seeds. A tree plant will grow into a tree
upto 16m if left undisturbed, but cultivated plants are generally pruned to waist height
for ease of plucking.
Tea is an aromatic beverage commonly prepared by pouring hot or boiling water
over cured leaves of the tea plant Camellia sinensis. It gives slightly bitter and
astringent flavour and is enjoyed world over. Tea originated in China as a medicinal
drink . It was first introduced to Portuguese priests and merchants in china during the
16th century. Taking tea became popular in Britian during the 17th century. The British
introduced it to India in order to compete with the Chinese monopoly on the product.
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Broadly speaking, tea can be categorised into non-fermented, semi-fermented, fullfermented, and post-fermented tea.The process of oxidation and fermentation in
harvested tea leaves makes a world of difference to the kinds and quality of tea
produced. This includes not only the taste, colour, aroma, and nutrition values of the
tea, but even the lifespan of the tea. For example, green tea, which is non-fermented,
lives a much shorter lifespan than the post-fermented Pu'er tea, which can be kept for
decades.
Processing of tea leaves
All tea varieties go through several key production steps:

Picking

Withering

Pan firing

Rolling

Fermentation

Roasting and sieving.
Picking- includes picking of green leaves, impurities like yellow leaves, old leaves, and
stalks have are picked out from the harvest. After picking, tea leaves have to go through
a withering and pan firing process.
Withering- means spreading out the plucked leaves to transpire to remove moisture
and promote enzyme activity to facilitate chemical changes within the leaves. This
process is necessary for the production of oolong, white, and black tea, where oxidation
is necessary to produce the chemical compounds that create their flavours and
characteristics.
Pan firing- For green and yellow tea, leaves are normally prevented from oxidation.
After harvest
fresh leaves are processed at high temperature to destroy enzyme
activity. Tea leaves become soft, tender, and easy to roll. Leaf juice is also enriched.
Rolling- Withered leaves are rolled to crush the cells with tensile force, stress, and
friction, so that juice is spilled over the leave surface to make it contract and twist. With
this process, tea leaves are moulded into long spirals.
Fermentation- During fermentation, the enzymes in tea are oxidised upon contact with
air. When leaves begin to oxidise, different chemical compound are produced, and the
leaves turn gradually from green to copper. By adjusting the duration and the
temperature/humidity of the oxidation process, tea makers are able to produce and
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control the taste, colour, and richness of their products. This step is vital to the aroma
and taste of tea and must be carried out by skilled hands.
Roasting and Sieving – After fermentation tea leaves are roasted at high temperature
for quick moisture evaporation. Enzyme activity is also inhibited to prevent tannin
oxidation, so that the desired colour, aroma, and taste can be achieved. Traditional
roasting is done on a big pan on coal fire. The final step of tea production is sieving.
With a bamboo sifter, workers sieve out the loose bits and then spread out the tea to
cool. The sieved tea leaves are now ready for grading and packing to head for different
markets.
Cultivation of Tea
Tea grows only in a warm environment. For this reason this plant is mainly found in
regions between 16 degrees south latitude and 20 degrees north latitude. Statistics of
the United Nations Food and Agriculture Organization show that there are 34 tea
producing countries in the world and the most important ones are distributed in Asia
and Africa.
1. Land cultivation, sowing, and irrigation
As a preparatory step, tea farmers weed and turn the soil so the roots of tea seedlings
can penetrate deep into the earth to draw moisture and nutrients. Then the plantation is
arranged into plots where seeds are sown. Three to five seeds are sown in evenly
distributed ditches, then covered with soil. After sowing, tea farmers have to water the
seeds every morning and evening before they take root. After germination, irrigation is
reduced to one every other day or even several days, just to keep the soil moist.
2. Plucking tea leaves
Terminal buds and new leaves can be plucked from the 3rd year inwards. A tea plant
yields best in the 5th to 7th year. Plucking is usually carried out at dawn before the
morning dew is dry. Pressing it between with the thumb and index finger, the bud is
gently plucked. Depending on the variety, the pluckers will pluck one bud and one leaf,
one bud and two leaves, or one bud and five leaves. The fresh buds and leaves must be
transported to the factory for processing immediately, otherwise tea quality will be
compromised. To speed up the process, tea factories are usually set close to plantations.
3. Continuation of life
Tea is a perennial deep-rooted crop which can live for several decades. Unlike other
short-lived crops that require replanting, tea plants develop new buds after each
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plucking to provide new yield. At the end of every harvest season, the plucking areas of
the plants are pruned to keep them neat and maintain the height at a controlled level.
After repeated plucking and pruning, the size of new leaves will be reduced and quality
deteriorates. The plant is then chopped off from the roots up, so new branches can
develop from the base. When they emerge, the tea plant is as good as new and ready for
another long cultivation cycle.
Uses of Tea
1.
Green tea helps to reduce the risk of cardiovascular diseases and some forms of
cancer
2.
It promotes oral health, reduces blood pressure and helps in weight control.
3.
It has antibacterial properties, anti-fibrotic and neuroprotective properties.
4.
Tea catechins have known anti-inflammatory and neuroprotective activities, and
have an affinity for cannabinoid receptors, which may suppress pain and nausea
and provide calming effects.
5.
Consumption of green tea is associated with lower risk of diseases that cause
functional disability such as Stroke, cognitive impairment and osteoporosis in the
elderly.
6.
Tea contains L-theanine whose consumption is strongly associated witgh a calm
but alert and focused, relatively productive mental state in humans.
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UNIT: V
i. Tissue Culture:
(i) Plant tissue culture: G Haberlandt also known as father of tissue culture used this
technique for the first time in 1902 and it is the most recent and promising method of
crop improvement technique where all other conventional methods of breeding fail. It
can be defined as the maintenance and growth of plant cells, tissues and organs in
vitro on a suitable culture medium. It is also called micropropagation used to
propagate plants under sterile conditions and to produce clones of plants.
Tissue culture technique is based on cellular totipotency of plant cells i,e the
ability of plant cells to generate into complete plant, a property found only in plant cells
and not in animal cells. A callus( undifferentiated mass of cells) phase is involved
before the cells can undergo redifferentiation leading to regeneration of a whole plant.

Differentiation: In plants when a meristematic cell attains its distinction in terms of
its structure and function, we say the cell is differentiated. For differentiation number
of changes occur in cellular metabolism of a meristematic cell. Like the smaller
vacuoles merge to form a large central vacuole, number of biochemicals relevant to
differentiated cell get accumulated, modification in cell wall and activation of genes
are some of features which a differentiated cell experiences in its journey.

Dedifferentiation, redifferentiation and morphogenesis: In plant tissue culture the
explants (pieces of differentiated tissue) are used for culture in culture medium. In the
first instance the differentiated tissue has to revert back to its meristematic nature
leading to the formation of callus. This is known as dedifferentiation. Whereas the
ability of these cells in the callus to form a whole plant in the next stage is referred as
redifferentiation.
To generate whole plant from callus, the differentiation of different organs like
shoot and root buds are first priority in tissue culture. Two pathways follow in the
callus for generation of these organs viz; shoot bud differentiation and somatic embryo
genesis. In former case monopolar structures are formed whereas in latter bipolar
structures are formed. This organ formation from callus is what we call morphogenesis
in microporpagation. For generation of shoot buds in callus relative ratio of cytokinin to
Auxin is considered important . High levels of cytokinin causes bud initiation while as
high auxin concentration cause rooting. Other factors like size and source of explant
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are considered to play vital role. In somatic embryogenesis( embryo formation from
sporophytic tissue) plant extracts, growth regulators and physiological state of calli are
considered important factors. It has been shown that nitrogen supplied in the form of
NH4+ ions or as casein hydrolysate stimulates somatic embryogenesis.

Application of plant tissue culture( conservation, agriculture and industry):
A. Conservation:
1.
Good quality and better yield varieties can be preserved for a long duration in
offsite collection like in cryopreservation which involves preservation of plant
material at very low temperature.
2.
Production of synthetic seeds ( somatic seeds are encapsulated in suitable matrix
e.g. sodium alginate along with other substances) can be stored upto a year without
loss of viability.
3.
Conservation of male sterile lines for production of hybrids though tissue culture
has been practised e.g in Pepper.
Agriculture:
1.
For seedless plant or plants producing abortive seeds like Banana, pineapple,
sugarcane etc it is good technique for their multiplication.
2.
Rapid multiplication of superior clones, disease free plants and their maintenance
is possible only though cultural techniques.
3.
Disease free plants can be cultured by micropropagation and micrografting.
4.
Production of synthetic varieties through protoplast fusion and somatic
hybridization which overcomes the barriers of species and genera.
5.
In
conventional hybridization techique the vigour for high quality, disease
resistance is generally lost after few generation, whereas the same vigour can be
maintained for long time through clonal propagation.
6.
In several crops desirable mutants have been isolated among haploids derived in
culture.
Industry
1.
Production of secondary metabolites: variety of chemicals used for variety of
purposes including pharmacy, medicine and industry are produced by plants. By
manipulating physiological and biochemical conditions the yield of these
chemicals can be increased.
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2.
Biotransformers: in this method low cost precursors are used as a substrate and are
transformed into value added high cost products. This is easily achieved by
cultured plant cells.
3.
Production of single cell proteins( SCP) is made possible though culture of single
cell.
4.
Observation of widespread interest in micropropagation several private companies
are involved in this venture for cloning plants from their own farm collection e.g.
manalaroo laboratory in Kerala for cloning Cordamom.
ii. Genetic Engineering:
Genetic engineering is the technology involved in synthesis of artificial genes ,
manipulation of genes though fusion, inversion, shifting of genes for improvement in
human beings , plants, animals and microbes. An important aspect of genetic
engineering is recombinant DNA technology, which is employed for combining DNA
from two different organisms to produce recombinant DNA. Paul Berg is the father of
Genetic engineering who was awarded Nobal Prize in 1980 for his pioneering work in
this field.
Tools and technique of Recombinant DNA technology : The various tools used in
this technology can be discussed under following heads:
1.
Enzymes: various types of enzymes and their functions are;
a.
Restriction endonuclease : cut DNA at specic sites
b.
DNA ligase: Join the cut DNA
c.
Exonuclease: digest the base pairs at 5/ and 3/ end of a single stranded DNA or at a
single stranded nicks or gaps in double stranded DNA.
d.
Endonuclease: cleave the double stranded DNA at any point except the ends.
e.
DNA polymerase: polymerises the DNA synthesis on DNA template.
f.
Reverse transcriptase: used to synthesis cDNA by using mRNA template.
2.
Vectors: They are those DNA molecules that can carry foreign DNA fragment
when inserted into it. It is also known as vehicle DNA. Vectors are grouped into
bacterial plasmids, bacteriophages, cosmids, plasmids etc.
3.
Desired DNA: This is DNA fragment of desired features and is loaded on vechicle
DNA to form recombinant DNA or chimeric DNA.
Technique involved : To begin with a desired DNA segment is identified with the help
of molecular probe usually using southern blotting technique. A specific Restriction
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enzyme is selected to isolate the desired DNA from the genome of an organism. Same
enzyme is then used to nick the vehicle DNA so that sticky ends with complementary
bases are maintained. In a culture medium the passenger DNA and vehicle DNA and
other ingredients like ligases, polymarases etc are also added. Chimeric or Recombinant
DNA is obtained from the culture apart from non fusing vectors.
Plasmids : plasmids like PBR322 , PBR324 , PC194 etc are the extra chromosomal, self
replicating and double stranded closed and circular DNA molecules present in the
bacterial cell. There are number of other features present in these plasmids which make
them suitable vectors in recombinant DNA technology. Some of them are as follows:
a. Plasmids with relaxed replication control are accumulated in very large numbers,
thus making them excellent tool for increased yield.
b. Restriction sites for insertion of desired DNA.
c. Antibiotic resistance for screening chimeric DNA.
d. Origin of replication for polymerase enzymes for self replication.
Application of genetic engineering:
1. Genomic and cDNA libraries: clones of thousands of genes of an organism are
made possible through recombinant DNA technology which may be obtained
either directly from its genome or from its isolated mRNA.
2. Isolation and sequencing of genes: for better understanding of structure of genes, it
is now possible to isolate the gene and also whole genome of an organism can be
sequenced by using various techniques of Genetic engineering.
3. Transgenic organisms: Genetic engineering has made it possible to tailor an
organism at genomic level according to its own requirements.
4. Molecular farming: one use of transgenic organisms is to produce relatively large
quantities of rare and expensive proteins for use in medicine.
5. Genetic screening: it is detection of mutant genes in an individual that reduces
suffering of victims of genetic disease e.g. Thalassemia, sickle cell anaemia.
6. It helps in possible improvement of genetics of a species known as Eugenics.
7. It helps families with genetic disorders about the possibilities of such disorders in
future babies which is known as genetic concelling.
8. Gene therapy: it the treatment of disease by replacing, altering or supplementing a
gene that is absent or abnormal.
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iii. Polymerase Chain Reaction (PCR):
PCR, polymerase chain reaction, is an in-vitro technique for amplification (Xeroxing or
making numerous copies) of a region of DNA whose sequence is known or which lies
between two regions of known sequence. It is rapid and versatile in-vitro method of cell
free cloning of DNA. The technique was formulated by Kary Mullis in 1985 for which
he was awarded the noble prize.
Working principle
As the name implies, it is a chain reaction, in which a small fragment of the DNA of
interest needs to be identified which serves as the template for producing the primers
that initiate the reaction. The method relies on thermal cycling, consisting of cycles of
repeated heating and cooling of the reaction for DNA melting and enzymatic
replication of the DNA. One DNA molecule is used to produce two copies, then four,
then eight and so forth. This continuous doubling is accomplished by specific proteins
known as polymerases, enzymes that are able to string together individual DNA
building blocks to form long molecular strands. To do their job polymerases require a
supply of DNA building blocks, i.e., the nucleotides consisting of the four bases
adenine (A), thymine (T), cytosine (C) and guanine (G). They also need a small
fragment of DNA, known as the primer, to which they attach the building blocks as
well as a longer DNA molecule to serve as a template for constructing the new strand.
If these three ingredients are supplied, the enzymes will construct exact copies of the
templates.
Steps
PCR is an iterative process, consisting of 3 steps; (a) denaturation of the template by
heat, (b) annealing of the oligonucleotide primers to the single stranded target
sequence(s), and (c) extension of the annealed primers by a thermostable DNA
polymerase.
Denaturation
This step is the first regular cycling event and consists of heating the reaction to 94–98
°C for 20–30 seconds. It causes DNA melting of the DNA template by disrupting the
hydrogen bonds between complementary bases, yielding single-stranded DNA
molecules. Double stranded DNA templates denature (i.e. the two strands are separated
into single stranded forms) at a temperature that is determined in part by their G+C
content. The higher the proportion of G+C, the higher the temperature required to
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separate the strands of template DNA. The longer the DNA molecules, the greater the
time required at the chosen denaturation temperature to separate the two strands
completely. If the temperature for denaturation is too low or if the time is too short at
AT rich regions of the template DNA will be denatured. When the temperature is
reduced later in the PCR cycle, the template DNA will reanneal into fully native
condition. In PCRs catalyzed by Taq polymerase, denaturation is carried out at 9495oC, which is the highest temperature that the enzyme can endure for 30 or more
cycles without sustaining excessive damage.
Annealing of primers to template DNA
The temperature used for the annealing (or binding) of primers to template strand is
critical. The reaction temperature is lowered to 50–65 °C for 20–40 seconds allowing
annealing of the primers to the single-stranded DNA template. This temperature must
be low enough to allow for hybridization of the primer to the strand, but high enough
for the hybridization to be specific, i.e., the primer should only bind to a perfectly
complementary part of the template. In PCR, efficiency and specificity are affected by
the annealing temperature. If the annealing temperature is too high, the oligonucleotide
primers anneal poorly, if at all to the template and the yield of amplified DNA is very
low. If the annealing temperature is too low, non specific annealing of primers may
occur, resulting in the amplification of unwanted segments of DNA.
Extension of oligonucleotide primers
It is carried out at or near the optimal temperature for DNA synthesis catalyzed by the
thermostable polymerase, which in the case of Taq polymerase is 72-78oC. At this step
the DNA polymerase synthesizes a new DNA strand complementary to the DNA
template strand by adding dNTPs that are complementary to the template in 5' to 3'
direction, condensing the 5'-phosphate group of the dNTPs with the 3'-hydroxyl group
at the end of the nascent (extending) DNA strand. The extension time depends both on
the DNA polymerase used and on the length of the DNA fragment to be amplied.
The number of cycles required for amplification depends on the number of copies
of template DNA present at the beginning of the reaction and the efficiency of primer
extension and amplification. Once established in the geometric phase, the reaction
proceeds until one of the components becomes limiting. At this point, the yield of
specific amplification products should be maximal, whereas nonspecific amplification
products should be barely detectable, if at all. This is generally the case after ~30 cycles
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in PCRs containing ~105 copies of the target sequence and Taq DNA polymerase
(efficiency~0.7). Atleast 25 cycles are required to achieve acceptable levels of
amplification of single copy target sequences in mammalian DNA templates.
Number of copies of DNA obtained after 'n' cycles = 2(n+1)
Fig. Components and working of PCR
Factors affecting polymerase chain reactions:
1. A thermostable DNA polymerase to catalyse template-dependent synthesis of
DNA: Depending on the ability, fidelity, efficiency to synthesize large DNA
products, a wide choice of enzymes is now available. For routine PCRs, Taq
polymerase (0.5-2.5 units per standard 25-50 uL reaction) remains the enzyme of
choice. The specific activity of most commercial preparations of Taq is ~80,000
units/mg of protein.
2. A pair of synthetic oligonucleotides to prime DNA synthesis: Design of the
oligonucleotide primer being the most important factor that influences the
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efficiency and specificity of the amplification reaction, careful designing of
primers is required to obtain the desired products in high yield, to suppress
amplification of unwanted sequences and to facilitate subsequent manipulation of
the amplified product. Since the primers so heavily influence the success or failure
of PCR protocols, it is ironic that the guidelines for their design are largely
qualitative and are based more on common sense than on well understood
thermodynamic or structural principles.
3. Deoxynucleoside triphosphates(dNTPs): Standard PCRs contain equimolar
amounts of all four dNTPs. Concentrations of 200-250μM of each dNTP are
recommended for Taq polymerase in reactions containing1.5 mM MgCl2. High
concentrations of dNTPs (>4mM) are inhibitory, perhaps because of sequestering
of Mg2+.
4. Divalent cations: All thermostable DNA polymerases require free divalent cationsusually Mg2+ for activity. Some polymerases will also work, albeit less efficiently
with buffers containing Mn2+. Calcium ions are quite ineffective. Because dNTPs
and oligonucleotides bind Mg2+, the molar concentration of the cation must exceed
the molar concentration of phosphate groups contributed by dNTPs and primers. It
is therefore impossible to recommend a concentration of Mg2+ that is optional in all
circumstances.
5. Buffer to maintain pH: Tris -Cl , adjusted to a pH between 8.3 and 8.8 at room
temperature is included in standard PCRs at a concentration of 10mM. When
incubated at 72oC (extension phase of PCR), the pH of the reaction mixture drops
by more than a full unit, producing a buffer whose pH is ~7.2.
6. Monovalent cations: Standard PCR buffer contains 50mM KCl and works well for
amplification of segments of DNA >500bp in length. Raising the KCl
concentration to ~70-100mM often improves the yield of shorter DNA segments.
7. Template DNA: Template DNA containing target sequences can be added to PCR
in single or double stranded form. Closed circular DNA templates are amplified
slightly less efficiently than linear DNAs. All though the size of the template DNA
is not critical, amplification of sequences embedded in high molecular weight
DNA(>10kb) can be improved by digesting the template with a restriction enzyme
that doesn't cleave within the target sequence.
Advantages
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PCR is automated, fast, reliable (reproducible) way of cloning. It has less chances of
contamination unlike other methods of cloning. A large number of samples can be
processes in a very short time. This technique is very sensitive, well-defined and easy
to follow. In short The speed and ease of use, sensitivity, specificity and robustness of
PCR has revolutionised molecular biology and made PCR the most widely used and
powerful technique with great spectrum of research and diagnostic applications.
Application:
PCR finds its application in a number of fields including genome mapping and gene
function determination, biodiversity studies (e.g. evolution studies), diagnostics
(prenatal testing of genetic diseases, early detection of cancer, viral infection etc.),
detection of drug resistance genes, forensic (DNA fingerprinting) sciences etc.
1.
Yield: crop yield can be increased by incorporating high yielding genes and
increasing gene dosage.
2.
Disease resistance: Bt-cotton which is resistant to Bollworm disease has been
commercially permitted for cultivation in India and number of other disease
resistant crops have been developed through resistant gene transfer.
3.
Nitrogen fixing: The nitrogen fixing genes when isolated would be introduced in
all types of valuable plants to do away with nitrogen fertilizers.
4.
Modified crops: each and every crop in future, would be modified to maximum
yield.
5.
Flavour saviour crop:
Transgenic Tomato insensitive to ethylene through
mutation of ethylene receptor gene has been created. It remains fresh even after
three months of storage.
6.
Resistance against stress: A number of genes responsible for providing resistance
against stress such as heat, cold, salt, heavy metals etc have been identified. With
this background transgenic plants resistant variety of stresses will be produced in
future.
7.
Nutritional quality: Tomatoes with elevated sucrose and reduced starch could also be
produced using sucrose phosphate synthase gene. Likewise starch content of potatoes
could be increased by 20-40% by using a bacterial ADP glucose pyrophosphorylase
gene (ADP GPPase).
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