Download plant physiology

Tree Physiology
UNIT – 1
1.1
INTRODUCTION
1.1 The science of plant physiology
1.2 Principles of plant physiology
The science of plant physiology
Physiology, study of the physical and chemical processes that take place in living
organisms during the performance of life functions. Three broad divisions are recognized: general
physiology, concerned with basic processes common to all life forms; the physiology and functional
anatomy of humans and other animals, including pathology and comparative studies; and plant
physiology, which include photosynthesis and other processes pertinent to plant life.
Plant physiology is the study of life processes of plants. In recent years, the physical
sciences like physics, chemistry, etc. has become very close to science of plant physiology. All these
have helped us a lot in studying and understanding the life processes of plants.
The study of plant life is sub-divided into various branches e.g. physiology, morphology,
anatomy, ecology, pathology and genetics. Every physiology process is conditioned by the anatomical
arrangement of the tissues and by the size, configuration and other structural features if the cells in
which it occurs. Further more, the coordinated development of cells and tissues of the plant is a complex
of physiological processes. Thus, the science of plant physiology and plant anatomy merges in the study
of plant growth.
#
Importance of plant physiology
1)
All heterotrophic organisms including human beings are dependent on green plants.
The plants provide food, energy, shelter, medicine, cloth and a good environment for
all living organism. In this respect, the knowledge in plant physiology is important to
make plant productive so that the live of all heterotrophy will be secured.
2)
The knowledge in plant physiology is applied for the increase in world agricultural
productivity. As we know the mechanism of photosynthesis, the absorption of light
energy and the change of light energy to chemical energy and the syntheses of
carbohydrates, it is possible to increase the net photosynthesis that results the
increase in productivity of plants.
3)
Plant growth regulators control plant growth and development. Plant physiology
studies the mechanisms of the action of these regulators.
4)
The advancement seen in plant biotechnology is possible only due to our increasing
knowledge on the physiology of plants. The techniques of plant tissue culture and
cell culture are developed by the knowledge of plant physiology. Now it is possible
to change cells genetically and grow plants from the transformed cells.
5)
It is possible to use the plants in better ways to get benefit by the practical
applications of plant physiology. We are able to get mere advantages from plants by
increasing plant productivity or decreasing environment pollution by the use of
plants.
#
Plant physiology as a science:
Plant physiology is an interdisciplinary (involving different areas of knowledge) science.
The development in plant physiology depends on the growth of other sciences. All chemical and
physical theories apply also in living system. The laws of physics and chemistry help to understand the
role of environment in the growth and development of plants. The environment affects the physiology
of plants. Techniques developed in physics and chemistry is often used in physiological experiments.
Krishna Ram Dhital. (2013) cell no:9841181926
-1-
Tree Physiology
The discoveries in other branches of biological sciences e.g. by chemistry cell and molecular biology,
etc. helped in the rapid development of plant physiology. All this modern branches are the branches of
physiology as they are developed after the extensive research in the physiology of organisms. The
development of these modern branches and physiology are dependent upon each other.
UNIT – 2
2.1
PHENOMENON OF WATER LOSS
2.1 Transpiration
2.1.1 Mechanism of transpiration
2.1.2 Types of transpiration
2.1.3 Mechanism of stomatal movement
2.1.4 Role of transpiration
2.1.5 Factors affecting the rate of transpiration
2.2 Guttation and bleeding
Transpiration
The loss of water from the living tissue of aerial parts of the plant in the form of water
vapor is termed as transpiration. In other words, the process of losing water in the form of water
vapor by aerial part of plant is the transpiration.
2.1.1 Mechanism of transpiration
Mechanism of transpiration occurs in two stages:
i.
Evaporation of water from the cell walls in the inter cellular spaces and
ii.
Diffusion of these water vapors of the intercellular spaces into the outside
atmosphere.
In the first step, water is evaporated from the turgid cells (expanded cells) and is
collected in the intercellular spaces increasing the water vapor pressure and lowering its DPD
(diffusion pressure deficit) and in the second step, this water diffuses through stomata, lenticels and
cuticles in the outer atmosphere because of low water vapor pressure and high DPD value outside.
The difference in the mechanism of three types of transpiration is that in stomatal
transpiration, pores controlled by guard cells are involved, in lenticular transpiration pores with
uncontrolled opening and closing are involved and in cuticular transpiration pores are not involved at
all.
#
1)
2)
3)
4)
5)
6)
7)
#
Transpiration is advantageous
It creates suction force and helps in the ascent of sap.
It affects the DPD helping diffusion through the cells.
It affects the absorption of water and minerals by roots.
It helps in evaporating excess amount of water.
It plays a necessary role in translocation of food from one portion of the plant to the
other.
It maintains suitable temperature for the leaves.
It influences the process of photosynthesis and respiration.
Transpiration is the main process for the dissipating the sun's excess energy falling
on leaf.
2.1.2 Types of transpiration
The leaves are the main sites of transpiration (the loss of water vapor from living
plants). The aerial parts of the plant lose more than 99% of water absorbed. Plants lose most of the
Krishna Ram Dhital. (2013) cell no:9841181926
-2-
Tree Physiology
water through stomata. However, loss of water vapor may take place from any part of a plant which is
exposed to the air.
Transpiration is of three types:
i.
Stomatal transpiration
ii.
Cuticular transpiration
iii.
Lenticular transpiration
2.1.2.1 Stomatal transpiration
Most of the transpiration from leaves occurs through stomata (minute and microscopic
pores in the epidermal surface of the leaf) is termed as stomatal transpiration. Maximum
diffusion of water (about 90%) vapor takes place through the stomata. Stomata are mostly situated
on leaves but in herbaceous stems also they are found in the epidermis.
2.1.2.2 Cuticular transpiration
Cuticle is a wax like layer of cutin covering the surface of leaves or epidermis of leaves
and herbaceous stem. Smaller amount of water vapor are lost from leaves by direct evaporation from
the epidermal cells through the cuticle is called cuticular transpiration. Its thickness varies from
plant to plant. Generally, the cutin layer is thicker in sun leaves of a plant of dry habitat in
comparison to shade leaves and plants of moist habitats. It involves up to 20% of the herbaceous
stems, flowers parts and fruits is of the cuticular type.
2.1.2.3 Lenticular transpiration
Lenticels are the areas in the bark (of fruits and woody stems) which are filled with
loosely arranged cells called complementary cells. Loss of water vapor also takes place through the
lenticels is called lenticular transpiration. It is insignificant as compared to the stomatal
transpiration. Only under very dry conditions, when the stomata are closed water can loss through
the lenticels about 0.1% of the total loss.
2.1.3 Mechanism of stomatal movement
The epidermal surface of a leaf bears a great number of minute and microscopic pores
called stomata and each stoma is bordered by two epidermal cells called guard cells. These guard
cells contain chloroplast and thickened cell wall and control the opening and closing of stomata by
changing their shape. The thick wall around stoma is less elastic than thin wall of the guard cells
towards subsidiary cells. The stomata are more frequently found on the under surface of leaves but
they are found on both surfaces of the leaves in many species.
The mechanism of stomatal movements depends on the turgid and flaccid state of guard
cells. As water enters guard cells, it becomes turgid and the increased pressure inside will press the
cell wall and cause the cell to expand. The thin wall towards subsidiary cells expands more than
thicker wall around the pore of stoma making the guard cell kidney shaped and the pore size increases
i.e. stoma opens. When guard cells lose water, the cells become flaccid and the thick walls of two
guard cells come together and stoma closes. The turgidity of guard cells is due to variation of osmotic
potential (OP) of guard cells. When OP of guard cells is higher than epidermal cells, endosmosis
takes place and guard cells becomes turgid when OP of guard cells becomes lower than that of
adjoining cells ex-osmosis takes place thus making guard cells flaccid. The size of pores depends upon
the degree of turgidity of guard cells. Stomata generally open during day time and close at night but
how the light controls opening and closing of stomata is still not certain.
The three principle factors affect the stomatal movement:
i.
Influence of the light factor
ii. Influence of the water factor
iii. Influence of temperature factor
Many theories in the variation of OP of guard cells are given by different workers which
are discussed below:
Krishna Ram Dhital. (2013) cell no:9841181926
-3-
Tree Physiology
1)
2)
3)
4)
Theory of photosynthesis in guard cells
Theory of starch
sugar inter-conversion
Theory of starch
glucose inter-conversion
Theory of potassium ion
2.1.3.1 Theory of photosynthesis in guard cells (von mohl, 1856)
According to this theory, the guard cells are different from other epidermal cells. Due to
the presence of chloroplast, the guard cells photosynthesize in the presence of light resulting in the
production of sugar due to which osmotic pressure of guard cells increases. The following sequence
explains the mechanism of stomatal opening:
LIGHT
PHOTOSYNTHESIS BY GUARD CELLS
FORMATION OF SUGAR
INCREASE OF OSMOTIC PRESSURE OF CELL SAP
ENDOSMOSIS
INCREASE IN TURGIDITY
OF GUARD CELLS
STOMA OPENS
Later, this theory was not accepted just because for sufficient accumulation of sugar to
bring about endosmosis and turgidity. The rate of photosynthesis of guard cells is too low.
2.1.3.2 Theory of starch
sugar inter-conversion
The starch
sugar conversion in light and dark is another possible theory that tries to
explain the mechanism of opening and closing of stomata. According to this theory, the enzymatic
conversion of starch to sugar is favored by high PH (stomata open in light). Low PH (stomata close
in night) favors the conversion of sugar to starch. During day time, CO2 produced during respiration
in guard cells is consumed in photosynthesis but CO2 accumulates during night. The accumulated
CO2 combines with water and produce carbonic acid making the cells sap acidic. Thus, during night,
PH of cell sap decreases and this favors the conversion of sugar to starch. The low sugar
concentration increases the water potential of cell sap thus guard cells loses water and stoma closes.
During daytime, CO2 won't accumulate and carbonic acid won't form that results the increase in PH
of cell sap and this favors the conversion of starch to sugar. Sugar reduces the water potential of cell
thus the guard cells gain water becoming turgid and stoma opens.
AT DAY
RESPIRATION
PHOTOSYNTHESIS
CO2
HIGH PH
STARCH
SUGAR
LOW PH
CO2
AT NIGHT
RESPIRATION
COMBINED WITH H2O AND PRODUCE
CARBONIC ACID REDUCING THE PH OF CELL SAP
This theory was explained by Scarth (1932) that how PH change in guard cells.
Later, this theory was criticized by Yin and Tung (1948) and Steward (1964) and
several workers proposed that conversion of starch into sugar is always insufficient to increase
osmotic pressure enough to get turgidity in guard cells.
Krishna Ram Dhital. (2013) cell no:9841181926
-4-
Tree Physiology
2.1.3.3 Theory of starch
glucose inter-conversion
With the discovery of presence of phosphorylase enzyme in guard cells, Yin and Tung
(1948) and Steward (1964) proposed a modified scheme for stomatal opening which is given below:
LIGHT
HIGH RATE OF PHOTOSYNTHESIS IN MESOPHYLL CELLS
FROM INTER CELLULAR SAPCES
INCREASE IN PH IN GUARD CELLS
STARCH INTO GLUCOSE
INCREASE IN OSMOTIC PRESSURE IN CELL SAP
GUARD CELL BECOME TURGID
STOMATA OPEN
REMOVAL OF CO2
CONVERSION OF
ENDOSMOSIS
STARCH
STOMATA CLOSE (PH =5)
STOMATA OPEN (PH =7)
GLUCOSE -1, PHOSPATE
PHOSPHOGLUCOMUTASE
GLUCOSE - 6, PO4
HEXOKINASE + ATP
PHOSPHATASE
GLUCOSE + INORGANIC PO4
O2 RESPIRATION
Figure: Steward's scheme for movement of stomata
2.1.3.4 Theory of potassium ion
The change in potential of guard cells is found to be controlled by the K+ ions
concentrations. The guard cells of open stomata have higher concentration of K+ than the accessory
cells. During night, K+ moves out of the guard cells increasing the water potential of guard cells and
decreasing the water potential of subsidiary cells. The guard cells loss water and become flaccid.
Thus, stomata close.
During light, the concentration of K+ increases (K+ ions are transported in guard cells
from subsidiary cells) and water potential of guard cells decreases due to the formation of potassium
salt and it gains water and turgid making the stoma opens. For this, ATP is required and this ATP is
synthesized in guard cells during day time using light energy by photophosphorylation.
2.1.4 Role of transpiration
Transpiration is a wasteful process. It is a necessary evil. It is a vital and unavoidable
phenomenon of plants. It also consumes energy and causes unnecessary absorption of excess water by
roots because the water absorbed by roots is continuously lost by transpiration. During the normal gas
exchange occurring in plants for photosynthesis and respiration, the water of plant cells evaporates and
losses by the process of transpiration. For the continuous supply of gases to the cell, the transpiration
process is not avoided. It is necessary evil in the sense that to get sufficient CO2 for photosynthesis,
stomata should open widely that results the heavy loss of water. That’s why; transpiration has both the
advantageous and disadvantageous roles which are as follows:
Krishna Ram Dhital. (2013) cell no:9841181926
-5-
Tree Physiology
1)
2)
3)
4)
5)
6)
Cooling effect:
Effect on growth and development
Effect on mineral salt absorption
Effect in energy dispersal and dissipating the sun's excess energy
It is unavoidable and dangerous
Other roles
2.1.4.1 Cooling effect
Transpiration is an evaporation process. The evaporation of water needs energy which is
provided by plant cells. As the sweating process from human skin makes the skin cool similarly
transpiration cools the leaf surface. Thus, the cooling effect of transpiration keeps the plant from
being overheated.
2.1.4.2 Effect on growth and development
The transpiration may have an influence on the growth of some plants. It is necessary
factor in the normal growth of the plant. When the rate of transpiration exceeds, water deficit may
occur and wilting may take place. If water deficit is carried to extreme, it may result in the death of
plant.
2.1.4.3 Effect on mineral salt absorption
Transpiration provides continuous supply of flowing water to the plant cells. Water and
mineral salt together in the soil absorb by root move up to the plant through xylem cells. The
continuous flow of water through xylem caused by transpiration is called transpiration stream
which is necessary to provide water continuously to the cells of different parts and helps in the
absorption and distribution of minerals to the different parts of the plants.
2.1.4.4 Effect in energy dispersal and dissipating the sun’s excess
energy
Transpiration is necessary for energy dispersal and it is used in the dissipation of excess
energy which the plant receives from the sun. Only less than 1% of the energy received from the
sun is used in the endothermic manufacture of carbohydrates.
2.1.4.5 It is unavoidable and dangerous
It is unavoidable because there is ingress (entrance) and egress (out) of O2 and CO2 by
the process of transpiration. However, the loss of water doesn't show any good purpose in plant
life. A large number of plants die every year because of exceeding the transpiration. It also
consumes energy and causes unnecessary absorption of excess water absorbed by roots.
2.1.4.6 Other roles
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
To help in evaporating excess amount of water
To affect DPD
To help in the ascent of sap
Influence the process of photosynthesis and respiration
For the quality of fruits
For drainage of soil water
Reduce yield
Fall of metabolism
2.1.5 Factors affecting the rate of transpiration
The following are the factors which affects the rate of transpiration:
1) External or environmental factors
2) Internal or morphological or structural factors
Krishna Ram Dhital. (2013) cell no:9841181926
-6-
Tree Physiology
2.1.5.1 External or environmental factors
The rate of transpiration is influenced greatly by several environmental factors. The most
important of these are: a) light b) temperature c) wind velocity d) water content of soil e)
atmospheric pressure f) atmospheric humidity g) sprays and dusts and h) vital activities.
2.1.5.1. i Light
The light has indirect effect upon transpiration. As light intensity increases, leaf absorbs
more energy. Only fraction energy is used in photosynthesis and the rest energy is converted into
heat energy which raises the temperature of leaf and increases transpiration. Light also influences
transpiration by its effect upon opening of stomata allowing transpiration proceeds.
2.1.5.1. ii Temperature
If all other factors are constant, an increase in temperature within a certain physiological
range increases the rate of transpiration. It is due to the effect of temperature on stomatal
movement and vapor pressure gradients in intercellular spaces. According to Vant Hoff's rule,
for every 10% rise in temperature, the rate of transpiration gets doubled.
2.1.5.1. iii Wind velocity
The increase in wind velocity increases the rate of transpiration by removing the water
vapor of the atmosphere outside the transpiration area and lowering the relative humidity.
However, the winds of much higher velocity retard the rate of transpiration by the closure of
stomata.
2.1.5.1. iv Water content
The rate of transpiration is also influenced by the availability of soil water to the roots of
plants and the efficiency of its absorption. The deficiency of water in the soil decreases the rate
of transpiration by decreasing the rate of its absorption.
2.1.5.1. v Atmospheric pressure
The reduction in atmospheric pressure results in an increase in the rate of transpiration.
E.g. the plants growing on hills show higher rates of transpiration because of low atmospheric
pressure.
2.1.5.1. vi Atmospheric humidity
The humidity of air surrounding the plant is important factor affect the transpiration rate.
The rate depends upon the differences in concentration of water molecule between intercellular
space of leaf and air near leaf. Exchange of water vapor between air around leaf and air space
within leaf occurs through stomata. The direction and net movement of water vapor depends on
the difference in the concentration of vapor within and outside the leaf. Normally, an increase in
the vapor pressure of the intercellular space would result in an increase in the rate of
transpiration, whereas a decrease in the vapor pressure of the intercellular spaces relative to that
of atmosphere would have the opposite effect. On the rare occasions, when the vapor pressures
of the atmosphere and of the intercellular spaces are equal, no transpiration will occur even if the
stomata are open. Change in temperature changes the relative humidity which controls the
transpiration rate. If the relative humidity decreases, the transpiration rate increases.
2.1.5.1. vii Sprays and Dusts
Sprays and dusts affect the rate of transpiration. Bordeaux mixture lowers the leaf
temperature but increases the permeability of the cuticle resulting cuticular transpiration at night.
2.1.5.1. viii Vital activities
They may also affect the rate of transpiration. E.g. heat produced by the transpiration
may increase the rate of transpiration.
Krishna Ram Dhital. (2013) cell no:9841181926
-7-
Tree Physiology
2.1.5.2 Internal or morphological or structural factors
They are also called plant factors. They include a) stomatal frequency b) root shoot ratio
c) leaf area and d) leaf structure which are described as follows:
2.1.5.1. i Stomatal frequency
It means number of stomata per unit area of leaf structure. It varies from plant to plant
and also depends upon the effect of environment. Stomata index is given by:
S
I=
Where S = number of stomata per unit area and E = number of epidermal
cells in
E+S
same unit area
If the stomata per unit area are more and opened, the rate of transpiration increases.
2.1.5.1. ii Root shoot ratio
If all the other conditions are constant for good transpiration, the efficiency of root
surface and leaf surface controls the rate of transpiration. If water absorption follows
transpiration, a water deficit will occur in the plant which will reduce transpiration.
2.1.5.1. iii Leaf area
It is assumed that the greater the leaf area, the greater will be the magnitude of water loss.
On a per unit area basis, smaller plants often transpire at a greater rate than larger plants.
The removal of leaves from a plant may increase the rate of transpiration per unit area of
that plant but the total loss is greater in the un-pruned trees. It is due to the pruned trees provide a
greater amount of water to a smaller number of leaves.
2.1.5.1. iv Leaf structure
Certain plants are adopted to reduce the rate of transpiration e.g. by reducing the size of
leaves. Leaves of xerophytic plants possess a thick cuticle, thick cell walls, well developed
palisade parenchyma, sunken stomata, a covering dead epidermal hairs. All these features reduce
the total evaporating surface e.g. needle like leaves in pine, spine like leaves in opuntia, etc.
1> Pine leaves
2> Evergreen leaves
3> Succulent leaves
Several other factors such as amount of spongy tissue, volume of intercellular space,
orientation of leaf and extent of root system etc. also the affect the rate of transpiration.
Krishna Ram Dhital. (2013) cell no:9841181926
-8-
Tree Physiology
2.2
Guttation and bleeding
2.2.1 Guttation
Some plants lose water in the form of liquid water in drops from their leaf margin when
transpiration is reduced to minimum. This phenomenon of water loss is known as guttation. In other
words, the loss of water in a liquid form along the margin of leaves is called guttation. The amount of
water lost by this process is negligible as compared to that lost by transpiration.
It occurs when the relative humidity of air is high around 100%, so that the evaporation
will be the least. In this condition more water accumulates inside the plant than the loss by
transpiration. The water droplets often come from special glandular structure called hydathodes
present at the tips of edges of leaves. Winter nights of tropical and some temperate regions where the
day temperature is relatively high have relative humidity of atmosphere almost 100%. At leaf margins
of grasses and some herbs of these areas water drops caused by guttation are often observed on winter
mornings before sunrise.
There are two types of hydathodes in plants.
i. Epithem hydathode
ii. Active hydathode
During night when transpiration is minimum water accumulated in xylem and develops a
pressure called root pressure which is the main cause of guttation. This pressure of xylem forces more
water in leaf during night than the loss by transpiration. The pressurized water of xylem is lastly
released as water droplets from the hydathodes. The hydathodes consist of loosely arranged cells
(epithem) at leaf margin. The cells are joined to xylem vessels.
Root pressure is reduced by cold dry aerated soil etc. and it brings down the guttation
rate. The mineral deficiency also reduces the guttation rate. The guttation water contains various kinds
of enzymes such as sugars, amino acids, organic acids, vitamins, mineral salts, etc.
Hydathode
Figure: Entire leaf showing guttation
2.2.2 Bleeding
If a plant is cut above from its base with a knife, the xylem sap is seen flowing out
through the cut end. This phenomenon is called exudation or bleeding. Priestly explained for the first
time that process of upward of flow of water is due to a hydrostatic pressure developed in the root
system.
#
Factors affecting stomatal movement
The environmental factors having the greatest influence on the opening and closing of
stomata are a) light, b) water, c) CO2 concentration and d) temperature.
a)
Light
Generally the stomata of a leaf open when exposed to light and remain opened under
continuous light unless some other factors become limiting. But the stomata will close to
darkness. The amount of light necessary to achieve maximum stomatal opening varies
Krishna Ram Dhital. (2013) cell no:9841181926
-9-
Tree Physiology
with species but it is less than for maximum photosynthetic activities e.g. a light intensity
of 250 foot candles is needed for maximum stomatal opening in tobacco leaf tissue. The
stomata of some plant species may be induced to open by bright moonlight.
b)
Water
Water deficit is created in the plant when the rate of transpiration exceeds. It may take
place even under conditions favoring good water absorption. The wilting of leaves is
visible to the eyes due to lack of water in the plant. The development of an internal water
deficit in a plant causes a DPD gradient between guard cells and the mesophyll and
epidermal cells. This gradient favors the movement of water out of the guard cells, the
reduced turgor causing the stomata to partially or completely close. Water deficit in a
plant can cause chemical changes in the guard cells. It is noted that grater the water
deficit, the sooner the stomata close.
c)
CO2 concentration
An increase in CO2 concentration of the air will cause stomata to close even in the light.
Stomatal closure can also be induced by breathing on leaves.
The CO2 concentration in the leaf intercellular spaces rather than in the external air
controls stomatal movements. Stomata close due to high concentration of CO2 and do not
open when CO2 is transferred to a free atmosphere in the dark. The assumption is that
CO2 concentration in the leaf intercellular spaces remain high. However, light causes the
stomata to open because the intercellular CO2 is consumed in photosynthesis.
Temperature
It is noted that an increase in temperature causes an increase in stomatal opening. The
stomata of some species e.g. cotton, camellia remain closed under continuous light when
temperatures are lower than 0°C. As temperatures are increased, stomatal opening also
increases.
d)
#
Some important questions
1)
Describe the mechanism of transpiration and explain that transpiration is
advantageous.
Describe the process of transpiration and its main role in plant life.
Explain the meaning of stomatal transpiration.
Explain the mechanism of opening and closing of stomata with suitable examples.
2)
3)
4)
UNIT – 3
ABSORPTION OF WATER LOSS
3.1 Root hairs and mycorrhizae
3.2 Mechanism of mineral salt absorption
3.2.1 Mechanism of passive absorption
3.2.2 Mechanism of active absorption
3.3 Factors affecting salt absorption
3.1
Root hairs and mycorrhizae
Root hair is the special modified cell of epidermis meant for the absorption of water. It is
specialized not only in the appearance but also its internal structure. The wall of root hair consists of
cellulose and pectic substances (calcium pectate). Both are highly hydrophilic in nature. These
substances have great capacity of water absorption.
The cell wall acts as permeable layer. Next to cell wall is plasma membrane enclosing
cytoplasm nucleus and vacuole. Vacuole is quite large in size so as to give peripheral arrangement of
cytoplasm. The role of vacuole during absorption of water is just like a controller.
Krishna Ram Dhital. (2013) cell no:9841181926
- 10 -
Tree Physiology
The roots of many species of plants are regularly infected with a mycelium of fungi. Such
a root together with its associated fungal hyphae is called mycorrhiza (fungous root). In other words, it
means the fungus root designated a symbiotic association between the root of higher plants and fungal
species. Roots which become infected with mycorrhizal fungi stop elongating and often branch
extensively. It is believed those mycorrhiza are present on the roots of the majority of vascular species.
Mycorrhiza may play a significant role in the absorption of mineral salt. Mycorrhizal fungi have also an
important role in ecosystem in soil having low mineral contents. There are two types of mycorrhiza
1)
Ectotropic mycorrhizae
2)
Endotropic mycorrhizae
3.1.1 Ectotropic mycorrhizae
In the ectotropic mycorrhiza, the mycelium is chiefly external root, having a web-like
mantle of hyphae. Some hyphae also penetrate into the root infecting principally the cortex. These are
found on many forest tree species such as oaks, beeches and many conifers. They are particularly
abundant on trees growing in soils rich in humus.
3.1.2 Endotropic mycorrhizae
In the endotropic mycorrhiza, the hyphae are intracellular, being found principally
within the cells of epidermis and the cortex. These are found on many species of orchid, heath, walnut,
etc. The fungous associates in such mycorrhiza are actually mostly microscopic molds.
Figure: Root hair and mycelium
3.2
Mechanism of mineral salt absorption
Early workers assumed that inorganic salts were passively carried into the plant with the
absorption of water. Soon it was discovered that these assumption could not adequately account for the
obvious differences in the salt composition of the plant tissues and the medium in which the plant grew.
It was commonly stated that the entrance of mineral salt into the peripheral cells of roots occurred by
diffusion. Although it is true that limited quantity of mineral salts do pass into some cells under some
conditions by diffusion. In general, this is a relatively unimportant mechanism of electrolyte absorption
compared with certain others. Various theories are proposed to explain the mechanism of mineral salt
absorption which can be divided into two broad categories. First is passive absorption and the second is
active absorption.
3.2.1 Mechanism of passive absorption
It includes the theories of diffusion, ion exchange, mass flow and Donnan equilibrium. It
is also called physical absorption. It may be defined as the absorption of solute by cells according to
ordinary laws of diffusion.
Krishna Ram Dhital. (2013) cell no:9841181926
- 11 -
Tree Physiology
3.2.1.1 Diffusion
Diffusion is the movement of substances from the region of their higher concentration to
the region of lower concentration. When a plant tissue that has been immersed into the relatively
concentration solution is transferred to a dilute salt solution. Some of the ions already absorbed by
immersed tissue diffuse rapidly out into external solution. If the tissue is replaced into concentration
salt ion will rapidly back into the tissue. It is thus evident that diffusion of ion can occur freely
between external solution and certain parts of tissue, which is caked free space. A plant root dipped
in a dilute solution transfers to concentration solution; the rate of absorption increase providing that
absorption is a diffusion process.
3.2.1.2 Mass flow theory (bulk flow)
According to this theory, ions are taken up by the roots along with the mass flow of water
under the influence of transpiration. Lopushinsky (1964) studied the uptake of radio active P32 and
Ca45, and found that an increase in the hydrostatic pressure (transpiration pull) increases ion uptake.
So transpiration effect on salt absorption is direct. However, both mass flow theory and direct
influence of transpiration fail to explain salt accumulation against osmotic gradient.
3.2.1.3 Ion exchange theory
Fundamentally, this theory consists in the exchange of anions or cat-ions from within
cells for ions of the same sign and equivalent charge in the environment of the absorbing cell. The
ions from external solution may exchange with the ions absorbed on the surface of the cell wall or
membranes of tissue. Just like ions exchange between soils colloids and soil solution, the cat-ions
may exchange with hydrogen ions and anions with hydroxyl ions absorbed on the surface of the
tissue and this ion exchange may allow greater absorption of ions from the external medium by free
diffusion. As hydrogen ion and hydroxyl ion are both readily available absorbed on the surface of
cell membrane, the cat-ions and anions are exchanged freely. This exchange process doesn't involve
the participation of the metabolic energy of aerobic respiration.
Experimentally, it can be shown in excised barely roots with the use of radioactive K+ in
which case K+ exchange place with non-radioactive K+ ions. Likewise negatively charged Cl- and
Br- are also exchanged without disturbing the electrical neutrality. It is believed that a similar
mechanism operates between soil solution and clay micelle which would allow for a greater
absorption of ions from the external medium than could normally be accepted for by free diffusion.
ClBr
+
Cl
K
ClK+
K+
BrK+
Br+
Cl
K
Br
K+
K+
BrK+
ClClK+
ClK+
+
+
K
Br
K
BrClK+
BrK+
K+
BrK+
Cl+
Cl
K
Cl
K+
K+
BrK+
BrClK+
BrK+
+
+
K
Br
K
ClFigure: Ion exchange theory (Negatively Cl- and Br- are exchanged without
disturbing the electrical neutrality)
3.2.1.3 Donnan equilibrium theory
This theory accounts for the effect of fixed or in-diffusible ions and assumes the cooperation of both electrical as well as diffusion phenomena (Donnan’s equilibrium) for the control
Krishna Ram Dhital. (2013) cell no:9841181926
- 12 -
Tree Physiology
of electro-chemical equilibrium. This theory assumes that certain fixed or in-diffusible ions (for
which cell membrane is impermeable) are present in a cell sap. In order to maintain an internal
balance, such ions would require ions of other charge. The following figure explains the basic
concept of Donnan’s equilibrium.
Cl- K+
RK+
RK+
K+
ClK+
R- K+
K+R- K+
R-
RK+
Cl-
K+
K+
R-
ClK+
ClK+
K+ K+
RK+RRK+
+
K
K+
K+R-
R-
Cl-
ClK+
Cl-
RK+RRK+ClRK+
K+
4 K+ + 4 Cl- : 8 R- + 8 K+
3 Cl- + 3 K+: 8 R- + 9 K+ + 1 ClFigure: Donnan equilibrium showing ion pair diffusion
In this case, ion exchange is prevented due to non-diffusible ions (R-) inside the cell. The
above equation is
Cl- (outside)
K+ (inside) which means that the ratio of number of Cl- ions is equal to
the
Cl (inside)
K+(outside)
ratio of K+
ions.
#
Objections
i.
In actual process, the rate of absorption of minerals is too rapid to be explained by
passive absorption.
ii.
No theory of passive absorption explains adequately the absorption and accumulation of
salts and ions against the osmotic gradients or against the law of diffusion.
iii.
It is experimentally proved that there is a close relationship between salt uptake and
metabolic activities.
3.2.2 Mechanism of active absorption
According to this concept of salt uptake, it is believed that this process is supported by
metabolic energy. The following are the several theories about active absorption.
3.2.2.1 The carrier concept theory
Honert (1937) proposed the carrier concept theory of mineral salt absorption. According
to this theory, the ion transport process is carried out by means of carriers which may be organic
molecules or vesicles. It is believed that ions undergo reversible binding with some constituents of
outer space designated as carriers and pass through the impermeable boundary between outer space
and inner space in the form of ion carrier complexes on reaching the inner space they are again
separated from carrier molecule. The direction of the movement of ion carrier complexes is from
outer space to inner space only and ion released in the inner space cannot move out and thus are
accumulated there.
This theory explains selectively, abundance of selected ions in the membrane and their
chemical affinities with the carrier molecules.
Krishna Ram Dhital. (2013) cell no:9841181926
- 13 -
Tree Physiology
Sometimes, the word translocator is used in place of carriers. The translocators are
specific large protein molecules. These can be identified the mirror images of one another of the
same compound.
cell membrane
movement across
XK
XK
Ion-carrier
combination
K+
Breakbown
Of complex
membrane
K+
Activation of carrier
X
External
X’
Movement across the cell membrane
YA
Internal
YA
A-
A-
Y
Y’
Figure: Explanation of carrier mechanism of ion uptake by cells (X and Y =
carriers; X’ and Y’ = precursors; XK and YA = carriers and complexes;
K+ = cation and A- = anion --- Street, 1952)
In the above diagram, X and Y are for carriers, X’ and Y’ are for precursors of carriers,
XK and YA are for carriers and complexes and K+ and A- are for cation and anion respectively. In
the process carrier precursors X’ and Y’ are activated to form carrier X and Y which combine with
ions to form ion carrier complexes XK and YA for carrying them to inner space and releasing them
over there. The metabolic energy is required in the process for regeneration of the carrier, and
carrier combination, transportation of ion carrier complex; break down of ion carrier complex and
movement of carrier precursors back. The scheme accounts for both cations and anions but in the
process separate carriers are involved and this is not accepted universally.
However, this concept has three important supporting evidences.
1)
Isotopic exchange
2)
Saturation effects
3)
Specificity
3.2.2.1. i Isotopic change
According to Epstein (1956), it is fact that not only back diffusion but also isotopic
exchange of the actively absorbed ions is prevented suggests a membrane highly impermeable to
free ions.
3.2.2.1. ii Saturation effects
This concept is also supported by the fact that when the concentration of the salt is
increased, the rate of absorption also increases. But after a certain period, the rate of absorption
Krishna Ram Dhital. (2013) cell no:9841181926
- 14 -
Tree Physiology
becomes constant with no further increase. This level of maximum rate of absorption can be
maintained for a relatively long period of time and this limit is reduced when the active carrier
sides become saturated with the ions. Further increase in the rate is stopped because all the
carrier sides remain occupied with ions and the constant rate is maintained because as soon as
carrier releases an ion in the inner space, it gets occupied by another ion for absorption.
3.2.2.1. iii Specificity
Selective absorption of roots also supports carrier concept. Ions are absorbed at different
rates and have different levels of accumulation in the root tissue. This suggests the presence of
specific carriers. Ions of dissimilar chemical behavior have weak and non existent specificity.
3.2.2.2 Protein-Lecithin theory
This theory believes in the participation of some amphoteric compound as carrier with
which both cat-ions and anions can combine (applicable to both cations and anions uptake).
According to Clarke (1956), the carrier can be a protein associate with phosphatide i.e. lecithin. In
the transport of ions, the lecithin is synthesized and hydrolyzed in a cyclic manner. Ions from outer
space are picked up by lecithin to produce a lecithine ion complex which moves in the inner space
and releases the ions on the hydrolysis of the complex. This process occurs in a cyclic manner as
explained in the following figure. During synthesis of lecithin ATP (metabolic energy) are required.
3.2.2.4 Cytochrome-pump theory
According to Lundegardh and Burstom (1933), a quantitative relationship exists
between anion absorption and respiration. When a plant is transferred from water to salt solution the
rate of respiration increases. It is called salt induced respiration. It is stated that anion absorption is
independent of cat-ion absorption and occurs by a different mechanism. Oxygen concentration
gradient exists from the outer surface to inner surface of a membrane favoring oxidation at the outer
surface and reduction at the inner surface of a membrane. The actual transport of anions occurs
through cytochrome system.
Protons (H+) and electrons (e-) are produced on the inner surface as a result of
dehydrogenase reaction. Via a cytochrome chain the electron moves outwards to be released to unite
Krishna Ram Dhital. (2013) cell no:9841181926
- 15 -
Tree Physiology
with proteins and oxygen from water, as a result of which the reduced ion of the cytochrome on the
outer surface get oxidized by losing the electron as follows:
(Fe++/ 2A-) + A(Fe+++/3 A-) + e-.
The oxidized ion of cytochrome on the inner surface gets reduced by taking an electron
released from dehydrogenase reaction. The anions picked up by oxidized ion of the cytochrome on
the outer surface are released on the inner side in the last step and in order to balance the potential
difference caused due to accumulation of anions on the inner surface, the cat-ions are absorbed
passively.
1/
4
A-
Fe++
Fe+++
Fe++
A-
e-
Fe+++
Fe++
Fe+++
e-
A-
ADehydrogenase reaction
H
A-
Cell internal
External Solution
A-
H+
O2
1/2H2O
M+
M+
Figure: Cytochrome-pump theory on salt absorption. Anions A- are actively
absorbed by means of a cytochrome pump and cations M+ are
passively absorbed (redrawn from Lundegardh, 1950)
#
i.
ii.
iii.
iv.
Objections
It is applicable only for anion and it has no proper explanation for cat-ion
absorption.
It cannot explain selectivity in ion absorption.
It only explains a common mechanism for all kinds of anion absorption.
An elaborated electron transport system.
3.2.2.5 ATP Theories
Specially, there are some ATP theories for the cat-ion uptake. According to these
theories, ion uptake into the cells is energized by ATP. The energy from hydrolysis of ATP
molecules can be made available to energize ion pumps through the action of enzymes. These
enzymes have been found present in the cell membrane. The role of ATP in cat-ion uptake can be of
two kinds i.e. by removal or by addition of phosphate group. In the first case, organic compound is
first phosphorylated which on dephosphorylation makes the organic compound capable to combine
with cat-ion. The cat-ion is released when phosphorylation occurs again. In the second case, the
phosphorylated organic compound with cat-ion and the cat-ion is released on hydrolysis of the
complex (dephosphorylation).
3.3 Factors affecting salt solution
There are several factors affecting salt absorption by roots. They are as follows:
1)
Temperature
Krishna Ram Dhital. (2013) cell no:9841181926
- 16 -
Tree Physiology
2)
3)
4)
5)
6)
PH
Value
Light
Oxygen tension
Interaction
Growth
3.3.1
Temperature
The increase in temperature increases both passive and active salt absorption processes
and a lowering in temperature decreases them. This is because the same relationship exists between
temperature and free diffusion involved in passive absorption and temperature and biochemical
reactions involved in active salt absorption.
3.3.2
PH Value
As the pH affects the availability of ions in the medium, it may indirectly affect
absorption also. When sufficient ions are present in the medium, the effects of pH over a
physiological range on salt absorption are not important. However, at P H values outside the
physiological range damage the plant tissue.
3.3.3
Light
As stomata opening allow more transpiration and increased mass flow and photosynthesis
provides energy and oxygen for salt uptake, light indirectly affects the rate of salt absorption by
affecting the opening and closing of stomata and the process of photosynthesis.
3.3.4
Oxygen tension
The deficiency of oxygen decreases salt absorption.
3.3.5
Interaction
The absorption of one ion may be influenced by the presence of other ion. The interaction
appears to be associated with the availability and specificity of binding sites on carriers.
3.3.6
Growth
Different types of growth affect salt absorption in different ways e.g. growth involving
increase in surface area, no of cells, carriers, volume of water uptake stimulate salt absorption.
#
Absorption of mineral salt by aerial organs
Under natural conditions absorption of mineral salt through the aerial organs of a plant
rarely occurs in appreciable quantities. Plants are some times “fertilized” by spraying the
aerial organs with dilute solutions, a practice which involves the absorption of solutes
directly through the leaves or stems. This practice has been followed must successfully as
a means of supplying certain micro-metabolic elements to plants.
#
General roles of the mineral elements in plants
Strictly speaking mineral elements as such do not influence the physiological processes
of plants. It is only when present in ionic form or as constituents of organic molecules
that they assume important roles in plant. Considered as one group or class of substances
found in plants, mineral elements function in a number of different ways.
1)
Constituents of protoplasm and cell wall.
2)
Influence on the osmotic pressure of plant cells.
3)
Influence on acidity and buffer action.
4)
Influence on the permeability of cytoplasmic membranes.
5)
Toxic effects of minerals elements.
6)
Antagonistic effects
7)
Catalytic effects
#
Some important questions
1)
Explain the mechanism of active absorption of mineral salts.
2)
Discuss the carrier concept of active uptake of minerals.
3)
Describe the concept and mechanism of active absorption of salt by plant roots.
4)
Explain the mechanism of passive absorption of mineral salts with suitable example.
Krishna Ram Dhital. (2013) cell no:9841181926
- 17 -
Tree Physiology
UNIT – 4 TRANSLOCATION
4.1
4.2
4.3
Ascent of sap
4.1.1 Vital theories
4.1.2 Physical force theories
4.1.3 Root pressure and atmospheric theories
4.1.4 Cohesion tension theory
Translocation of mineral salts
Translocation of organic food (Phloem transport
theories)
Translocation; the movement of water which takes place in water conducting tissues i.e.
xylem tissue etc is called translocation. In other words the movement of organic and inorganic solutes
from one part of the plant to another is called translocation or transport. The water is absorbed by root
hairs of the plants from where it reaches xylem via cortical cells and passage cells and through xylem.
Then, it reaches to the top of the plant where it is transpired by leaves and used for other metabolic
activities.
4.1
Ascent of sap
The upward movement of water from stem base to the top of the tree is called ascent of
sap. In other words, the process of transportation of water from root to aerial parts of plant is called the
ascent of sap. During ascent of sap, mineral ions are also transported with water. The movement of
water to the top of tall trees is still not understood. Sometimes it covers a height of 400 ft. against
gravitational pull. Various theories have been purposed to explain the mechanism involved in the ascent
of sap. They are as follows:
4.1.1 Vital theories
According to this theory, the living cells of stem play an important role in the ascent of
sap. The scientists believed that living tissues are involved in the ascent of sap. The supporters of this
theory are as follows.
4.1.1.1 Westermeir (1883):
He stated that the force for upward conduction of water in the stem is affected in wood
parenchyma and that the tracheids and vessels acted as water reservoirs.
4.1.1.2 Godlewski (1884):
He proposed relay pump theory. According to this theory, the living tissues in the xylem
bring about a pumping action in a upward direction. Here, the xylem tracheids and vessels act as
water reservoirs.
4.1.1.3 Janse (1887):
He supported the theory of Godlewski and showed that if the lower portion of a branch is
killed, the leaves above are affected within a few days. However, this theory has little experimental
background.
4.1.1.4 J. C. Bose (1923):
He proposed pulsation theory of ascents of sap so it is called pulsation theory of Bose.
According to this theory, it is observed that pulsatory activities by the innermost cortical cells lying
just outside the indodermis. Factors such as temperature, anesthetics, poison, etc. were also found
Krishna Ram Dhital. (2013) cell no:9841181926
- 18 -
Tree Physiology
influencing pulsatory activities. He applied his observation explaining the phenomenon of ascent of
sap. According to him, the pulsatory activities of cell absorb water from the outside and pump the
same into the vessels.
4.1.1.5 Benediots (1927):
He showed that the rate of ascent of water is 8000 to 30,000 time higher than pulsation
rate as showed by boss. Bose theory was also supported by Molisch (1929). He observed electric
activity of inner cortical cells increased when treated with chemicals.
#
Objections
Strasburger (1891) and Overton (1911) experimentally demonstrated that the ascent of
sap can take place in segments of xylem in which living cells have been killed by heat or
poisons. It means the water solutes in the soil can be ascended to the top of the tree
through dead cells which rejected the vital theories on ascent of sap.
4.1.2 Physical force theories
Physical force theories believe that living cells are not involved in the ascent of sap. It is
purely a physical phenomenon. Many physical theories have been proposed to explain the mechanism
of ascent of sap. Some of them are as follows:
4.1.2.1 Capillary force theory
According to Boehm (1809), the water rises in the narrow tubes due to the force of
surface tension. This phenomenon is observed commonly when a straw pipe is dipped in a cann of
soft drink. The suggestions of some investigators are that water moves through the Lumina of the
tracheids and vessels as a result of capillary action. As the xylem vessels are narrow and comparable
to the straw, the capillary force is responsible for ascent of sap.
#
i.
ii.
iii.
Objections
Capillary force is inversely proportional to diameter, larger the diameter lower will be
the height to which water can rise is not applicable to all plants.
Vessels do not have uniformity on account of various thick rings.
Soil water is not directly connected with xylem vessel.
4.1.2.2 Imbibitional theory
It is assumed that water moves upward in the stem entirely through the wall of xylem
elements and trachieds due to the process of imbibition. Although the movement of water through
imbibition by hydrophilic colloids is extremely slow and negligible but the ambibitional pressure
(100 to 1000 atms) is quite adequate to carry water to any distance. However this theory was
discarded as it became evident that water can rise through the wall due to imbibitional pressure up to
a certain height but it moves rapidly and rises up through the lumen of xylem elements rather than its
wall. It means the walls of xylem do not carry water.
4.1.3 Root pressure and atmospheric theories
4.1.3.1 Root pressure theory
It was first observed by Josheph Priestley and explained by Stephen Hales (1727). The
stump of recently felled tree of de-topped herbaceous plant will often give visual evidence of root
pressure. Xylem sap under pressure may be observed exuding out of the cut-end of the stump. It is a
hydrostatic pressure developed in the roots due to accumulation of absorbed water called root
pressure. This root pressure forces water into xylem from adjoining cells resulting it in upward flow.
Stephen Hales observed a pressure of 1 atm in root (10 ft.).
In view of Stocking (1956), root pressure is a pressure developing in tracheary elements
of xylem as a result of metabolic activities of the roots. The root pressure is therefore is referred to as
an active process. It should be clearly understood however that the movement of water up the stem
Krishna Ram Dhital. (2013) cell no:9841181926
- 19 -
Tree Physiology
as result of root pressure is due to osmotic mechanisms (passive), which are created as a result of
active absorption of salt by the roots. The root pressure is referred to as an active process in the sense
that living roots are essential for it to occur. For example:
Glass tube
Water level
Rubber Sleeve
Tomato Stem
A)
Figure: A method of demonstrating root pressure
A)
B)
B)
Tomato plant just after decapitation
Tomato plant sometime after decapitation
If a well watered tomato plant is de-topped and the stem attached with a rubber sleeve to
a glass tube containing some water, the manifestation of this pressure can be observed. The above
figure shows that in such a combination water is actually pushed up the glass tube.
#
i.
ii.
iii.
#
i.
ii.
iii.
iv.
v.
vi.
vii.
Factor affecting root pressure
It is concerned with metabolic activities of cell.
It is also affected by obstruction of mineral salts.
It is largely affected by energy supply through respiration.
Objections
No root pressure observed in Gymnosperm which includes tallest tress of the world but
the ascent of sap is high in tall tress
Water continuous rise upward even in the absence of living roots.
Magnitude of root pressure is no sufficient to account for ascent of sap in such tallest
tress like eucalyptus, etc.
In plants root pressure is always below 2 atoms and therefore water due to root pressure
can rise up only to 21 meters if all favorable conditions are available. So it cannot be
applied to the tall trees
Ascents of sap can take place even in the absence of living cells while root pressure is
active process linked by supply of energy.
The amount of water exuded by root pressure is very low.
Root pressure has some role in young plants not in the higher plants.
4.1.3.2 Atmospheric pressure theory
It is thought that atmospheric pressure is responsible for upward movement of water or
for ascent of sap. Water moves up in the xylem vessel to fill up gap in fall of atmospheric pressure
caused due to loss of water during transpiration.
#
i.
Objection
For the operation of atmospheric pressure, the lower end of plant should be free surface.
But this is not found in plant, xylem elements do not directly open into soil water.
Krishna Ram Dhital. (2013) cell no:9841181926
- 20 -
Tree Physiology
ii.
It is not applicable because there is not vaccum at the upper end of plant even if vaccum
is created at transpiring end, the maximum height to which water can rises in just over 30
ft.(10 m) as plants are far more taller.
4.1.4 Cohesion-tension theory
This theory was developed by Dixon and Jolly (1894) and was supported by Curtis and
Clark (1951), Milburn and Johnson (1966). It is also called cohesion hypothesis, theory of
cohesive force, Dixon and Jolly theory of cohesion. It is the most accepted theory which explains the
lifting of water also in tall plants. The transpiration process is responsible for the rising of water from
roots to leaves. The xylems of root and stem are joined and have a continuous column of water from
deepest root to the top of the tree. As the water is lost from leaf cells by transpiration, the water
potential of leaf cell becomes lower and water flow to it from adjoining cells. The flow of water from
one cell to other continues and lastly water flow from xylem to its neighboring cells. The loss of water
from xylem creates tension or pulling force in water column. The force is transmitted to the root and it
increases absorption. Hydrogen bonds formed between water molecules of water column hold the
whole water column. The force holding the water molecules together is called cohesion (attraction
between similar molecules which cannot be easily separated from one another).
As the water molecules flow from xylem to adjoining cell, whole water column is pulled
upward. The cohesive force of water measured up to 350 atms is much in access of the minimum
required for the ascents of sap in the tallest trees. In short we can say cohesion-tension theory means
water forms a continuous column from base of the plant to its top and remains under cohesive tension
due to transpiration pull and according to need water is being pulled up to the top of the tree.
Evaporation
transpiration
twig
Porous pot
Water
Mercury
A)
B)
Figure: A) Demonstration of cohesion-tension theory by physical system
(evaporation of water from porous pot pulls up the column of mercury
due to tension)
B) Demonstration of the same using using plant in stead of porous pot
(mercury column rises)
In the above experiment a porous pot is joined to one end of glass tube and the tube and
pots are filled with water. When the other end of the tube is dipped in the mercury as shown in above
figure, the level of mercury starts rising. As water evaporates through minute pores of pot, it pulls the
whole column of water causing the rise in the level of mercury. The rise in mercury level is much
higher than the rise due to atmospheric pressure. Similar result is obtained when a plant twig is
attached of one end of tube in stead of porous pot. In this case, the mercury is also pulled upward.
#
Characteristics of cohesion-tension theory
i.
A continuous column is formed by water from the base of the plant to its top.
ii.
Due to transpiration, water is lost from mesophyll cells because of which a pulling force
develops and puts these cells under tension.
Krishna Ram Dhital. (2013) cell no:9841181926
- 21 -
Tree Physiology
iii.
The continuous column is not broken due to cohesive property of water molecules
however the tension causes a break in water column.
iv.
The transpiration pull is transmitted to the root region to regulate absorption.
#
Evidences in support of Dixon theory
i.
It is purely a physical process which doesn’t require metabolic energy. Even if required
energy it is negligible because for hundred meters rise of 1 ml water only 0.5 calorie
energy is needed.
ii.
Osmotic pressure of mesophyll cells has been recorded up to 20 atoms which is quite
sufficient of ascent of sap.
iii.
Osmotic pressure of leaf shows diurnal variation. It is maximum in noon when the water
contain of leaf is minimum. Such condition indicates that it is connected with upward
movement of sap water.
iv.
Different workers have reported the value of osmotic pressure of cell sap in different
plant in order of trees>shrubs>herbs.
v.
The movement of water under tension is quite rapid which can be shown by cutting the
stem of a wilted plant under water.
#
Objection to cohesion-tension theory
The only one major objection of this theory is due to variation of temperature during day
and night and in vessels of larger diameter, there are fair chances of gas bubbles entering
in water column from soil with water which may break the continuity of water column or
due to pressure of dissolved gas due to cavitations
#
Note:
The water molecules stick and form water column in xylem with tensile strength. It
prevents the column to break when the molecules move upward. The molecules of water
also stick to the vessel wall by a force called adhesion. It prevents the water column to
flow downward. Water in xylem moves upward by the combine effect of cohesion and
adhesion.
#
Mechanism of ascent of sap
The loss of water from the surface of leaf mesophyll cells due to transpiration reduces the
water amount and causes an increase in the osmotic pressure of these cells. Thus a
reduced water potential is developed in mesophyll cells, i.e. DPD increases. Water from
the adjacent cells and ultimately from the conducting tissue is pulled to meet this loss of
water and as result a pull is developed in the mesophyll cells and xylem cells of the leaf.
Now water present in the xylem cells is placed under tension which is ultimately
transmitted to the root through the stem trachieds.
This downward transmission of tension is because of cohesive properties of a continuous
water column moves upward by mass flow due to transpiration pull and simultaneously
the process of ascent of sap is accomplished.
4.2 Translocation of mineral salts
As a rule translocation in a general direction of roots to leaves or other apical regions is
termed upward translocation. For many years it was universally agreed that upward translocation of
mineral salt occurred through the xylem, although it now appears that the situation is not quite this
simple.
Studies of the sap from xylem vessels show that it usually contains at least traces of both
organic and inorganic solutes. However, the concentration of inorganic constituents in the xylem sap is
relatively higher than that of organic solutes. Further more the concentration of mineral salts are
commonly present in the sap of vessels at seasons when upward flow of water is occurring at its most
rapid rates. At such time, the xylem sap contains little or no organic material in solution. Presence of
dissolved mineral salts at such seasons is presumptive evidence that at least some of them are
translocated upward in the plant through the xylem.
Generally, the circulation of mineral salts takes place by vascular elements. Use of
radioactive traces, several different pathways for the translocation of mineral salts have been discovered,
which are discussed below.
Krishna Ram Dhital. (2013) cell no:9841181926
- 22 -
Tree Physiology
4.2.1 Translocation of mineral salts in xylem
It is observed that increase in transpiration increase the salt absorption uptake. This
observation has been made with tomato plant by Arnon et.al. They found that radioactive phosphate
traveled upward to the tip of the tomato plant much more rapidly under condition favoring rapid
transpiration, such as a bright sunlight. It has been also shown that transpiration by leaf is inhibited
(checked )by covering the leaf with polythene bag, translocation of mineral salts that particular leaf
reduced considerably. It indicates that the upward movement of salt is through the xylem.
Stout and Hoagland (1939); they were the first to employ the radio active tracers in
experiments to ascertain path of upward translocation of mineral salts in plants. Small plant of willow
root in sand or solution cultures was used. Certain branches of the plant was “stripped” by cutting
longitudinal slits 9 inch long opposite sides of the stem and then carefully pulling the bark away from
the wood but leaving it attached ends. A sheet of impervious waxed paper i.e. paraffined paper was
then inserted between the phloem (the bark) and xylem. This treatment resulted in no visible signs of
injury to the plants during the course of an experiment or the methods used were such that the
continuity of the bark and the xylem was undisturbed and the plant was left intact. Radio active ions of
potassium, sodium or bromide were introduced in the rooting medium. After a few hours say 5 hours
under conditions favorable to transpiration, distribution of the tracer ions in the stem was ascertained
by measuring the quantity of radio active elements in ashed segments of the xylem and phloem of the
stem above, below and in the region where the xylem and phloem were separated with paraffined
paper.
By the results, K42 was found to be relatively abundant in both the bark and the wood
above and below the section of the willow stem in which the xylem and phloem had been separated by
paraffined. With in this latter segment of the stem, however almost all of the tracer elements was
located in the xylem. In the intact branch there was no such marked difference in the distribution of the
radio active potassium. Similar results were obtained with the other plants used.
It is obvious that mineral salt absorbed by the roots were translocated upward at relatively
rapid rate through the xylem. During their upward passage some of the mineral salts moved laterally
from the xylem to the phloem. However the lateral movement of salts from xylem to phloem was
intercepted by interposing impermeable barrier between these two tissues, practically all of the tracer
elements were found in the xylem.
These results show that the upward movement of mineral salts occurs in the xylem but
the possibility of some such movement in the phloem also is not excluded even though it is clear that
lateral movement from xylem to phloem takes place readily. Similar experiments of Gustafson (1939)
also indicate that a limited amount of upward translocation or radio active phosphate may occur
through the phloem.
Bark (phloem)
paraffined paper
Wood (xylem)
Figure: Tracer technique of Stout and Hoagland
There is no doubt that upward translocation of mineral salts occurs in the xylem and it is
the main pathway along which general upward movement from roots to leaves occurs. The favorable
conditions to mineral salt translocation in the xylem include high rate of transpiration, high
Krishna Ram Dhital. (2013) cell no:9841181926
- 23 -
Tree Physiology
concentration of mineral salts in the substrate and the prevalence in the root cells of metabolic
condition which favor rapid movement of minerals salts from the absorbing cells to xylem.
Some upward translocation of mineral salt also occurs in the phloem under certain
circumstances but it is occurred at slower rates than in the xylem. The mineral salts are more likely to
move in the phloem where in organic than when in inorganic combination. Entrance of such elements
as nitrogen, phosphorus and sulfur into organic combination in the root cells may favor their upper
translocation in the phloem.
Clements and Engard (1938), Phillis and Mason (1940) and others have stated that
ringing stems of various species doesn’t prevent upward movement of mineral salt through the plant
and the upward translocation of mineral salts can occur in the xylem.
4.2.2 Lateral translocation of mineral salts
In addition to upward translocation of mineral salts, there is also lateral movement
between vascular tissues. Generally the xylem tissue is separated from the phloem tissue by layer of
living cells which constitute the cambial tissue. It is thought that the cambial tissue may regulate to
some extent, the amount of salt carried up in the transpiration stream. It is obvious that if the upward
movement of the salt was not regulate in some manner, the salt need to be certain area of the plant
need to be accommodated. The cambium is positioned in such a manner as to make it available both
metabolically and physically for regulation of upward, downward and lateral movement of mineral
salt. Biddulph has suggested that the active accumulation of salt by the cambial cell may act as a
deterrent (limit ) against an ‘indiscriminate’ sweep of salt upward in the transpiration stream.
On the other hand, if the element should present in low concentration in the phloem, the
active accumulation of the element and its lateral translocation into the phloem would be enhanced.
4.2.3 Translocation of mineral salt in the phloem
Curtis (1950) employed the techniques of intercepting the xylem versus intercepting the
phloem to ascertain whether mineral salts moved into growing, defoliated stems of Sumac through the
xylem or through the phloem. Relative to the quantity which moved through check-stems a much
larger proportion of mineral salts or nitrogenous compounds was translocated through the stems in
which the xylem was intercepted than through those in which the phloem was intercepted. This
experiment proves that the movement of mineral salts into woody shoots can also occur in the phloem.
Mason and Philis performed similar experiments on cotton plants, which also lead to the
conclusion that upward translocation of nitrogenous compound can also occur in the phloem.
4.2.4 Outward movement of mineral salt from leaves
In studied done on the mineral nutrition of leaves of deciduous plant, it has been shown
that just prior to the abscission, there is movement of mineral nutrient out of leaf. Among the mineral
nutrients moving out of the leaves are nitrogen, potassium, phosphorus, sulfur, chlorine, and under
certain condition iron and magnesium. These remaining include calcium, boron, manganese and
silicon.
4.3
Translocation of organic food (Phloem transport theories)
Movement of organic and inorganic solutes from one part of the plant to another is called
translocation, transport or conduction of solutes. These terms are generally restricted to movements of
solutes in the tissues of the phloem and xylem in which the distance through which they are transported
is usually very great in proportion to the size of the individual cells. They are not used to refer to the cell
movement of solutes which may occur in any part of the plant.
The movement of solutes from the place of manufacture to the other parts of the plant
through special conducting or transporting tissues is called translocation of organic food. If the
movement of these solutes (food) takes place in phloem tissues then it is called phloem transport.
According to classical concept the inorganic solute substances are carried in xylem
vessels along with ascending sap of water in the transportation stream whereas phloem is considered as
the downward translocation of organic foods. The sieve tubes of phloem are considered as the sole path
Krishna Ram Dhital. (2013) cell no:9841181926
- 24 -
Tree Physiology
of downward translocation of solutes. Thus, it can be easily said that the longitudinal translocation of
organic food in stem with upward and downward translocation takes place in phloem. The mechanism of
organic food translocation can be divided as short distance translocation and long distance translocation.
There are several theories about mechanism of phloem transport.
#
evidences supporting the translocation of organic food
through phloem
I
exudation from incision in bark
A careful examination reveals that the exudates come from sieve tubes element.
II
ringing or girdling experiment
The ringing of phloem breaks the downward movements of carbohydrates and
caused marked accumulation of sugar above the ring. These results can be explained in
terms of phloem transports of sugar in downward direction.
III
analysis of sap from aphide style
The aphide sucks sap by penetrating its stylet portion to an individual sieve tube.
The droplets are collected and analyzed chemically. The result supports that translocation
of solute takes place through phloem.
IV
evidences from tracer technique
The use of tracer technique support to the fact that phloem is the tissue through
which organic compounds are translocated downwards.
#
General aspects of the translocation of solutes in
plants
From the time that a young plant starts to grow until its death, a more or less continuous
movement of solutes takes place through the conducting elements of every organ of the plant. In
a very young seedling, foods are usually translocated upward in the growing stems and
downward in the developing roots from the storage tissues of seed. As soon as the photosynthesis
in the developing seedling becomes sufficiently high, at least part of photosynthate moves in a
downward direction from the leaves towards the roots while some of it is often moving upward
toward apical meristems. As soon as the developing roots make effective contact with the
substrate absorption of mineral salts begins and large part of them moves upward direction
through plant. At least some of the solutes absorbed from the soil containing nitrogen,
phosphorus or sulfur often react within the root cells with organic compounds descended in to
the roots from the leaves. The resulting chemically more complex compound such as amino
acids and acid amides may then be translocated in the reverse direction from the roots in to the
aerial parts of the plant.
Mineral salts absorbed by the roots are mostly translocated to young leaves and other
growing organs of the plants. All of them do not remain in the organ into which they are first
translocated. Some of them which move into a leaf or a flower petal may get sooner or later.
Move out of such a lateral organ back into stem.
Through the above discussion, the patterns of translocation of plants are complex and
may be different at different stages in the life history of the plants. Many kinds of solutes are
being translocated in various directions within the plants. Certain routes are as follows.

Downward translocation of organic solutes from the leaves to the other parts of the plant.

Upward translocation of organic solutes to growing regions.

Upward translocation of mineral salts from roots to aerial parts.

Outward translocation of mineral salts from leaves and other lateral organs into stems.

Lateral transfer of solutes within stems.
#
General aspects of phloem translocation
Krishna Ram Dhital. (2013) cell no:9841181926
- 25 -
Tree Physiology
Nine-tenths or even more of the substances translocated in the phloem are carbohydrates.
Although this has been shown experimentally, one could assume that this statement is true after
considering that the bulk of the plant is composed of carbohydrate materials.
The nitrogenous compound like amino acids and amides are translocated out of senescent
leaves and flowers and relocated in younger plant. Further more the movement of these
nitrogenous compounds takes place primarily in the phloem. We should also consider the
direction of movement of these substances discussed below:
1)
2)
3)
Direction of movement
Bidirectional movement
Lateral movement in a tangential movement
Lateral movement in a radial direction
Bidirectional movement:
The movement of organic materials in the plant is bidirectional i.e. substances are
translocated in opposite direction in the stem simultaneously. Materials prepared by
photosynthesis (Photosynthate) moving out of the leaves may be translocated in the direction of
the root or it may move toward growing points where flowers or fruits are in the process of
developing. The mobilization of organic materials in storage organs e.g. taps roots, tubers, etc.
for the nourishment of seedling growth is generally in an upward direction. The re-translocation
of materials out of aging leaves and into young leaves is an upward movement.
An experiment of phloem transport carried out in bean plants has shown that leaves
nearest the roots transport metabolites primarily to the root, leaves nearest the top of the plant
transport to the stem apex and leaves in an intermediate position transport metabolites in both
direction.
The organic materials move in both directions in the stems simultaneously has been
shown with the use of radio active tagging techniques but it is unsolved that whether materials
move in different directions in different phloem ducts or in the same ducts simultaneously.
However, Biddulph and Cory demonstrated that bidirectional movement in bean plants took
place in separate phloem bundles.
*
Lateral movement in a tangential movement
Several studies of translocation patterns showed that materials moving in the phloem
ducts generally move in a linear fashion i.e. sugars moving out of a leaf into the main
translocation stream will move both up and down the stem in line with the supplying leaf. Very
little tangential movement takes place. It is commonly observed that annual rings of trees
directly under large branches or less competition side are considerably wider than the opposite
side. Defoliation of one side of a plant will cause asymmetrical growth (reduced toward
defoliated side).
*
Lateral movement in a radial direction
Radial transfer form the phloem to the xylem tissues has been observed in a wide variety
of plant. Loss of labeled metabolites from the phloem to the xylem through radial transport has
been shown in the bean plant to reach values of 25% or more as compared to their concentration
in the phloem.
*
4.3.1 Protoplasmic streaming theory (short distance translocation)
This theory was proposed by De Vries (1885) and was supported by Curtis (1935).
According to this theory, the movement of organic solutes is caused by a combination of diffusion and
cytoplasmic streaming. Protoplasmic streaming is observed in several cells including sieve element is
responsible for movement of organic solutes. This theory assumes that solute particles are caught up in
Krishna Ram Dhital. (2013) cell no:9841181926
- 26 -
Tree Physiology
the circulating cytoplasm of the sieve tube element and carried from one end to the cell to the other.
These particles pass across sieve plate areas through pore by diffusing through cytoplasmic strands
connecting one element to another. Thus solute translocation is bidirectional and it is combination of
both diffusion and protoplasmic streaming. According to some physiological protoplasmic streaming,
solute translocation is continuous from sieve tube to another sieve tube through pores. It is
bidirectional in the sense that two substances can be moved in the sieve elements in the opposite
direction depending upon their own concentration gradient.
#
1.
2.
3.
Objections
It is not observed in matured sieve tubes
The maximum rate of protoplasmic streaming in higher plants is 5 cm/ hr. while the rate
of translocation of organic solutes is 150 cm/hr.
Fluorescent dye injected into sieve tube doesn’t show movement in up and down
direction as shown is more according to their own concentration gradient and
protoplasmic streaming.
Krishna Ram Dhital. (2013) cell no:9841181926
- 27 -
Tree Physiology
4.3.2 Pressure flow theory or Munch hypothesis (long distance
translocation)
The physiological basis of pressure flow hypothesis has been put forwarded by Munch in
1930 and modified by Crafts (1961) and Esau (1966) on the assumption that a turgor pressure
gradients exists between the supplying (source) and receiving (sink) tissues. In other words, in this
system there is a unidirectional flow of solutes and water through the sieve ducts driven by a turgor
pressure gradient. During photosynthesis, the cell sap concentration of mesophyll cells will be high.
As a result osmotic pressure increases and causes absorption of water. These two factors are combined
and a high turgor pressure is produced in mesophyll cells which are interconnected through small
pores called plasmodesmata. These connections occur from the cell wall of mesophyll cells and
ultimately to sieve tubes in which solution is forced down due to turgor pressure. At last we can say
organic solutes move from a source( point of manufacture of organic food) to sink(consumptive
region) in a mass flow of solution which is caused by the entry of water in the source due to osmosis
and resulting in increasing turgor pressure.
The principle outline of this theory is very simple. It is discussed as follows.
Semi permeable
membrane
C
Water
A
B
Concentrated sugar solution
Dilute sugar solution
Figure: Experimental representation of Munch mass flow hypothesis
Let assume that A and B are osmometers permeable only to water. These two
osmometers are connected by tube C containing water to a closed system. The osmometer A contains
concentrated sugar solution while B contains only water. The both osmometers are dipped in water.
“A” corresponds to mesophyll cells in leaf (supply end), B to the root (receiving end), tube C to
phloem with sieve tubes and water filled vessels to the xylem vessel.
The chamber A will absorb water rapidly in large quantity and result in a high turgor
pressure. Therefore the solution will flow in mass from A to B via C under turgor pressure gradient
until the solution in both chambers become equal. Thereafter the process will stop itself. This process
can be regulated if a continuous supply of sugar solution to chamber A is maintained. The water from
chamber B will diffuse into water vessel and again pass through to chamber A. Almost similar
mechanism is seen in plants.
CO2
Water lost in transpiration
Krishna Ram Dhital. (2013) cell no:9841181926
- 28 -
Root
Vessels
Movement of water
Cambium
Sieve tube
Dissolved organic solute
High O.P. mesophyll cells (Sugar from Photosynthesis)
Tree Physiology
cells
Sugar consumed
Water from soil
Figure: Diagram of Munch hypothesis
i.
ii.
iii.
iv.
v.
Objections to pressure flow theory
This theory accounts only in unidirectional movement.
Bidirectional movement of solute which can occur in the same tube is impossible in mass
or pressure flow theory.
Osmotic pressure supply and consumption end don’t always assure the positive gradient.
Rate of movement of water and solute should be similar according to mass flow but
actually they are different.
Vacuoles of the adjacent sieve tube cell aren’t continuous.
4.3.3 Transcellular streaming (short distance translocation)
It is the modification of cytoplasmic streaming. According to Thaine (1964), sieve tubes
possess tubular strands which are continuous from one sieve to another sieve tube through sieve pore.
These strands move up and down. Metabolic energy is used for this purpose.
He defined transcellular streaming as the movement of the particulate and fluid
constituents of cytoplasm through linear files of longitudinally oriented plant cells. These transcellular
strands are proteinaceous and characteristic microtubules to afford rhythmic contraction. This theory
explains the phenomenon of bidirectional translocation. However electronic microscope has failed to
spot transcellular strands.
4.3.4 Activated diffusion process (short distance translocation)
According to Mason and Phillis there is an activation of diffusion process inside the
phloem because i) cell to cell movement of organic compound from phloem tube into sink is enhanced
by supply of ATP ii) assimilates move from mesophyll cell into sieve tube against concentration
gradient and sieve plate inhibit mass flow.
According to them protoplasm of sieve tube accelerates diffusion of solutes by
a)
Utilizing metabolic energy
b)
By activating diffusion molecules of sieve tubes and
c)
By decreasing the resistance offered by protoplasm of sieve tube
d)
Due to lack of experienced evidence this theory was discarded.
#
1)
2)
Some important questions
Describe the process of translocation of organic food materials in plants and explain the
bidirectional translocation.
Describe physical force theories and cohesion tension theory of ascent of sap.
Krishna Ram Dhital. (2013) cell no:9841181926
- 29 -
Tree Physiology
3)
4)
5)
6)
What is ascent of sap? Discuss different theories of ascent of sap.
What is phloem transport? Describe Munch hypothesis and its role in phloem transport.
What is ascent of sap? Describe the ascent of sap in higher plants.
Describe the various theories of translocation of food in plant.
UNIT – 5
PHOTOSYNTHESIS
5.1 Mechanism of photosynthesis
5.2 Light reaction (hill reaction or photo-chemical
reaction)
5.3 Dark reaction (Calvin’s cycle or c-3 cycle)
5.4 C-4 cycle
5.5 Cam pathway
Photosynthesis: the process in which certain carbohydrates are synthesized from carbon
dioxide and water by chlorophyllus cells in the presence of light, oxygen and water being the by
products is generally called photosynthesis or carbon assimilation. In other words, it is the process in
which green parts of plant mostly leaves manufacture food materials (carbohydrates) from CO2 and
H2O in the presence of sunlight
It is one of the most important physiochemical processes of the world on which the
existence of life on earth depends. It is the ability of green plants only to utilize light energy to produce
carbon containing organic material from stable inorganic matter by photosynthetic process. For most
plants, CO2 is the source of carbon but aquatic plants may obtain carbon from dissolved hydrogen
carbonate ions.
The process of photosynthesis can be shown by the following equation.
Light energy
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2 (Ruben and Kamen-1941)
Chlorophyll
Photosynthesis converts carbon of CO2 to complex organic compounds which are used by
both autotrophic and heterotrophic organisms as carbon and energy source. The carbons of organic
compounds are brought back to the atmosphere in the form of CO2 by the process of respiration or
burning. Both photosynthesis and respiration of living organism contribute in cycling of carbon in
nature.
CO2 of air
and water
Photosynthesis and chemosynthesis
Respiration
i.
ii.
Organic
compounds in
heterotrophs
organisms
Organic
compounds in
autotrophic
organisms
Feeding
Figure: A simple carbon cycle
The process of photosynthesis consists of two phases
Light dependent phase(light reaction or hill reaction or photochemical reaction)
Light independent phase(dark reaction or Blackman’s reaction)
Krishna Ram Dhital. (2013) cell no:9841181926
- 30 -
Tree Physiology
5.1
mechanism of photosynthesis
Photosynthesis is an oxidation-reduction process between carbon dioxide and water in
which water is oxidized and CO2 is reduced to carbohydrate level, the H2O and O2 being by products.
The reduction of CO 2 to carbohydrate level needs assimilatory powers such as ATP and NADPH + H+.
Reduction of CO2 occurs in dark but the production of assimilatory power is light dependent. The
external source of energy is light; therefore the overall process is a photochemical reduction of CO2. The
capture of the photon and the conversion of its light energy to chemical energy is the unique property of
plants.
Photosynthetic reaction is reverse of respiration and oxygen evolved during the process
doesn’t come from CO2.
Photosynthesis
6H2O +
6H2O
C6H12O6
+
6O2
Respiration
The mechanism of photosynthesis is discussed briefly under the following sub-headings.
1)
2)
3)
4)
5)
6)
7)
8)
9)
Role of chloroplast pigment
Other protoplasmic factors
Carbon dioxide absorbing mechanisms of leaves
The light and dark reaction of photosynthesis
Source of the oxygen released in photosynthesis
The carbon pathway in photosynthesis
Variant mechanisms of photosynthesis
Quantum (electro-magnetic energy) requirement
Induction period
5.1.1 Role of chloroplast pigment
The role of chloroplast can be divided into two parts:
1)
They absorb certain wave lengths of radiant energy
This energy is either converted into other wave lengths used in photosynthesis or
transferred the absorbed directly to compounds involved in the reaction.
2)
They act in the capacity of the catalyst at some stages of photosynthetic process
The first role is obvious in the sense that neither CO2 nor H2O absorbs radiant energy in
the visible range. There is no change in the proportion of chlorophyll after a period of active
photosynthesis. Therefore, no destruction of chlorophyll occurs during the process and indirectly
support the concept of catalytic roles of chlorophyll in photosynthesis
5.1.2 Other protoplasmic factors
The complete process of photosynthesis in vitro is accomplished by the use of
chlorophyll solution or by means of chloroplast which have been isolated from cells. The complete
process occurs only in intact (complete) chlorophyll containing cells, indicating that other constituents
of the living cells system besides chlorophylls are essentials for the occurrence of photosynthesis.
5.1.3 Carbon dioxide absorbing mechanism of leaves
The actual participation in the overall mechanism of the process, CO2 may be absorbed
and accumulated in the cells of leaves and other plant organs in considerable quantities. Green leaves
can absorb much more CO2 than can be accounted for by simple solution cells sap. This process is
reversible and is not related to the presence of chlorophyll or the occurrence of photosynthesis because
it occurs equally well in non-green organs and in the dark. At least three mechanism of accumulation
of CO2 have been shown to operate in sunflower leaves.
Krishna Ram Dhital. (2013) cell no:9841181926
- 31 -
Tree Physiology
i.
ii.
iii.
Solution in the cell sap
Reaction with soluble phosphates
Reaction with insoluble carbonates e.g. calcium carbonate
5.1.4 The light and dark reaction of photosynthesis
There are two types of reactions in photosynthesis. One of the reactions involved in
photosynthesis is of a purely chemical type. This fact was first pointed out by Blackman, this reaction
is often called Blackman reaction or dark reaction since it doesn’t require light. It may take place in
either the light or the dark.
A chemical reaction which takes place only at the expense of absorbed light is called a
photochemical reaction or light reaction. The photosynthesis involves such a reaction or reactions only
in the light.
5.1.5 Source of the oxygen released in photosynthesis
Knowledge of the source of the oxygen released in photosynthesis is one of the keys to
understand the mechanism of the process. In the past many investigators assumed that oxygen released
can form the CO2 but Ruben et .al (1941) have showed that the O2 set free in the process comes from
water molecules and not from CO2 molecules. The necessary finding is that more water molecules
must participate in the overall photosynthetic reaction and more O2 is released than can be provided by
only six molecules.
The following equation is there for a more accurate representation of the overall reaction
Light energy
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2
Chlorophyll
5.1.6 The carbon pathway in photosynthesis
The progress made in tracing the pathway of carbon through the photosynthetic
mechanism has seen by the use of radio-active isotopes. When a suspension the alga chlorella was
allowed to photosynthesize for the very short period of five seconds in the presence of CO2 made with
radio-active C14 isotope, a very large proportion of labeled carbon was found by subsequent analysis
of the cells to be present in the compound phospho-glyceric acid. This phosphogylceric acid is the
principal compounds containing radio-active carbon after very short period of photosynthesis in
soybean leaves (Aronoff and Vernon-1950). It is a intermediate key in photosynthesis. It is also a
known intermediate compound in the process of respiration. Radio-active carbon also appears in
sucrose molecules after relatively short periods of photosynthesis.
5.1.7 Variant mechanism of photosynthesis
Although it is known that the process of photosynthesis follows the same general course
in all of the higher plants, but different mechanisms occur in some of the species of the lower plant
Phyla. Investigations have shown that in the photosynthesis of the anaerobic green and purple
bacteria, the reaction is similar to that of the higher plants except that substances other than water act
as hydrogen “donors”. E.g. green sulfur bacteria containing the pigment bacterioviridin can use
hydrogen sulfide as a source of hydrogen in photosynthesis. The overall reaction being
6CO2 + 12H2S
C6H12O6 + 6H2O + 12S
5.1.8 Quantum (electro-magnetic energy) requirement
The quantum requirement of photosynthesis i.e. the number of quanta required for the
reduction of each molecule of CO2 is an important consideration in attempting to visualize the
mechanism of the process. If the radient energy required in the synthesis of one mol of glucose (673
kg cal) be divided by the no of carbon atom is found to be approximately three times that of one
quantum at 600 m (3.27*10-12 erg). The absolute minimum light requirement of photosynthesis would
be about 3 quanta per molecule of CO2 reduced. However 4 hydrogen atoms must receive sufficient
energy to move them from water to CO2 in this process and since a quantum is an indivisible unit, it
Krishna Ram Dhital. (2013) cell no:9841181926
- 32 -
Tree Physiology
appears that the theoretical minimum requirement would actually be about 4 quanta per molecule of
CO2 reduced.
According to Franck’s (1949), there are 8 intermediate steps in photosynthesis, each
requiring one quantum for its activation.
5.1.9 Induction period
When illumination of a plant starts, the rate of photosynthesis is at first low and gradually
increases until it reaches the maximum value for the prevailing conditions. The length of this
“induction period” is typically 1-3 minutes but it may be shorter or longer depending upon the
species, its previous treatment and the prevailing environmental conditions. The causes of an initially
low rate of photosynthesis are complex and are not the same under all conditions but the existence of
such induction periods must be united with any postudated mechanisms of photosynthesis.
5.2 Light reaction (hill reaction or hydrogen transfer phase)
Light reaction takes place in the grana of chloroplast in the presence of light. It
was studied by Robert Hill (1937), therefore it is also called Hill reaction. He demonstrated that isolated
chloroplasts when illuminated in the presence of hydrogen accepter i.e. ferricyanide can produce
oxygen in absence of CO2. This ferricyanide is reduced to ferrocyanide by photolysis of water. It
explains water is used as a source of electrons for CO2 fixation and O2 is evolved as a byproduct. It is
light dependent reaction and it is also called primary process of photosynthesis or photo stage or
hydrogen transfer phase.
According to Ruben, Randall and Kamen (1941), the oxygen evolved during
photosynthesis comes from water and not from carbon dioxide. Chloroplasts when illuminated can break
water molecule and produce oxygen (photolysis of water).
Light energy
6CO2
+
12H2O
C6H12O6
+
6H2O +
6O2
Chlorophyll
Light / chloroplast
4H+ + O2
2H2O
The light reaction phase of photosynthesis is a complicated process therefore it is
discussed in the following steps
i) Red drop, Emersion effect and two pigment systems.
ii) Production of assimilatory powers.
5.2.1 Red drop, emersion effect and two pigment systems
It can also be divided into two categories. They are
5.2.1.1 Red drop and emersion effect
Emerson and Lewis (1943) working on quantum yield of photosynthesis in
monochromatic light (single color) of different wave lengths, they observe sharp decline at wave
length greater than 680 nm in the red zone. This decline is called red drop. Eight years later,
Emerson and Chalmers found that red drop could be brought back to full efficiency by
simultaneously providing shorter wave lengths of light. This increased in photosynthetic activity in
successive application of beams of light of different wave lengths is called Emerson effect.
5.2.1.2 Two pigment system
With the discovery of red drop and Emerson effect, it was concluded that at least two
pigment systems are involved in photosynthesis. These are known as pigment system-I (PS-I) and
pigment system-II (PS-II).
Krishna Ram Dhital. (2013) cell no:9841181926
- 33 -
Tree Physiology
Each pigment system contains about 200 chlorophyll molecules that trap light energy. In
each pigment system, a molecule of chlorophyll- “a” is the primary pigment. The other chlorophyll
molecules are known as accessory pigment as they absorb light energy and transfer to chlorophyll“a” i.e. the reaction centre. The accessory pigments include carotenoides, phycobillins and
chlorophyll – “b”, “c” and “d”. In PS-I, chlorophyll – “a” is P700 which absorb light at a wave
length of 700 nm and PS-II has P680 which utilizes light of wave length 680 nm. In both systems, all
pigments absorb light energy. However, all accessory pigment transfer absorbed light energy to
reaction centre. The primary pigment collects all the energy and initiates the reaction of
photosynthesis.
5.2.2 Production and assimilatory powers
It can also be divided into two steps. They are
5.2.2.1 ETS in photosynthesis or reduction of NADP
The process of reduction of NADP into NADPH + H+ is called electron transfer system
(ETS) in photosynthesis. It includes photo excitation of chlorophyll- “a” and hydrolysis of water or
photo-oxidation of water
Light
4H2O
4H+ +
4OHChlorophyll
4OH-
2H2O +
O2
+
4e-
Fd – NADP+ reductase (oxidation)
Fd (reduction)
NADPH
+
+H
Or
NADH2
Fd – NADP+ reductase (reduction)
Fd (oxidation)
NADP+
Or
NAD+
The mechanism of NADP+ reduction in photosynthesis occurs into three photochemical
steps: a) photochemical reduction of ferredoxin b) reoxidation of ferredoxin by Fd – NADP+
reductase and c) reoxidation of ferredoxin-NADP+ reductase by NADP+
or NAD+
5.2.2.2 Photophosphorylation or formation of ATP in photosynthesis.
The process of formation of ATP from ADP and inorganic phosphate (pi) utilizing light
energy is called photophosphorylation.
Light
ADP
+
Pi
ATP
Chlorophyll
There are three types of photophosphorylation. They are as follows:
1.
Non cyclic photophosphorylation
In this process, the movement of electron is non-cyclic i.e. the flow of electron is
unidirectional. It means the electron released by the chlorophyll doesn’t return to it. It occurs in
the higher green plants and involves both PS – I and PS – II.
The energy rich electron which releases from water passes through the ETS and at every
step it releases energy. At steps of transfer where the released energy is sufficient to bind
inorganic phosphate with ADP, the ATP synthesizes, otherwise this energy is wasted in the form
of heat.
In this process, the unidirectional flow of electron i.e. the electron donated by PS – II
after passing through PQ, cyt. b6, cyt. f, PC and PS – I, reaches Fd and finally utilizes to reduce
Krishna Ram Dhital. (2013) cell no:9841181926
- 34 -
Tree Physiology
NADP . The reduced NADP is utilized for the reduction of CO2 to carbohydrate level. In this
way, it starts from PS – II and is drained off in the carbohydrates produced by CO2 reduction. So
the ATP synthesis resulting from this manner of non-cyclic electron flow is called non-cyclic
photophosphorylation. In this system, two molecules of ATP are formed per two molecules of
NADP+ reduced or one molecule of O2 is evolved for two molecules of water oxidized.
2ADP + 2Pi + 2NADP+ + 2H2O
2ATP + 2NADPH + H+ + O2
+
2.
Cyclic photophosphorylation
It is another pathway of ATP formation by photosynthetic tissues, which involves only
PS-I and wavelength of light greater than 680 nm. It is one of the ways of excluding non cyclic
photophosphorylation by illuminating chloroplasts with wavelength of light greater than 680 nm.
It is found in lower plants e.g. algae. This type of photophosphorylation was demonstrated by
Frenkel. Electrons used in this process don’t come from water i.e. water is not oxidized and O2
isn’t evolved in this process.
In this cycle, P700 of PS-I receiving light energy becomes activated and donates electrons.
The donated electrons are taken up by FRS or Fd. The excited electrons from Fd, unable to pass
the electrons to NADP+, may return back to P700 through three transfer steps i.e. Cyt.b6, Cyt. f
and PC. The synthesis ATP may occur between Fd and Cyt.b6 and between Cyt.b6 and Cyt.f. In
this way, the synthesis of ATP as a result of this cyclic transport of electrons is called cyclic
photophosphorylation.
FRS
e-
-0.6
Fd
eFd – NADP
-0.4
PQ
e-
ee-
E’o in volts
-0.2
Cyt. b6
ATP
e-
0.0
0.2
e-
ADP + Pi
NADPH2
ADP + Pi
e-
Cyclic
photophosph
-orylation
NADP+
Cyt. f
0.4
ATP
0.6
Pigment
system II
0.8
Cl-
H2O
Non - Cyclic
photophosph
orylation
Mn++
673 nm
½ O2 + 2h+
ePC
e-
e-PS – I
P700
683 nm
light
Light
Figure: Schematic representation of cyclic and non – cyclic
photophosphorylation
3.
Pseudo-cyclic photophosphorylation
Arnon and his co- workers (1954) observed that even in the absence of CO2 and
NADP, if chlorophyll molecules are illuminated, they can produce ATP from ADP and Pi in the
presence of FMN. It involves the reduction of FMN with the production of O2. The source of
both hydrogen (electrons) for the reduction of FMN and O2 for its oxidation is water. The FMN
Krishna Ram Dhital. (2013) cell no:9841181926
- 35 -
Tree Physiology
with help of light and chlorophyll breaks and rejoins the water molecule. The formation of ATP
during the process depends upon the reduction of FMN.
Illuminated chloroplast
FMN + H2O
FMNH2 + 1/2O2
ADP + Pi
ATP 5.3
Dark reaction (Calvin’s cycle
or c3 cycle)
The second type of reaction of photosynthesis is independent of light, so it is called dark
reaction. The reactions run in light as well as in dark. It was established by Blackman (1905) and
sometime called Blackman’s reaction. It is also called non-photo chemical process or secondary process
of photosynthesis or synthesis stage or carbon assimilation phase.
In this process CO2 is reduced by utilizing the products of primary process i.e. ATP and
NADPH2 forming during light reaction and carbohydrate is produced. It occurs in the stoma region of
chloroplast.
It is also called carbon fixation process. There are three types of photosynthetic pathways
or three modes of Carbon fixations well established in plants are as follows.
i.
Calvin’s cycle or C3 cycle
ii. Hatch and Slack cycle or C4 cycle
iii. CAM Pathways or CAM cycle
The plants exhibiting these pathways are respectively known as C 3 plants, C4 plants and
CAM plants.
5.3.1 Calvin’s cycle or C3 cycle
It is also called carbon assimilation path of carbon in photosynthesis, reductive pentose
phosphate path or cycle (pp pathway) etc. Calvin and his co- workers traced the path of carbon in
photosynthesis by using radio active isotopes 14C in carbon dioxide. They advanced their studies while
working on unicellular green alga chlorella pyrenoidosa. Various steps involve in the dark fixation of
CO2 through the Calvin’s cycle are as follows.
i.
Carboxylation
Carbon dioxide is first accepted by ribulose- 1, 5- diphosphate (RuDP) and forms an
unstable 6 carbon compound. This 6 carbon compound immediately splits to form 2 molecule of
3-phosphoglyceric acid or phosphoglycerate (PGA). The reaction takes place in the presence of
Rubisco.
rubisco
RuDP +
CO2 +
H2 O
2PGA
Reduction
Phosphogyceric acid is reduced to phosphoglyceraldehyde (PGAL) by NADPH + H+.
This reaction is regulated by the ATP and enzyme triose phosphate dehydrogenase.
ATP + Triose phosphate
PGA + NADPH + H+
3 – Phosphoglyceraldehyde + ADP
+NADP
dehydrogenase
( PGAL)
ii.
Isomerisation
The phosphoglyceraldehyde molecule is converted into its isomers dihydroxy acetone
phosphate (DiHAP) in the presence of enzyme triose phosphate isomerase.
Triose phosphate
3 – Phosphoglyceraldehyde
DiHAP
Isomerase
iv.
Hexoses formation
iii.
Krishna Ram Dhital. (2013) cell no:9841181926
- 36 -
Tree Physiology
One molecule of three phosphoglyceraldehyde combines with one molecule of dihydroxy
acetone phosphate to form fructose – 1, 6 – diphosphate in the presence of enzyme aldolase.
aldolase
3 – Phosphoglyceraldehyde + DiHAP
diphospahte
Fructose – 1, 6 -
Fructose – 1, 6 diphosphate converted into glucose in the following steps.
Fructose – 1, 6 – diphosphate
phosphatase
Fructose – 6, phosphate
phosphofructase
Fructose – 1, phosphate
mutase
Glucose – 1, phosphate
v.
a)
b)
isomerase
Glucose + Pi
Hence Glucose is the main photosynthetic products but it is stored in the form of Sucrose
and starch when condensed.
Regeneration of RuDP (Ribulose diphosphate)
RuDP is regenerated through a series of reaction in the following steps.
Fructose-6-phosphate and phosphoglyceraldehyde combine and break into erythrose-4phosphate and xylulose-5-phosphate in the presence of enzyme transketolase.
transketolase
Fructose -6- phosphate + phosphoglyceraldehyde
erythrose-4phosphste + xylulose -5- phosphate.
Erythrose -4- phosphate combines with a molecule of dihydroxy acetone phosphate to
form sedoheptolase -1,7- diphosphate in the presence of enzyme aldolase.
aldolase
Erythrose -4- phosphate + DiHAP
sedoheptolase -1,7diphosphate.
c)
From sedoheptulose -1,7- diphosphate, one phosphate is removed in the presence of the
enzyme phosphotase to form sedoheptulose -7- phosphate.
phosphatase
Sedoheptulose – 1,7 – diphosphate
sedoheptulase – 7 –
phosphate.
d)
Sedoheptulose -7- phosphate combines with phosphoglyceraldehyde and form xylulose 5- phosphate and Ribose -5- phosphate in the presence of transketolase.
transketolase
Sedoheptulose -7 – phosphate + phosphoglyceraldehyde
xylulose-5
–phosphate + ribose -5 – phosphate.
Krishna Ram Dhital. (2013) cell no:9841181926
- 37 -
Tree Physiology
e)
Both these compounds convert into ribulose- 5- phosphate in the presence of enzyme
phosphopentose isomerase.
phosphopentose
Xylulose- 5- phosphate
ribulose -5- phosphate and
isomerase
phosphopentose
Ribose- 5- phosphate
ribulose- 5 - phosphate
Isomerase
f)
Ribulose–5-phosphate forms ribulose–1,5-diphosphate utilizing ATP obtained by
phosphorylation in the presence of phosphopentosekinase.
phosphopentokinase
Ribulose- 5- phosphate + ATP
ribulose- 1,5 – diphosphate
5.3.2 Hatch and Slack cycle or C4 cycle or c4 plants
It is also called Dicarboxylic acid pathway or Beta- carboxylation or Cooperative
synthesis. Kortschak et.al (1954) reported the formation of dicarboxylic acid (C4) as primary products
of photosynthesis, while tracing the the path of carbon in sugarcane leaves. It was again confirmed in
several other plants by Hatch and Slack in 1966 and proposed the C4 cycle or Hatch & Slack cycle.
It is observed in many Monocots and Dicot families’ i.e Gramineae, Cyperaceae, Compositae,
Chenipodiaceae etc. However wheat, barely, oat and rice are C3 plants, though these are the members
of Gramineae. The plants which possess the C4 pathway are called C4 plants.
The first reaction in this pathway involves the phosphorylation of pyruvic acid to give
phosphoenolpyruvate (PEPA), which is then carboxylated to yield oxaloacetic acid. This oxaloacetic
acid can then be transaminated to give aspartic acid or reduced to give malic acid.
Typically, C4 plants possess two types of chloroplasts located in two kinds of cells.
Leaves of C4 plants have a parenchyma sheath arranged around the vascular bundles.The sheath cells
having large chloroplasts lack grana and contain starch grains (Bundle sheath cell chloroplast). On
the other hand, the mesophyll cells of the leaves possess smaller chloroplasts containing grana and do
not accumulate starch (Mesophyll cell chloroplast). The mesophyll chloroplasts are the site for the
conversion of pyruvic acid to malic or aspartic acid depending upon the species. Chloroplasts of the
sheath cells contain an enzyme that catalyzes the oxidative decarboxylation of malic acid to give
pyruvic acid.
It is proposed that malic acid or aspartic acid in some plants is translocated via
plasmodesmata from mesophyll chloroplasts to sheath chloroplasts, where both acids are
decarboxylated to give pyruvic acid. The pyruvic acid, in turn is translocated back to mesophyll
chloroplasts. The CO2 and NADPH formed in the decarboxylation of the C4 acids are used in the
Calvin cycle, which is found in the sheath chloroplasts but not in the mesophyll chloroplasts. This
pathway is shown in the following figure.
ATP AMP+ iPP CO2
NADPH
NADP+
Pyruvic acid
acid
PEPA
Oxaloacetic acid
Aspartic acid or Malic
(Reactions in mesophyll chloroplast)
Aspartic acid or Malic
acid
Pyruvic acid
Oxaloacetic acid
Malate
Krishna Ram Dhital. (2013) cell no:9841181926
- 38 -
Tree Physiology
CO2
Pyruvate
Reactions in bundle sheath chloroplast
CO2
NADPH
NADP+
C3 cycle
Figure: Hatch and slack pathway
#
i.
ii.
iii.
iv.
v.
vi.
#
i.
ii.
iii.
iv.
v.
vi.
#
Some peculiarities and importance of C4 plants
They are tropical plants.
The most distinguishable anatomical features of leaves of C4 plants are the presence of
bundle sheath cell chloroplast.
The bundle sheath cell may lack grana in their chloroplast while mesophyll cells have
developed grana and chloroplast.
The arrangement of chloroplast containing bundle sheath cells around vascular bundle is
another characteristic feature of C4 plant.
The kranz type (twisted) arrangement of mesophyll cell around bundle sheath cells is
significant in C4 plant.
Bundle sheath cells generally have abundant starch grains as compared to mesophyll
cells.
Significance of C4 plants
The plant operating C4 dicarboxylic acid path can perform normal photosynthesis even in
low CO2 concentration and higher intensity of light.
They are able to trap CO2 both from outside and that released internally due to respiration
because of highly efficient PEPA carbosylase.
Effect of water stress is minimized by the C4 plants because bundle sheath cells lie closed
to the surface of water supply.
It is found that C4 plants mostly live in area having excess salts, high temperature of low
water availability.
Because of their higher photosynthetic capacity and adoptability to adverse
environmental conditions, some of the C4 plants are serious weeds e.g. amaranthus,
salsols, etc.
The concentric arrangement of tissues provides small surface area in relation to volume.
This geometry reduce transpiration surface and allow better utilization of available water.
Difference between C3 and C4 plants
C3 plants
C4 plants
i. C3 cycle is found in all photosynthetic plant.
C4 cycle is found only is certain tropical
plants.
ii. The CO2 acceptor is Ribulose – 1, 5 DP.
CO2 acceptor is phosphoenolpyruvate.
iii. The CO2 absorption is less.
The CO2 absorption is high.
iv. The first stable product is phosphoglyceric acid.
The first stable product is oxaloacetate.
v. All cells have 1 type of chloroplast in
The cells have different type of chloroplast
photosynthesis.
i.e.mesophyll cells and bundle sheath.
vi. It has two pigment systems. PS – I and PS – II.
In the bundle sheath cells, the PS – II is
absent.
Krishna Ram Dhital. (2013) cell no:9841181926
- 39 -
Tree Physiology
The Calvin cycle enzymes are absent in
Chloroplast.
Photo-respiration is absent.
The CO2 concentration in leaf remains low.
Both C3 and C4 cycles are found.
ATP requirement for 1 mole of glucose is 30
vii. The Calvin cycle enzymes are present in
mesophyll mesophyll chloroplast.
viii. Photo-respiration is present.
ix. The CO2 concentration in leaf remains high.
x. Only C3 cycle is found.
xi. ATP requirement for 1 mole of glucose is 18
ATP. ATP.
xii. The optimum temperature for the process is
10 – 45 ºC.
xiii. Net rate of photosynthesis in full sunlight is
The optimum temperature is 30 – 45 ºC.
Net rate of Photosynthesis in full sunlight is
15 – 35 mg/dm2/hr. 40 – 80 mg/dm2/hr.
5.3.3 CAM Pathways of Cam cycle (Crassulacean acid metabolism)
Certain plants belonging to crassulaceae, cactaeae, orchidaceae, etc. show diurnal pattern
of organic acid formation. All such plants are called CAM plants. Most of them possess the succulent
habit.
Malic acid is formed in the night by the carboxylation of PEPA in the presence of
enzyme PEPA carboxylase. This phase is completed in two steps.
I.
PEPA fixes Carbon dioxide and is converted into oxaloacetic acid and
II.
The oxaloacetic acid is converted into malic acid by the enzyme malic dehydrogenase.
The leaves of CAM plants also possess enzymes of Calvin cycle. The CO2 fixed in malic
acid during dark period is ultimately converted to hexoses or carbohydrates. The CAM cycle is as
follows:
CO2
PEPA
Oxlaloacetic acid
Malic acid
Dark
Starch
Malic acid
RuDP
C3 cycle
Pyruvic acid
1 CO2
PGA
Sugar
PEPA
starch
Figure: CAM cycle
The two cycles (CAM and Calvin) occur in the mesophyll cells and there is no
differentiation between the types of cells as found in C4 plants.
#
Factors affecting the rate of photosynthesis
Krishna Ram Dhital. (2013) cell no:9841181926
- 40 -
Tree Physiology
i.
ii.
Internal factors.
External or environmental factors.
Internal factors:
a)
Chlorophyll
The amount of chlorophyll present in the leaf has a direct relationship with the rate of
photosynthesis because it is the pigment which is photoreceptive and is directly in trapping the
light energy.
b)
Protoplasmic factors
Photosynthesis doesn’t start immediately after the appearance of chlorophyll in young
seedlings because there is some unknown factors affecting the rate of photosynthesis called
protoplasmic factor.
c)
Photosynthetic enzyme systems
The nature and amount of enzymes play a direct role on the rate of photosynthesis.
d)
Leaf resistance
Both transpiration and photosynthesis show close dependence upon leaf resistance
because both water and carbon dioxide exchanges are under stomatal control.
e)
Demand for photosynthate
Rapidly growing plants show increased rate of photosynthesis in comparison to matured
plants. If the area of leaf surface is reduced, carbon dioxide assimilation per leaf increases.
Alternatively, if the demand for photosynthate is lowered by removal of meristem, the
photosynthetic rate declines.
f)
Role of hormones
Some hormones increase the photosynthetic rate and carboxylating activity.
g)
Genetic controls
Genetic control of photosynthesis can be exerted on both the carbon dioxide fixing
system and the carbon dioxide transport system.
h)
Leaf age
The newly expanding leaves show maximum photosynthetic activity as the leaves
achieve full size.
External factors:
Concept of limiting factors
According to this concept, there is a minimum, optimum and maximum for each factor in
relation to photosynthesis e.g. with any given species, there may be a minimum temperature below
which no photosynthesis takes place, optimum temperature at which the highest rate takes place and a
maximum temperature above which no photosynthesis will take place. These relationships are shown
graphically in the following figure:
Photosynthesis rate
Optimum
Krishna Ram Dhital. (2013) cell no:9841181926
- 41 -
Tree Physiology
Minimum
maximum
Limiting factor
Figure: Graphic representation of concept of the three cardinal points
Light
Light affects the rate of photosynthesis in various ways.
a) The intensity of light
With the increase in the light intensity, the rate of photosynthesis increases.
i.
ii.
iii.
iv.
v.
vi.
vii.
#





b) Wave length of light
For photosynthesis the visible range or spectrum (between 350 to 750 nm) is essential.
The maximum photosynthesis is shown to occur in the red part of the spectrum with the
next peak in blue part and minimum in the green region.
c) Duration of light
It is assumed that a greater quantity of photosynthesis will take place in a plant exposed
to longer periods of light.
d) Photo-oxidation
When the light intensity for the photosynthesizing tissue is increased beyond a certain
limit, the cells of that organ become vulnerable to chlorophyll – catalyzed photooxidations due to many more chlorophyll molecules becomes exited then can possibly be
utilized, causing a damaging effect.
Carbon dioxide
The carbon concentration affects the rate of photosynthesis markedly. If the
carbon dioxide supply is increased, the rate of photosynthesis increases markedly.
Temperature
The effect of temperature on the rate of photosynthesis is little then on other processes.
Very high and very low temperature affects the photosynthetic rate adversely.
Water
It has an indirect effect on the rate of photosynthesis. In scarcity of water, cells become
flaccid. Depending upon the availability of water, the rate of photosynthesis may be
decreased for 10 – 90 %.
Oxygen
It is a byproduct of photosynthesis. Both oxygen evolution and carbon dioxide
assimilation were reduced in the presence of atmospheric oxygen.
Mineral nutrient elements
Some mineral nutrient elements e.g. mg, Cu, affect the rate of photosynthesis indirectly.
Osmotic relations
It also affects the rate of photosynthesis indirectly because of the availability of water.
Some important questions
Differentiate between light and dark reaction, and C-3 and C-4 cyc le.
Explain photophosphorylation and ETS in photosynthesis.
Explain the light reaction of photosynthesis with the help of suitable diagram.
Explain the dark reaction of photosynthesis with the help of a well labeled diagram.
Describe the dark reaction of photosynthesis in C-3 plants.
Krishna Ram Dhital. (2013) cell no:9841181926
- 42 -
Tree Physiology
UNIT – 6
RESPIRATION
6.1 Mechanism of respiration
6.2 Glycolysis
6.3 Krebs cycle
6.4 Fermentation
6.5 Electron transport chain
6.6 pentose phosphate shunt
Respiration
It is a metabolic process in which biological oxidation of photosynthetic products
(carbohydrates, proteins and fats) is obtained to release CO2, water and energy.
During the process of photosynthesis, light energy is converted in to chemical energy and
stored in the bonds of complex organic molecules. The major portion of stored energy in plants is found
in the form of carbohydrates i.e. starch and glucose. The breaking of the carbon- carbon bonds of such
compounds releases a considerable amount of energy for utilization by the plant.
Respiration is the vital process which occurs in all living cells of the plant. All living
cells need continuous supply of energy to perform various functions.
The overall respiration process may be represented as
C6H12O6 + 6H2O + 6O2
6CO2 + 12H2O + energy (686 kcal)
On the basis of availability of oxygen, respiration can be divided into two categories
i)
aerobic respiration
It takes place in the presence of O2 and the stored food gets completely oxidized into CO2
and H2O
C6H12O6 + 6O2
ii)
6CO2 + 6H2O +
686 kcal
anaerobic respiration
It takes place in the absence of oxygen. The stored food is completely oxidized and
instead of CO2 and H2O, certain other compounds are also formed. This type of
respiration is common in micro-organisms i.e. Yeasts.
C6H12O6
2C2H5OH + 2CO2 + 56 Kcal
Respiration is a complex process which includes
Absorption of oxygen.
Oxidation of food i.e. carbohydrate
Release of energy.
Formation of intermediate products.
Liberation of CO2 and water.
Loss in weight in plants as a result of oxidation.
i.
ii.
iii.
iv.
v.
vi.
#
i)
ii)
iii)
iv)
v)
Significance of respiration
It releases energy which is consumed in various metabolic processes of plants.
It brings about the formation of other necessary compounds participating as important
cell
constituents.
It converts insoluble food into soluble form.
It liberates CO2 and plays an important role in the balance of CO2 in the nature .
It converts stored energy ( potential energy ) into usable energy ( kinetic energy )
Krishna Ram Dhital. (2013) cell no:9841181926
- 43 -
Tree Physiology
6.1
Mechanism of respiration
The energy need for each living cell is fulfilled generally by respiration. The
carbohydrates, fats or proteins are broken down in respiration for the release of energy. Proteins are used
up as respiratory substrate only when carbohydrates and fats are not available.
According to Blackman, the respiration in which carbohydrates are used as respiratory
substrate is called floating respiration and if proteins are used then it is called protoplasmic
respiration.
The reaction may be.
C6H12O6 + 6CO2
6CO2 + 6H2O + ENERGY
Most of the energy released in respiration is lost in the form of heat. Some energy is used
to produce ATP. All complex carbohydrates are firstly converted into hexoses before entering into
respiratory process. The oxidation of glucose (in cell) to CO2 and H2O consists of two phases –
glycolysis and Krebs cycle.
6.2
Glycolysis (emp pathway i.e. embden–meyerhof paranas pathway,
cytoplasmic respiration, common respiratory pathway )
The stepwise degradation from glucose to pyruvic acid is called Glycolysis. It is the
initial step common for all types of respiration and do not require oxygen. It is the term used to describe
sequential series of reactions that starts with a hexose sugar and ends with pyruvic acid.
The equation for overall reaction may be written as
C6H12O6
2C3H4O3 + 4H
It states simply that one molecule of glucose is converted to two molecules of pyruvic
acid. However, glycolysis is not a one step reaction but a series of closely integrated reactions that led
eventually to pyruvate. The reactions of glycolysis occur in the cytoplasm.
It may be divided into two major steps
i)
The conversion of glucose to fructose – 1, 6 – diphosphate.
Three reactions called activation reactions occur in the conversion of glucose to
fructose -1, 6- diphosphate. They are as follows
hexokinase
 6C6H12O6
glucose – 6- phosphate.
ATP
ADP
phosphoglucoisomerase

Glucose -6 – phosphate

Fructose -6- phosphate
+ ADP
fructose -6- phosphate.
phosphofructokinase
ATP
fructose-1,6- diphosphate
ADP
ii)
The splitting of fructose-1,6-diphosphate into two 3-carbon compounds ( 3phosphoglyceraldehyde and dihydroxyacetonephosphate )
It involves the splitting of fructose- 6 – diphosphate into two 3-carbon compounds.
Aldolase catalyses this reaction and the products formed are interconvertible i.e equilibrium
exists between the two 3-carbon compounds catalyzed by the enzyme phosphotriose isomerase.
Krishna Ram Dhital. (2013) cell no:9841181926
- 44 -
Tree Physiology
aldolase
Fructose- 1,6- diphosphate
3-phosphoglyceraldehyde +
dihydroxyacetone
phosphate.
The 3- phosphoglyceraldehyde is converted to 1, 3 diphosphoglyceric acid. This reaction
is catalyzed by the enzyme phosphoglyceraldehyde dehydrogenase.
Phosphoglyceraldehyde
3- phosphoglyceraldehyde
1, 3 – diphosphoglyceric
acid
Dehydrogenase
The continual conversion of 3- phosphoglyceraldehyde to other intermediates of the
glycolytic pathway causes a shift in the equilibrium between 3- phosphoglyceraldehyde and
dihydroxyacetone phosphate. Thus, with the continuous conversion to other glycolytic
intermediates more dihydroxyacetone phosphate is converted into 3- phosphoglyceraldehyde.
The consumption of pi in the oxidation of 3- phosphoglyceraldehyde is important to
plant because in the next reaction the phosphate attaches itself with ADP to synthesis ATP where
1,3 diphosphoglyceric acid is converted into 3- phosphoglyceric acid in the presence of
phosphoglycerickinase. The production of ATP in glycolysis is called transphosphorylation or
glycolytic substrate phosphorylation.
Phosphoglyceric kinase
1, 3 - diphosphoglyceric acid + ADP
3
phosphoglyceric acid.
The 3-phosphoglyceric acid is transformed to 2- phosphoglyceric acid in the presence of
enzyme phosphoglyceromutase.
phosphoglyceromutase
3-phosphoglyceric acid
2-phosphoglyceric acid
Lastly in the presence of enolase, 2- phosphoglyceric acid results in the formation of
phosphoenolpyruvic acid which is then converted into pyruvic acid in the presence of ADP and
pyruvic kinase.
enolase
2-phosphoglyceric acid
2-phosphoenol pyruvic acid
Mg
++
Pyruvic kinase
2-phosphoenol pyruvic acid
pyruvic acid + ATP
Krishna Ram Dhital. (2013) cell no:9841181926
- 45 -
Tree Physiology
6.3
Krebs cycle
In 1973, H.A. Krebs proposed a cycle about the mechanism of respiration is called Krebs
cycle. It is also called tricarboxylic acid cycle or citric acid cycle or mitochondrial respiration or
oxidation of pyruvate or organic acid cycle etc.
The glycolysis and fermentation are relatively insufficient processes for the release of
energy. However, under aerobic conditions, the pyruvic acid i.e the final product of glycolysis can
undergo decarboxylation and with CoA from Acetyle CoA, which is the connecting link between the
glycolysis and the Krebs cycle. The Krebs cycle or citric acid cycle, so named because of the cyclic
manner in which the starting compound oxaloacetic acid is formed. By the Krebs cycle, pyruvic acid is
oxidized to CO2 and H2O. The complete oxidation of glucose to CO2 and H2O may occur through the
mediation of glycolysis, the Krebs cycle and ETS. By the association of ETS, the Krebs cycle can
produce 24 ATP molecules which is more efficient in the release of energy than either glycolysis or
fermentation. It requires the presence of oxygen and confined to mitochondria.
Glucose
Aconitase
Citric acid (6C)
1
Pyruvic acid
CoA
Cis- Aconitic acid (6C)
- H2O
2
2
citrate synthatase
+ H2O
aconitase
CO2
NADP+
Acetyle CoA
Isocitric acid (6 C )
Mg ++
oxaloacetic acid ( 4C )
Isocitric
acid
NADH+H+
dehydrogenase
2H
8
2H
Malic dehydrogenase
3
NADPH+H+
Oxalosuccinic acid ( 6C )
NAD+
carboxylase
Malic acid ( 4C )
CO2
Fumarase
FADH2
7
Alfa – ketoglutaric acid ( 5C )
NAD +
Succinic dehydrogenase
CoA
2H
6
Alfa- ketaglutaric dehydrogenase
5
CoA
CO2
NADP +
Fumaric acid ( 4 C )
2H
FAD
4
H+
Succinic acid
( 4C )
Succinyl CoA ( 4C )
succinyl thiokinase
Krishna Ram Dhital. (2013) cell no:9841181926
- 46 -
Tree Physiology
Figure: - Metabolites and reactions of the Krebs cycle
Krebs cycle can be explained under the following subheadings
a.
Formation of citric acid
The first reaction of this cycle is the condensation of Acetyle CoA with oxaloacetate to
form citric acid and release CoA. It is the reaction in which 4 carbon dicarboxylic acids is
converted into a six carbon tricarboxylic acid catalyzed by the enzyme citrate synthatase.
Oxaloacetic acid + Acetyle CoA
citric acid + CoA
b.
Regeneration of oxaloacetic acid
Through a series of reactions involving 4 oxidation steps and three molecules of water,
oxaloacetic acid is regenerated from citric acid. In the process, two molecules of CO2 and H
atoms are produced. The reactions which show the regeneration of oxaloacetic acid from citric
acid are shown in the above diagram.
The reversible interconversions of the first three acids of Krebs cycle i.e. citric acid, cisaconitic acid, and isocitric acids are catalyzed by the same enzyme, aconitase. The first reaction
involves a hydration of citric acid to form cis- aconitic acid and the second reaction is for the
dehydration of cis- aconitic acid to yield isocitric acid.
The first oxidation step of the Krebs cycle is that isocitric acid is converted into
oxalosuccinic acid in the presence of isocitric acid dehydrogenase and NADP+. The next
reaction of the Krebs cycle involves the decarboxylation of oxalosuccinic acid to form alfaketoglutaric acid in the presence of the enzyme carboxylase. The alfa- ketoglutaric acid is the
key compound in the metabolism, which plays an important role in the synthesis of amino acids.
The alft- ketoglutaric acid is oxidized to form succinyl CoA in the presence of alfaketoglutaric dehydrogenase and in the presence of succinyl thiokinase, the succinyl CoA is
converted into succinic acid. The succinic acid is oxidized to form fumaric acid in the presence
of succinic dehydrogenase and it represents the third oxidation step of the Krebs cycle. The
fumaric acid is hydrated in the presence of fumarase to yield malic acid. The malic acid is
converted into oxaloacetic acid in the presence of malic dehydrogenase and it is the fourth
oxidation step of the Krebs cycle. Thus, the regeneration of oxaloacetic acid completes the cycle.
6.4
Fermentation
It is an anaerobic respiration in which glucose is only partially oxidized. It is the major energy
yielding process of a variety of micro- organisms called anaerobes.
The overall reaction for fermentation is
C6H12O6
2CH3 – CH2OH + 2CO2
It means one molecule of glucose is converted to two molecules of ethanol and two molecules of
carbon dioxide. It is the sequential series of reaction that occurs in the absence of oxygen.
There are two common types of fermentation.
i ) Alcoholic fermentation.
In the alcoholic fermentation, pyruvic acid is converted into ethanol and CO2. It involves
two steps.
Pyruvate decarboxylase
a) Pyruvic acid
acetaldehyde + CO2
Alcohol dehydrogenase
b) Acetaldehyde +
NADH2
Ethanol (CH3 CH2OH )
Krishna Ram Dhital. (2013) cell no:9841181926
- 47 -
Tree Physiology
ii ) Lactic acid fermentation
In the lactic acid fermentation, pyruvic acid is metabolized to lactic acid in the presence
of enzyme lactic acid dehydrogenase and NADH2.
Lactic acid dehydrogenase
Pyruvic acid + NADH +
H+
Lactic acid + NAD
This type of breakdown isn’t much familiar in higher plants but is very common in
animal tissues. There is very little difference between the fermentation and glycolysis because most of
the intermediate reactions are found in both pathways. In glycolysis, glucose is converted to pyruvate
during the process of fermentation. But in fermentation, the process goes one step further and converts
pyruvate to ethanol and CO2. The best known of the fermenting organisms are the Yeasts.
6.5
Electron transport chain ( ets chain or respiratory chain or
oxidative phosphorylation )
The hydrated co-enzymes NADH + H+ and FADH2 produced in different reactions of
respiration store most of the free energy of glucose released during glycolysis and Krebs cycle. These
NADH2 and FADH2 molecules are oxidized by oxygen in oxidative phosphorylation releasing energy.
The free energy released during oxidation from a single molecule of NADH2 and FADH2 is utilized to
synthesis several molecules of ATP from ADP and Pi. The electrons are transferred through a series of
electron carrier is called ETS chain. The electron is lastly transferred to oxygen molecule and the
oxygen molecule is reduced to H2O. The ETS chains are found in the inner membrane of mitochondrion.
For aerobic organisms, it is essential that the enzymes of the Krebs cycle are associated
with ETS chain. The ETS chain consists of a sequential series of cytochrome enzymes capable of
passing electrons from one to another. Electrons taken up by hydrogen acceptors i. e NADP, NAD, FAD
is the oxidation steps of respiration are ultimately transferred to ETS where they are passed "down hill
“a chain of cytochrome enzymes. Most important to the living cell is the fact that with each step in this
system, the energy level of the electron is lowered, the energy difference being transformed into
phosphate bond energy by the conversion of ADP to ATP.
ATP
AH2
NAD+
ATP
FAD
UQ
2cyt b
Fe3+
ATP
2cyt.c
Fe3+
2 cyt a
Fe3+
2 Cyt a3
Fe3+
½
O
2
A
FADH2
NADH2
ADP+iP
UQH2
2 cyt b
Fe2+
2cyt.c
Fe 3+
333+
2 Cyt
a Fe2+
ADP+iP
2 Cyt.
a3 Fe2+
ADP+iP
2H+
Figure – Electron transport chain
A condensation of the complete oxidation of glucose to CO2 and H2O will show that
there is a net gain of 38 ATP.
Krishna Ram Dhital. (2013) cell no:9841181926
- 48 -
O2 -
H2O
Tree Physiology
6.6
Pentose phosphate shunt
It is also called hexose monophosphate shunt or direct oxidation pathway or oxidative
pentose phosphate pathway (PPP) or phosphogluconate pathway or HMP pathway.
The major pathway for the aerobic respiration of glucose is through glycolysis and Krebs cycle
but there exists an alternative pathway in many organisms. This pathway requiring the presence of
oxygen is called pentose phosphate shunt. In this shunt, glucose molecule after its phosphorylation is
directly oxidized to 6- phosphogluconic acid and then to Ribulose -5- phosphate. Electrons released in
between these two steps reduce NADP to NADPH2, each molecule of which after passing through ETS
system produces 3 molecules of ATP. The remaining carbon skeleton is thus processed through to
produce hexose sugar again. A summary representation of the pathway is
Glucose-6-P isomerase
6 Glucose -6 –phosphate + 12 NADP + 6 H2O
6CO2
5 Fructose-6-phosphate +
+
+12H+
12NADPH
+H3PO4
Glucose
ATP
ADP
(Dihydroxyacetone-P) Glucose-6-P
acid(Gluconate-6-P)
7
6
9
Fructose-1, 6-dip
8
NADP+
NADPH2
6- Phosphogluconic
Glucose-6-P dehydrogenase
6-phosphogluconic acid
dehydrogenase
NADP+
NADPH2
CO2
2
epimerase
Xylulose-5-P
Ribulose – 5- P
+
transketolase
phosphoriboisomerase
Transketolase
3
transaldolase sedoheptulose-7-P
Ribose-5-P
+ 3- phosphoglyceraldehyde
4
5
4
Glyceraldehyde +Fructose-6-P
-3-P
Fructose-6-P +
Erythrose- 4-P
1
+
Figure: - Pentose phosphate cycle
Some of the Ribulose-5-phosphate is isomerized to Ribose-5-phosphate in the presence
of enzyme phosphoriboisomerase and some of the Ribulase-5-phosphate catalyzed by the enzyme
epimerase produces Xylulose -5-phosphate. In the presence of the enzyme transketolase, Xylulose -5phosphate and Ribose-5- phosphate react to produce sedoheptulase-7-phosphate and 3phosphoglyceraldehyde and they both are reduced into Erythrose-4-phosphate and Fructose-6phosphate. At the same time, Xylulose-5-phosphate and Erythrose-4-phosphate combine to produce
another molecule of Fructose-6-phosphate and Glyceraldehyde-3-P under the influence of the same
enzyme transketolase. Thus the cycle is completed with the production of Fructose-6-phosphate as
shown in the above figure.
Krishna Ram Dhital. (2013) cell no:9841181926
- 49 -
Tree Physiology
#
Factors affecting the rate of respiration
1.
External factors
i) Temperature
The chemical reactions of respiration are sensitive to temperature changes as with all
chemical reactions. At 0º celcious, the rate of respiration becomes very low. As the temperature
rises, the rate of respiration also rises. A maximum rate is between 35º celcious to 45º celcious.
The optimum temperature of respiration is near about 30º c.
ii) Light
With the increase in light, the rate of respiration increases. It is because photosynthesis
occurs in light and more respiratory material is available and light also increases. The
temperature affecting the opening of stomata due to which exchange of gases is facilitated which
increases the rate of respiration.
iii) Oxygen concentration of atmosphere
The presence and absence of oxygen determine the type of respiration i.e. either aerobic
or anaerobic. The presence of O2 is necessary for Krebs cycle acceptor of electrons in the ETS.
Although its presence in atmosphere in large quantity, it doesn’t affect the rate too much. When
O2 content is reduced to 1%, the rate of respiration reaches its minimum.
iv) Carbon dioxide concentration
The concentration of CO2 has an inverse relationship with the rate of respiration. The
CO2 content of the atmosphere doesn’t affect the rate of respiration but if the concentration
increases the rate of respiration falls down and that is why seeds fail to germinate.
v) Water
The shortage of water reduces the rate of respiration because it maintains the turgidity of
water of the cells and provides the medium for many respiratory and enzymatic reactions.
vi) Injury
Injury increases the rate of respiration. When a portion of a plant is injured the
conversion of starch into sugar increases in that portion.
vii) Effect of certain chemical substances
Many chemicals such as carbon monoxide, cyanides, azides etc and some anesthetics
such as chloroform, either formaldehyde, etc. if provided in very small quantities these
substances increases the rate of respiration initially. If the concentration of such chemicals is
increased, there is a fall in the rate of respiration.
viii) Mechanical effects
The mechanical stimulation such as rubbing the leaves or bending the leaves by land
increases the rate of respiration.
2.
Internal factors
i) Protoplasmic factors
The rate of respiration depends on the quantity and quality of protoplasm present in the
cell. Younger cell have more active protoplasm which respire more rapidly than the older cells
having lesser quantity of protoplasm.
ii) Concentration of respiratory material
The rate of respiration depends much on the presence of respiratory material e.g. the rate
of respiration increases after the photosynthesis, respiratory rate is higher.
#
Some important questions

Explain the process of glycolysis and give the balance sheet of ATP.

Explain Krebs cycle with the help of a diagram.

Explain various steps of ATP production during aerobic respiration.

Write short notes on
a) Glycolysis
b) ETS
c) Emp – Pathway
Krishna Ram Dhital. (2013) cell no:9841181926
- 50 -
Tree Physiology
NITROGEN METABOLISMS
7.1 Concept of N2 fixation (physical and biological)
7.2 Metabolism of Nitrite and nitrate ions
7.3 Transamination reaction
UNIT – 7
Nitrogen metabolism
Nitrogen is an important element for plant life. Leaves consist of about 1 – 15 % nitrogen
of their dry weight.
Nitrogen metabolism is one of the important aspects of the plant life on which animal life
also depends directly or indirectly. It is as the series of biochemical changes taking place inside or
outside the plant body which results in the construction of complex nitrogenous food from its simpler
derivatives and the destruction of complex nitrogenous food into its components. That’s why it includes
both anabolic (constructive) e.g. nitrogen fixation, amino–acid synthesis and protein synthesis and,
catabolic (destruction) process e.g. proteolysis, de-nitrification and nitrification, etc. The both processes
of nitrogen metabolism occurring in nature, a continuous supply of nitrogen to plants and other living
organism is maintained.
The complete story of Nitrogen in relation to plant life involves a whole series of events,
some of which occur in the cells of micro-organisms of the soil and some in the tissues of the higher
plants.The series of reactions or events of nitrogen metabolism make a sort of cycle and a supply of
nitrogen from atmosphere and its return back to atmosphere is called “nitrogen cycle”.
Plants
animals
Excretion urea
Photosynthesis
Nitrogen fixing
bacteria and
algae
Bacteria
Bacteria and
fungi of decay
nitrate
electrification and photochemical
fixation, etc.
organic residues,
N2 in air
amino – acids
nitrate bacteria
denitrifying bacteria
ammonia
nitrate
nitrite bacteria
volcanic action
Figure: Nitrogen cycle
7.1
concept of Nitrogen fixation
It is the phenomenon of conversion of free nitrogen into nitrogenous salts to make it
available for absorption by plants. The fertility of the soil increases due to some bacteria found in root
nodules of leguminous crops and soil and these bacteria are capable of fixing atmosphere nitrogen.
Nitrogen fixation may be
i.
Physical Nitrogen fixation
ii.
Biological Nitrogen fixation
Krishna Ram Dhital. (2013) cell no:9841181926
- 51 -
Tree Physiology
7.1.1 Physical nitrogen fixation
It starts with the combination of atmospheric nitrogen with oxygen under the influence of
electric discharge and thunder to produce nitric oxide which is then oxidized to nitrogen peroxide in
the presence of oxygen.
electric discharge
N2
+
O2
Nitric oxide (2NO)
Thunder
oxidation
N2
+
O2
Nitrogen peroxide (2NO2)
The nitrogen peroxide combines with rain water to form nitrous acid and nitric acid
which come ground along with rains.
2NO2 + rainwater
HNO2 +
HNO3
The alkali radicals of the soil react with nitric acid (HNO3) to produce nitrites which are
soluble in water and can be absorbed by the roots of plants.
Ca or K salts + HNO3
Ca or K nitrites
7.1.2 Biological nitrogen fixation
It may be two types:
a)
Symbiotic N2 fixation
This type of nitrogen fixation is carried out by bacteria i.e. Rhizobium found in the root
nodules of leguminous plants. It has two types: fast growers and slow growers. Rhizobia are
found in the soil of crop fields where bacteria and plants both affect each other’s growth.
Nodule formation being a complicated process is initiated by the infection of root hair by
free living Rhizobia which can’t digest cellulose and enter the root hair from the tip region
having cellulose absent. The process of nodule formation is very peculiar. The nitrogen fixation
involves (either symbiotic or asymbiotic) the reduction of atmosphere nitrogen to ammonia
(NH3) by the enzyme nitrogenase. Nitrogenase is made up of two protein components i.e. Mo –
Fe protein and Fe – protein. Nitrogenase is extremely sensitive to oxygen.
b)
Asymbiotic N2 fixation
Many free living bacteria i.e. Azotobacter chroococcum, A. gilis, chromatium, etc. and
blue green algae i.e. synechococcus, oscillatoria erythraea, calothrix, etc. are capable to fix
atmospheric nitrogen. This type of nitrogen fixation where ATP is involved is called asymbiotic
nitrogen fixation.
#
The origin of Nitrogenous compounds in the soil
Plants absorb nitrogen from the inorganic forms either as nitrate or as ammonium. Nitrate
is first reduced to ammonium level in order to be absorbed by the plants. Nitrogen occurs in the
soil as inorganic i.e. nitrates and nitrites of Ca, K, etc. and organic i.e. proteins. Oxidation of
ammonia to nitrate in the soil may occur through the meditation of two groups of bacteria
nitrosomonas and nitrobacter. The energy needed for the growth of these organisms is obtained
through the oxidation of ammonia or of nitrate. They both are auto-tropic bacteria requiring only
inorganic materials for growth carried out by a wide variety of soil organisms.
Nitrogen compounds are continuously being lost from the soil by leaching action of rains
and by the removal of the plant cover through fire or other agencies. Large quantity of Nitrogen
are lost every year from cultivated soils as constituents of crops harvested there-from. However,
the soils show no equivalent depletion of their N2 supply. Since there is no N2 in the rocks from
which the soils are derived. It is obvious that the supply of Nitrogenous compounds in the soil
must be replenished by the activities of certain soil organisms i.e. the N2 fixing bacteria.
Krishna Ram Dhital. (2013) cell no:9841181926
- 52 -
Tree Physiology
Various processes of N2 fixation and other phenomenon influencing the soil N2 supply are as
follows:
a)
N2 fixatoion
Two groups of bacteria are able to fix atmospheric Nitrogen in organic compounds.
i) Certain saprophytic bacteria obtained their energy from dead organic matter in the
soil and
ii) Symbiotic N2 fixing bacteria which live in the roots of leguminous plants.
Most of the N2 fixation by saprophytic bacteria is brought about by two groups of
organisms a) The Azotobactor (aerobic bacteria) and b) Clostridium (anaerobic bacteria). They
both are common in well aerated soils. The aerobic forms occur around the surface of the soil
particals where the anaerobic forms are found in regions of the soil in which oxygen content has
been depleted by respiration. These bacteria combine the gaseous N2 of the air with carbohydrate
compounds obtained from the soil. The Azotobactor is usually absent from soils more acid than
PH 6 but Clostridium can tolerate soil acidities as great as PH 5. They both can operate effectively
in dry soils.
Symbiotic N2 fixation is the result of the activities of species Rhizobium being rodshaped bacteria that enter the roots of legumes by the way of root hairs and cause the formation
of nodules on the roots. Each species of Rhizobium infects the roots of only certain leguminous
species. These baeteria live inside the nodules and there synthesize organic N2 compounds from
the carbohydrates of the host and gaseous N2 of the air. For example, Soybean plants are
estimated to utilize about 90 % of the N2 fixed by the bacteria in the nodules on their roots.
Symbiotic N2 fixation is depressed if inorganic N2 compounds are present in abundance in the
soil around the roots of the legume plant
b)
Denitrification
It is the process in which the conversion of nitrates and nitrites into ammonia, nitrous
oxide and nitrogen is involved. A large number of organisms are capable of reducing nitrates to
nitrites and ammonia. This occurs commonly in the tissues of higher plants. Certain soil
organisms can reduce nitrates to molecular nitrogen and these organisms are called denitrifying
bacteria (Bacterium denitrificans etc). It is effective when an abundant supply of carbohydrates
is present in the soil. It doesn’t normally occur in well cultivated soil. In denitrification, through
a series of reaction the nitrates are reduced to ammonia and free nitrogen released back to
atmosphere, completing nature’s complex nitrogen cycle. This process is necessay to supply
ammonia for the metabolic process and to maintain the balance of nitrogen in the atmosphere.
c)
Ammonification
A large number of organic compounds contained in the soil is decomposed to release
ammonia which is then oxidized to produce nitrates under the influence of certain microorganisms, the process of release ammonia is known as ammonification. In the process of decay,
the complex nitrogenous compounds present in dead plant and animal tissues are broken down
into a number of simpler forms, most of the N2 being released in the form of Ammonia.
Ammonification isn’t the result of the activities of a single group of bacteria but also be brought
by a large number of different microorganisms (Actinomyces and filamentous fungi). The
bacteria called ammonifying bacteria e.g. Bacillus vulgaris, Bacillus ramosus, etc. are
responsible for the release of ammonia. They are also called saprophytic bacteria.
The formation of ammonia is influenced by
i.
Available carbohydrate supply
ii.
Chemical composition of the nitrogenous materials.
iii.
The organism involved.
iv.
The acidity, aeration, moisture content of the soil.
These ammonifying bacteria utilize carbohydrates as a source of energy.
d)
Nitrification
The ammonia formed in the decomposition of proteins and other organic nitrogenous
compounds may be acted upon by the nitrifying bacteria and transformed in two steps
Krishna Ram Dhital. (2013) cell no:9841181926
- 53 -
Tree Physiology
Amonia is first oxidized to nitrites by Nitrosomnas and nitrosococcus (bacteria).
nitromonas
2NH3 + 3O2
2HNO2 (Nitrites)
And nitrosococcus
Nitrites are then oxidized to nitrates by nitrobactor
Nitrobactor
2HNO2 + O2
2HNO3 (nitric acid)
+
2H2O
and
The conversion of ammonia to nitrite and then to nitrate is called nitrification. or the
process of oxidation of ammonia to nitrates is called nitrification. They both types of bacteria
involved are chemosynthetic in their metabolism and use the energy obtained from the oxidation
of ammonia or nitrites in the synthesis of carbohydrates from carbon dioxide and water. The soil
conditions favoring for Nitrification are

PH value on the alkaline side of neutrality.

The absence of large amounts of carbohydrates in the soil.

Good aeration.

Sufficient moisture.
Ammonification and nitrification are found to be maximum when the soil temperatures
between 30ºC to 35ºC, in alkaline soils, soils with lesser carbohydrate and sufficient moisture
and aeration.
e)
Rain as a source of n2 compounds
Small amounts of inorganic N2 compounds reach the soil from the atmosphere. Oxides
are formed during electrical storms and these are brought into the soil by the rain. Ammonia also
escapes from various sources into the atmosphere and may be returned to the soil in solution in
raindrops.
sulphates
Carbohydrates
Plant
proteins
Amino
acids
Animal
proteins
N2 fixation
Redn
by green
plants
death & decay
denitrification
Free N2
of atm
nitrates
Plant and animal
resources
electrical fixation
Nitrites
NH3
ammonification
nitrification
Figure – A typical Nitrogen cycle
Krishna Ram Dhital. (2013) cell no:9841181926
- 54 -
Tree Physiology
7.2
Metabolism of Nitrite and nitrate ions (Nitrogen assimilation)
The atmospheric nitrogen is fixed in the soil in the form of nitrates which are observed by
the plants and converted into ammonia through a series of steps catalyzed by various enzymes.
NO3
Nitrate
Ammonia
NO2
Nitrite
HNO
Hyponitrite
NH2OH
Hydroxylamine
NH3
FAD – Flavine Adenine Dinuleotide
7.2.1 Nitrate reduction
Nitrate reductase is a metalloflavoprotein that catalyzes the reduction of nitrate to nitrite
seen in a highly purified from soybean leaves and neurospora. The enzyme system includes a reduced
pyridine nucleotide (NADPH or NADP) as an electron donor FAD as a prosthetic group and
molybdenum as an activator. Electrons are passed from reduced pyridine nucleotide to FAD, giving
reduced FAD (FADH2). The electrons are in turn passed from FADH2 to oxidized Mo, resulting in
reduced Mo, which in turn passes electrons to nitrate reducing it to nitrite.
NADH+ H+
NADPH + H+
FAD
REDUCED Mo + 2H+
NO3¯
NAD+
FADH2
OXIDIZED Mo
NO2¯
+ H2O
NADP+
Figure: The sequence of electron transport in nitrate reduction catalyzed by
nitrate reductase
It is one reason that in Mo deficient soils plants accumulate nitrogen.
7.2.2 Nitrite reduction
Nitrite reductase, which is also a metalloflavoprotein, catalyzes the reduction of nitrite to
ammonia. Compounds intermediate between nitrite and ammonia are not free but are thought to be
bound to nitrate reductase. Reduced ferredoxin or reduced pyridine nucleotide act as electron donors to
nitrite reductase and ATP is necessary for this activity.
2HNO3
+
2H2O
2NH3
+
3O2
This process is found more rapidly in the presence of light.
7.2.3 Reduction of hydroxylamine
An enzyme from neurospora and higher plants called hydroxylamine reductase which
requires manganese for its activity.
Mn
+
NH2OH + NADH + H +
NH3 + NAD + H2O
The ammonia combines with organic acid to produce amino acids.
7.3
Transamination reaction
The most important reaction in amino acid synthesis, which involves the transfer of an
amino group of an amino acid is called transmination.
The enzymes which catalyze transamination reactions are called transaminases. The
enzyme catalyzing the transfer of an amino group of glutamic acid (substrate) to the carbonyle group of
oxalo-acetate to form asparatate (product) is called glutamic – aspartic transaminase.
The transamination reactions also involve the participation of pyridoxal phosphate of
pyridoxamise phosphate as a co-enzyme. The pyridoxal phosphate accepts an amino group from amino
acid to form pyridoxamine phosphate releasing the corresponding keto acid product. The pyridoxamine
Krishna Ram Dhital. (2013) cell no:9841181926
- 55 -
Tree Physiology
phosphate then passes the amino group to another keto group forming a next amino acid and
regenerating pyridoxal phosphate. The reaction is as follows:
Glutamate
aspartate
pyridoxal phosphate enzyme complex
α- ketoglutarate
pyridoxamine enzyme complex
oxalo-
acetate
Before leaving the synthesis of amino acids we should first discuss the amides
asparagines and glutamine. The compounds asparagines and glutamine have been found in high
quantities in many plants and appear to function in the transport and storage of nitrogen. In the synthesis
of glutamine the hydroxyl group of one of the carboxyle groups of glutamic acid is replaced by an NH2
group. The reaction is catalyzed by the enzyme glutamine synthetase activated by metal cofactor Mg 2+.
ATP is also required.
Glutamine synthetase
Glutamate + ATP + NH3 +
glutamine + ADP + iP
Mg2+
The synthesis of asparagines from aspartate takes place in the same manner requiring a
metal activator and ATP. This reaction is catalyzed by asparagines synthetase.
#


Some important questions
Describe biological nitrogen fixation and the mechanism of NO3¯ reduction in plants.
Write short notes on
i.
Nitrate reduction
ii.
Physical nitrogen fixation
iii.
Nitrogen fixation
iv. Biological nitrogen fixation
Krishna Ram Dhital. (2013) cell no:9841181926
- 56 -
UNIT – 8
Tree Physiology
PLANT GROWTH AND DEVELOPMENT
8.1 Concept of growth and development
8.2 Photoperiodism
8.3 Vernalization
8.4 Properties and roles of phytochromes
8.5 The florigen concept
8.1
Concept of growth and development
The two processes growth and development are different. Growth indicates increase in
bulk, size and weight, and development includes differentiation, flowering, pollination and fertilization
leading to reproduction. These two processes can precede independent of each other.
8.2
Photoperodism (periodicity of light)
It is a kind of photo-biological process in which the response of a plant to the relative
lengths of light and dark periods is studied. In other words, it is the phenomenon in which the influence
of day length on plants is studied.
The duration of the dark period is much more important than the duration of the light
period. Intensity and quantity of the light can be modifying features in the magnitude of the response or
play an important role. The total quality of light received can have an influencing effect. Any response
by a plant to the duration and order of sequence of light and dark periods is called a photoperiodic
response.
Plants respond to alterations of light and dark periods in a variety of ways. Flowering,
vegetative, growth, inter-node elongation, seed germination are the examples of photoperiodic responses
discovered in plants. Thus, the photoperiod (light period) affects various plant metabolisms including
the vegetative growth and the reproductive activities of plants, the process called photoperiodism.
According to need of photoperiod, plants can be classified into
i.
ii.
iii.
Short day plant (SDP) or long night plants
Long day plant (LDP) or short night plants
Day neutral plant (DNP) or intermediate plants
8.2.1 Short day plant
It is such a plant which flowers only under short day conditions. For flowering, short day
plant requires the day length which is less than a certain critical length. It is also called long night
plant. If the dark period is less than a critical length, flowering in short day plants won’t occur. These
plants are also not capable of flowering if short dark and short light are provided alternatively. Day
length in excess of this critical point will keep the short day plant vegetative.
According to Hillman (1959), the short day plants are able for flowering even if kept
continuously in the dark but provided with sucrose. It means light period is required only for short day
plants for photosynthesis. Flowering can be induced in short day plants during long days by increasing
the dark period. Some examples if short day plants are soybean, Maryland mammoth, potato,
sugarcane, aster, dahlia, cosmos, strawberry, etc.
8.2.2 Long day plant
It is such a plant which requires photoperiod of more than a critical length, it may vary
from 14 to 18 hours. The best flowering of long day plants usually occurs in continuous light. They
require either no dark period or a very short dark period for flowering. A flash of light given to long
day plants during long dark periods can induce flowering in them even during short day periods. A
long day plant requiring 16 hours of light period in 24 hours can be made to flower if it is provided
with a cycle of 8 hours of light period and 4 hours of dark period. Long day plants are also called short
night plants because flowering in them is inhibited not because of the short light periods but because
of the too long dark periods. Some examples are radish, sugar beet, wheat, oats, spinach, etc.
Krishna Ram Dhital. (2013) cell no:9841181926
- 57 -
Tree Physiology
8.2.3 Day neutral plant
Day neutral plants flower after a period of vegetative growth, regardless of the
photoperiod. Their flowering is not affected by the length of the day. They can flower in any periods.
Some examples are tomato, cucumbers, cotton, pea, sunflower, maize, etc.
8.3
Venalization (transformation of winter forms into spring)
All plants won’t flower when kept to the correct photoperiod. In many plants,
temperature has a great influence on the initiation and development of reproductive organs. The
influence of temperature on flowering in case of annual plants is secondary to that of light; the effect
may be more metabolic than catalytic.
Vernalization is defined as the method of inducing early flowering in plants by
pretreatment of their seeds at very low temperatures. According to Chouard (1960), it is the”
acquisition or acceleration of the ability to flower by chilling treatment”. In many plants especially
the biennials and perennials, the flowering is stimulated by exposure of whole plant or plant parts to low
temperature called vernalization which is coined first time in 1920 by Russian scientist Lysenko.
It is used only for the cold induced processes that promote the flowering in plants. For
many plants, it is an absolute requirement for flowering. The effective temperature for vernalization lies
some degree above freezing to 15ºC depending on plant species. Many winter annuals and biennials
require vernalization. Winter annuals are normally vernalized as seedlings and biennials are vernalized
after the first season of growth.
The practical utility of vernalization is
i.
Crops can be produced earlier i.e. a crop can be harvested much earlier than the control
crop.
ii.
Crops can be grown in the regions where they don’t naturally reproduce and
iii.
Plant breeding work can be accepted.
#
Theory of Vernalization
The certain stages are to be crossed for the development of a plant before it can actually
start maturity. In the process, the completion of one stage is absolutely necessary for the
commencement of the next one. Each stage requires suitable temperature humidity, light,
aeration, etc. Lack of any suitable conditions, further development of plant is checked.
They are as follows:
I)
Vernalization stage or Lysenko stage
This is the first stage which is quite necessary for the development of plant. Conditions
are
a)
Temperature
Low temperature ranging from 0ºC to 20ºC is required for this stage.
b)
Moisture
Like temperature, availability of moisture is also important for the completion
of next stage.
c)
Aeration
Like temperature and moisture, proper aeration is very important.
d)
Time
It varies with environment conditions and depends on the nature of plant itself.
Response of Vernalization also decreases if the period of Vernalization is
interrupted by period of heat treatments.
II)
Photo stage
When the Vernalization stage is complete, the plant passes through the next stage called
photo-stage. It is related with the effect of the relative length of day and night on producing
flowers.
Krishna Ram Dhital. (2013) cell no:9841181926
- 58 -
Tree Physiology
III)
Third stage
It is quite necessary for seed formation and it is associated with gametogenesis. The
photoperiod requirement for the commencement of this stage is a little shorter than required for
the completion of photo-stage.
#
Vernalization process
For Vernalization the seeds are allowed to germinate for some time and then are given
cold treatments by keeping them at 0ºC to 5ºC. The period of cold treatment varies from few
days to many weeks according to species. After the cold treatment, seedlings are allowed to dry
for sometime and then sown. They should not be sown immediately after the cold treatment.
The drying period also should be a very long.
The response to Vernalization also depends on the duration for which and the
temperature at which the seed is subjected to Vernalization. In some plants, vernalization can be
affected only after some vegetative growth has taken place.
Photoperiodic effect which not only prepares the plant to flower but also initiates
flowering, vernalization merely prepares the plants for the flowering. Not only the seeds but
isolated embryos also be vernilized.
8.4
Properties and roles of phytochromes
In plants, there is a kind of blue green pigment which absorbs light i.e. either red
light (660 nm) or far red light (730nm) effective in causing photomorphogenesis (controlling the
appearance or development of plants) and its absorbing region is closely linked with protein, such
a pigment is called phytochrome.
The red light and far red light absorbing forms are named as Pr and Pfr respectively. The
Pr form changes to Pfr in red light and Pfr changes to Pr by the application of far red light. Many
physiological processes in plants are controlled by phytochrome. It also controls the flowering of plants.
It shows relatively weak absorption of blue rays. The Pfr suppresses the flower formation in SDP
whereas Pfr stimulate the flower formation in LDP and how Pfr affects the flowering of LDP is still not
known well. It differs from chlorophyll in its absorption spectrum. For example, phytochrome extracted
from the shoots of dark grown maize seedlings was a blue green chromoprotein and chromophore.
Pr (P660)
Absorbance
.
600 nm
Pfr (P730)
730 nm
400
500
600
700
800 (wavelength- nm)
Figure – absorption spectra of the two forms of phytochrome Pr and Pfr
Krishna Ram Dhital. (2013) cell no:9841181926
- 59 -
Tree Physiology
#
I.
Two forms of phytochrome or properties of phytochrome
The Pr form
It is an inactive form which doesn’t show phytochrome mediated response and absorbs
red light and gets converted into active form Pfr is called Pr form.
The Pfr form
It is an active form which shows phytochrome mediated response and absorbs far red
light and gets converted into Pr form in the darkness slowly.
II.
absorbs
sunlight
red light
changes to
Pr
Pfr
Changes quickly
changes slowly
far red light
total dark
Exposed to
Figure: – Phytochrome concept
The two forms are photochemically interconvertible. Phytochrome is a conjugated
protein consisting of a water soluble protein and a chromophone
#
Roles or activities of phytochrome
Phytochrome plays important roles to mediate several processes which are discussed
below
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Flowering
Phytochrome pigments are playing a deciding role in flowering. Pfr is retained for
a longer time under long day photoperiods and stimulates flowering of LDP and
suppresses flowering of SDP. Under snort photoperiods, Pr is retained for a
longer time which stimulates flowering of SDP and inhibits the flowering of LPD.
DNA and protein synthesis
Red light induces DNA and protein synthesis studied in pea stems.
Chloroplast development
Red light induces the chloroplast development process and synthesis of
photosynthetic enzymes.
Water uptake
The uptake of different substances e.g. water, auxin, acetate etc is regulated by
the role of red light of phytochrome.
Seed germination and dormancy
Phytochrome inducds seed germination.
Pollen germination
Red light has found accelerating pollen tube emergence and elongation.
Nitrate reductase
Phytochrome has found regulating the enzyme activity which is used un the
conversion of Nitrate into Nitrite.
Peroxidase
Peroxidase activity was controlled by phytochrome.
Krishna Ram Dhital. (2013) cell no:9841181926
- 60 -
Tree Physiology
ix.
Other roles

Elongation of leaf, stem and petiole.

Development of roots,rhizome and bulb

Differentiation of stomata and trachery elements.

Participating in respiration.

Fat degradation.

Photoperiodism.

Degradation of protein.
8.5
The florigen concept
Florigen is a kind of hormone produced in leaf for inducing flower formation. The effect of
photoperiod doesn’t lie in the flowering apex but it is always seen in young expanded leaf. A plant
without leaf is insensitive to photoperiod. Plants having leaf kept in correct photoperiod induces the
plant to flower. The light induction is received by leaf but the response is seen in apical meristem of
shoot buds. The response of plant to photoperiodic induction is transmitted from leaf to bud. It is
possible only when a kind of flowering hormone is produced in leaf during inductive cycle, such a
concept is called florigen concept.
The response of hormone i.e. flowering is seen when the hormone is translocated from leaf to
meristem. If the translocation of florigen from one branch of a plant to other branches applies, the whole
plant flowers eventhough only one branch is kept at right photoperiod.
#
Some important questions

Describe the properties and roles of phytochromes.

Write short notes on
a) Vernalization.
b) Photoperiodism.
c) Phytochromes.
d) Differentiate between LDP and SDP.
e) Properties of phytochromes.
Krishna Ram Dhital. (2013) cell no:9841181926
- 61 -
Tree Physiology
UNIT– 9
9.1
SEED PHYSIOLOGY
9.1 physiology of seed germination
9.2 seed dormancy and viability
9.3 methods of breaking seed dormancy
Physiology of seed germination
The resumption of active growth on the part of the embryo resulting in the breaking of the
seed coats and the emergence of the young plant is called seed germination.
The seeds of many plants will germinate as soon as ripe if environmental conditions are suitable.
The initial step in germination is the imbibitions of water by the various tissues within the seed. This
generally results in an increase in its volume which causes the permeability of oxygen and carbon
dioxide. The swelling of the seed often breaks the seed coat but in some species, this doesn’t occur until
the emergence of primary root.
Enzymes become activated with an increase in the hydration of the cells. Stored foods either in
the endosperm or in cotyledons are digested and the soluble products of the digestion process are
translocated towards the growing points of the embryo. After digestion, a large portion of the fats
present are converted into soluble carbohydrates which are again converted into starch or other
carbohydrates digested during the process. These compounds aren’t consumed in respiration but are
utilized in the synthesis of the organic nitrogenous compound i.e. amino acid of the growing embryo.
The actual physiology of seed germination are concerned with two principle groups of seeds
which are as follows
1)
Seeds in which the cotyledons emerge
Germination is initiated by a marked swelling of the seed breaking the seed coat. This is
followed by the emergence of the primary root developing from the lower end of the hypocotyle
and is the first structure of the embryo to make contact with the external environment. The
primary root grows downward in the soil producing lateral roots and root hairs. The hypocotyle
then elongates rapidly pulling the cotyledons upward out of the soil into the air and the plumule
comes out. The plumule then begins active growth giving rise to the stem and foliage leaves of
the seedling. The food used during germination is derived from the thick cotyledons. E.g. Lima
bean.
Figure: - Different stages in the germination of a seed of Lima bean.
Krishna Ram Dhital. (2013) cell no:9841181926
- 62 -
Tree Physiology
Seeds in which the cotyledons don’t emerge.
The seed of the pea is structurally very similar to that of the bean but its germination
behavior is very different. Elongation of the hypocotyle doesn’t occur and the cotyledon remains
in the seed. The primary root elongates early in the process of germination much as in the bean.
The plumule is elevated through the soil by rapid elongation of the epicotyl which is the first
internode.
Many monocotyledons also show this type of germination behavior e.g. red oak.
2)
Figure: - Different stages in the germination
#
Environmental conditions necessary for germination
The seeds of all species of plants require external conditions before germination occur.
They are

Water
A low water content is one of the prominent characteristics of the dormant seeds of most
plant species. The physiological processes of living cells occur largely in an aqueous medium
and germination can’t occur unless the seed can absorb water from its environment. The
absorption of water initiates a series of physical and chemical processes which result in the
emergence of the embryo from the seed. Soil moisture contents need not be high for germination
to occur. The amount of water absorbed during germination by the seeds of different species
varies within wide limits.

A suitable temperature
The seeds of any species will germinate within a certain range of temperatures, but at
temperatures above or below this range no germination will occur. As a rule, the seeds of
temperature plants require relatively low temperatures as compared to tropical or sub-tropical
species. The optimum temperature required for seed germination shows variation from species to
species. For example, the range of temperatures for germination of seeds of maize lies between
5ºC to 45ºC.
Krishna Ram Dhital. (2013) cell no:9841181926
- 63 -
Tree Physiology
Oxygen
The oxygen requirement of different seeds varies considerably but most seeds germinate
at a oxygen level which is lower than the O2 level present in the atmosphere. However the
requirement of O2 in germination phase of seeds is higher than the growing plants due to seed
coat, a permeability barrier. Most seeds require an active aerobic oxidation system for
germination and that is why fully submerged seeds show a relatively lower percentage of
germination.

Light
Many kinds of seeds germinate better when exposed the light than when kept in total
darkness. Some epiphytes have seeds which fail to germinate unless exposed to the light. The
germination of the seeds of some species e.g. onion and Lilly, etc. appears to be retarded by
exposure to light. The germination of the seeds of some species of grass is influence by light.

9.2
Seed dormancy and viability
Many kinds of seeds fail to germinate even if placed under favorable conditions i.e. all
environmental conditions for germination. In such seeds, resumption of growth by the embryo is
arrested or inhibited (checked) by condition within the seeds themselves. Such seeds are called dormant
seeds. The state of inhibited growth of dormant seeds as a result of internal causes is called seed
dormancy. In other words, the phenomenon shown by dormant seeds is called seed dormancy.
Dormancy seed is caused by various factors which are as follows: i.
Seed coats impermeable to water.
ii.
Mechanically resistant seed coats or physically resistant to embryo expansion.
iii.
Seed coats impermeable to oxygen.
iv.
Rudimentary embryos or immature embryo.
v.
Dormant embryos.
vi.
Germination inhibitors (specific light requirements and specific temperature
requirements)
Seed dormancy is broken by various ways depending on the cause of dormancy. The
dormancy caused by hard and impermeable seed coat is remained by the process scarification
(mechanical or chemical means). Immature embryos in seed grow during storage to maturity, the
process is known as “ after ripening”. Such seeds germinate only after certain period of storage. Some
seeds need some period of low temperature under moist condition for their germination, such cold
treatment is known as stratification or chilling. Naturally such seeds germinate only after the cold
winter.
Seed viability (longevity of seeds)
It is the life span of seeds and it varies from a few weeks to many years depending upon
the species, storage conditions and environmental to which the seeds are subjected. The life span of the
seeds can be increased by keeping them under suitable storage conditions.
Some of the seeds of a number of species of wild plants will remain viable for 50 years or
more. It is specially found in hard coated species. As a general rule, only seeds with a pronounced
dormancy remain viable very many years in nature. The seeds of many weed species are long-lived as
compared with the seeds of most crop plants.
#
Method of prolonging seed viability
Seeds of many species rapidly loose their capacity to germinate if stored in an
unfavorable environment. The critical factors in determining the viable period of most seeds are
moisture and temperature, although the oxygen and carbon dioxide content of the surrounding
atmosphere are also of importance. In general, seeds remain viable for longer periods of time at low
temperatures ( 5ºC ) than at room temperature and low relative humidity are usually more favorable for
prolonging the life of seeds the high ones.
Krishna Ram Dhital. (2013) cell no:9841181926
- 64 -
Tree Physiology
The fluctuations of temperature or relative humidity in storage rooms aren’t conductive to
prolonging the viability of seeds. The variation in temperature and humidity shorten the seed viability
significantly. Longevity of many seeds seems correlated with respiration rates of the embryo and low
temperatures, and low seed moisture content are essential to maintain seed viability for long periods of
time.
There are internal factors responsible for the influence the longevity of seeds in storage i.e. the
stage of maturity, the moisture content of the seed at the time of storage and genetic factors etc in
determining the viability of the seeds.
9.3
Methods of breaking seed dormancy
Methods have been devised either the dormancy the many kinds of seeds can be broken or the
length of the dormant period in many other kinds can be shortened. However the methods employed for
the breaking of dormancy vary depending upon its cause. Methods which can be used for breaking the
dormancy of one species may be totally ineffective when used with seeds of another species and
sometimes may even prolong dormancy.
Some methods are as followsi.
Scarification
It is the process by which the dormancy caused by hard and impermeable seed coat is
removed mechanically or chemically. Strong mineral acids are used to interrupt seed dormancy caused
by resistant or impermeable seed coats. It is essential that any method used to interrupt seed coat
dormancy shouldn’t be injurious to the embryo. Under natural conditions, dormancy of such seeds is
broken by the slow decay of the seed coats by the action of alternate freezing and thawing (warmth of
weather)
ii.
Stratification (low temperature)
Some seeds need some period of low temperature under moist condition for their
germination. Such cold treatment is called stratification. Many seeds germination occur rapidly when
they are stratified in moist peat at low temperatures than at higher temperatures. Temperature between
5ºC and 10ºC for two or three months are effective with conifer seeds. Low temperature combined with
moisture have been found to reduce the period of “After ripening” in seeds of many species.
Alternating temperature
It is another method of interrupting the dormancy of the seeds by alternating relatively
low or high temperatures. The temperature extremes of such treatments may not differ by more than
10ºC or 20ºC and both are above the freezing point. The dormancy of some seeds may be interrupted by
alternate freezing and thawing but it is harmful to other species. This type of treatment is used with
seeds in which dormancy is inherent in the embryo.
iv.
Light
Light is the means of breaking dormancy of certain species of seeds which need light for
the germination. Light improves germination at low temperatures.
v.
Pressures
When the pressure was applied for some periods in seeds of some species, the
germination of the seeds was increased considerably. The effect of the pressure persists after the seeds
have been dried and stored and changes in the permeability of the seed coats to water.
vi.
Growth regulators
Growth regulators are used in the development of roots on cuttings and to increase the number of roots.
These treatments have suggested the possibility of interrupting the dormancy of seeds or improving their
germination by similar means.
#
Some important questions

Physiology of seed germination

Seed germination

Method of breaking seed dormancy
iii.
Krishna Ram Dhital. (2013) cell no:9841181926
- 65 -
Tree Physiology
1.2
principles of plant physiology
The principal of plant physiology are remarkably uniform and apply equally throughout the wide
range of plant form. “The plant physiology is the study of how plant functions, very broadly speaking it
is the study of the life processes involved in the plant growth, development, and plant behaviors. It
includes the internal mechanism by which plant carries on its many complex synthetic chemical
processes and the ways in which these processes are integrated.
Minerals and water taken up by the roots are transported and made available to the leaves, while
simultaneously the photosynthetic product of the leaves is moved downward to the roots.
Transportation of materials within the plant is another important aspect of plant physiology.
Plant metabolism: the synthesis and inter-conversion of complex molecules from the simple
raw materials of plant nutrition is also a part of plant physiology.
The important events of development of plant such as seed germination, flower formation.,
fruit production, seed formation and their mechanism and plant growth and its interaction by the
chemical messenger substance make up the another important part of study of plant physiology.
Plant in relation to the environment such as climatic influence as temperature, light and rainfall
together with factor of soil determines not only the nature of agricultural product but also in large
vegetation type.
It also studies the plant and water relationship.
Reference books:
Plant physiology (S.N. Pandey and B.K. Sinha)
Plant physiology (R. M. Devlin)
Plant physiology (Meyer Anderson)
Plant physiology (Tribikram Bhattarai)
“Best of luck”
Krishna Ram Dhital. (2013) cell no:9841181926
- 66 -
Tree Physiology
Krishna Ram Dhital. (2013) cell no:9841181926
- 67 -