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PLANT WATER RELATIONS
Transpiration is the process of water loss from the stomata of plants. Plants vary
in their transpiration rates. Individual crop plants such as corn and sunflower
can transpire from 0.5 gallon to more than 1 gallon (2 to 5 liters) per day. Large
trees can transpire more than 50 gallons (200 liters) per day. Transpiration has
several important functions including cooling of the plant, movement of nutrients
within the plant, and uptake of mineral nutrients. Transpiration is driven by the
sun’s energy and a moisture-concentration gradient. Water moves along a
gradient from the roots, where there is the most water (or a high concentration) to
the leaves, where there is the least water (or a low concentration). Transpirational
pull creates suction that pulls available water from the soil. Root hairs that
develop behind the meristematic region of a root tip absorb water from the soil.
Water molecules and any dissolved minerals pass through the epidermal cell
membrane of the root hair. As it moves from the epidermal cells to the xylem,
water passes through the root cortex, endodermis, and pericycle. Water travels
through the xylem tissue to the veins of the leaves and finally into the mesophyll.
The plant will close its stomata and wilt if the available soil moisture is
inadequate to support transpiration. Transpiration is affected by soil moisture
content, high air temperatures, low water concentration in the air, and air
movement. Therefore, on hot, dry, windy days with low relative humidity, we can
expect plants to use more water. Factors that affect transpiration include the
following:
■ The number of stomata a plant has and their size can influence transpiration.
■ The process of transpiration is dependent on the presence of soil moisture.
Therefore, climate and local weather that provides water and agronomic factors
(e.g., weed control, crop residues, or fallowing) that minimize water loss
enhance transpiration.
■ Water evaporation within the leaf and transpiration are nearly doubled for every
10° rise in air temperatures within an air temperature range of 50–86°F (10–
30°C).
■ Air-moisture content (or humidity) influences water loss because water
molecule change from liquid to gas is dependent on the concentration of
water molecules in the air. A greater concentration of water molecules in the
air suppresses the escape of additional water molecules to the air.
■ Wind or air movement pulls water vapor away from the leaf surface and
increases the transpiration pull.
Transpiration facilitates the absorption of many essential minerals that are
soluble in water. Minerals are moved throughout the plant within the water
stream.
Evapotranspiration
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Evapotranspiration (ET) is the plant’s total water use. As the name suggests, ET
includes the actual water loss in transpiration as well as water that is evaporated
from the surface of the leaves or from the soil surrounding the plant. A plant’s
total water use varies greatly. The ET ratio (weight of water required to produce
the weight of the crop’s dry matter) depends on several factors.
The dominant process in water relations of the whole plant is the absorption of
large quantities of water from the soil, its translocation through the plant, and its
eventual loss to the surrounding atmosphere as water vapor. Of all the water
absorbed by plants, less than 5 percent is actually retained for growth and even
less is used biochemically. The balance passes through the plant to be lost as
water vapor, a phenomenon known as transpiration. Nowhere is transpiration
more evident than in crop plants, where several hundred kilograms of water may
be required to produce each kilogram of dry matter and excessive transpiration
can lead to significant reductions in productivity. A single, 14.5m open-grown
silver maple tree may lose as much as 225 liters of water per hour. Were it not for
transpiration, for example, a single rainfall might well provide sufficient water to
grow a crop. As it is, the failure of plants to grow because of water deficits
produced by transpiration is a principal cause of economic loss and crop failure
across the world. Thus, on both theoretical and practical grounds, transpiration
is without doubt a process of considerable importance.
Transpiration is defined as the loss of water from the plant in the form of water
vapor. Although a small amount of water vapor may be lost through small
openings (called lenticels) in the bark of young twigs and branches, the largest
proportion by far (more than 90%) escapes from leaves. Indeed, the process of
transpiration is strongly tied to leaf anatomy. The outer surfaces of a typical
vascular plant leaf are covered with a multilayered waxy deposit called the
cuticle. The principal component of the cuticle is cutin, a heterogeneous polymer
of long-chain—typically 16 or 18 carbons—hydroxylated fatty acids. Because
cuticular waxes are very hydrophobic, they offer extremely high resistance to
diffusion of both liquid water and water vapor from the underlying cells.
The cuticle thus serves to restrict evaporation of water directly from the outer
surfaces of leaf epidermal cells and protects both the epidermal and underlying
mesophyll cells from potentially lethal desiccation. The integrity of the epidermis
and the overlying cuticle is occasionally interrupted by small pores called
stomata (sing. stoma). Each pore is surrounded by a pair of specialized cells,
called guard cells. These guard cells function as hydraulically operated valves
that control the size of the pore. The interior of the leaf is comprised of
photosynthetic mesophyll cells. The somewhat loose arrangement of mesophyll
cells in most leaves creates an interconnected system of intercellular air spaces.
This system of air spaces may be quite extensive, accounting for up to 70 percent
of the total leaf volume in some cases. Stomata are located such that, when open,
they provide a route for the exchange of gases (principally carbon dioxide, oxygen,
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and water vapor) between the internal air space and the bulk atmosphere
surrounding the leaf. Because of this relationship, this space is referred to as
substomatal space.
The cuticle is generally impermeable to water and open stomata provide the
primary route for escape of water vapor from the plant. Transpiration may be
considered a two-stage process: (1) the evaporation of water from the moist cell
walls into the substomatal air space and (2) the diffusion of water vapor from the
substomatal space into the atmosphere. It is commonly assumed that
evaporation occurs primarily at the surfaces of those mesophyll cells that border
the substomatal air spaces. However, several investigators have proposed a more
restricted view, suggesting instead that most of the water evaporates from the
inner surfaces of epidermal cells in the immediate vicinity of the stomata Known
as peristomal evaporation, this view is based on numerous reports indicating
the presence of cuticle layers on mesophyll cell walls. In addition, mathematical
modeling of diffusion in substomatal cavities has predicted that as much as 75
percent of all evaporation occurs in the immediate vicinity of the stomata. The
diffusion of water vapor from the substomatal space into the atmosphere is
relatively straightforward.
Once the water vapor has left the cell surfaces, it diffuses through the
substomatal space and exits the leaf through the stomatal pore. Diffusion of
water vapor through the stomatal pores, known as stomatal transpiration,
accounts for 90 to 95 percent of the water loss from leaves. The remaining 5 to 10
percent is accounted for by cuticular transpiration. Although the cuticle is
composed of waxes and other hydrophobic substances and is generally
impermeable to water, small quantities of water vapor can pass through. The
contribution of cuticular transpiration to leaf water loss varies considerably
between species. It is to some extent dependent on the thickness of the cuticle.
Thicker cuticles are characteristic of plants growing in full sun or dry habitats,
while cuticles are generally thinner on the leaves of plants growing in shaded or
moist habitats. Cuticular transpiration may become more significant, particularly
for leaves with thin cuticles, under dry conditions when stomatal transpiration is
prevented by closure of the stomata.
DIFFERENCES IN VAPOR PRESSURE
According to Fick’s law of diffusion, molecules will diffuse from a region of high
concentration to a region of low concentration, or, down a concentration gradient.
Because vapor pressure is proportional to vapor concentration, water vapor will
also diffuse down a vapor pressure gradient, that is, from a region of high vapor
pressure to a region of lower vapor pressure. In principle, we can assume that the
substomatal air space of a leaf is normally saturated or very nearly saturated
with water vapor. This is because the mesophyll cells that border the air space
present a large, exposed surface area for evaporation of water. On the other hand,
the atmosphere that surrounds the leaf is usually unsaturated and may often
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have very low water content. These circumstances create a gradient between the
high water vapor pressure in the interior of the leaf and the lower water vapor
pressure of the external atmosphere. This difference in water vapor pressure
between the internal air spaces of the leaf and the surrounding air is the driving
force for transpiration.
TRANSPIRATION IS INFLUENCED BY ENVIRONMENTAL FACTORS
The rate of transpiration will naturally be influenced by factors such as humidity
and temperature, and wind speed, which influence the rate of water vapor
diffusion between the substomatal air chamber and the ambient atmosphere.
Fick’s law of diffusion tells us that the rate of diffusion is proportional to the
difference in concentration of the diffusing substance. It therefore follows that the
rate of transpiration will be governed in large measure by the magnitude of the
vapor pressure difference between the leaf and the surrounding air.
EFFECTS OF HUMIDITY
Humidity is the actual water content of air, which, as noted earlier, may be
expressed either as vapor density (g m−3) or vapor pressure (kPa). In practice,
however, it is more useful to express water content as the relative humidity
(RH). Relative humidity is the ratio of the actual water content of air to the
maximum amount of water that can be held by air at that temperature.
Expressed another way, relative humidity is the ratio of the actual vapor pressure
to the saturation vapor pressure. Relative humidity is most commonly expressed
asRH×100, or percent relative humidity. Air at 50 percent RH by definition
contains one-half the amount of water possible at saturation. Its vapor pressure
is therefore one-half the saturation vapor pressure. Note also that a 10◦C rise in
temperature nearly doubles the saturation vapor pressure. Relative humidity and
temperature also have a significant effect on the water potential of air. As
indicated earlier, the vapor pressure of the substomatal leaf spaces is probably
close to saturation most of the time.
Even in a rapidly transpiring leaf the relative humidity would probably be greater
than 95 percent and the resulting water potential would be close to zero. Under
these conditions, the vapor pressure in the substomatal space will be the
saturation vapor pressure at the leaf temperature. The vapor pressure of
atmospheric air, on the other hand, depends on both the relative humidity of the
air and its temperature. Humidity and temperature thus have the potential to
modify the magnitude of the vapor pressure gradient, which, in turn, will
influence the rate of transpiration.
WHAT IS THE EFFECTS OF TEMPERATURE?
Temperature modulates transpiration rate through its effect on vapor pressure,
which in turn affects the vapor pressure gradient. As long as the stomata remain
open and a vapor pressure gradient exists between the leaf and the atmosphere,
water vapor will diffuse out of the leaf. This means transpiration may occur even
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when the relative humidity of the atmosphere is 100 percent. This is often the
case in tropical jungles where leaf temperature and, consequently, saturation
vapor pressure is higher than the surrounding atmosphere. Because the
atmosphere is already saturated, the water vapor condenses upon exiting the
leaf, thereby giving substance to the popular image of the steaming jungle.
ADHESION AND SURFACE TENSION
If a glass capillary tube is inserted into a volume of water, water will rise in the
tube to some level above the surface of the surrounding bulk water. This
phenomenon is called capillary rise, or simply capillarity. Capillary rise is due
to the interaction of several forces. These include adhesion between water and
polar groups along the capillary wall, surface tension (due to cohesive forces
between water molecules), and the force of gravity acting on the water column.
Adhesive forces attract water molecules to polar groups along the surface of the
tube. When these water-to-wall forces are strong, as they are between water and
glass tubes or the inner surfaces of tracheary elements, the walls are said to be
wettable. As water flows upward along the wall, strong cohesive forces between
the water molecules act to pull the bulk water up the lumen of the tube. This will
continue until these lifting forces are balanced by the downward force of gravity
acting on the water column.
ABSORBTION AND TRANSPORT WATER OF WATER BY ROOT
Roots have four important functions. Roots (1) anchor the plant in the soil; (2)
provide a place for storage of carbohydrates and other organic molecules; (3) are
a site of synthesis for important molecules such as alkaloids and to the stem
virtually all the water and minerals taken up by plants. The effectiveness of roots
as absorbing organs is related to the extent of the root system.
RADIAL MOVEMENT OF WATER THROUGH THE ROOT
Once water has been absorbed into the root hairs or epidermal cells, it must
traverse the cortex in order to reach the xylem elements in the central stele. In
principle, the pathway of water through the cortex is relatively straightforward.
There appear to be two options: water may flow either past the cells through the
apoplast of the cortex or from cell to cell through the plasmodesmata of the
symplast. In practice, however, the two pathways are not separate. Apoplastic
water is in constant equilibration with water in the symplast and cell vacuoles.
This means that water is constantly being exchanged across both the cell and
vacuolar membranes. In effect then, water flow through the cortex involves both
pathways. The cortex consists of loosely packed cells with numerous intercellular
spaces. The apoplast would thus appear to offer the least resistance and probably
accounts for a larger proportion of the flow. In the less mature region near the tip
of the root, water will flow directly from the cortex into the developing xylem
elements, meeting relatively little resistance along the way. Moving away from the
tip toward the more mature regions, water will encounter the endodermis.
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Suberization of the endodermal cell walls imposes a permeability barrier, forcing
water taken up in these regions to pass through the cell membranes. While the
endodermis does increase resistance to water flow, it is far from being an
absolute barrier. Indeed, under conditions of rapid transpiration the region of
most rapid water uptake will shift toward the basal part of the root.
NUTRIENT UPTAKE, DEFICIENCY AND SYMPTOMS
THE SOIL AS A NUTRIENT RESERVOIR
Soils vary widely with respect to composition, structure, and nutrient supply.
Especially important from the nutritional perspective are inorganic and organic
soil particles called colloids. Soil colloids retain nutrients for release into the soil
solution where they are available for uptake by roots. Thus, the soil colloids serve
to maintain a reservoir of soluble nutrients in the soil.
COLLOIDS ARE A SIGNIFICANT COMPONENT OF MOST SOILS
It is noted that the mineral component of soils consists predominantly of sand,
silt, and clay, which are differentiated on the basis of particle size. The three
components are easily demonstrated by stirring a small quantity of soil into
water. The larger particles of sand will settle out almost immediately, leaving a
turbid suspension. Over the course of hours or perhaps days, if left undisturbed,
the finer particles of silt will settle slowly to the bottom as well and the turbidity
will in all likelihood disappear. The very small clay particles, however, remain in
stable suspension and will not settle out, at least not within a reasonable time
frame. Clay particles in suspension are not normally visible to the naked eye—
they are simply too small. They are, however, small enough to remain suspended.
These suspended clay particles can be detected by directing a beam of light
through the suspension.
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The suspended clay particles will scatter the light, causing the path traversed by
the light beam to become visible. Particles that are small enough to remain in
suspension but too large to go into true solution are called colloids and the lightscattering phenomenon, known as the Tyndall effect, is a distinguishing
characteristic of colloidal suspensions Clay is not the only soil component that
forms colloidal particles. Many soils also contain a colloidal carbonaceous
residue, called humus. Humus is organic material that has been slowly but
incompletely degraded to a colloidal dimension through the action of weathering
and microorganisms. In a good loam soil, the colloidal humus content may be
substantially greater than the colloidal clay content and make an even greater
contribution to the nutrient reservoir
COLLOIDS PRESENT A LARGE NEGATIVELY CHARGED SURFACE AREA
The function of the colloidal soil fraction as a nutrient reservoir depends on two
factors: (1) colloids present a large specific surface area, and (2) the colloidal
surfaces carry a large number of charges. The charged surfaces in turn reversibly
bind large numbers of ions, especially positively charged cations from the soil
solution. This ability to retain and exchange cations on colloidal surfaces is the
single most important property of soils, in so far as plant nutrition is concerned.
Because of their small size, one of the distinguishing features of colloids is a high
surface area per unit mass, also known as specific surface area.
THE ANION EXCHANGE CAPACITY OF SOIL COLLOIDS IS RELATIVELY LOW
The soil colloids are predominantly negatively charged and, consequently, they do
not tend to attract negatively charged anions. Although some of the clay minerals
do contain cations such as Mg2+, the anion exchange capacity of most soils is
generally low. The result is that anions are not held in the soil but tend to be
readily leached out by percolating ground water. This situation has important
consequences for agricultural practice. Nutrients supplied in the form of anions,
in particular nitrogen (NO− 3 ), must be provided in large quantity to ensure
sufficient uptake by the plants. As a rule, farmers sometimes find they must
apply at least twice—sometimes more—the amount of nitrogen actually required
producing a crop. Unfortunately, much of the excess nitrate is leached into the
ground water and eventually finds its way into wells or into streams a
NUTRIENT UPTAKE BY PLANTS
In order for mineral nutrients to be taken up by a plant, they must enter the root
by crossing the plasma membranes of root cells. From there they can be
transported through the symplast to the interior of the root and eventually find
their way into the rest of the plant. Nutrient uptake by roots is therefore
fundamentally a cellular problem, governed by the rules of membrane transport.
Membrane transport is inherently an abstract subject. That is to say,
investigators measure the kinetics of solute movement across various natural and
artificial membranes under a variety of circumstances. Models are then
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constructed that attempt to explain these kinetic patterns in terms of what is
currently understood about the composition and architecture of membranes. As
our understanding of membrane structure has changed over the years, so have
the models that attempt to interpret how solutes cross these membranes. There
are, however, three fundamental concepts—simple diffusion, facilitated
diffusion, and active transport—that have persevered, largely because they have
proven particularly useful in categorizing and interpreting experimental
observations. These three concepts now make up the basic language of transport
across all membranes of all organisms.
ROLE OF MACRO AND MICRO NUTRIENTS
ESSENTIAL FOR PLANT GROWTH AND DEVELOPMENT
Most plants require a relatively small number of nutrient elements in order to
successfully complete their life cycle. Those that are required are deemed to be
essential nutrient elements. Essentiality is based primarily on two criteria
formulated by E. Epstein in 1972. According to Epstein, an element is considered
essential if (1) in its absence the plant is unable to complete a normal life cycle, or
(2) that element is part of some essential plant constituent or metabolite. By the
first criterion, if a plant is unable to produce viable seed when deprived of that
element, then that element is deemed essential. By the second criterion, an
element such as magnesium would be considered essential because it is a
constituent of the chlorophyll molecule and chlorophyll is essential for
photosynthesis.
Similarly, chlorine is essential because it is a necessary factor in the
photosynthetic oxidation of water. Most elements satisfy both criteria, although
either one alone is usually considered sufficient. The essential elements are
traditionally segregated into two categories: (1) the so-called macronutrients and
(2) the trace elements or micronutrients. The distinction between macro- and
micronutrients simply reflects the relative concentrations found in tissue or
required in nutrient solutions and does not infer importance relative to the
nutritional needs of the plant. The first nine elements are called macronutrients
because they are required in large amounts (in excess of 10 mmole kg−1 of dry
weight). The macronutrients are largely, but not exclusively, involved in the
structure of molecules, which to some extent accounts for the need for large
quantities. The remaining eight essential elements are considered micronutrients.
Micronutrients are required in relatively small quantities (less than 10 mmole
kg−1 of dry weight) and serve catalytic and regulatory roles such as enzyme
activators. Some macronutrients, calcium and magnesium for example, serve as
regulators in addition to their structural role.
DEFICIENCY
To summarize, most plants absorb nitrogen from the soil solution primarily as
inorganic nitrate ion (NO−3) and, in a few cases, as ammonium (NH+4 ) ion. Once
in the plant, NO−3 must be reduced to NH+4 before it can be incorporated into
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amino acids, proteins, and other nitrogenous organic molecules. Nitrogen is most
often limiting in agricultural situations. Many plants, such as maize (Zea mays),
are known as ‘‘heavy feeders’’ and require heavy applications of nitrogen fertilizer.
The manufacture and distribution of nitrogen fertilizers for agriculture are, in
both energy and financial terms, an extremely costly process. Nitrogen is a
constituent of many important molecules, including proteins, nucleic acids,
certain hormones (e.g., indole-3-acetic acid; cytokinin), and chlorophyll.
Nutrient Functions and Deficiency Symptoms
Most overt symptoms of nitrogen deficiency are a slow, stunted growth and a
general chlorosis of the leaves. Nitrogen is very mobile in the plant. As the older
leaves yellow and die, the nitrogen is mobilized, largely in the form of soluble
amines and amides, and exported from the older leaves to the younger, more
rapidly developing leaves. Thus the symptoms of nitrogen deficiency generally
appear first in the older leaves and do not occur in the younger leaves until the
deficiency becomes severe. At this point, the older leaves will turn completely
yellow or brown and fall off the plant. Conditions of nitrogen stress will also lead
to an accumulation of anthocyanin pigments in many species, contributing a
purplish color to the stems, petioles, and the underside of leaves. The precise
cause of anthocyanin accumulation in nitrogen-starved plants is not known. It
may be related to an overproduction of carbon structures that, in the absence of
nitrogen, cannot be utilized to make amino acids and other nitrogen-containing
compounds. Excess nitrogen normally stimulates abundant growth of the shoot
system, favoring a high shoot/root ratio, and will often delay the onset of
flowering in agricultural and horticultural crops. Similarly, a deficiency of
nitrogen reduces shoot growth and stimulates early flowering.
PHOSPHORUS
Phosphorous is available in the soil solution primarily as forms of the polyprotic
phosphoric acid (H3PO4). A polyprotic acid contains more than one proton, each
with a different dissociation constant. Soil pH thus assumes a major role in the
availability of phosphorous. At a soil pH less than 6.8, the predominant form of
phosphorous and the form most readily taken up by roots is the monovalent
orthophosphate anion (H2PO− 4). Between pH 6.8 and pH 7.2, the predominant
form is HPO2− 4, which is less readily absorbed by the roots. In alkaline soils (pH
greater than 7.2), the predominant form is the trivalent PO3−4 , which is
essentially not available for uptake by plants. The actual concentration of soluble
phosphorous in most soils is relatively low—on the order of 1 M—because of
several factors. One factor is the propensity of phosphorous to form insoluble
complexes. At neutral pH, for example, phosphorous tends to form insoluble
complexes with aluminum and iron, while in basic soils calcium and magnesium
complexes will precipitate the phosphorous. Because insoluble phosphates are
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only very slowly released into the soil solution, phosphorous is always limited in
highly calcareous soils uptake by plants. Organic phosphorous must first be
converted to an inorganic form by the action of soil microorganisms, or through
the action of phosphatase enzymes released by the roots, before it is available for
uptake.
In addition, plants must compete with the soil microflora for the small amounts
of phosphorous that are available. For these sorts of reasons, phosphorous,
rather than nitrogen, is most commonly the limiting element in natural
ecosystems. One of the more successful strategies developed by plants for
increasing the uptake of phosphorous is the formation of intimate associations
between roots and soil fungi, called mycorrhiza. In the plant, phosphorous is
found largely as phosphate esters—including the sugar-phosphates, which play
such an important role in photosynthesis and intermediary metabolism. Other
important phosphate esters are the nucleotides that make up DNA and RNA as
well as the phospholipids present in membranes. Phosphorous in the form of
nucleotides such as ATP and ADP, as well as inorganic phosphate (Pi),
phosphorylated sugars, and phosphorylated organic acids also plays an integral
role in the energy metabolism of cells. The most characteristic manifestation of
phosphorous deficiency is an intense green coloration of the leaves. In the
extreme, the leaves may become malformed and exhibit necrotic spots. In some
cases, the blue and purple anthocyanin pigments also accumulate, giving the
leaves a dark greenish-purple color.
Like nitrogen, phosphorous is readily mobilized and redistributed in the plant,
leading to the rapid senescence and death of the older leaves. The stems are
usually shortened and slender and the yield of fruits and seeds is markedly
reduced. An excess of phosphorous has the opposite effect of nitrogen in that it
preferentially stimulates growth of roots over shoots, thus reducing the
shoot/root ratio. Fertilizers with a high phosphorous content, such as bone meal,
are often applied when transplanting perennial plants in order to encourage
establishment of a strong root system.
POTASSIUM
Potassium (K+) is the most abundant cellular cation and so is required in large
amounts by most plants. In agricultural practice, potassium is usually provided
as potash (potassium carbonate, K2CO3). Potassium is frequently deficient in
sandy soils because of its high solubility and the ease with which K+ leaches out
of sandy soils. Potassium is an activator for a number of enzymes, most notably
those involved in photosynthesis and respiration. Starch and protein synthesis
are also affected by potassium deficiency.
PLANTS AND INORGANIC NUTRIENTS
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An important function of inorganic nutrients is in regulating the osmotic
potential of cells. As an osmoregulator, potassium is a principal factor in plant
movements, such as the opening and closure of stomatal guard cells and the
sleep movements, or daily changes in the orientation of leaves. Because it is
highly mobile, potassium also serves to balance the charge of both diffusible and
non diffusible anions. Unlike other macronutrients, potassium is not structurally
bound in the plant, but like nitrogen and phosphorous is highly mobile.
Deficiency symptoms first appear in older leaves, which characteristically develop
mottling or chlorosis, followed by necrotic lesions (spots of dead tissue) at the leaf
margins. In monocotyledonous plants, especially maize and other cereals, the
necrotic lesions begin at the older tips of the leaves and gradually progress along
the margins to the younger cells near the leaf base. Stems are shortened and
weakened and susceptibility to root-rotting fungi is increased. The result is that
potassium-deficient plants are easily lodged.
TOXICITY OF MICRONUTRIENTS
As a group, the micronutrient elements are an excellent example of the dangers of
excess. Most have a rather narrow adequate range and become toxic at relatively
low concentrations. Critical toxicity levels, defined as the tissue concentration
that gives a 10 percent reduction in dry matter, vary widely between the several
micronutrients as well as between plant species. As noted earlier, critical
concentrations for copper, boron, and zinc are on the order of 20, 75, and 200 g
g−1 dry weight, respectively. On the other hand, critical toxicity levels for
manganese vary from 200 g g−1 dry weight for corn, to 600 gg−1 for soybean,
and 5300 gg−1 for sunflower. Toxicity symptoms are often difficult to decipher
because an excess of one nutrient may induce deficiencies of other nutrients. For
example, the classic symptom of manganese toxicity, which often occurs in
waterlogged soils, is the appearance of brown spots due to deposition of MnO2
surrounded by chlorotic veins. But excess manganese may also induce
deficiencies of iron, magnesium and calcium. Manganese competes with both Iron
and magnesium for uptake and with magnesium for binding to enzymes.
Manganese also inhibits calcium translocation into the shoot apex, causing a
disorder known as ‘‘crinkle leaf.’’ Thus the dominant symptoms of manganese
toxicity may actually be the symptoms of iron, magnesium, and/or calcium
deficiency.
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FLOWERS AND SEED DORMANCY
Sexual reproduction in all higher plants begins with the formation of flower buds
(the precise terms for this are flower bud induction and differentiation) . The
process ends with seed maturation and dispersal. Annual plants accomplish this
cycle in 1 year; biennials, in 2 years or growing seasons. Perennials repeat the
cycle year after year once they have reached reproductive maturity. [n most trees
and shrubs, a 2-year cycle is common. In the first year, flower buds form
anywhere from the middle to near the end of the growing season. In the second
year, flowers bloom, and pollination and fertilization occur. The embryo grows
rapidly, and seeds mature by summer or early fall. Most pines follow a 3-year
cycle and some other plants yet another cycle. Flower Structure and Development
As mentioned above, flowers develop from flower buds, and you can distinguish
flower buds from vegetative buds (which produce leaves and stems) by their
appearance and location.
Flower buds are usually larger than vegetative buds and in some species, such as
flowering dogwood, have distinctive shapes. Watching for flower-bud formation in
annual crops is important because it puts you on alert that your plants will be
flowering soon. And locating the flower buds on woody plants helps you confirm
that a plant is sexually mature and also lets you know where you'll be able to find
fruit and seeds to collect later in the season. Flowers come in an amazing variety
of forms and sizes. It's important to recognize what type of flowers a plant has
when you want to save seed from it, especially if you need or want to handpollinate it (we explain more about hand-pollination. There are some basic
structures that are the building blocks of all flowers. The typical flower of an
angiosperm has the following parts.
i Stigma, style, and ovary: the female parts of a flower
• Pistil: the collective term for the stigma, style, and ovary. The word calpel is also
used to describe the stigma, style, and ovary. Some flowers may have multiple
pistils.
• Anther: a sac that contains pollen
• Filament: a stem like structure upon which the anthers are attached
• Stamen: the collective term for the anther and filament; the male sexual organ
of a
Flower: The sexual parts of the flower are usually surrounded by the petals,
collectively called the corolla. Outside the corolla are the sepals (petallike
structures collectively called the calyx. Both the corolla and the calyx are
nonsexual parts of a flower. Flowers are usually borne on a stalk called a pedicel
or peduncle. A flower that contains all these parts is called a complete flower. Any
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type of flower that is lacking one or more of the basic parts is called an
incomplete flower. And as you might have guessed, botanists have come up with
a specific term to describe each variation in flower structure. On the next page,
you'll find some of those descriptive terms.
PERFECT FLOWER
PERFECT: A perfect flower has both male and female parts but may not have all
the nonsexual parts. Such flowers are also called bisexual or hermaphroditic.
Perfect flowers are not necessarily self-fertile; i.e., they may not be able to fertilize
themselves and produce seed. Many plants with perfect flowers are self sterile
and thus require pollen from another plant in order for fertilization to occur.
IMPERFECT: An imperfect flower has only one type of sexual organ, or, if both
types are present, only one is functional. These flowers are also called unisexual.
Seed Dormancy
Seed dormancy normally refers to the failure of viable seeds to germinate under
favorable germination conditions. Presence of dormancy is beneficial to the extent
that it prevents sprouting during open storage or in rains during harvesting. Most
seeds are dormant at the time of harvest. This dormancy is due to improper
development of the embryo, a seed coat impermeable to water and gases, and the
presence of inhibitors. The seed coat acts as a barrier. to water, oxygen uptake,
and radicle emergence. It also contains inhibitors and sometimes carries
pathogens affecting the germination process. Softening of the seed coat occurs
over time in different species due to hydration and dehydration, freezing and
thawing, soil acidity, and attack by microorganisms. Seed scarification, physically
or chemically, softens the seed coat, facilitating easy absorption of moisture and
exchange of gases. Hard seeds are commonly found in species of Leguminosae,
Compositae, and Malvaceae. Legume seed hardness develops on exposure to low
humidity or very dry conditions. In cases of embryo dormancy, seeds fail to
germinate even after removal of the seed coat. Such seeds are commonly found in
temperate fruits and require cold stratification for germination. Most of these
seeds are stored at 3 to 10°C for various periods to eliminate the dormancy.
Embryo dormancy lasts for various periods in different genotypes. Sometimes
seeds possess both a seed coat and innate dormancy. In beetroot seeds,
germination inhibitors present in the seed coat prevent the germination process.
Chemicals, such as hydrogen cyanide, ammonia, and ethylene inhibit
germination. The naturally occurring inhibitors are cyanide, ammonia, alkaloids,
organic acids, essential oils, and phenolic compounds. In enforced dormancy
seeds fail to germinate due to non availability of parameters, such as absence of
light or low temperatures. Such seeds germinate under suitable light or
temperature conditions.
SEED GERMINATION AND FACTORS AFFECTING SEED GERMINATION
13
The resting embryo emerges as a young plant upon germination under favorable
conditions. The embryo absorbs moisture, activates the metabolism, and grows;
the young seedling emerges after rupturing the seed coat. In many cases, fresh
seeds germinate readily, whereas others require after-ripening to eliminate the
dormancy. Certain changes occur during after-ripening that facilitate the
germination process. On imbibition, the cell becomes more turgid and facilitates
the exchange of gases. This activates the enzyme system that breaks the stored
food material and translocates it to the growing point. Normally, the radicle
emerges first by rupturing the seed coat; later, both root and shoot systems
develop, and the young seedling becomes independent and synthesizes its own
food material for growth and development.
Initial Viability
Seed begins to deteriorate on separation from the mother plant. Seed
deterioration is promoted under unfavorable storage conditions, such as high
seed moisture and temperatures, and increases with the length of the storage
period. However, metabolic processes are reduced under low storage temperature,
low seed moisture, and high carbon dioxide contents. Seed viability decreases
under improper storage conditions. The viability loss rate varies in different
species. In certain species seeds are viable for a very short period. Fresh and non
dormant seeds exhibit higher germination, although it subsequently declines with
improper storage.
When a living seed fails to germinate even when provided with the normal
conditions necessary for germination, such a seed is said to be dormant.
i. Causes of seed dormancy
1. Presence of an impermeable testa that may prevent intake of water and
probably oxygen or.
2. The presence of growth inhibitors in the seed.
3. Alternatively, it may be caused by the need for cold treatment or for exposure
to certain photoperiods before the seed can germinate.
4. The embryo is still immature and has not yet reached its full development at
the time of harvest.
5. Very high temperatures during seed maturity may induce dormancy in some
species
ii. Measures of overcoming Seed Dormancy
a. After ripening treatment
One type of primary dormancy is characterized by immature embryos. Although
the seeds are shed by the plant, the embryo must continue to develop before
14
germination will occur. Problems of immature embryos will be overcome if the
seeds receive appropriate after ripening treatment. Often high temperatures are
required for after-ripening of certain palm seed require 38°C to 40°C for three
months.
b. Stratification
Seeds of many plants require moist chilling conditions for a period of time to
render them capable of germination. This process is called stratification.
Chilling is usually 0°C- 10°C for 7 to 180 days. For example, apple seeds require
up to 60 days in moist medium at 3°C to 5°C to overcome dormancy.
c. Scarification
Seeds of some plant species have a very hard covering that may prevent them
from germination unless treated. Hard seed covering can prevent absorption of
water and gaseous exchange or may physically prevent the embryo from growing
and emerging through the seed coat. It is therefore necessary to make these
covering weaker or pervious to water and gases through the process of
scarification.
d. Light
Is essential for seed germination of certain species of lettuce. (positively
photoblastic) If light is required seeds should be sown shallowly. Some species
such as tomato and some lilies are negatively photoblastic (their germination is
inhibited by light) and should be sown more deeply for good germination.
e. Vernalization of seeds Temperatures affect the time of flowering in many
plants. Winter varieties of cereals crops head normally from spring sowing and
behave like spring varieties when the seeds are germinated at temperatures above
freezing point before they are sown. This process is called Vernalization.
iii. Methods of seeds scarifications
a. Seeds can be scarified by soaking them in concentrated sulpuric acid for a
period ranging from few minutes to an hour or more.
b. A hot water soak is another method of scarifying seeds, first heating water to
boiling point add seeds and remove them when the water is cool 12-24 hours
later.
c. Dry heat can be used to rupture seed coats of some species.
d. Mechanical scarifier can be used to scratch the seed coats. When scarified seed
deteriorate rapidly, they should be planted immediately.
15
e. Aging brings about slow natural deterioration of the seed coat in dry storage. In
an experiment with alfalfa, one half of the impermeable seeds germinated after
one and half years while all germinated after eleven years in storage.
f. Alternate freezing and thawing sometimes stimulates germination of hard seeds
of alfalfa and sweet clover.
g. The germination of hard seeds of alfalfa or clover can also be achieved by
exposure for 1 to 1.5 seconds or less to infrared rays of 1180 millimicrons wave
length or by exposure to a few seconds to high frequency electric energy.
Iv Chemicals
Several chemicals, either dissolved in acetone or in water, promote the
germination process. The most commonly used chemicals are hydrogen peroxide,
ethyl alcohol, thiourea, sodium hypochlorite, potassium nitrates, gibberellins,
and cytokinins. Gibberellic acid substitutes for the light requirements and
promotes seed germination.
NITOGEN METABOLISM AND FIXATION
Nitrogen is one of the most important elements for plants and is the most limiting
nutrient in terrestrial environments. Much of the world’s nitrogen is in the
atmosphere, where it is the most abundant gas (see Chapter 10). Symbiotic
nitrogen fixation is a process that makes atmospheric nitrogen available to some
plants. Plants from the legume family (Fabaceae) commonly engage in symbiotic
nitrogen fixation. However, some nonlegume species (mostly trees and shrubs)
and freeliving algae are also able to conduct biological fixation. This mutually
beneficial (symbiotic) partnership occurs between legumes and bacteria
collectively known as rhizobia. The legume plant supplies nutrients and energy
to the bacteria that reside in root nodules. A bacterial enzyme, nitrogenase,
16
converts nitrogen from the soil into ammonia (NH3), which is reduced to
ammonium (NH4) that is used by the legume plant to form amino acids and
protein. This process is costly for the plant, and if nitrogen levels in the soil are
high, plants may reduce N2 fixation levels.
Rhizobia that are present in the soil, or supplied in inocula to the seed, infect
plant root hairs and stimulate development of tumor-like nodules on the roots. A
specific rhizobial species is required for a given legume species. For example,
bacteria infecting and nodulating white clover will not effectively nodulate
soybean. The amount of fixed nitrogen varies depending on the symbiosis (Table
8-4). Some of the fixed nitrogen can be transferred to nonfixing plants grown in
mixtures or may be used by subsequent crops in crop rotations.
Nodule Formation
The following is the process of nodule formation, as shown in Figure 8-7:
1. Root hairs grow. They release root exudates (flavonoids, sugars, amino acids,
and so on), which attract specific rhizobia to the root.
2. The rhizobia attach to the root hair surface.
3. The root hair curls, entrapping the rhizobia.
4. The rhizobia digest the cell wall and form an infection thread into the center
of the root. Within the thread, the bacteria divide and increase in number.
5. The rhizobia induce division of the root cells.
6. A nodule is formed from the protrusion of the root cells to the surface of the
root.
7. Within the individual root cells, the bacteria become devoid of a cell wall and
become bacteroids, which develop the nitrogenase enzyme and fix atmospheric
nitrogen.
Nodule shapes vary and can be the elongated lobes found in roots of alfalfa and
the clovers, or they can be round like those found on birdsfoot trefoil and
soybean (Figure 8-8). Lobed nodules are perennial. They overwinter and fix
nitrogen for more than one growing season, whereas round nodules die and
reform on roots each year. Upon dissection, active regions of nodules will be
observed to contain a pink pigment, leghemoglobin, that is responsible for oxygen
regulation in the nodule. Nitrogen that is fixed by the bacteroids enters the
plant’s vascular system and can be transported throughout the plant
Nitrogen fixation enables the nitrogen in the air to be used for plant growth
In a closed ecological system, the nitrate required for plant growth is derived
from the degradation of the biomass. In contrast to other plant nutrients (e.g.,
phosphate or sulfate), nitrate cannot be delivered by the weathering of rocks.
Smaller amounts of nitrate are generated by lightning and carried into the soil by
rain water (in temperate areas about 5 kg N/ha per year). Due to the effects of
17
civilization (e.g., car traffic, mass animal production, etc.), the amount of nitrate,
other nitrous oxides and ammonia carried into the soil by rain can be in the
range of 15 to 70 kg N/ha per year. Fertilizers are essential for agricultural
production to compensate for the nitrogen that is lost by the withdrawal of
harvest products. For the cultivation of maize, for instance, per year about 200 kg
N/ha have to be added as fertilizers in the form of nitrate or ammonia. Ammonia,
the primary product for the synthesis of nitrate fertilizer, The majority of
cyanobacteria and some bacteria are able to synthesize ammonia from nitrogen in
air. A number of plants live in symbiosis with N2-fixing bacteria, which supply
the plant with organic nitrogen. In return, the plants provide these bacteria with
metabolites for their nutrition. The symbiosis of legumes with nodule-inducing
bacteria (rhizobia) is widespread and important for agriculture.
Legumes, which include soybean, lentil, pea, clover, and lupines, form a large
family (Leguminosae) with about 20,000 species. A very large part of the legumes
have been shown to form a symbiosis with rhizobia. In temperate climates, the
cultivation of legumes can lead to an N2 fixation of 100 to 400 kg N2/ha per year.
Therefore legumes are important as green manure; in crop rotation they are an
inexpensive alternative to artificial fertilizers. The symbiosis of the water fern
Azolla with the cyanobacterium Nostoc supplies rice fields with nitrogen. N2fixing actinomycetes of the genus Frankia form a symbiosis with woody plants
such as the alder or the Australian casuarina. The latter is a pioneer plant on
nitrogen-deficient soils.
Legumes form a symbiosis with nodule-inducing bacteria
Initially it was thought that the nodules of legumes were caused by a plant
disease, until their function in N2 fixation was recognized by H. Hellriegel and H.
Wilfarth in 1888. They found that beans containing these nodules were able to
grow without nitrogen fertilizer. The nodule-inducing bacteria include, among
other genera, the genera Rhizobium, Bradyrhizobium, and Azorhizobium and are
collectively called rhizobia. The rhizobia are strictly aerobic gram-negative rods,
which live in the soil and grow heterotrophically in the presence of organic
compounds. Some species (Bradyrhizobium) are also able to grow autotrophically
in the presence of H2, although at a low growth rate. The uptake of rhizobia into
the host plant is a controlled infection. Nitrogen fixation enables the nitrogen
in the air to be used for plant growth which are part of signal transduction chain.
In this way the “key” induces the root hair of the host to curl and the root cortex
cells to divide, forming the nodule primordium.
After the root hair has been invaded by the rhizobia, an infection thread forms,
which extends into the cortex of the roots, forms branches there, and infects the
cells of the nodule primordium. A nodule thus develops from the infection
thread. The morphogenesis of the nodule is of similarly high complexity to that of
any other plant organ such as the root or shoot. The nodules are connected with
18
the root via vascular tissues, which supply them with substrates formed by
photosynthesis. The bacteria, which have been incorporated into the plant cell,
are enclosed by a peribacteroid membrane (also called a symbiosome
membrane), which is formed by the plant. The incorporated bacteria are thus
separated from the cytoplasm of the host cell in a so-called symbiosome. In the
symbiosome, the rhizobia differentiate to bacteroids. The volume of these
bacteroids can be 10 times the volume of
individual bacteria. Several of these bacteroids are surrounded by a peribacteroid
membrane. Controlled infection of a host cell by rhizobia is induced by an
interaction with the root hairs. The rhizobia induce the formation of an infection
thread, which is formed by invagination of the root hair cell wall and protrudes
into the cells of the root cortex. In this way the rhizobia invaginate the host cell
where they are separated by a peribacteroid membrane from the cytosol of the
host cells. The rhizobia grow and differentiate into large bacteroids. Plants
improve their nutrition by symbiosis with fungi, plant growth is limited by the
supply of nutrients other than nitrate (e.g., phosphate). Because of its low
solubility, the extraction of phosphate by the roots from the soil requires very
efficient uptake systems. For this reason, plant roots possess very high affinity
transporters, with a half saturation of 1 to 5 mM phosphate, where the
phosphate transport is drive Nitrogen fixation enables the nitrogen in the air to
be used for plant growth by proton symport, similar to the transport of nitrate. In
order to increase the uptake of phosphate, but also of other mineral nutrients
(e.g., nitrate and potassium), most plants enter a symbiosis with fungi. Fungi are
able to form a mycelium with hyphae, which have a much lower diameter than
root hairs and which are therefore well suited to penetrate soil particles and to
mobilize their nutrients. The symbiotic fungi (microsymbionts) deliver these
nutrients to the plant root (macrosymbiont) and are in turn supplied by the plant
with substrates for maintaining their metabolism.
The supply of the symbiotic fungi by the roots demands a high amount of
assimilates. For this reason, many plants make the establishment of the
mycorrhiza dependent on the phosphate availability in the soil. In the case of a
high phosphate concentration in the soil, when the plant can do without, it treats
the fungus as a pathogen and activates its defense system against fungal
infections. The arbuscular mycorrhiza is widespread. The arbuscular mycorrhiza
has been detected in more than 80% of all plant species. In this symbiosis the
fungus penetrates the cortex of plant roots and forms there a network of hyphae,
which protrude into cortical cells and form there treelike invaginations, which are
termed arbuscules or form hyphal coils. The boundary membranes of fungus
and host remain intact.
19
ENZYMES
Most enzymes are proteins; some (ribozymes) are nucleic acids (RNA). Enzymes
have enormous catalytic power; they greatly enhance the rate at which specific
chemical reactions take place. The metabolism of a plant is organized and
controlled specifically at the points where catalysis takes place. Therefore, one of
the most important foundations in all of biology is that life is a series of chemical
processes that enzymes regulate.
An enzyme (Gr. enzymos, leavened) is a biological catalyst that can accelerate a
specific chemical reaction by lowering the required activation energy but is
unaltered in the process. In other words, the same reaction would have occurred
to the same degree in the absence of the catalyst, but it would have progressed at
a much slower rate. Because it is unaltered, the enzyme can be used over and
over.
An enzyme is extremely selective for the reaction it will catalyze. The reactants of
enzymatic reactions are called substrates. The precise “fit” between an enzyme
and its specific substrate is crucial to cell metabolism. For example, cellular
concentrations of many reactants must be kept at low levels to avoid undesirable
side reactions. At the same time, concentrations must remain high enough for the
required reactions to occur at a rate compatible with life. Metabolism proceeds
under these seemingly conflicting conditions because enzymes channel molecules
into and through specific chemical pathways. With the exception of the several
digestive enzymes that were the first to be discovered (e.g., pepsin and trypsin),
all enzyme names end with the suffix ase and are named after their substrate.
ENZYME STRUCTURE
Enzymes are three-dimensional globular protein molecules or nucleic acids with
at least one surface region having a crevice or pocket. This crevice occupies only a
small portion of the enzyme’s surface and is known as the enzyme’s active site.
This site is shaped so that a substrate molecule (or several substrate molecules,
depending on the reaction) fits into it in a very specific way and is held in place
by weak chemical forces, such as hydrogen bonds. Binding of the substrate to the
enzyme changes the enzyme’s shape, a phenomenon called induced fit. Induced
fit is like a clasping handshake. The active site’s embrace of the substrate brings
chemical groups of the active site into positions that enhance their ability to work
on the substrate and to catalyze the chemical reaction. When the reaction is
complete, the product (new compound) of the catalyzed reaction is released, and
the enzyme resumes its initial conformation (shape), ready to catalyze another
chemical reaction.
ENZYME FUNCTION
20
When a substrate molecule binds to an enzyme’s active site, an enzymesubstrate complex (ES) forms. This is the essential first step in enzyme catalysis
and can be summarized as follows:
Enzyme + substrate
Eenzyme- substrate complex
Eproducts +enzyme
Once the unstable, high-energy ES forms, amino acid side groups of the enzyme
are placed against certain bonds of the substrate. These side groups stress or
distort the substrate bonds, lowering the activation energy needed to break the
bonds. The bonds break, releasing the substrate, which now reacts to produce
the final product and release the enzyme.
FACTORS AFFECTING ENZYME ACTIVITY
Any condition that alters the three-dimensional shape of an enzyme also affects
the enzyme’s activity. Two factors that affect enzyme activity are temperature and
pH. The shape of a protein or nucleic acid is determined largely by its hydrogen
bonds. Temperature changes easily disrupt hydrogen bonds. For example, most
higher vertebrates, such as birds and mammals, have enzymes
that function best within a relatively narrow temperature range (between 35 and
40° C). Below 35° C, the bonds that determine protein shape are not flexible
enough to permit the shape change necessary for substrate to fit into a reactive
site. Above 40° C, the bonds are too weak to hold the protein in
proper position and to maintain its shape. When proper shape is lost, the enzyme
is, in essence, destroyed; this loss of shape is called denaturation.
Most enzymes also have a pH optimum, usually between 6 and 8. For example,
when the pH is too low, the H_ ions combine with the R groups of the enzyme’s
amino acids, reducing their ability to bind with substrates. Acidic environments
can also denature enzymes not adapted to such conditions.
Some enzymes, however, function at a low pH. For example, pepsin (the enzyme
found in the stomach of mammals) has an optimal pH of approximately 2. Pepsin
functions at such a low pH because it has an amino acid sequence that
maintains the ionic and hydrogen bonds, even in the presence of large numbers
of H_ ions (low pH). Conversely, trypsin is active in the more basic medium (pH 9)
found in the small intestine of mammals.
COFACTORS AND COENZYMES
Cofactors are metal ions, such as Ca2_, Mg2_, Mn2_, Cu2_, and Zn2_. Many
enzymes must use these metal ions to change a nonfunctioning active site to a
functioning one. In these enzymes, the attachment of a cofactor changes the
shape of the protein and allows it to combine with its substrate. The cofactors of
other enzymes participate in the temporary bonds between the enzyme and its
substrate when the enzyme-substrate complex forms.
Coenzymes are nonprotein, organic molecules that participate in enzymecatalyzed reactions, often by transporting electrons, in the form of hydrogen
21
atoms, from one enzyme to another. Many vitamins (e.g., niacin and riboflavin)
function as coenzymes or are used to make coenzymes. Just as a taxi transports
people around a city, so coenzymes transport energy, in the form of hydrogen
atoms, from one enzyme to another One of the most important coenzymes in the
cell is the hydrogen acceptor nicotinamide adenine dinucleotide (NAD_), which
is made from a B vitamin. When NAD_ acquires a hydrogen atom from an
enzyme, it reduces to NADH. The electron of the hydrogen atom contains energy
that the NADH molecule then carries. For example, when various foods are
oxidized in the cell, the cell strips electrons from the food molecules and transfers
them to NAD_, which reduces to NAD
22
PHYSIOLOGICAL BASIS OF YIELD
Introduction
Plant production is driven by photosynthesis. Key elements in the system are
(i)
(ii)
(iii)
(iv)
the interception of photosynthetically active radiation (PAR, 400-700 nm
spectral band)
The use of that energy in the reduction of CO2 and other substrates
(photosynthesis),
Incorporation of assimilates into new plant structures (biosynthesis and
growth), and
Maintenance of plant as living unit.
Achieving high yield is conceptually simply to maximize the extent and duration
of radiation interception; use captured energy in efficient photosynthesis;
partition new assimilates in ways that provide optimal proportions of leaf, stem,
root, and reproductive structures; and maintain those at minimum cost.
Crop yield comprises only a portion of biomass that accumulates over a crop
cycle. Effective root and canopy systems (including stem structure for foliage
display), for example, generally must be established before onset of reproductive
effort. In addition, cost of maintenance increases as vegetative biomass
accumulates during the season. Because crops are at the mercy of spatial and
temporal variations in weather, plant spacing, and supplies of water and
nutrients, and in occurrence of pests and disease, flexibility in morphogenesis
and acclimation of physiological systems is a key requirement for achieving high
and stable performance. Whether biological efficiency of these processes has, or
might be, improved through breeding are important questions.
Biomass and Other Morphological Traits
Just as the impact of Green Revolution can be attributed mostly to improved
partitioning of products of photosynthesis to grain yield, progress in yield is
strongly associated with improved Harvest Index. Morphological traits associated
with increased yield potential include grain number and HI Even if HI could be
raised to 60% from its current maximum value (50%), it implies that yields could
only be increased by a further 20% using HI as a selection criterion, unless total
crop biomass is also raised. Furthermore, improved partitioning by
greater reduction in plant height is unlikely since research suggests that optimal
plant heights have already been achieved. Some studies have shown
increased biomass to be associated with yield increases.
23
Photosynthesis and Related Traits
By definition, improved yield cannot be attributed to better overall radiation use
efficiency (RUE) in cases where total biomass has not been improved. (RUE in a
crop context represents ratio of total energy present in crop's biomass to that of
solar energy incident on crop across its growth cycle. Expression of higher
photosynthetic rate in absence of significant changes in biomass could be a
pleiotropic effect of improved partitioning to yield driven by high demand for
assimilates during grain filling. Canopy temperature depression is a
direct function of evapotranspiration rate, which itself is determined largely by
stomatal conductance. These traits could also be pleiotropic effects of genetic
variability among lines for a number of physiological and metabolic processes
including sink strength, photosynthetic rate, vascular capacity, and
hormonal signals.
Improvement in Nutrient Use Efficiency
Genetic gains in Nutrient Use Efficiency (NUE), defined as grain yield per unit of
Nutrient available to plant. Improvement in NUE has been associated with
improvements in both total N uptake, as well as efficiency of utilization in terms
of grain yield.
Adaptation to Density
Idea that higher yield potential could be achieved by designing a plant type that is
well adapted to commercial practice of sowing high density monocultures was
introduced 30 yr ago. Improvement in yield potential would appear to be more a
function of improved adaptation to canopy microenvironment, rather than macro
environmental factors such as climate. Several studies have shown that
selection for yield potential in early generations can be enhanced by reducing
interplant competition between crops.
Source and Sink
It is widely believed that yield gains are most likely to be achieved by
simultaneously increasing both source (photosynthetic rate) and sink
(partitioning to grain) strengths. While most experiments indicate that yield is
primarily limited by growth factors prior to anthesis, source capacity may have
become more limited in modern cultivars. For example, experiments indicated
that, while sink capacity has been improved in post–Green Revolution
period, improvement has also resulted in modern lines that are now more source
limited than those in previous eras. Reproductive stages of development, from
24
initiation of floral development to anthesis, are pivotal in determining yield
potential,
Source and Sink: seed Size
Genetic progress in yield potential is strongly associated with increases in grain
number while weight per grain has generally declined. Nonetheless, some studies
have shown increased kernel weight has contributed to improved yield potential
in irrigated wheat. Understanding the physiological and genetic basis of potential
kernel size remains an obvious challenge for yield improvement. An important
question is whether grain weight potential can be increased independently of
increases in grain number. Simplistically, it can be argued that this inverse
relationship is a necessary tradeoff when more grains are competing for limited
assimilates during grain filling. However, studies that have examined
the relationship between kernel sizes and number at different spike positions
using lines from different eras conclude that size of kernels at low potential
weight spikelet positions are independent of kernel number or year of release. It is
suggested that grain weight is colimited by both source and sinks, such that
grain weight potential would be most likely determined during spike growth,
resulting in different potential sizes at different spike positions. Realization of
potential would be determined by assimilate availability during grain filling.
Photosynthetic Production
Whether a canopy (amount of leaf area, LAI, and its manner of display) is optimal
for photosynthesis in a particular environment is reciprocally linked with
development and properties of individual leaves, including their longevity.
According to a leaf's position in canopy, variations occur in the components of its
photosynthetic system, its acclimation to changing conditions, and its
protection from excess photon flux density (PFD).
Leaf Components
Solar-energy-capturing apparatus of higher plants is located in thylakoid
membranes of chloroplasts. It consists of light-harvesting antennae complexes
composed of carotenoids and chlorophylls a and b connected to Photosystem (PS)
I and II reaction centers, a cytochrome b6f complex, and ATP synthase.
The b6f complex transfers electrons from PSII, the water-oxidizing center, to PSI
leading to NADP+ reduction. The proton gradient that develops across thylakoid
between an interior lumen and the exterior stroma is employed by ATP synthase
(coupling factor complex, CFo-CF1) to produce ATP from ADP.
Leaf Angle
25
The erectophile leaf canopy has been proposed as a trait that could increase crop
yield potential by improving light use efficiency in high radiation environments. A
number of studies support the hypothesis. More erect leaf posture was associated
with higher grain number and higher stomatal conductance. Erect leaf varieties
show a more even distribution of photosynthetic rate throughout the canopy, as
well as higher rates of stem photosynthesis than non erect leaf varieties.
Stem Reserves and Green Leaf Area Duration
There are a number of additional physiological traits that have implications for
yield potential and are related to increasing assimilate availability (i.e., source).
One is ability to reach full ground cover as early as possible after emergence to
maximize interception of radiation. Another is remobilization of soluble
carbohydrates (stem reserves) during grain filling. A third is ability to maintain
green leaf area duration ("stay-green") throughout grain filling. Stem reserves
apparently make a greater contribution to performance in relative low-yielding
lines where contrasting lines have been examined. It is suggested that use of stem
reserves and stay-green may be mutually exclusive, since loss of chlorophyll and
stem reserve mobilization seem to be consequences of plant senescence. A greater
understanding of genetics of these traits is called for to establish potential for
breaking such linkage. As yield potential is raised by improving
reproductive sinks, extra assimilates gained by increasing early ground cover
could contribute to increased stem reserves and be tapped at later reproductive
stages to enhance potential kernel number and size.
Acclimation
Crop plants are exposed to widely fluctuating conditions of light and temperature,
and supplies of water and nutrients, and have evolved with a leaf-level
photosynthetic apparatus that is highly flexible in structure and activity.
Depending upon environment, leaves develop with different numbers and sizes of
cells, different numbers of chloroplasts per cell, and with variations in amounts
and proportions of thylakoid and carbon-reduction-cycle components. Changes in
these factors seem more related to photosynthetic activity per unit C and N
invested in leaf structure than per unit leaf area. Acclimative changes depend on
light environment and position in the canopy and continue on a time scale of
days to weeks throughout the life of a leaf.
Acclimation to light is proportional to mean daily irradiance of leaf rather than to
peak irradiance. This ability is important because new leaves generally emerge at
top of a crop in full sun and later are submerged into shade of canopy as other
leaves develop above them. C3 leaves in full sun typically have more of their leaf N
involved in electron transport and carbon reduction and less in light harvesting
26
(and fewer grana stacks) than is case for shade leaves. These properties also vary
with depth within the leaf from its sunlit surface. For young crops with small
leaf area, increasing leaf area for greater radiation interception provides more
benefit than increasing photosynthetic capacity of existing leaves (through greater
N content per unit leaf area).
Radiation-Use Efficiency
Crop growth rate and yield are functions of canopy photosynthesis and they
generally correlate poorly if at all with maximum photosynthesis rates of
individual leaves. As a result, crop physiologists have sought other measures that
would relate yield and canopy photosynthesis. Light-conversion efficiency
( radiation-use efficiency, RUE) has received most attention. RUE is measured and
reported in various units, e.g., g new biomass produced MJ-1 radiation
intercepted or absorbed by leaves.
27
Photosynthesis
All living organisms are composed, among other things of organic compounds.
These organic compounds are derived from simple inorganic substances present
in the physical environment of living things.
Photosynthesis is a biological process whereby the Sun’s energy is captured and
stored by a series of events that convert the pure energy of light into the free
energy needed to power life. This remarkable process provides the foundation for
essentially all life and has over geologic time profoundly altered the Earth itself. It
provides all our food and most of our energy resources. Only plant out of all the
living things that which have a green pigment called chlorophyll, are capable of
carrying out this transformation of inorganic substances (Carbon dioxide and
water into organic compound)e.g. carbohydrates.
Carbon dioxide which is present in the air, and in water in dissolved form,
diffuses into the cells of land and water plants and where these cells contain
chlorophyll the CO2 is transformed to carbohydrates. This transformation occurs
in the presence of and with the participation of water and light.
CO2 + 2H2O --------------------(CH2O) + O2 + H2O
Carbon dioxide + water---------------- Carbohydrate + Oxygen + Water
The overall reaction is represented by the general equation above. Oxygen is
evolved during the process and water is also a product. Light is the source of
energy utilized in photosynthesis.
A fundamental principle of photochemistry requires that for light to be active in
chemical reaction, it must first be absorbed. In green plants, the pigments that
serve for the absorption of light for photosynthetic purposes are the chlorophylls.
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The physical radiant energy that is trapped by the chlorophylls is converted to
chemical energy, and stored in the organic compounds synthesized.
Chlorophylls are insoluble in water but soluble in organic solvents and is of three
types a, b and c. Chlorophylls a is the major and essential photosynthetic and is
coloured bluish green while chlorophyll b is olive/yellow green. These pigments
are contained in specialized structures called chloroplasts in algae and higher
plants.
Also present in the photosynthetic apparatus is another group of pigments called
caratenoids which include carotenes and xanthophylls and are brightly coloured.
The carotene is coloured orange while the xanthophylls are yellow in colour.
These pigments may absorb light energy but this has to be passed to chlorophyll
a before it can be used in photosynthesis and so these pigments are called –
accessory pigments.
The kind of light absorbed by carotenoids is such as may damage the chlorophyll
pigment. Carotenoids therefore protect chlorophylls from photo destruction.
Approximately 10% of the photosynthetic activity that occurs on earth is carried
out by land plants. The remaining 90% occurs in phytoplankton, i.e. marine and
fresh water algae.
The process of photosynthesis consist of two phases.
a) Light dependent phase or light reaction
b) Light independent phase or dark reaction
Light Reaction: Adenosine triphosphate- ATP is formed in the light by the
photosynthetic apparatus. This activity is called photophosphorylation and is a
major activity of light reaction. The electron generated as a result of photolysis of
water or has to reach chlorophyll through several electron carriers. Water
photolysis occurs thus:
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H2O-------------H+ +OH
40H---------------2H2O + O2 + 4e
Water photolysis is one of light reaction activities with Oxygen evolved and water
produced. Since the electrons from chlorophyll are initially excited and energized
on the absorption of light, they posses high energy. They lose this energy in little
bits as inorganic phosphate (pi) during their transport. An enzyme, ATPase, takes
advantage of this energy loss by the electrons, using the energy Pi to drive ATP
formation
ADP + Pi --------------------ATP
Where ADP = adenosine diphosphate and Pi – inorganic phosphate.
It should be noted that during light reactions, two assimilatory products ATP and
reduced NADPH2 are formed for use in the Dark Reaction of photosynthesis.
Dark Reaction: The dark reaction of photosynthesis refers to the fixation /
reduction / assimilation of carbon dioxide to yield organic compound and
carbohydrates. For substance to be fixed there must be an acceptor molecule. In
the cycle, as proposed by Calvin (1950) the acceptor molecule is Ribulose
diphosphate (RuDP) generated in light from Ribulose (5PO4) with ATP used.
The Calvin cycle suggested that RuDP accepts a molecule of CO2 under the
influence of RUDP- carboxylase enzyme to form an unstable C6 intermediate
(Ribulose diphosphoric acid) which immediately splits to form two molecules of
3-phosphoglyceric acid (3C-PGA). PGA is the first detectable stable organic
compound formed during the fixation of CO2 by the mechanism.PGA is then
reduced under the influences of Triose phosphate dehyrogenase and light to form
3-phosphoglyceraldehydrade and Dihydraxyacetone phosphate, both of which are
triose phosphates.
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This reduction requires reduced NADPH2 and ATP as assimilatory products. The
two trioses formed condense under the influence of an Adolase enzyme to form
Fructose-1, 6-diphosphate which is further catalysed by a phosphate enzyme to
form Fructose-6-phosphate sugar. To form molecule of fructose hexose sugar, the
above cycle must go through thrice. Some of the PGA molecules are regenerated
in light with ATP used to form Ribolose diphosphate (RuDP) for further CO2
fixation. In this Calvin cycle, light intervenes twice and the first stable compound
formed is a C3 compound hence the reference C3 pathway.
There is also a C4 pathway. C4 Pathway in C4-Plants: It was discovered by Hatch
and Slack (1966) that during photosynthesis CO2 fixation in certain tropical
monocots, the first stable compound formed were C4 organic acids like
Oxaloacetate, aspartate and malate rather than phosphoglyceric acid (PGA) which
is a C3 compound.
It was discovered that tropical monocots contain an enzyme system that can
catalyse CO2 fixation at much lower concentrations of the gas than in Calvin (C3)
cycle. The CO2 acceptor molecule here is phosphoenol Pyruvate (PEP). It is
carboxylated under the influence of phosphoenol pyruvate carboxylase (PEPC) to
form Oxaloacetate (OA) is mesophyll chlorophyll chroplasts.
Oxaloacetate is then reduced to Malate or Asparatae under the influence of
Malate dehydrogenase. Malate is transported to bundle sheath chloroplasts and
there
undergoes
decarboxylation
and
dehydrogenation
reactions
to
form
pyruvate. Pyruvate is then transported back to mesophyll chloroplasts and under
the influence of Pyruvate Orthophosphate dikinase (PODK) is energized with the
use of ATP to form/regerate phosphoenol Pyruvate (PEP).
CO2 in bundle sheath chloroplasts is fixed by Calvin C3 enzymes to form
carbohydrate. The importance of this pathway is that CO2 at very low
concentrations, i.e. 0-10ppm is fixed to form Oxaloacetate, Malate or Asparatate
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at much higher concentration in mesophyll chloroplasts. The C4 pathway has
also been detected in such dicots as Amaranthus, Euphorbia etc.
The majority of the world’s plants originating in temperate regions of the world
(e.g., alfalfa, wheat, soybean, smooth bromegrass) and a few subtropical species
(e.g., cotton, tobacco, peanut) are C3 plants. Photosynthesis in C3 plants is most
effective
at
temperatures
from
10–25°C
and
decreases
thereafter.
C3
photosynthesis is inefficient in warm, high-tropical climates, and when CO2
levels in the leaf are low such as occurs during drought when stomata are
partially closed.
This is because C3 species have a version of respiration called photorespiration
that can use up a significant portion of the carbon energy fixed during
photosynthesis.
three-carbon
compound
(phosphoenol
pyruvate.
The
first
compounds formed are the four-carbon compounds malate and aspartate. These
compounds are then transferred to bundle sheath cells that surround the
vascular system. In these cells, CO2 is released to the Calvin cycle that then
generates glucose and other sugars.
Only about 10 percent of plant species are C4 plants, but some are very
significant for world agriculture. Examples of C4 plants include corn, sorghum,
amaranths, and millet. Many prairie grasses also have the C4 cycle including big
bluestem, Indiangrass, and switchgrass. Crassulacean acid metabolism (CAM)
plants have yet another type of carbon fixation.
CAM plants, like C4 plants, are adapted to hot temperatures. They use
the C3 and C4 pathways like C4 plants, but they use the C4 pathway at night
and the
C3 pathway in the day. This enables these plants to preserve water because they
open their stomata only at night. Examples of CAM plants include cacti and
pineapple. About 3–4 percent of plant species are CAM plants, but the most
significant types of plants for crop production are C3 and C4 plants.
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RESPIRATON
The conversion of sugars formed through photosynthesis to energy (ATP) for use
in metabolism of living cells is called respiration. Plants use this energy for cell
maintenance, growth, and building new tissues. One molecule of glucose formed
during photosynthesis results in the production of 36 ATP molecules. The overall
process of respiration occurs in both the cytoplasm and the mitochondria—
small, microscopic organelles within the cytoplasm of living plant cells. The
overall reaction that occurs in respiration is
C6H12O6 _ 6 O2 6 CO2 _ 6 H2O _ Energy (ATP and heat)
Respiration, like photosynthesis, is a complex process with many steps. In
summary, glucose and oxygen are transformed into carbon dioxide, water, and
energy. The four primary processes of respiration include the following:
1. Glycolysis, which occurs in the cell’s cytoplasm. Glucose is split into two
threecarbon molecules (i.e., pyruvate). This reaction also produces ATP, NADH,
and water. ATP and NADH are both high-energy compounds.
2. Pyruvate enters the mitochondria and is split into CO2 and acetyl coenzyme A
(acetyl CoA). This reaction also produces NADH.
3. The Krebs cycle uses acetyl CoA to produce ATP and NADH, and it releases
CO2 and water. The NADH will be used in the next step to generate more ATP.
4. Oxidative phosphorylation is the final step of energy formation
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GROWTH AND GROWTH REGULATION
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