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Transcript
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 1 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, 2 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 3 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 4 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. 5 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. 6 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 7 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 8 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 9 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 10 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. 11 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 12 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. 28 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: 29 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. 30 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 31 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. 32 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 33 GROWTH AND GROWTH REGULATION 34