Crop Physiology PDF

Crop Physiology Crop Physiology Author TNAU  Index Lecture Lecture Name Page No 1 Introduction - Importance of crop physiology in agriculture. 5-9 2 Role and significance of water -...

Crop Physiology Crop Physiology Author TNAU  Index Lecture Lecture Name Page No 1 Introduction - Importance of crop physiology in agriculture. 5-9 2 Role and significance of water - diffusion, imbibition, 10-20 osmosis and its significance,plasmolysis. 3 Field capacity, Available soil water and permanent wilting 21-27 point 4 Absorption of water – mode of water absorption – active 28-40 and Passive absorption and factors affecting absorption 5 Translocation of solutes - phloem and xylem transport. 41-48 6 Transpiration - types - Steward’s theory of mechanism - 49-62 significance, factors affecting transpiration and guttation - antitranspirants. 7 Mineral nutrition - introduction - criteria of essentiality of 63-67 elements - macro, secondary and micronutrients - soil less culture - sand and hydroponics. 8 Mechanism of uptake - physiological role of nutrients. 68-78 9 Foliar diagnosis - nutritional and physiological disorders - 79-92 foliar nutrition-fertigation 10 Photosynthesis - requirements of photosynthesis - light, 93-110 CO2, pigments and H20. 11 Photosynthetic pathways - c3, c4 and cam 111-126 12 Respiration - Glycolysis, TCA and Pentose Phosphate 127-131 Pathway. 13 Krebs’ cycle / citric acid cycle /tca cycle 132-144 14 Protein and fat synthesis 145-150 15 Photoperiodism - short day, long day and day neutral plants 151-156 - phytochrome. Role of phytochrome in flowering and regulation of flowering. 16 Transmission of stimulus - theories of flowering. 157-164 17 Source sink relationship - yield components - harvest index 165-168 and its importance 18 Plant growth 169-173 19 Growth analysis 174-179 20 Plant growth regulators 180-194 21 Practical application of plant growth regulators in crop 195-197 productivity 22 Environmental stresses 198-211 23 Seed germination 212-215 24 Abscission and senescence 216-219 25 Global warming - physiological effects on crop 220-229 Productivity  Crop Physiology 01. INTRODUCTION The spectacular diversity of plant size and form is familiar to everyone. In nature all plants carry out similar physiological processes. As primary producers, plants convert solar energy to chemical energy. Being non motile, plants must grow toward light, and they must have efficient vascular systems for movement of water, mineral nutrients, and photosynthetic products throughout the plant body. Green land plants must also have mechanisms for avoiding desiccation. The meaning of Plant Physiology refers to “the science of properties and functions in normal conditions”. The aim of the Plant Physiology has been described as early as the early 20th Century by the Russian Plant Physiologyist, V.I. Palladin as : “Which is to gain a complete and thorough knowledge of all the Phenomena occurring in plants, to analyse complex life processes. So as to interpret them in terms of simpler one and reduce them finally to the principles of physics and chemistry”. Nevertheless, Noggle and fritz (1983) described the Plant Physiology as “the science concerned with processes and functions, the response of plants to changes in environment and the growth and development that results from responses Crop physiology is concerned with the processes and functions of the crops at cellular, sub-cellular and whole plant levels in response to environmental variables and growth. In short, physiology is the study of functional aspects of crop plants. Cell Plants are multicellular organisms composed of millions of cells with specialized functions. At maturity, such specialized cells may differ greatly from one another in their structures. However, all plant cells have the same basic eukaryotic organization: They contain a nucleus, a cytoplasm, and sub cellular organelles, and they are enclosed in a membrane that defines their boundaries. www.AgriMoon.Com 5  Crop Physiology In plants, cell migrations are prevented because each walled cell and its neighbor are cemented together by a middle lamella. As a consequence, plant development unlike animal development, depends solely on patterns of cell division and cell enlargement. Plant cells have two types of walls: primary and secondary. Primary cell walls are typically thin and are characteristic of young, growing cells. Secondary cell walls are thicker and stronger than primary walls and are deposited when most cell enlargement has ended. Secondary cell walls owe their strength and toughness to lignin, a brittle, glue-like material. The evolution of lignified secondary cell walls provided plants with the structural reinforcement necessary to grow vertically above the soil and to colonize the land. Plant anatomy There are two categories of seed plants, gymnosperms and angiosperms. Gymnosperms are the less advanced type. Angiosperms, the more advanced type of seed plant which dominate the landscape. About 250,000 species are known, but many more remain to be characterized. The major innovation of the angiosperms is the flower; hence they are referred to as flowering plants. Three major tissue systems are found in flowering plants; in all plant organs contain dermal tissue, ground tissue, and vascular tissue. The vegetative body is composed of three organs: leaf, stem, and root. The primary function of a leaf is photosynthesis, that of the stem is support, and that of the root is anchorage and absorption of water and minerals. Leaves are attached to the stem at nodes, and the region of the stem between two nodes is termed the internode. The stem together with its leaves is commonly referred to as the shoot. www.AgriMoon.Com 6  Crop Physiology Plant parts (Source: Plant Physiology by Taiz and Zeiger) www.AgriMoon.Com 7  Crop Physiology Plant growth is concentrated in localized regions of cell division called meristems. Nearly all nuclear divisions (mitosis) and cell divisions (cytokinesis) occur in these meristematic regions. In a young plant, the most active meristems are called apical meristems; they are located at the tips of the stem and the root At the nodes, axillary buds contain the apical meristems for branch shoots. Lateral roots arise from the pericycle, an internal meristematic tissue Proximal and overlapping the meristematic regions are zones of cell elongation in which cells increase dramatically in length and width. Cells usually differentiate into specialized types after they elongate.The phase of plant development that gives rise to new organs and to the basic plant form is called primary growth. Primary growth results from the activity of apical meristems, in which cell division is followed by progressive cell enlargement, typically elongation. After elongation in a given region is complete, secondary growth may occur. Secondary growth involves two lateral meristems: the vascular cambium (plural cambia) and the cork cambium. The vascular cambium gives rise to secondary xylem (wood) and secondary phloem. The cork cambium produces the periderm, consisting mainly of cork cells. The architecture, mechanics, and function of plants depend crucially on the structure of the cell wall. The wall is secreted and assembled as a complex structure that varies in form and composition as the cell differentiates. Without a cell wall, plants would be very different organisms from what we know. Indeed, the plant cell wall is essential for many processes in plant growth, development, maintenance, and reproduction: Plant cell walls determine the mechanical strength of plant structures, allowing those structures to grow to great heights. Cell walls glue cells together, preventing them from sliding past one another. This constraint on cellular movement contrasts markedly to the situation in animal cells, and it dictates the way in which plants develop www.AgriMoon.Com 8  Crop Physiology A tough outer coating enclosing the cell, the cell wall acts as a cellular “exoskeleton” that controls cell shape and allows high turgor pressures to develop. Plant morphogenesis depends largely on the control of cell wall properties because the expansive growth of plant cells is limited principally by the ability of the cell wall to expand. The cell wall is required for normal water relations of plants because the wall determines the relationship between the cell turgor pressure and cell volume The bulk flow of water in the xylem requires a mechanically tough wall that resists collapse by the negative pressure in the xylem. The wall acts as a diffusion barrier that limits the size of macromolecules that can reach the plasma membrane from outside, and it is a major structural barrier to pathogen invasion. Much of the carbon that is assimilated in photosynthesis is channeled into polysaccharides in the wall. During specific phases of development, these polymers may be hydrolyzed into their constituent sugars, which may be scavenged by the cell and used to make new polymers www.AgriMoon.Com 9  Crop Physiology 02. Role and significance of water Water is said to be the liquid of life. Because, life is originated in organs, environmental and in the course of evolution it became fully dependent upon water in a number of ways. Water is one of the most plentiful chemicals available in the earth and the chemical formula is H20. It is a tiny V-shaped molecule contains three atoms do not stay together as the hydrogen atoms are constantly exchanging between water molecules The water molecule consists of an oxygen atom covalently bonded to two hydrogen atoms. The two O—H bondsform an angle of 105° (Figure ). Because the oxygen atom is more electronegative than hydrogen, it tends to attract the electrons of the covalent bond. This attraction results in a partial negative charge at the oxygen end of the molecule and a partial positive charge at each hydrogen. Water has special properties that enable it to act as a solvent and to be readily transported through the body of the plant. These properties derive primarily from the polar structure of the water molecule. The Polarity of water molecules gives rise to hydrogen bonds The Polarity of water makes an excellent solvent The Thermal properties of water result from hydrogen bonding The Cohesive and adhesive properties of water are due to hydrogen bonding www.AgriMoon.Com 10  Crop Physiology Importance of water to plants Water typically constitutes 80 to 95% of the mass of growing plant tissues. Water is the main constituent of protoplasm comprising up to about 90-95 per cent of its total weight. In the absence of water, protoplasm becomes inactive and is even killed. Different organic constituents of plants such as carbohydrates proteins, nucleic acid and enzymes etc. Lose their physical and chemical properties in the absence of water. Water participates directly in many metabolic processes. Inter conversion of carbohydrates and organic acids depend upon hydrolysis and condensation reaction. Water increases the rate of respiration. Seeds respire fast in the presence of water. Water is the source of hydrogen atom for the reduction of CO2 in the reaction of photosynthesis. www.AgriMoon.Com 11  Crop Physiology Water acts as a solvent and acts as a carrier for many substance. If forms the medium in which several reactions take place. Water present in the vacuoles helps in maintaining the turgidity of the cells which is a must for proper activities of life and to maintain this from and structure. Water helps in translocation of solutes In tropical plants, water plays a very important role of thermal regulation against high temperature. The elongation phase of cell growth depends on absorption of water. www.AgriMoon.Com 12  Crop Physiology Properties of water 1. Solvent for electrolyte & non electrolyte 2. High specific heat 3. High latent heat of vaporization (540 cal g-1) 4. Cohesive and Adhesive Properties 5. High surface tension 6. High Tensile Strength 7. Stabilizes temperature 8. Transparent to visible radiation 9. Low viscosity HOW IT HELPS IN PLANTS? WATER PLAYS A CRUCIAL ROLE in the life of the plant. For every gram of organic matter made by the plant, approximately 500 g of water is absorbed by the roots, transported through the plant body and lost to the atmosphere. Even slight imbalances in this flow of water can cause water deficits and severe malfunctioning of many cellular processes. Thus, every plant must delicately balance its uptake and loss of water. www.AgriMoon.Com 13  Crop Physiology Diffusion, osmosis and imbibitions The movement of materials in and out of the cells in plants taken place in a solution or gaseous form. Although the exert process of this is not very clear, three physical process are usually involved in it. They are diffusion, osmosis and imbibition. The movement of particles or molecules from a region of higher concentrations to a region of lower concentration is called as diffusion. The rate of diffusion of gases is faster than liquids or solutes. The diffusion particles have a certain pressure called as the diffusion pressure which is directly proportional to the number as concentration of the diffusing particles. These forms the diffusion takes place always from a region of higher diffusion pressure to a region of lower diffusion pressure (i.e) along a diffusion pressure gradient. The rate of diffusion increases if, i) Diffusion pressure gradient is steeper ii) Temperature is increased www.AgriMoon.Com 14  Crop Physiology iii) Density of the differing particles is lesser iv) Medium through which diffusion occurs is less concentrated. Diffusion of more than one substance at the same time and place may be at different rates and in different direction, but is independent of each other. A very common example of this is the gaseous exchange in plants. Beside osmotic diffusion the above mentioned simple diffusion also plays a very important role in the life of the plants. - It is an essential step in the exchange of gases during respiration and photosynthesis - During passive salt uptake, the ions are absorbed by diffusion - It is important in stomatal transpiration as the last step in the pollen, where diffusion of water vapour from the interrelation space into the outer atmosphere occurs through open stomata. Osmosis The diffusion of solvent molecules into the solution through a semi permeable membrane is called as osmosis (some times called as Osmotic diffusion). In case there are two solutions of different concentration separated by the semi permeable membrane, the diffusion of solvent will take place from the less concentrated suitable into the more concentrated solution till both the solutions attain equal concentration. www.AgriMoon.Com 15  Crop Physiology Osmotic pressure As a result of the separation of solution from its solvent (or) the two solutions by the semi permeable membrane, a pressure is developed in solution to the pressure by dissolved solutes in it. This is called as osmotic pressure (O.P). OP is measured in- terms of atmospheres and is directly proportional to the concentration of dissolved solutes in the solution. More concentration solution has higher O.P. O.P of a solution is always higher than its pure solvent. www.AgriMoon.Com 16  Crop Physiology During osmosis, the movement of solvent molecules taken place form the solution whose osmotic pressure is lower (i.e less concentration as hypotonic) into the solution whose osmotic pressure is higher (i.e, more concentrated as hypertonic). Osmotic diffusion of solvent molecules will not take place if the two solutions separated by the semipermeable membrane are of equal concentration having equal Osmotic pressures (i.e., they are isotonic). In plant cells, plasma membrane and tonoplant act as selectively permeable or differentially permeable membrane. End-osmosis Of a living plant cell is placed in water or hypotonic solution whose O.P is lower than cell sap, water in-terms into the cell sap by osmosis and the process is called end osmosis. As a result of entry of water with the cell sap, a pressure is developed which press the protoplasm against the cell wall and become turgid. This pressure is called a turgor pressure. Consequence of the turgor pressure is the wall pressure which is exerted by the elastic cell wall against the expanding protoplasm. At a given time, turgor pressure (T.P) equals the wall pressure (W.P). T.P = W.P www.AgriMoon.Com 17  Crop Physiology Exosmosis If on the other hand, the plant cell is placed in hypertonic solution (whose O.P is higher than cell sap) the water cover out the cell sap into the outer solution and the cell becomes flaccid. This process is known as exosmosis. Cell (or) tissue will remain as such in isotonic solution. Significance of osmosis in plants 1. Large quantities of water are absorbed by roots from the soil by osmosis 2. Cell to cell movement of water and other substances dissolve is involves osmosis 3. Opening and closing of stomata depend upon the turgor pressure of guard cells 4. Due to osmosis, the turgidity of the cells and hence the shape or from of them organs is maintained. 5. The resistance of plants to drought and frost increases with increase in osmotic pressure to later cells 6. Turgidity of the cells of the young seedling allows them to come out of the soil. www.AgriMoon.Com 18  Crop Physiology Imbibition Certain substances if placed in a particular liquid absorb it and swell up. For example, when some pieces of grass or dry wood or dry seeds are placed in water they absorb the water quickly and swell up considerably so that their volume is increased. These substances are called as imbibants and the phenomenon as imbibition, certain force of attraction is existing between imbibants and the involved substance. In plants, the hydrophilic colloids viz., protein and carbohydrates such as starch, cellulose and pectic substance have strong altercation towards water. Imbibition plays a very important role in the life of plants. The first step in the absorption of water by the roots of higher plants is the imbibition of water by the cell walls of the root hairs. Dry seeds require water by imbibition for germination. As a result of imbibition, a pressure is developed which is called as imbibition pressure or matric potential (ψm). It is analogous to the osmotic potential of a solution. With reference to pure water, the values of ψm are always negative. The water potential of an imbibant is equal to its matric potential plus any turgor or other pressure (pressure potential) which may be imposed upon the imbibant. ψw = ψm + ψP If the imbibant is unconfined to turgor or such pressure, the equation can be significant to ψw =ψm www.AgriMoon.Com 19  Crop Physiology Plasmolysis When a plant cell or tissue is placed in a hypertonic solution water cover out from the cell sap into the outer solution of exosmosis and the protoplasm begins to sprinkler or contract. The protoplasm separate from the cell wall and assures a spherical form and them phenomenon is called plasmolysis. Incipient plasmolysis is stage where protoplasm begins to contract from the cell wall. If a plasmolysed cell in tissue is placed in water, the process of endosmosis take place. Water enters into the cell sap, the cell becomes turgid and the protoplasm again assumes it normal shape and position. This phenomenon is called deplasmolysis. Advantages of plasmolysis 1. It indicates the semi permeable nature of the plasma membrane. 2. It is used in determine the osmotic pressure of the cell sap. 3. Plasmolysis is used in salting of meat and fishes. Addition of concentrated sugar solution to jam and jellies check the growth of fungi and bacteria which become plasmolysed in concentrated solution. www.AgriMoon.Com 20  Crop Physiology 03. Field capacity, Available soil water and permanent wilting point Field capacity or water holding capacity of the soil After heavy rain fall or irrigation of the soil some water is drained off along the slopes while the rest percolates down in the soil. Out of this water, some amount of water gradually reaches the water table under the force of gravity (gravitational water) while the rest is retained by the soil. This amount of water retained by the soil is called as field capacity or water holding capacity of the soil. Field capacity is affected by soil profiles soil structure and temperature. The effective depth of a soil, as determined by physical and chemical barriers, together with the clay content of the soil within that depth, determine the water holding capacity of the profile, and how much of the water is available to plants. Effective soil depth varies between plant species. Wheat is used as the benchmark plant in this assessment. Available water holding www.AgriMoon.Com 21  Crop Physiology capacity rankings are estimated from soil texture, structure and stone content within the potential root zone of a wheat plant. Water-holding capacity is controlled primarily by soil texture and organic matter. Soils with smaller particles (silt and clay) have a larger surface area than those with larger sand particles, and a large surface area allows a soil to hold more water. In other words, a soil with a high percentage of silt and clay particles, which describes fine soil, has a higher water- holding capacity. The table illustrates water-holding-capacity differences as influenced by texture. Organic matter percentage also influences water-holding capacity. As the percentage increases, the water-holding capacity increases because of the affinity organic matter has for water. It is the water content of the soil after downward drainage of gravitational water. It is the capillary capacity of a soil. It is the upper limit of soil water storage for the plant growth. At field capacity, the soil water potential is –0.1 to –0.3 bars. www.AgriMoon.Com 22  Crop Physiology Water potential Every component of a system possesses free energy capable of doing work under constant temperature conditions. For non-electrolytes, free energy / mole is known as chemical potential. With refuse to water, the chemical potential of water is called as water potential. The chemical potential is denoted by a Greek letter Psi (ψ). For pure water, the water potential is Zero. The presence of solute particles will reduce the free energy of water or decrease the water potential. Therefore it is expressed in vegetative value. It is therefore, water potential of solution is always less than zero so in negative value. For solutions, water potential is determined by three internal factors i.e. ψw = ψw + ψs + ψp ψS = is the solute potential or osmotic potential ψp = pressure potential or turgor potential www.AgriMoon.Com 23  Crop Physiology ψw = is the matric potential. Matric potential can be measured for the water molecules adhering on the soil particles and cell wall. In plant system, the matric potential is disregarded. Therefore, ψw = ψs + ψp Osmotic pressure Osmotic pressure is equivalent to osmotic potential but opposite in sign.Osmotic pressure in a solution results due to the presence of solutes and the solutes lower the water potential. Therefore osmotic pressure is a quantitative index of the lowering of water potential in a solution and using thermodynamic terminology is called as osmotic potential. Osmotic pressure and osmotic potential are numerically equal but opposite in sign. Osmotic pressure has positive sign Osmotic potential has negative sign (ψs) For eg. IA OP = 20 atm. ψw = - 20 atm Turgor pressure In plant cell, the turgor pressure results due to the presence of water molecules is turgor pressure. The potential created by such pressures is called presure potential (ψp) In a normal plant cell, the water potential ψw = ψs + ψp – partially turgid cell (High) ψw = Zero - Fully turgid cell (Highest) ψw = ψs - Flaccid cell or plasmolysed cell (Lowest) www.AgriMoon.Com 24  Crop Physiology Water relation Water forms the major constituent of living (cells) things and the cells originated in a highly aqueous medium and all the vital processes of the life are carried out in it. Besides, water predominately arts as a source of hydrogen to plants and is released by the photolysis of water during photosynthesis. In living tissue, water is the medium for many biochemical reactions and extraction process. Inorganic nutrients, photosynthesis, bases and hormones are all transported in aqueous solution. Evaporation of water can control the temperature of leaf on canopy soil nutrients are available to plant roots only when dissolved in water. In short, water is essential for life and plays a unique role in virtually all biological process. Example: There are 2 cells A and B in contact with each other, cell A has a pressure potential (turgor pressure) of 4 bars and certain sap with an osmotic potential of -12 bars. Cell B has presume potential of 2 bars and certain sap with osmotic potential of -5 bars. Then, ψw of cell A = ψs + ψp = -12 + (+4) = -8 bars ψw of cell B = -5+(+2) -3 bars Hence, water will move from cell B to cell A (i.e., towards lower or more negative water potential) with a form of (-8-(-3) = -5 bars. Diffusion Pressure Déficit (DPD) (Suction pressure) Diffusion pressure of a solution is always lower than its pure solvent. The difference between the diffusion pressure of the solution and its solvent at a particular temperate and www.AgriMoon.Com 25  Crop Physiology atmosphere conditions is called as diffusion pressure deficit (D.P.D). If the solution is more concentrated D.P.D increases but it decreases with the dilution of the solution, D.P.D of the cell sap or the cells is a measure of the ability of the cells to absorb water and hence is often called as the suction pressure (S.P). It is related with osmotic pressure (O.P) and turgor pressure (T.P) of cell sap and also the wall pressure (W.P) as follows. D.P.D. (S.P) = O.P – W.P But (W.P) = T.P D.P.D = O.P – T.P Due to the entry of the water the osmotic pressure of the cell sap decreases while its turgor pressure is increased so much so that in a fully turgid cell T.P equals the O.P O.P = T.P = D.P.D = O In fully plasmolysed cells: T.P = O So D.P.D = O.P D.P.D. incase of plant cells is not directly proportional to their osmotic pressure or the concentration of the cell sap but depend both on O.P and T.P. Higher osmotic pressure of the cell sap is usually accompanied by lower turgor pressure so that its D.P.D is greater and water enters into it. But sometimes it is possible that two cells are in contact with each other one having higher O.P and also higher turgor pressure than the other cell and still its does not draw water. It is because of its lower D.P.D., no matter is O.P is higher. Cell a Cell b O.P = 25 atm. O.P = 30 atm T.P = 15 atm. T.P =10 atm. A D.P.D = 10 atm. D.P.D = 30 atm. www.AgriMoon.Com 26  Crop Physiology Cell a Cell b O.P = 35 atm. O.P = 40 atm T.P = 10 atm. T.P = 20 atm. B D.P.D = 25 atm. D.P.D = 20 atm. Entry of water into the cell depends on D.P.D and not on O.P only www.AgriMoon.Com 27  Crop Physiology ABSORPTION OF WATER – MODE OF WATER ABSORPTION – ACTIVE AND PASSIVE ABSORPTION AND FACTORS AFFECTING ABSORPTION. PRELUDE OF WATER POTENTIAL Most organisms are comprised of at least 70% or more water. Some plants, like a head of lettuce, are made up of nearly 95% water. When organisms go dormant, they loose most of their water. For example, seeds and buds are typically less than 10% water, as are desiccated rotifers, nematodes and yeast cells. Earth is the water planet (that's why astronomers get so excited about finding water in space). Water is the limiting resource for crop productivity in most agricultural systems LEARN MORE ABOUT WATER POTENTIAL In general, water always moves down its water potential gradient from areas of higher water potential to areas of lower water potential. Water potential is typically measured as the amount of pressure needed to stop the movement of water. The unit used to express this pressure is the megapascal (MPa). The three factors that most commonly determine water potential are WHAT IS WATER POTENTIAL? Water potential is the potential energy of water relative to pure free water (e.g. deionized water) in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension. Water potential is measured in units of pressure and www.AgriMoon.Com 28  Crop Physiology is commonly represented by the Greek letter (Psi). This concept has proved especially useful in understanding water movement within plants, animals, and soil. Components of water potential Much different potential affect the total water potential and sum of these potentials determines the overall water potential and the direction of water flow: = 0 + + p + s+ v+ m Where: 0 is the reference correction, is the solute potential, p is the pressure potential, s is the gravimetric component, v is the potential due to humidity, and m is the potential due to matrix effects (e.g., fluid cohesion and surface tension.) COMPONENT OF WATER POTENTIAL 1. Pressure potential Pressure potential is based on mechanical pressure, and is an important component of the total water potential within plant cells. Pressure potential is increased as water enters a cell. As water passes through the cell wall and cell membrane, it increases the total amount of water present inside the cell, which exerts an outward pressure that is retained by the structural rigidity of the cell wall. The pressure potential in a living plant cell is usually positive. In plasmolysed cells, pressure potential is almost zero. Negative pressure potentials occur when water is pulled through an open system such as a plant xylem vessel. Withstanding negative pressure potentials (frequently called tension) is an important adaptation of xylem vessels. 2.Solute potential Pure water is usually defined as having a solute potential ( ) of zero, and in this case, solute potential can never be positive. The relationship of solute concentration (in molarity) to solute potential is given by the van 't Hoff equation: = miRT Where m - The concentration in molarity of the solute, i - The van 't Hoff factor, the ratio of amount of particles in solution to amount of formula units dissolved, R - The ideal gas constant, and T is the absolute temperature. www.AgriMoon.Com 29  Crop Physiology 3. Matrix potential When water is in contact with solid particles (e.g., clay or sand particles within soil) adhesive intermolecular forces between the water and the solid can be large and important. The forces between the water molecules and the solid particles in combination with attraction among water molecules promote surface tension and the formation of menisci within the solid matrix. Force is then required to break these menisci. The magnitude of matrix potential depends on the distances between solid particles--the width of the menisci and the chemical composition of the solid matrix. In many cases, matrix potential can be quite large and comparable to the other components of water potential discussed above. It is worth noting that matrix potentials are very important for plant water relations. Strong (very negative) matrix potentials bind water to soil particles within very dry soils. Plants then create even more negative matrix potentials within tiny pores in the cell walls of their leaves to extract water from the soil and allow physiological activity to continue through dry periods. 4. Gravity (Ψg): Contributions due to gravity which is usually ignored unless referring to the tops of tall trees. ABSORPTION OF WATER We know from a very early age that plants obtain water through their roots, though it is not perhaps until our school biology lessons that we learn of the important role that water plays in the process of photosynthesis. Most of the water absorption is carried out by the younger part of the roots. Just behind the growing tip of a young root is the piliferous region, made up of hundreds of projections of the epidermal tissue, the root hairs. STRUCTURE INVOLVED IN WATER ABSORPTION In higher plants water is absorbed through root hairs which are in contact with soil water and form a root hair zone a little behind the root tips. Root hairs are tubular hair like prolongations of the cells of the epidermal layer (when epidermis bears root hairs it is also known as pilloferous layer of the roots. The walls of root hairs are permeable and consist of pectic substances and cellulose which are strongly hydrophilic in nature root hairs contain vacuoles filled with cell sap. When roots elongate, the older root hairs die and new root hairs are developed so that they are in contact with fresh supplies of water in the soil. Lateral Movement of water is achieved through root. This can described as follows: www.AgriMoon.Com 30  Crop Physiology ROOTS Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it's important to understand plant root systems (Fig 1) because they have a pronounced effect on a plant's size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation. Fig 1. Diagrammatically the internal structure of a typical root Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, support the stem, and store food. In some plants, they can be used for propagation. STRUCTURE OF ROOTS Internally, there are three major parts of a root (Fig 2): The meristem is at the tip and manufactures new cells; it is an area of cell division and growth. Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorption. As they grow, they push the root through the soil. The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue. A root's epidermis is its outermost layer of cells (Fig 2). These cells are responsible for absorbing water and minerals dissolved in water. Cortex cells are involved in moving water from the epidermis to the vascular tissue (xylem and phloem) and in storing food. Vascular tissue is located in the center of the root and conducts food and water. www.AgriMoon.Com 31  Crop Physiology Fig 2. Cross section of roots Fig 3. Structure of root hair www.AgriMoon.Com 32  Crop Physiology Externally, there are two areas of importance: the root cap and the root hairs (Figure 3). The root cap is the root's outermost tip. It consists of cells that are sloughed off as the root grows through the soil. Its function is to protect the root meristem. Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root's growing tip. They generally appear as fine down to the naked eye. Their function is to increase the root's surface area and absorptive capacity. Root hairs usually live 1 or 2 days. When a plant is transplanted, they are easily torn off or may dry out in the sun. WATER MOVEMENT MECHANISM IN PLANTS In plants, following two pathways are involved in the water movement. They are (1) Apoplastic pathway (2) Symplastic pathway (3) Transmembrane pathway 1. Apoplastic pathway (Fig 4) The apoplastic movement of water in plants occurs exclusively through the cell wall without crossing any membranes. The cortex receive majority of water through apoplastic way as loosely bound cortical cells do not offer any resistance. But the movement of water in root beyond cortex apoplastic pathway is blocked by casparian strip present in the endodermis. Fig 4 www.AgriMoon.Com 33  Crop Physiology 2. Symplastic pathway (Fig 5) The movement of water from one cell to other cell through the plasmodesmata is called the symplastic pathway of water movement. This pathway comprises the network of cytoplasm of all cells inter-connected by plasmodermata. Fig 5 3. Transmembrane pathway (Fig 6) In plant roots, water movement from soil till the endodermis occurs through apoplastic pathway i.e. only through cell wall. The casparian strips in the endodermis are made-up of wax -like substance suberin which blocks water and solute movement through the cell wall of the endodermis. As a result water is forced to move through cell membranes and may cross the tonoplast of vacuole. This movement of water through cell membranes is called transmembrane pathway. Fig 6. www.AgriMoon.Com 34  Crop Physiology Following schematic diagram showing the apoplastic and symplastic pathway of water movement through root (Fig 7) Fig 5 Apoplastic (Red) and symplastic (Blue) and transmembrane (green) pathways of movement of substances in a plant cell With the help of the following schematic arrow flow chart, you can understand the path of water from soil to root xylem. MECHANISM OF WATER ABSORPTION 1. Active absorption of water In this process the root cells play active role in the absorption of water and metabolic energy released through respiration is consumed active absorption may be of two kinds. Steps involved in the active osmotic absorption of water First step in osmotic the osmotic absorption of water is the imbibition of soil water by the hydrophilic cell walls of root hairs. Osmotic pressure of the cell sap of root hairs is usually higher than the OP of the soil water. Therefore, the DPD and suction presume in the root hairs become higher and water from the cell walls enters into them through plasma membrane by osmotic diffusion. As a result, OP, suction pressure and DPD of root hairs how become lower, while their turgor pressure is increased. www.AgriMoon.Com 35  Crop Physiology Now the cortical cells adjacent to root hairs have high OP, SP & DPD in comparison to the root hairs. Therefore, water is drawn into the adjacent cortical cells from root hairs by osmotic diffusion. In the same way, by cell to cell osmotic diffusion gradually reaches the inner most cortical cells and the endodermis. Osmotic diffusion of water into endodermis takes place through special thin walled passage cells because the other endodermis cells have casparian strips on thin walls which are impervious to water. Water from endodermis cells is down into the cells of pericycle by osmotic diffusion which now become turgid and their suction pressure in decreased. In the last step, water is drawn into xylem from turgid pericycle cells (In roots the vascular bundles are radical and protoxylem elements are in contact with pericycle). It is because in the absence of turgor presume of the xylem vessels, the SP of xylem vessels become higher than SP of the cells of the pericycle when water enters into xylem from pericycle a presume is developed in the xylem of roots which can raise the water to a certain height in the xylem. This pressure is called as root pressure. (A) Osmotic absorption Water is absorbed from the soil into the xylem of the roots according to osmotic gradient. www.AgriMoon.Com 36  Crop Physiology Likewise, water moves by osmotic diffusion and reaches endodermis Endodermis water moves thro’ passage cell (because casparian cell) Now water reaches pericycle, pericylce becomes turgid and their DPD is decreased Last step, water is drawn into xylem from turgid pericycle cells (protoxylem in contact) Pressure is developed in the xylem of root by water entry – Root pressure (B) Non-osmotic absorption Water is absorbed against the osmotic gradient. Sometimes, it has been observed that absorption of water takes place even when OP of soil water is high than OP of cell sap. This type of absorption which is non-osmotic and against the osmotic gradient requires the expenditure of metabolic energy probably through respiration. 2. Passive absorption of water It is mainly due to transpiration, the root cells do not play active role and remain passive. STEPS: Transpiration creates tension in water in the xylem of the leaves Tension is transmitted to water in xylem of root thro’ xylem of stem and water rises upward to reach transpiring surface Hence soil water enters cortical cells thro’ root hairs to reach xylem of roots to maintain the supply of water. The force for entry of water in leaves is due to rapid transpiration and root cells remain passive www.AgriMoon.Com 37  Crop Physiology 2. Passive absorption of water Passive absorption of water takes place when rate of transpiration is usually high. Rapid evaporation of water from the leaves during transpiration creates a tension in water in the xylem of the leaves. This tension is transmitted to water in xylem of roots through the xylem of stream and water rises upward to reach the transpiring surfaces. As the results soil water enters into the cortical cells through root hairs to reach the xylem of roots to maintain the supply of water. The force of this entry of water is created in leaves due to rapid transpiration and hence, the root cells remain passive during this process. External factors affecting absorption of water 1. Available soil water Sufficient amount of water should be present in the soil in such form which can easily be absorbed by the plants. Usually the plants absorb capillary water i.e water present in films in between soil particles other forms of water in the soil eg. Hygroscopic water, combined water, gravitational water etc. is not easily available to plants. Increased amount of water in the soil beyond a certain limit results in poor aeration of the soil which retards metabolic activities of root cells like respiration and hence, the rate of water absorption is also retarded. 2. Concentration of soil solution Increased concentration of soil solution (due to presence of more salts in the soil) results in higher OP. If OP of soil solution will become higher than the OP of cell sap in root cells, the water absorption particularly the osmotic absorption of water will be greatly suppressed. Therefore, absorption of water is poor in alkaline soils and marshes. 3. Soil air Absorption of water is retarded in poorly aerated soils because in such soils deficiency of O 2 and consequently the accumulation of CO2 will retard the metabolic activities of roots like respiration. This also inhibits www.AgriMoon.Com 38  Crop Physiology rapid growth and elongation of the roots so that they are deprived of fresh supply of water in the soil. Water logged soils are poorly aerated and hence, are physiologically dry. They are not good for absorption of water. 4. Soil temperature Increase in soil temperature up to about 30°C favours water absorption. At higher temperature water absorption is decreased. At low temperature also water absorption decreased so much so that at about 0°C, it is almost decreased. This is probably because at low temperature. 1. The viscosity of water and protoplasm is increased 2. Permeability of cell membrane is decreased 3. Metabolic activity of root cells are decreased 4. Root growth and elongation of roots are checked. Quiz 1. Roots have a root cap, but lack nodes and never bear leaves or flowers directly 2. The meristem is at the tip and manufactures new cells; it is an area of cell division and growth. 3. Behind the meristem is the zone of elongation 4. The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue. 5. Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root's growing tip. 6. The movement of water from one cell to other cell through the plasmodesmata is called the symplastic pathway of water movement. 7. The casparian strips in the endodermis are made-up of wax -like substance suberin which blocks water and solute movement through the cell wall of the endodermis. www.AgriMoon.Com 39  Crop Physiology www.AgriMoon.Com 40  Crop Physiology 05. TRANSLOCATION OF SOLUTES Translocation of organic solutes The movement of organic food materials or the solutes in soluble form one place to another in higher plants is called as translocation of organic solutes Directions of translocation Translocation of organic solutes may take place in the following directions. 1. Downward translocation Mostly, the organic material is manufactured by leaves and translocated downward to stem and roots for consumption and storage. 2. Upward translocation It takes place mainly during the germination of seeds, tubers etc. When stored food after being converted into soluble form is supplied to the upper growing part of the young seedling till it has developed green leaves. Upward translocation of solutes also takes place through stem to young leaves, buds and flowers which are situated at the tip of the branch. 3. Lateral translocation Radical translocation of organic solutes also takes place in plants from the cells of the pith to cortex. Path of the translocation of organic solutes 1. Path of downward translocation Downward translocation of the organic solutes takes place through phloem. This can be proved by the ringing experiment. 2. Path of upward translocation Although translocation of organic solutes take place through phloem, but under certain conditions it may take place through xylem. 3. Path of lateral translocation Lateral translocation from pith to cortex takes place through medullary rays. www.AgriMoon.Com 41  Crop Physiology Mechanism of translocation Various theories have been put forward to explain the mechanism of phloem conduction. Among them Munchs’ (1930) hypothesis is mot convincing. Munchs mass flow on pressure flow hypothesis According to this hypothesis put forward by Much (1930) and others, the translocation of organic solutes takes place though phloem along a gradient of turgor pressure from the region of higher concentration of soluble solutes (supply end) to the region of lower concentration (consumption end). The principle involved in this hypothesis can be explained by a simple physical system as shown in Fig. Two members X and Y permeable only to water and dipping in water are connected by a tube T to form a closed system membrane X contains more concentrated sugar solution than in membrane Y. Due to higher osmotic presence of the concentrated sugar solution in the membrane X, water enters into it so that its turgor pressure is increased. The increase in turgor pressure results in mass flow of sugar solution to membrane Y though the T till the concentration of sugar solution in both the membrane is equal. In the above system it could be possible to maintain continuous supply of sugars in membrane X and its utilization on conversion into insoluble form in membrane Y, the flow of sugar solution from X to Y will continue indefinitely. According to this theory, a similar analogous system for the translocation of organic solutes exists in plants. As a result of photosynthesis, the mesophyll cells in the leaves contain high concentration of organic food material in them in soluble form and correspond to membrane X or supply end. www.AgriMoon.Com 42  Crop Physiology The cells of stem and roots where the food material is utilized or converted into insoluble form correspond to membrane Y or consumption end. While the sieve tubes in phloem which are placed and to end correspond to the tube T. Mesophyll cells draw water from the xylem of the leaf due to higher osmotic pressure and suction presume of their sap so that their turgor pressure is increased. The turgor presume in the cells of stem and the roots is comparatively low and hence, the soluble organic solutes begin to flow en mass from mesophyll through phloem down to the cells of stem and the roots under the gradient of turgor presume. In the stem and the roots, the organic solutes are either consumed or converted into insoluble form and the excess water is released into xylem through cambium. XYLEM TRANSPORT ASCENT OF SAP The water after being absorbed by the roots is distributed to all parts of the plants. In order to reach the topmost part of the plant, the water has to move upward through the stem. The upward movement of water is called as Ascent of sap. www.AgriMoon.Com 43  Crop Physiology Ascent of sap can be studied under the following two headings. 1. Path of ascent of sap 2. Mechanism of ascent of sap. 1. Path of ascent of sap Ascent of sap takes place through xylem. It can be shown by the experiment. A leafy twig of Balsam plant (it has semi transpiration stem) is cut under water (to avoid entry of air bubble through the cut end of the stem) and placed in a beaker containing water with some Eosine (a dye) dissolved in it. After sometimes coloured lines will be seen moving upward in the stem. If sections of stem are cut at this time, only the xylem elements will appear to be filled with coloured water. www.AgriMoon.Com 44  Crop Physiology 2. Ringing experiment A leafy twig from a tree is cut under water and placed in a beaker filled with water. A ring of bark is removed from the stem. After sometime it is observed that the leaves above the ringing part of the stem remain fresh and green. It is because water is being continuously supplied to the upper part of the twig through xylem. B. Mechanism of ascent of sap In small trees and herbaceous plants, the ascent of sap can be explained easily, but in tall trees like Eucalyptus and conifers reaching a height of 300-400 feet), where water has to rise up to the height of several hundred feet, the ascent of sap, it feet, becomes a problem. To explain the mechanism of Ascent of sap, a number of theories have been put forward. a. vital theory b. root pressure theory c. physical force theory d. transpiration pull and cohesion of water theory A. Vital theories According to vital theories, the ascent of sap is under the control of vital activities in the stem. 1. According to Godlewski (1884) – Ascent of sap takes place due to the pumping activity xylem tissues which are living. 2. According to Bose (1923) – upward translocation of water takes place due to pulsatory activity of the living cells of the inner must cortical layer just outside the endodermis. B. Root pressure theory Although, root pressure which is developed in the xylem of the roots can raise water to a certain height but does not seem to be an effective force in ascent of sap due to the following reasons. Magnitude of root pressure is very low (about 2 atmos). www.AgriMoon.Com 45  Crop Physiology Even in the absence of root pressure, ascent of sap continues. For example, when leafy twig is cut under water and placed in a beaker full of water it remains fresh and green for sufficient long time. C. Physical force theories Various physical forces may be involved in ascent of sap. 1. Atmospheric pressure This does not seem to be convincing because it cannot act on water present in xylem in roots Incase it is working, and then also it will not be able to raise water beyond 34. 2. Imbibition Sachs (1878) supported the view that ascent of sap could take place by imbibition through the walls of xylem. But imbibitional force is insignificant in the A. of sap because it takes place through the lumen of xylem elements and not through walls. 3. Capillary force In plants the xylem vessels are placed one above the other forming a sort of continuous channel which can be compared with long capillary tubes and it was thought that as water rises in capillary tube due to capillary force in the same manner ascent of sap takes place in the xylem. D. Transpiration pull and cohesion of water theory This theory was originally proposed by Dixon and Jolly (1894) later supported and elaborated by Dixon (1924). This theory is very convincing and has now been widely supported by many workers. Although H- bond is very weak (Containing about 5 K –cal – energy) but they are present in enormous numbers as incase of water, a very strong mutual force of attraction or cohesive force develops between water molecules and hence they remain in the form of a continuous water column in the xylem. The magnitude of this force is very high (up to 350 atm), therefore the continuous water column in the xylem cannot be broken easily due to the www.AgriMoon.Com 46  Crop Physiology force of gravity or other abstractions offered by the internal tissues in the upward movement of water. The adhesive properties of water i.e. attractions between the water molecules and the containers walls (here the walls of xylem) further ensure the continuity of water column in xylem. When transpiration takes place in the leaves at the upper parts of the plant, water evaporates from the intercellular spaces of the leaves to the outer atmosphere through stomata. More water is released into the intercellular spaces from mesophyll cells. In turn, the mesophyll cells draw water from the xylem of the leaf. Due to all this, a tension is created in the xylem elements of the leaves. This tension is transmitted downward to water in xylem elements of the root through the xylem of petiole and stem and the water is pulled upward in the form of continuous unbroken water column to reach the transpiring surfaces up to the top of the plant www.AgriMoon.Com 47  Crop Physiology www.AgriMoon.Com 48  Crop Physiology 06. TRANSPIRATION Although large quantities of water are absorbed by plant from the soil but only a small amount of it is utilized. The excess of water is lost from the aerial parts of plants in the form of water vapours. This is called as transpiration. Transpiration is of three types 1. Stomatal transpiration Most of the transpiration takes place through stomata. Stomata are usually confined in more numbers on the lower sides of the leaves. In monocots. Eg. Grasses they are equally distributed on both sides. While in aquatic plants with floating leaves they are present on the upper surface. 2. Cuticular transpiration Cuticle is impervious to water, even though, some water may be lost through it. It may contribute a maximum of about 10% of the total transpiration. 3. Lenticular transpiration Some water may be lost by woody stems through lenticells which is called as lenticular transpiration. Mechanism of stomatal transpiration The mechanism of stomatal transpiration which takes place during the day time can be studied in three steps. i. Osmotic diffusion of water in the leaf from xylem to intercellular space above the stomata through the mesophyll cells. ii. Opening and closing of stomata (stomatal movement) iii. Simple diffusion of water vapours from intercellular spaces to other atmosphere through stomata. ♦ Inside the leaf the mesophyll cells are in contact ♦ With xylem, and on the other hand with intercellular space above the stomata www.AgriMoon.Com 49  Crop Physiology ♦ When mesophyll cells draw water from the xylem they become turgid and their diffusion pressure deficit (DPD) and osmotic pressure (OP) decreases with the result that they release water in the form of vapour in intercellular spaces close to stomata by osmotic diffusion. Now in turn, the O.P and D.P.D of mesophyll cells become higher and hence, they draw water form xylem by osmotic diffusion. Opening and closing of stomata (Stomatal movement) The stomata are easily recognized from the surrounding epidermal cells by their peculiar shape. The epidermal cells that immediately surround the stomata may be similar to other epidermal cells or may be different and specialized. In the latter case, they are called as subsidiary cells. The guard cells differ from other epidermal cells also in containing chloroplasts and peculiar thickening on their adjacent surface (in closed stomata) or on surfaces. Consequent to an increase in the osmotic pressure (OP) and diffusion pressure deficit (DPD) of the guard cells (which is due to accumulation of osmotically active substances), osmotic diffusion of water from surrounding epidermal cells and mesophyll www.AgriMoon.Com 50  Crop Physiology cells into guard cells follows. This increase the turgor pressure (TP) of the guard cells and they become turgid. The guard cells swell, increase in length and their adjacent thickened surfaces starch forming a pore and thus the stomata open. On the other hand, when OP and DPD of guard cells decrease (due to depletion of osmotically active substances) relative to surrounding epidermal and mesophyll cells, water is released back into the latter by osmotic diffusion and the guard cells become flaccid. The thickened surfaces of the guard cells come close to each other, thereby closing the stomatal pore and stomata. Osmotic diffusion of water into guard cells occur when their osmotic pressure increases and water potential decreases (i.e become more negative) related to those of surrounding epidermal and mesophyll cells. The guard cells become flaccid when their osmotic pressure decreases relative to the surrounding cells (Movement of water takes place from a region of higher water potential to a region of lower water potential. These may be several different agents or mechanisms which control stomatal movements. Hydrolysis of starch into sugars in guard cells Synthesis of sugars or organic acids in them The active pumping of K+ ions in the guard. 1. Hydrolysis of starch into sugars in guard cells Starch – sugar Inter conversion theory This classical theory is based on the effect of pH on starch phosphorylase enzyme which reversibly catalyses the conversion of starch + inorganic phosphate into glucose -1 phosphate. During the day, pH is guard cells in high. This favours hydrolysis of starch (which is insoluble into glucose -1- phosphate (which is soluble) so that osmotic pressure is increased in guard cells. www.AgriMoon.Com 51  Crop Physiology Consequently water enters, into the guard cells by osmotic diffusion from the surrounding epidermal and mesophyll cells. Guard cells become turgid and the stomata open. During dark, reverse process occurs. Glucose 1- phosphate is converted back into starch in the guard cells thereby decreasing osmotic pressure. The guard cell release water, become flaccid and stomata become closed. Light high pH Starch +Pi Glucose-1-phosphate (Insoluble) Dark low pH (Soluble) According to Steward 91964), the conversion of starch and inorganic phosphate into glucose-1-phosphate does not cause any appreciable change in the osmotic pressure because the inorganic phosphate and glucose-1-phosphate are equally active osmotically. In this scheme he has suggested that, Glucose-1-phosphate should be further converted into glucose and inorganic phosphate for the opening of stomata. Metabolic energy in the form of ATP would be required for the closing of stomata which probably comes through respiration. Starch pH 5.0 pH 7.0 Glucose-1-phosphate Hexokinase + ATP Glucose-6-phosphate O2 Resp. Phosphatase Glucose + Pi www.AgriMoon.Com 52  Crop Physiology 2. Synthesis of sugars or organic acids in Guard cells During day light photosynthesis occurs in guard cells as they contain chloroplast. The soluble sugars formed in this process may contribute in increasing the osmotic potential of guard cells and hence resulting in stomatal opening. However, very small amounts of soluble sugars (osmotically active) have been extracted from the guard cells which are insufficient to affect water potential. As a result of photosynthesis CO2 concentration in guard cells decreases which leads to increased pH up of organic acids, chiefly malic acid during this period in guard cells. The formation of malic acid would produce proton that could operate in an ATP-driven proton K+ exchange pump moving protons into the adjacent epidermal cells and K ions into guard cells and thus may contribute in increasing the osmotic pressure of the guard cells and leading to stomatal opening. Reverse process would occur in darkness. 3. ATP –Driven proton (H+) – K exchange pump mechanism in Guard cells According to this mechanism, there is accumulation of K+ ions in the guard cells during day light period. The protons (H+) are ‘pumped out’ from the guard cells into the adjacent epidermal cells and in exchange K+ ions are mediated through ATP and thus are an active process. ATP is generated in non-cyclic photophosphorylation in photosynthesis in the guard cells. The ATP required in ion exchange process may also come through respiration. The accumulation of K ion is sufficient enough to significantly decrease the water potential of guard cells during day light. Consequently, water enters into them from the adjacent epidermal and mesophyll cells thereby increasing their turgor pressure and opening the stomatal pore. Reverse situation prevails during dark when stomata are closed. There is no accumulation of ‘K’ in g cells in dark. www.AgriMoon.Com 53  Crop Physiology (iii) The last step in the mechanism of transpiration is the simple diffusion of water vapours from the intercellular spaces to the atmosphere through open stomata. This is because the intercellular spaces are more saturated with moisture is comparison to the outer atmosphere in the vicinity of stomata. Significance of Transpiration Plants waste much of their energy in absorbing large quantities of water and most of which is ultimately lost through transpiration. Some people thin that – Transpiration as advantageous to plant. Others regard it as an unavoidable process which is rather harmful. Advances of transpiration 1. Role of movement of water Plays an important role in upward movement of water i.e Ascent of sap in plants. 2. Role in absorption and translocation of mineral salts Absorption of water and mineral salts are entirely independent process. Therefore transpiration has nothing to do with the absorption of mineral salts. However, once mineral salts have been absorbed by the plants, their further translocation and distribution may be facilitated by transpiration through translocation of water in the xylem elements. 3. Role of regulation of temperature Some light energy absorbed by the leaves is utilized in photosynthesis; rest is converted into heat energy www.AgriMoon.Com 54  Crop Physiology which raises their temperature. Transpiration plays an important role in controlling the temperature of the plants. Rapid evaporation of water from the aerial parts of the plant through transpiration brings down their temperature and thus prevents them from excessive heating. Transpiration as a necessary evil 1. When the rate of transpiration is high and soil is deficient in water, an internal water deficit is created in the plants which may affect metabolic processes 2. Many xerophytes have to develop structural modification and adaptation to check transpiration. www.AgriMoon.Com 55  Crop Physiology 3. Deciduous tress has to shed their leaves during autumn to check loss of water. But, in spite of the various disadvantages, the plants cannot avoid transpiration due to their peculiar internal structure particularly those of leaves. Their internal structure although basically mean for gaseous exchange for respiration, P.S. etc. is such that it cannot check the evaporation of water. Therefore, many workers like Curtis (1926) have called transpiration as necessary evil. www.AgriMoon.Com 56  Crop Physiology Factors affecting transpiration rate A. External factors 1. Atmospheric humidity In humid atmosphere, (when relative humidity) is high), the rate of transpiration decreases. It is because atmosphere is more saturated with moisture and retards the diffusion of water vapour from the intercellular spaces of the leaves to the outer atmosphere through stomata. In dry atmosphere, the RH is low and the air is not saturated with moisture and hence, the rate of transpiration increases. 2. Temperature An increase in temperature brings about an increase in the rate of transpiration by 1. lowering the relative humidity 2. Opening of stomata widely 3. Wind www.AgriMoon.Com 57  Crop Physiology i. When wind is stagnant (not blowing), the rate of transpiration remains normal ii. When the wind is blowing gently, the rate of transpiration increases because it removes moisture from the vicinity of the transpiration parts of the plant thus facilitating the diffusion of waster vapour from the intercellular spaces of the leaves to the outer atmosphere though stomata. iii. When the wind is blowing violently, the rate of transpiration decreased because it creates hindrance in the outward diffusion of water vapours from the transpiring part and it may also close the stomata. 4. Light Light increases the rate of transpiration because, In light stomata open; It increases the temperature In dark, due to closure of stomata, the stomatal transpiration is almost stopped. 5. Available soil water Rate of transpiration will decrease if there is not enough water in the soil in such from which can be easily absorbed by the roots. 6. CO2 An increase in CO2 concentration in the atmosphere (Ova the usual concentration) more so inside the leaf, leads towards stomatal closure and hence it retards transpiration. B. Internal factors 1. Internal water conditions It is very essential for transpiration. Deficiency of water in the plants will result in decrease of transpiration rate. Increase rate of transpiration containing for longer periods often create internal water deficit in plants because absorption of water does not keep pace with it. www.AgriMoon.Com 58  Crop Physiology 2. Structural features The number, size, position and the movement of stomata affect rate of transpiration. In dark stomata are closed and stomatal transpiration is checked. Sunken stomata help in reducing the rate of stomatal transpiration. In xerophytes the leaves are reduced in size or may even fall to check transpiration. Thick cuticle on presence of wax coating on exposed parts reduces cuticles transpiration. www.AgriMoon.Com 59  Crop Physiology Antitranspirants A number of substances are known which when applied to the plants retard their transpiration. Such substances are called as antitranspirants. Some examples of antitranspirants are colourless plastics, silicone, oils, low viscosity waxes, phenyl mercuric acetate, abscisic acid, CO2, etc. Colourless plastic, silicone oils and low viscosity waxes belong to one group as these are sprayed on the leaves, form after film which is permeable to O2 and CO2 but not to water. Fungicide phenyl mercuric acetate, when applied in low concentration (10-4 m), it exercised a very little toxic effect on leaves and resulted in partial closure of stomatal pores for a period of two weeks. Similarly ABA a plant hormone also induces stomatal closure. CO2 is an effective antitranspirants. A little rise in CO2 concentration from the natural 0.03% to 0.05% induces partial closure of stomata. Its higher concentration cannot be used which results in complete closure of stomata affecting adversely the photosynthesis and respiration. GUTTATION www.AgriMoon.Com 60  Crop Physiology In some plants such as garden nasturtium, tomato, colocasia etc, water drops ooze out from the uninjured margins of the leaves where a main vein ends. This is called as guttation and takes place usually early in the morning when the rate of absorption and root pressure are high while the transpiration is very low. The phenomenon of guttation is associated with the presence of special types of stomata at the margins of the leaves which are called as water stomata or hydathodes. Each hydathode consists of a water pore which remains permanently open. Below this there is a small cavity followed by a loose tissue called as epithem. This epithem is in close association with the ends of the vascular elements of veins. Under high root pressure the water is given to the epithem by the xylem of the veins. From epithem www.AgriMoon.Com 61  Crop Physiology water is released into the cavity. When this cavity is completely filled with watery solution, the later begins to ooze out in the form of watery drops through the water pore. Difference between transpiration and Guttation Transpiration Guttation 1. Water is lost from aerial parts of plants Watery solution oozes out from uninjured in the form of invisible water vapours margins of aerial leaves only 2. Transpiration occurs mostly through It occurs only through hydathodes (water stomata. It may also takes place through stomata) cuticle and lenticels 3. It takes place throughout the day, its rate It takes place only early in the morning being maximum at noon. when root pressure and the rate of water absorption are higher www.AgriMoon.Com 62  Crop Physiology 07. MINERAL NUTRITION The term, mineral nutrient is generally used to refer to an inorganic ion obtained from the soil and required for plant growth. The chemical form in which elements are applied to plants is called as nutrient. Nutrition may be defined as the supply and absorption of chemical compounds needed for plant growth and metabolism The nutrients indispensable for the growth and development of higher plants are obtained from three sources viz., atmosphere, water and soil. The atmosphere provides carbon and oxygen as carbon dioxide. Carbon is reduced during photosynthesis and oxygen is utilized during aerobic respiration. Soil provides the mineral ions. Essential elements The term essential mineral element was proposed by Arnon and Stout (1939). These are the composition of both macro and microelements, in the absence of any one of these elements the plant cannot maintain its normal growth and develops deficiency symptoms, affects metabolism and die prematurely. Of the many elements that have been detected in plant tissues, only 16 are essential for all higher plants. They are C, H, O, N, P, K, Ca, Mg, S, Zn, Cu, Fe, Mn, B, Cl and Mo. In the absence of each of the essential elements, plants develop deficiency symptoms characteristic of the deficient element and die prematurely. Macronutrients The nutrient elements which are required for the growth of plants relatively in larger quantities are called as major nutrients or macronutrients. The major elements required for growth of plants are C, H, O, N, P, K, Ca, Mg and S. Among these nutrients, C, H and O are taken up by the plants from the atmosphere and water. The N, P, K, Ca, Mg and S are taken up by the plants from the soil and they are applied in the form of chemical fertilizers either through the soil or foliage. Micronutrients The nutrient elements which are required comparatively in small quantities are called as minor or micro nutrients or trace elements. The micronutrients required for the plant growth are Zn, Cu, Fe, Mn, Mo, B and Cl. www.AgriMoon.Com 63  Crop Physiology Tracer elements or labeled elements The nutrient elements that are required for plants are some times labeled and used to study their movement or tracing out the involvement of such nutrients in metabolism in different organs of plants, are called as tracer elements. They may either be stable or radio active types and they are also called as isotopic elements. 15 E.g. Stable isotopes: N, 12C, 31P Radio active : 14C, 32P, 65Zn, 56Fe, 60Co, etc. Hidden hunger When the plants are not able to meet their requirement either one or more of these essential elements, the plants will undergo starvation for such elements. At the initial stage of deficiency of such elements plants will not show any characteristic symptoms which could be exhibited morphologically and due to want of those elements some activities of plants would rather be affected and the internal deficiency is called as Hidden hunger. General role of essential elements In general, an element is essential to the life of a higher green plant for one or more of the following three reasons. 1. It may perform a nutritive role by being a component of one or more of the major classes of plant constituents. 2. It may be a catalytic role either as an action for of an enzyme or as an integral component of an enzyme. 3. It may function as a free ion and thereby exert a balancing role in maintaining electro- neutrality within plant cells (e.g. Potassium). Criteria for essentiality of elements The demonstration of the essentially several elements (macro and micronutrients), especially, micronutrients is rather very difficult. In view of the technical difficulties associated with demonstrating the essentiality of elements required in very small amounts, Arnon and Stout (1939) suggested the adoption of the following three criteria of essentiality for judging the exact status of a mineral in the nutrient of a plant. www.AgriMoon.Com 64  Crop Physiology 1. The element must be essential for normal growth or reproduction and the plant processes cannot proceed without it. 2. The element cannot be replaced by another element. 3. The requirement must be direct i.e., not the result of some indirect effect such as relieving toxicity caused by some other substance. Another recent suggestion to the criteria of essentiality is that some elements might better be called functional or metabolic elements rather than essential elements. This is intended to indicate that an element that is metabolically active, functional or metabolic may or may not be essential. For example in chlorine-bromine, chlorine is designated as a functional element rather than an essential element as chlorine can be substituted with bromine. Based on the mobility in phloem, elements are also classified into three types. 1. Mobile elements : N, K, P, S and Mg 2. Immobile elements : Ca, Fe and B 3. Intermediate : Zn, Mn, Cu, Mo Functions of elements Protoplasmic elements : N, P, S Balancing elements : Ca, Mg, K – counteract to toxic effects of other minerals by causing ionic balance. Frame work elements : C, H2O – as they are the constituents of carbohydrates that form cell walls. Catalytic elements : Mn, Cu, Mg, etc. SOIL LESS GROWTH OR HYDROPONICS The practice of growing plants in nutrient enriched water without soil is called as soil less growth or hydroponics. However, the term hydroponics is now being applied to plants rooted in sand, gravel or other similar matter which is soaked with a recycling flow of nutrient – enriched water. www.AgriMoon.Com 65  Crop Physiology According to a recent limited nations report on hydroponics: In area of tropics, where the water deficiency is the limiting factor in crop production, the soil less methods hold out much promise because of the more economical use of water. The report also indicated that in some areas, lack of fertile soil or very thin soil layers may also move soil less methods worth serious consideration. Besides these the other advantages of growing cucumbers, egg plants, peppers, lettuces, spinach and other vegetables hydroponically under controlled environment are 1. The regulation of nutrients www.AgriMoon.Com