Plant Water Relation PDF

6 Plant Water Relation Can you recall? Curiosity Box : 11.. 1. Which are the various parts of plant body? 1. What is hydrogen bond? 2. What are the functions of various parts of 2. What are the meanings of spec...

6 Plant Water Relation Can you recall? Curiosity Box : 11.. 1. Which are the various parts of plant body? 1. What is hydrogen bond? 2. What are the functions of various parts of 2. What are the meanings of specific heat, plant body? heat of vaporization and heat of fusion? 3. Which plant tissues are involved in 3. What are adhesive and cohesive forces? transport of water and minerals? Water molecules have good adhesive and cohesive forces of attraction. Due to high surface tension and high adhesive and cohesive Use your brain power force, it can easily rise in the capillaries. It is You know that we need a water pump therefore, a significant molecule that connects to lift water at top of the building. but, how physical world with biological processes. does plants lift the water from soil upto 6.2 Water absorbing organ: canopy without any pump? Root : Plant obtains variety of substances like Root is the main organ of water and water, minerals, nutrients, food and gases mineral absorption. In terestrial plants, plants like O2 and CO2, from its surroundings. absorb water in the form of liquid from the soil Productivity in plants is mainly affected by the however, epiphytic plants like orchids absorb non-availibility of water. water vapours from air with the help of epiphytic Water is considered as ‘elixir of life’. roots having special tissue called velamen. Water constitutes almost 90 to 95% of most Typical root is divisible into four different plant cells and tissues. Water helps the cells regions. In the zone of absorption, epidermal to maintain turgidity and shape. It shows cells (epiblema cells) form unicellular hair like following properties due to which it has great extensions called root hairs. biological importance. Maturation zone 6.1 Properties of water: Root hair zone It is in the liquid form at room temperature Root hair and is the best solvent for most of the solutes. Zone of elongation It is inert inorganic compound with neutral pH Meristematic region when in pure form. Due to this, water is best Root cap transporting medium for dissolved minerals and food molecules. It is best aqueous medium Fig. 6.1 a. : Root tip showing root hair zone for all biochemical reactions occurring in A root hair cell the cells. It is an essential raw material for Mitochondria Cell membrane photosynthesis. Water has high specific heat, high heat of vaporization and high heat of Cell wall fusion. Due to this, it acts as thermal buffer. Nucleus Vacuole Cytoplasm These various properties are due to hydrogen Root epithelial (epiblemal) bonds between the water molecules. cell Fig. 6.1 b. : Structure of root hair 119 Structure of root hair : a. Imbibition: Root hair is cytoplasmic extension Imbibition is swelling up of hydrophillic (prolongation) of epiblema cell. Each root hair colloids due to adsorption of water. Substance may be approximately 1 to 10mm long and that adsorbs water / liquid, is called as imbibant tube like structure. It is colourless, unbranched, and water/ liquid, that gets imbibed is called short-lived (ephemeral) and very delicate. It as imbibate. The root hair cell wall is made has a large central vacuole surrounded by thin up of pectic compounds and cellulose which film of cytoplasm, plasma membrane and thin are hydrophillic colloids. During Imbibition, cell wall, which is two layered. Outer layer is water molecules get tightly adsorbed without composed of pectin and inner layer is made up the formation of solution. Imbibition continues of cellulose. Cell wall is freely permeable but till the equilibrium is reached. In other words, plasma membrane is selectively permeable. water moves along the concentration gradient. 6.3 Water available to roots for absorption: Imbibition is significant in soaking of Plants absorb water from the rhizosphere seeds, swelling up of dried raisins, kneading of (the microenvironment surrounding the root). flour etc. Water present in the soil occurs as gravitational (free) water, hygroscopic water, combined Use your brain power water and capillary water. Water percolates Why do the wooden doors become deep, due to the gravity, in the soil, is called very hard to close and open in rainy season? ‘gravitational water’. This is not available to plants for absorption. Fine soil particles b. Diffusion: Diffusion means to disperse. imbibe/ adsorb water and hold it. This is called Diffusion can be defined as the movement of ‘hygroscopic water’. Roots cannot absorb it. ions/ atoms/ molecules of a substance from the Water present in the form of hydrated oxides region of their higher concentration to the region of their lower concentration. The movement of silicon, aluminum, etc., is called ‘combined is due to the kinetic energy of the molecules. water’. It is also not available to plants for Diffusion continues till an equilibrium is absorption. Some amount of water is held in reached. Thus, water passes into the cell by pores present between the neighbouring soil diffusion through a freely permeable cell wall. particles, due to capillarity. This is called Water is now at the interface of cell wall and capillary water that is avilable for absorption. plasma membrane. 6.4 Absorption of water by roots from soil: Diffusion results in the diffusion pressure Root hair absorbs water by employing three (D. P.) which is directly proportional to the physical processes that occur sequentially- viz. number of diffusing particles. Diffusion imbibition, diffusion and osmosis. pressure of pure solvent (pure water) is always Activity : Try this at your home. A. Take 10 ml of pure water in a suitable glass vessel and put 2 - 3 raisins in it. Observe the changes in raisins since the time you put them in water till they become fully swollen i.e. turgid. Why did raisins become turgid? B. Take 10 ml of pure water and add 5 gms of either sugar or salt to it. Let it dissolve and then put the same turgid raisins in it and observe the changes in raisins. What changes did occur in raisins and why? Discuss your observations with your teachers. 120 more than the diffusion pressure of solvent c. Osmosis : It is a process by which water enters in a solution. The difference in the diffusion into the cytoplasm of the root hair cell. Osmosis pressures of pure solvent and the solvent in a is a special type of diffusion of solvent through solution is called Diffusion Pressure Deficit a semipermeable membrane. The cytoplasm (DPD) or Suction Pressure (SP). The term of root hair cell contains minerals, sugars, was coined by B.S. Meyer (1938). Now a etc. In other words, solution inside the cell is days, term water potential is used for DPD. In more concentrated (stronger) than outside the colloquial language, the term DPD is actually cell (weaker). Therefore, solvent from weaker the thirst of a cell with which it absorbs water solution enters into cytoplasm (i.e. to stronger from the surroundings. Water arround cell solution) of cell through a semipermiable wall has more diffusion pressure than cell plasma membrane. This migration of solvent is sap. Due to this, water moves in the cell by called Osmosis. diffusion. Diffusion is significant in plants in Thus, water at the interface of cell wall and the absorption of water, minerals, conduction plasma membrane, enters into the cytoplasm of of water against the gravity, exchange of gases the root hair cell due to osmosis. and transport and distribution of food. With respect to the concentration and osmotic migration, three types of solutions are Outside of cell Water molecules Water-selective recognized viz, pore (aquaporin) i. Hypotonic (weak solution or strong solvent) having low osmotic concentration. ii. Hypertonic (strong solution or weak solvent) having high osmotic concentration. iii. Isotonic having such a concentration of solution where there is neither gain nor loss of water in an osmotic system. In other words, concentration outside and inside the cell is same. Osmosis is of two types viz, Exosmosis and Endosmosis. Exosmosis : It is the diffusion of solvent from the cell outside. It causes flaccidity of cell. Cytoplasm Membrane bilayer Endosmosis : It is the diffusion of the solvent into the cell. It causes turgidity of cell i.e. Fig. 6.2 : Diffusion of water into plant cell across the plasma membrane cytoplasm becomes turgid. Turgidity increases the turgor pressure (T. P.) of the cell. T. P. is the pressure exerted by turgid cell sap on to the cell membrane and cell wall. In a fully Use your brain power turgid cell, DPD is zero. Cell wall being thick 1. When you burn an incense stick in one and rigid, exerts a counter pressure on the cell corner of room, its fragrance spreads all sap. This is called Wall pressure (W. P.). In a over the room in a short time. How does fully turgid cell, T. P. = W. P. but operating in it happen? opposite direction. 2. How does the water come out through Osmotic pressure (O. P.) : The pressure the surface of porous earthen pot? exerted due to osmosis is osmotic pressure. 121 Osmotic pressure is a pressure of the solution, 6.5 Water Potential (ȥ): which is required in opposite direction, so as to According to the principle of stop the entry of solvent molecules into the cell. thermodynamics, every component of a system More simply, osmotic pressure of a solution is having a definite amount of free energy is equivalant to the pressure which must be which is used to do work. Osmotic movement exerted upon it to prevent flow of solvent across of water is on the basis of free energy. Free a semipermeable membrane. energy per molecule in a chemical system, is Therefore, D. P. D. = O. P. - T. P. called its chemical potential. ? ( T. P. = W. P.) Chemical potential of water is called water ?D. P. D. = O. P. - W. P. potential. It is represented by Greek letter psi In a flaccid cell, T. P. is zero ? DPD = OP (ȥ). Water potential of protoplasm is equal but In a turgid cell, DPD is zero ? TP = OP opposite in sign to DPD. It has negative value. The unit of measurement is in bars/ pascals/ Do you know ? atmospheres. Water potential of pure water is always Improtance of T. P. : It keeps cells and zero. Addition of any solute in it, decreases its organelles stretched; provides support to psi (ȥ) value. Therefore, it has negative value. the non-woody tissues; essential for cell enlargement during growth; maintains shape D. P. D. is now termed as water potential. of cell and facilitates opening and closing of O. P. is now termed as osmotic potential. T. P. is now termed as pressure potential. It stoma. has always positive value. Improtance of Osmosis : It is responsible for absorption of water into root; maintains Water always flows from less negative turgidity of cell; facilitates cell to cell potential to more negative water potential movement of water; offers resistance to (i.e. from high water potential area to low drought, frost, etc; also helps in the drooping water potential area). Difference between of leaflets and leaves in vicinity of “touch me water potential of the adjacent cells decides not” plant. movement of water through plasmodesmata across the cells. Facilitated diffusion : The passive absorption Factors affecting water absorption: of solutes when mediated by a carrier, is i. Presence of capillary water is essential. called Facilitated diffusion. Particles that ii. Rate of water absorption is maximum at are lipid soluble can easily diffuse through soil temperature between 200 to 300C. lipoproteinous cell membrane. The diffusion iii. High concentration of solutes in soil water of hydrophilic solutes has to be facilitated reduces the rate of absorption of water. because their diffusion across the membrane iv. Poorly aerated soil shows poor absorption is difficult. Membrane proteins provide rate. such sites for facilitated diffusion. These v. Increased transpiration accelarates the rate proteins are aquaporins and ion- channels. of absorption of water in the irrigated soil. These proteins help move substances across 6.6 Plasmolysis: membranes without the expenditure of energy. Exo-osmosis in a living cell when placed Concentration gradient must be present for the in hypertonic solution, is called plasmolysis. molecules to be diffused through facilitated During plasmolysis, protoplast of cell shrinks diffusion. and recedes from cell wall. Thus, cell becomes 122 flaccid. In a plasmolysed cell, a gap is Absorption of water being a continous developed between cell wall and the protoplast. process, a sort of hydrostatic pressure is This gap is filled up by outer solution. developed in living cells of root. This is called In a plasmolysed cell, T. P. is always zero. root pressure. It is due to root pressure, water When such cell is placed in hypotonic solution, from pericycle is not only forced into the xylem, endo-osmosis occur, making cell turgid. This but also conducted upwards against the gravity. is called deplasmolysis. In a fully turgid cell Pathway of water across the root essentially T. P. = O. P. hence, DPD is always zero. occurs in two ways viz, apoplast and symplast. 6.7 Path of water across the root (i.e. from epiblema upto xylem in the stelar region) : Pathways Water is absorped by root hair cell Apoplast Symplast through imbibition diffusion osmosis, Vacuolar Transmembrane sequentially. Consequently the cell becomes turgid. Its turgor pressure increases, but its DPD When some amount of water passes value decreases. However, the immidiately across the root through the cell wall and the adjacent cortical cell inner to it, has more DPD intercellular spaces of cortical cells of root, it value, because its O. P. is more. Therefore, is then called apoplast pathway. This pathway cortical cell will suck water from the turgid root occurs up to endodermis. hair cell. It then becomes turgid. The flaccid The apoplastic (non-living) pathway root hair cell now absorbs water from soil. provides a route toward the vascular stele Water from the turgid cortical cell is sucked through free spaces and cell walls of the by inner cortical cell and the process goes on. epidermis and cortex. An additional apoplastic Thus, a gradient of suction pressure (DPD) is route that allows direct access to the devloped from cells of epiblema to the cortex xylem and phloem is along the margins of of the root. Consequently water moves rapidly secondary roots. Secondary roots develop across the root through loosely arranged living from the pericycle, a cell layer just inside the cells of cortex, followed by passage cells of endodermis. The endodermis is characterized endodermis (in monocot roots) and finally into by the Casparian strip, a suberized layer that the cell of pericycle. Protoxylem is in the close forces all to move in the symplast in order to proximity with pericycle. enter the vascular system. Since secondary Endodermis Pericycle Phloem roots grow through the endodermis, a direct pathway to the xylem and phloem is available that bypasses the Casparian strip and allows Symplastic to enter the vascular system without moving path into the symplast (living tissue). When water passes across from one living cell to other living cell through plasmodesmata, then it is called symplast pathway. It is also Xylem called transmembrane pathway. Cortex Casparian Apoplastic strip 6.8 Mechanism of absorption of water : path Mainly, there are two ways/ modes of Fig. 6.3 : Pathways for water uptake absorption of water viz, passive absorption and by the root active absorption. 123 a. Passive absorption : a hydrostatic pressure, called root pressure, It is the main way of absorbing water is developed in root cells. This root pressure through the roots and not by the roots from soil forces water from pericycle to xylem and then into the plant. The driving force is transpiration upwards to the stem. pull and it thus proceeds through DPD gradient. 2. Non-osmotic absorption : Kramer and There is no expenditure of energy (ATP) as Thimann (1959) proposed this theory. water moves in accordance to the concentration Sometimes, water is absorbed from soil against gradient. Hence, it is passive absorption. About the concentration gradiant. Such absorption 98% of the total water absorbed in plants, occur requires an expenditure of energy released passively. Passive absorption occurs during during respiration, directly. Poor supply of day time when transpiration is in progress. It oxygen retards water absorption. Moreover stops at night when transpiration stops. low temperature retards water uptake because Rapid transpiration creates a tension in the of decrease in the rate of respiration. Use of xylem vessel due to negative water potential. metabolic inhibitors also retards the rate of This tension is transmitted to xylem in the respiration and thus the water uptake. roots. Consequently water is pulled upwards 6.9 Translocation of water: passively. The transport of water with dissolved During passive absorption, no ATP is minerals from root to other aerial parts like utilized. Obviously, the rate of respiration is not stem and leaves, against the gravity, is called affected. In plants, water is mainly absorbed translocation or ascent of sap. passively. Translocation of water occurs through b. Active absorption : the lumen of conducting elements of xylem- Here, water is absorbed due to activity tracheids and vessels, in all vascular plants. of roots. Root cells play active role in the Ringing experiment has proved that xylem is absorption of water. The driving force is the root the path of ascent of sap. pressure developed, in the living cells of root. Several mechanisms/ theories have Active absorption occurs usually at night when been put forth to explain the mechanism of transpiration stops due to closure of stomata. As translocation of water. The theories include- water absorption is against the DPD gradient, vital force theory, relay pump theory, physical there is expenditure of ATP (energy) generated force theory, root pressure theory, etc. We shall through the respiratory activity of cells. consider following three theories : Active absorption may be of two kinds a. Root Pressure Theory (Vital Theory) : viz, osmotic and non-osmotic : According to this theory, the activity 1. Osmotic absorption : Atkins and Priestly of living cells of root is responsible for (1922) proposed that water is absorbed from soil translocation of water. J. Pristley proposed this into xylem of the root according to the osmotic theory. When a stem of potted plant is cut few gradient. To create osmotic conditions, there is inches above the soil by a sharp knife, xylem an expenditure of energy. But such absorption sap is seen flowing out/ oozing out through the does not directly require an expenditure of cut end. This exudation at the cut end of stem is energy. a good proof for the existence of root pressure. A gradient of DPD develops from cell of As water absorption by roots is constant and epiblema to pericycle due to activity of living continous process, a hydrostatic pressure is cells of root. As the process is continuous, developed in the living cells of cortex of root. 124 This is termed as root pressure by S. Hales. It is b. Capillarity theory (physical force theory): due to root pressure water along with dissolved According to this theory, physical forces minerals is not only forced into xylem but it is and dead cells are responsible for ascent of sap. also conducted upwards against the gravity. This theory was put forth by Bohem in Root pressure seems to be largely an (1863). Wick dipped in an oil lamp, shows osmotic phenomenon and its development is an capillarity due to which oil is raised upwards. active process. The value of root pressure is +1 The conduction of water in a straw dipped in to +2 bars which is enough to pump water to water, is raised to a certain height because of a height of 10 to 20 meters. The factors like capillarity. The height to which water is raised oxygen, moisture, temperature of soil, salt depends on the diameter of the straw. contents, etc. influence the root pressure. Capillarity is because of surface tension, Manometer to measure and forces of cohesion (attraction between like exudation pressure from molecules) and adhesion (attraction between cut stump Manometer unlike molecules). Xylem vessel/ tracheid with Final level its lumen is comparable with straw. Water Initial level column exist because of combined cohesive and adhesive forces of water and xylem wall, Stump due to capillarity. It is because of capillarity water is raised or conducted upwards against the gravity, to few centimeters only. Objections/ Limitations of capillarity theory: Few important objections are : Fig. 6.4 : Experiment to demonstrate root pressure i. Capillary tube (xylem) must be continously and completely hollow from one end to the Objections/ limitations of root pressure other end but tracheids in the xylem show theory: closed end-walls. Although, ascent of sap takes place due to root pressure, there are certain objections ii. The lower end of capillary tube i.e. xylem must be in direct contact with soil water. raised, such as - However, there exists a barrier of root i. It is not applicable to plants taller than 20 cortex between xylem and soil water. meters. iii. Narrower the capillary tube, greater is the ii. Ascent of sap can also occur even in the height to which water column is raised. absence of root system. Thus, taller trees should show xylem iii. Root pressure value is almost nearly zero in vessels with very narrow bore (diameter). taller gymnosperm trees. However, in nature the tall trees show iv. In actively transpiring plants, no root xylem vessels having wider bore. pressure is developed. Hence, to sum up capillarity can not be the v. Xylem sap under normal condition is under sole mechanism to explain ascent of sap in all tension i.e. it shows negative hydrostatic the plants of varying heights. pressure or high osmotic pressure. c. Cohesion- tension theory (Transpiration To sum up therefore, root pressure is not pull theory) : the sole mechanism explaining the ascent of This is presently widely accepted theory sap in all plants of varying heights.. explaining ascent of sap in plants. It was put 125 forth by Dixon and Jolly (1894). This theory is iii. If plant leaves are smeared with vaseline in based on two principles. order to stop transpiration, even then ascent Cohesion and adhesion, and transpiration of sap occurs. pull : iv. Ascent of sap also occurs in deciduous A strong force of attraction between water plants that have shed all of their leaves. molecules, is called cohesive force. While These observations point to the fact that a strong force of attraction between water besides physical forces, activity of living cells molecules and lignified wall of lumen of xylem seems to be necessary for lifting the water vessel, is called adhesive force. column up. Due to combined cohesive and adhesive 6.10 Transport of mineral ions : forces a continous water column is developed Soil serves as main source for minerals. (formed) in the xylem right from root upto the Minerals constitute most commonly occuring tip of the topmost leaf in the plant. solid, crystalline inorganic materials obtained Transpiration pull : The transpiration pull from earth’s crust. Minerals play an important developed in the leaf vessel is transmitted down role in the day to day life of plant. Minerals are to root and thus accounts for the ascent of sap. absorbed by plants in the ionic (disolved) form, Excess water is lost in the form of vapour, mainly through roots and then transported. mainly through the stomata found on leaf. This water loss increases D. P. D. of mesophyll Do you know ? cells. These cells withdraw water ultimately from xylem in the leaf. In otherwords, due to Minerals that play important role in continous transpiration, a gradient of suction the day to day life, are called essential pressure (i.e. D. P. D.) is developed right from elements. About 36 to 40 elements are guard cells up to the xylem in the leaf. This incorporated in the plant’s life. Some minerals like C, H, O, P, N, S, Mg will create a tension (called negative pull or required in large quantity, are called transpiration pull) in the xylem. Consequently, macro elements. While minerals like Cu, water column is pulled out of xylem. Thus, Co, Mn, B, Zn required in small quantity, water is pulled upwards passively against the are called micro elements. gravity leading to the ascent of sap. Objections/ Limitations of transpiration pull The analysis of plant ash demonstrates theory: that minerals are absorbed by plants from soil and surroundings. Absorption of minerals is i. For transpiration pull to operate, water independent of that of water. column should be unbroken and continous. However, due to temperature fluctuations Absorbed mineral ions are pulled in during day and night, gas bubbles may enter upward direction along with xylem sap because in water column breaking the continuity. of transpiration pull. This could be understood when the ascending sap is analysed. Mineral ii. This mechanism assumes that tracheids are ions are needed in the areas of the plant viz. more efficient than the vessels, as their end apical, lateral, young leaves, developing walls support water column. flowers, fruits, seeds and storage organs. However, vessels are more evolved Hence, from the source (root), these are pulled than tracheids and are more efficient. and transported ascendingly through the sap and gets unloaded by fine veins through the 126 process of diffusion in the vicinity of cells. sink. This movement of food from one part to Cells uptake them actively. the other part, is called translocation of food. Soil would not be the only source for mineral Path of translocation : Food is to be uptake. Mineral ions can be remobilized within translocated to longer distances in higher the parts of the plant. Older parts (like leaves in plants. Hence plants must have adequate deciduous plants) export their ions to younger channels for the transport of food. Sieve tubes leaves before the fall. Most readily mobilized and vessels are structurally ideally suited ions are like phosphorus, sulphur, nitrogen for longitudinal (vertical) translocation. The and potassium but the ions from structural ringing experiment, structure and distribution components like calcium is not remobilized. of phloem, chemical analysis of phloem sap and use of isotope 14C, clearly point out that the Internet my friend phloem tissue is primarily responsible for flow of food in longitudinal downward direction. 1. The general roles of minerals in the life The horizontal (lateral) translocation occurs of plants. from phloem to pith or phloem to cortex via 2. The role of essential minerals in the day medullary rays in the stem. to day life of plants. Food is always translocated in the form of Analysis of xylem exudate also shows sucrose (soluble form) and always along the that some nitrogen travels as inorganic ions concentration gradient from source to sink. The whereas much of it is carried in the organic transport of food occurs in vertical and lateral form like amino acids and related compounds. direction. Small amount of inorganic molecules of Vertical translocation : In vertical phosphorus and sulphur are also carried. It was (longitudinal) transport, food is translocated a belief earlier that xylem transports inorganic in downward direction from leaves (source) to and phloem transports organic molecules. stem and root (sink). It also occurs in upward However, it is not correct because some direction during germination of seed, bulbils, exchange of materials also occurs between corm, etc. Upward translocation also occurs xylem and phloem. from leaves to growing point of stem, to developing flowers and fruits situated near the Do you know ? ends of the branches of stem. Different modes of passive absorption and active absorption of minerals in plants. Carrier concept of active absorption. Sieve plate 6.11 Transport of food : All the plant parts require continous Companion cell supply of food (photosynthate) for nutrition Sieve tube and developement. In higher plants, there is a element great differentiation and division of labour. Sieve tube Chloroplasts are confined to green cells of leaves where food is synthesized. The non-green parts like root and stem must received food from Cytoplasm leaves. The part where food is synthesized is called source and while part where it is utilized, Fig. 6.5 : L. S. of sieve tube is called sink. Food has to travel from source to 127 Lateral translocation : It occurs horizontally/ 6.12 Transpiration : laterally across the root and stem. When food Plants absorb water constantly and is translocated from phloem to pith, it is called continously. Hardly 5% of the total water radial translocation and from phloem to cortex, absorbed by roots that is utilised for cell it is called tangential translocation. expansion and plant growth. Remaining 95% The transport of food through phloem is water becomes surplus which is then lost into bidirectional. Phloem sap contains mainly the atmosphere, through its aerial parts. Hardly water and food in the form of sucrose. But 1% of surplus water is lost in the form of liquid sugars, amino acids and hormones are also and 99% of surplus water, is lost in the form of transported through phloem. vapour. The loss of water in the form of liquid Mechanism of sugar transport through is called guttation. It occurs through special phloem : structures called water stomata or hydathodes. The loss of water in the form of vapour is called Several mechanisms/ theories like transpiration that occurs through leaves, stem, diffusion, activated diffusion, protoplasmic flowers and fruits. Most of the transpiration streaming, electro-osmosis, pressure-flow, etc. occurs through the leaves (called foliar are put forth. The most convincing theory is transpiration). The actual water loss during Munch’s pressure flow theory or mass flow transpiration occurs through three main sites hypothesis. - cuticle, stomata and lenticels. Accordingly, Ernst Munch proposed that photosynthetic three types of transpiration are recognized viz. cell synthesizes glucose. Hence, its osmotic cuticular, stomatal and lenticular. concentration increases. Due to endo-osmosis water from surrounding cells and xylem, Water evaporates from the leaves is absorbed. The cell becomes turgid. Due to increase in turgor pressure, sugar from Veins carry water photosynthetic cell is forced ultimately into the into the leaves sieve tube of the vein. This is called loading of Vein. At the sink end, root cell utilizes sugar and also polymerizes excess sugar into the starch. Its osmotic concentration is lowered. Water is drawn up Exo-osmosis occurs. Water in the root cell is the stem to the leaves lost to surrounding cells, thereby decreasing the turgidity of cell. Turgor pressure is lowered. Hence, a turgor pressure gradient is developed Roots take up water from sieve tube in the leaf to the root cell. from the soil Consequently, food is translocated along the concentration gradient, passively. This is Vein unloading. At the sink end sugar is used and excess water exudes into the xylem. Fig. 6.6 : Transpiration Main objection to this theory is that this i. Cuticular transpiration: mechanism does not explain bidirectional Cuticle is a layer of waxy substance- cutin, transport of food. More over, according to present on outer surface of epidermal cells of Munch, pressure flow is purely a physical leaves and stem. Cuticular transpiration occurs process. by simple diffusion and contributes 8-10% of the total transpiration. Cuticular transpiration 128 occurs throughout the day and its rate is inversely proportional to thickness of cuticle. Do you know ? ii. Lenticular transpiration: The number of stomata per unit area of Lenticels are small raised structures leaf, is called stomatal frequency. composed of loosely arranged complementary The correlation between the number of cells. Each lenticel is a porous tissue consisting stomata and number of epidermal cells of cells with large intercellular spaces in per unit area, is called stomatal index (I). the periderm of the secondarily thickened 6.13 Structure of stomatal apparatus : organs and the bark of woody stems and roots Typical stomatal apparatus consists of two of dicotyledonous flowering plants. Lenticels guard cells, stoma and accessory cells. are present in bark of old stem and pericarp of woody fruits but are absent in leaves. Epidermal cells Subsidiary cell Lenticular transpiration contributes only Inner thick wall about 0.1-1.0% of total transpiration. Rate of Lenticular pore Nucleus Loose complementary Stoma pore cells Guard cell Fig. 6.8 (a) : Structure of guard cell Guard cells Stomatal pore Fig. 6.7 : Structure of lenticel lenticular transpiration is very slow. It also occurs throughout the day. Chloroplast iii. Stomatal transpiration: Fig. 6.8 (b) : Open and closed stoma Stomata are minute apertures formed of two guard cells and accessory cells. They are Stomata are minute, elliptical pores located in the epidermis of young stem and bounded by two kidney/ dumbbell shaped leaves. Leaves generally show more number guard cells. Guard cell is a type of epidermal of stomata on the lower surface. Depending tissue which may be called as modified, upon distribution of stomata on leaves, leaves epidermal parenchyma cell. They are kidney- are categorized into three types namely shaped in dicotyledons and dumbbell-shaped epistomatic- on upper epidermis (Hydrophytes- in grasses (monocotyledons/monocots). Epidermal e.g. Lotus), hypostomatic- on lower epidermis cells (Xerophytes- e.g. Nerium) and amphistomatic- Subsidiary on both surfaces (Mesophytes- e.g. Grass). cells Stomatal transpiration occurs only during Chloroplast daytime. (Exception: Desert plants). Guard cells 90 to 93% of total transpiration occurs Stomatal aperture through stomata and that too during day time A. Kidney shaped B. Dumbbell shaped only. Fig. 6.9 : Types of guard cells 129  In Cyperus, both kidney- and dumbbell- According to theory of proton transport shaped guard cells are present. (Levitt-1974), stomatal movement occurs due Guard cells are living, nucleated cells with to transport of protons H+ and K+ ions. During unevenly thick walls. Inner wall (wall facing daytime, starch is converted into malic acid. stoma) of guard cells is thick and inelastic, and Malic acid dissociates to form Malate and its lateral (outer) wall is thin and elastic. Guard protons. Protons are transported to subsidiary cells contain few chloroplasts which are capable cells and K+ ions are imported from them. of poor photosynthesis. Guard cells have ability Potassium malate is formed that increases to change their size and form due to which stoma osmolarity and causes endosmosis. Uptake of opens (widens) or closes (narrows). K+ ions is always accompanied with Cl¯ ions. Stoma is an elliptical pore formed due At night, uptake of K+and Cl- ions is to specific arrangement of guard cells. It is prevented by abscissic acid, changing the through the stoma, excess water is lost in the permeability of guard cells. Due to this guard form of vapour. cells become hypotonic and thereby become Accessory cells : These are specialized flaccid. epidermal cells surrounding the guard cells. Significance of Transpiration : Their number is variable and are the reservoirs Advantages: of K+ ions. These are also called subsidiary cells. i. It removes excess of water. Opening and Closing of Stoma : ii. It helps in the passive absorption of water Opening and closing of stoma is controlled and minerals from soil. by turgor of guard cells. During day time, iii. It helps in the ascent of sap. guard cells become turgid due to endosmosis. iv. As stomata are open, gaseous exchange Thus turgor pressure is exerted on the thin required for photosynthesis and respiration, walls of guard cells. Being elastic and thin, is facilitated. lateral walls are stretched out. Due to kidney v. It maintains turgor (turgidity) of the cells. or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma. During vi. Transpiration helps in reducing the night time, guard cells become flaccid due to temperature of leaf and in imparting exosmosis. Flaccidity closes the stoma almost cooling effect. completely. Endosmosis and exosmosis occur Disadvantages: due to diurnal changes in osmotic potential of Excessive transpiration leads to wilting guard cells. Different theories are proposed to and injury in the plant. It may also lead to the explain diurnal changes in osmotic potential. death of the plant. According to starch-sugar inter- Transpiration is ‘A necessary evil’ : conversion theory (Steward 1964), during day For stomatal transpiration to occur, stoma time, enzyme phosphorylase converts starch must remain open, during day time. When to sugar, thus increasing osmotic potential stomata are open then only the gaseous exhange of guard cells closing entry of water thereby needed for respiration and photosynthesis, will guard cells are stretched and stoma widens. take place. If stomatal transpiration stops, it will The reverse reaction occurs at night brining directly affect productivity of plant through the about the closure of stoma. loss of photosynthetic and respiratory activity. Phosphorylase (Day) Hence for productivity, stomata must remain Starch Sugar open. Consequently transpiration can not be Night avoided. Hence, Curtis (1926) regarded (Stoma closes) (Stoma opens) transpiration as ‘a necessary evil’. 130  Activity : Prepare stomatal frequency chart for any six angiospermic plants in your area. Plant Name Details 131  Exercise Q. 1 Multiple Choice Questions 1. In soil, water available for absorption 8. Water absorption takes place through by root is................................ a. gravitaional water a. lateral roots b. root cap b. capillary water c. root hair d. primary root c. hygroscopic water 9. Due to low atmospheric pressure the d. combined water rate of transpiration will............. 2. The most widely accepted theory for a. increase ascent of sap is.............. b. decrease rapidly a. capillarity theory c. decrease slowly b. root pressure theory d. remain unaffected c. diffusion 10. Osmosis is a property of.................. d. transpiration pull theory a. solute b. solvent 3. Water movement between the cells is c. solution d. membrane due to............. Q. 2 Very short answer questions. a. T. P. b. W. P. 1. What is osmotic pressure? c. D.P.D. d. incipient plasmolysis 2. Name the condition in which protoplast 4. In guard cells, when sugar is converted of the plant cell shrinks. into starch, the stomatal pore............. 3. What happens when a pressure greater a. closes almost completely than the atmospheric pressure is applied b. opens partially to pure water or a solution? c. opens fully 4. Which type of solution will bring about d. remains unchanged deplasmolysis? 5. Which type of plants have negative root 5. Surface tension is due to.............. pressure? a. diffusion b. osmosis 6. In which conditions transpiration pull c. gravitational force d. cohesion will be affected? 6. Which of the following type of solution 7. Mention the shape of guard cells in has lower level of solutes than the Cyperus. solution? 8. Why do diurnal changes occur in a. Isotonic b. Hypotonic osmotic potential of guard cells? c. Hypertonic d. Anisotonic 9. What is symplast pathway? 7. During rainy season wooden doors warp Q. 3 Answer the following questions. and become difficult to open or to close 1. Describe mechanism for absorption of because of............... water. a. plasmolysis b. imbibition 2. Discuss theories of water translocation. c. osmosis d. diffusion 3. What is transpiration? Describe mechanism of opening and closing of stomata. 132 4. What is transpiration? Explain role of Q. 4 Long answer questions. transpiration. 1. Describe structure of root hair. 5. What is significance of transpiration? 2. Write on journey of water from soil to Explain root pressure theory and its xylem in roots. limitations. 3. Explain cohesion theory for translocation 6. Explain capillarity theory of water of water. translocation. 4. Write on the mechanism of opening and 7. Why is transpiration called ‘a necessary closing of stoma. evil’? 5. Explain the active absorption of 8. Explain movement of water in the root. minerals. 9. Define and or explain the terms: Osmosis, diffusion, plasmolysis, Project : imbibition, guttation, transpiration, 1. Prepare powerpoint presentation for ascent of sap, active absorption, DPD, different types of transpiration. turgor pressure, water potential, wall pressure, root pressure. 10. Distinguish between a) Osmotic pressure and turgor pressure b) Diffusion and osmosis. 133