PLANT - WATER REALTIONS

Cells are functional and structural units of all organisms. In plants, these cells have cell wall and protoplast. Each cell has its own internal environment. Plasmodesmata connect the protoplasm of adjacent cells. Water is major raw material absorbed by plants and it is essential for all physiological activities of plants. Important roles of water in plant are: - * It acts as solvent. * Helps in uptake and distribution of solutes & minerals for plant growth. * Has a key role in photosynthesis. * Acts as source of oxygen * WATER POTENTIALS: - Plants need free energy to grow and reproduce. Free energy is the potential to do work. The potential energy of water is known as'Water Potential'. Water moves from greater water potential area to lesser water potential. Water potential is measured in relative quantity. It is expressed as the difference between the potential of a solution in a given state and the potential of same solution in standard state. Symbol: -It is represented by greek letter, Psi (ψ). Unit: -Its measurement unit is'Pascal', Pa. 1 Mpa (1 Megapascal) = 10 bars. Factors affecting water potential: -3 factors are there, which affect water potential Amount of solute, pressure and gravity. So, water potential (w) of any solution is the sum total of these three potentials and can be represented as: - w =S +P +g WhereS ='Solute potential' or'Osmotic potential'; presence of solute particle decreases the water potential of pure water. P ='Pressure potential' or'Hydrostatic pressure'. IfP is positive, w gets raised. IfP is negative. w gets reduced. PositiveP is called'Turgor pressure'. g ='Gravity potential'.It depends on height of water, density of water and acceleration due to gravity. In plant cell,g is negligible. So, it is ignored. The equation, thus gets simplified as: - equation, thus w =S +P. According to this equation, If water enters into cell, thep increases. Sow also increases. Thus difference between inside and outside. i.e. w is reduced. If solute concentration increase thenS decreases. Sow also decreases. A water potential gradient develops and water enters into cell from outside. Remember that water always moves from highw to low w. 1.2. Absorption & Movement of Water: - Roots absorb water from the soil in plants. Root hairs present on the root increase the surface area for absorption. Water moves from root hairs to cortex and then to xylem. It moves by two pathways: - * Apoplast pathway: - In this, water moves exclusively through cell wall. Cortex cells are loosely packed here, so water flows without any resistance till endodermis, where casparian strips block the flow. Major part of water flows via apoplast. * Symplast pathway: -In this, water flows from cell to cell through plasmodesmata. So symplast has a network of cytoplasm of all cells interconnected by plasmodesmata. Beyond endodermis, water flows through cell membranes. This is called'Transmembrane Pathway' Water may also cross tonoplast. DIFFUSION: -It is defined as the movement of molecules from the region of higher concentration to the region of lower concentration. A state of equilibrium is reached when there is no further movement of molecules. In plants, uptake of water, gases and solutes occurs by diffusion. PERMEABILITY: -The extent to which a membrane permits or restricts the movement of a substance is called Permeability. It depends upon (i) Composition of membrane and (ii) Chemical nature of solute. It can be measured by calculating the rate at which solute passes through membrane under specific conditions. OSMOSIS: -The movement of water or solvent from a region of higher diffusion pressure to lower diffusion pressure through a semi-permeable membrane is called as'Osmosis'. Demonstration of Osmosis by Thistle Funnel Experiment: - Tie the mouth of thistle funnel with a semi permeable membrane (egg membrane). Put sugar solution inside it. Dip it in a beaker containing water with the help of stand. After some time, water rises in the thistle funnel. This is due to diffusion of water into thistle funnel through semipermeable membrane. i.e. by Osmosis. Osmotic Pressure (O.P): -It is the external pressure which must be applied to stop entry of water/solvent due to osmosis. O.P. is measured by instrument called'Osmometer'. Its unit is Pascal, Pa (1 Pa = 1 Newton/m2) Osmotic Potential / Solvent Potential: -(S)- It is defined as the amount by which water potential is reduced as a result of presence of solute. As chemical potential of water is taken as zero, so the value of solute potential always comes out to be negative. S = - where - means negative of osmotic pressure. Reverse Osmosis: -If more than required osmotic pressure is applied, then water flows out from the solution (in thistle funnel) back into the beaker (containing water). This is Reverse Osmosis. It is used for removing salts from saline water. Factors affecting Osmosis: - (i) Concentration of dissolved solutes in a solution (ii) Pressure difference Plasmolysis: -Shrinkage of the protoplast of a cell from its cell wall when it is placed in a hypertonic solution is called Plasmolysis. Hypertonic solution has more solutes than solvent (water). So water moves out from cell. This causes shrinkage. Imbibition: -It is the phenomenon of absorption of water or any other liquid without forming a solution. Material which absorbs water & swells up is called an Adsorbant. e.g Wood, Seeds, Agar-Agar. These are also known as'Imbibants'. Due to imbibition (water absorption), things swell up & lot of pressure is built. This is called Imbibition pressure / Matric Potential. ( water potential of matrix) Conditions important for imbibition: - * Water potential gradient between adsorbant and liquid. * Affinity between adsorbant and imbibed liquid. * Materials which have negative water potential allow rapid imbibition of water. 1.3 THEORIES OF WATER TRANSLOCATION: - The movement of water from the roots to the tip of plant is called water Translocation. There are 3 main theories for water translocation: - * Root Pressure Theory: -given by Priestley (1916). Root pressure is the pressure developing in elements of xylem due to metabolic activities of root. Water is absorbed by root hairs from the soil. If a plant stem is cut just above soil, water oozes out of it. This is due to root pressure. Mineral ions from soil accumulate in xylem of roots and lower the osmotic potential which lowers the water potential. So water flows into the root. * Capillarity Theory: -When water rises up in small tubes with small diameter which are kept in vessel filled with water, this is called'Capillarity'. Rise in water is due to forces of adhesion and cohesion. In this theory, first water is taken in because of adhesive forces (between water & xylem vessel walls). Then cohesive forces act between water molecules and so water moves up. * Cohesion Theory: -Given by Henry Dixon in 1914. It is the most accepted theory. It states there is a continuous water column in plants because of cohesive forces between water molecules. When water evaporates from leaf, then the water potential decreases. Water from adjacent cells replaces the loss. Then water from xylem compensates this loss. So there is a continuous water column. The pull exerted on the water filled in xylem is known as'Transpiration Pull'. It results in upward movement of water. 1.4. Transpiration: - Definition: -Loss of water, in the form of vapours, from the aerial parts of the plant is known as Transpiration. Most of the transpiration occurs through pores called'Stomata' present on the leaf. Significance: -(i) It results in movement of water. Because when transpiration occurs water is absorbed from soil. (ii) If extra water is absorbed, then transpiration removes it. * It causes cooling effect on leaves (as transpiration involves evaporation which results in cooling). * Increases the absorption of mineral nutrients. Factors affecting Transpiration: - * Humidity: -Rate of Transpiration is inversely proportional to relative humidity. * Temperature: -Transpiration increases with increases in temperature. Higher temperature open the stomata. Lower temperatures close the stomata. * Wind: -Transpiration increases with the wind speed (upto 20-30 km/hr) but if wind speed is too high, then stomata close and become dry. This decreases transpiration. * Atmospheric Pressure: -Low atmospheric pressure increases rate of transpiration * Light: -Stomata open in presence of light & close in darknes. So transpiration is more in light & lesser in darkness. 1.5. Opening & Closing of Stomata: - Definition: -Stomata are tiny pores located on the surface of epidermal layers of leaves. Size of stomatal pore: -3 - 12 mm wide, 10-14 mm long. Number of pores: -1000-60,000 per cm2 of leaf surface. Location: -These occur on upper as well as lower surface of leaf. In monocots, their number is same on both the surfaces but in Dicots, these are more on lower surface. Mechanism of opening and closing of stomata: -Each stomata has two guard cells surrounding the pore. These control opening and closing of stomata. When the guard cells are turgid (i.e. full of water), the stomatal pore opens. When guard cells are flaccid (i.e. have very less water), the pores close. Guard cells take solutes from neighbouring epidermal & mesophyll cells. This lowers osmotic potential and water potential. It creates a gradient and water moves into guard cells which makes them turgid.Major solute taken in is k+ (potassium). Rise in k+ level causes stomatal opening. Decrease in k+ level closes the stomata. Factors affecting stomatal movement: - * Light: -Stomata generally open in light & close in dark. * Temperature: - High temperature (above 35° C) can open stomata even in darkness. Lower temperature (less than 8° C) closes the stomata. * Water: -If less water is available & transpiration is high, there is'water stress' or'water deficit' in plants. It closes stomata. * pH: -High pH opens stomata. Low pH closes it * CO2: -Stomata close when concentration of CO2 is high. These open when concentration of CO2 is low.