Important terms:
1. Anti –transpirants: Chemical substances that reduce transpiration without affecting the gaseous exchange.
2.Adhesion: The attraction between the molecules of dissimilar substances.
3. Apoplast (Munch 1931): Refers to non-living complex of the plant made of cell walls, intracellular spaces and water filled xylem channels.
4. Cohesion: The attraction between the molecules of the same substances.
5. Capillary Water: The water held against the force of gravity in the capillary like spaces between the soil particles.
6. Diffusion: Movement of particles, ions or molecules from the region of their higher concentration to the region of their lower concentration.
7. Diffusion Pressure Deficit (DPD): The amount by which diffusion pressure of water in a solution is lower than that of pure water.
8. Guttation:The exudation of liquid drops from the margins of leaves through hydathodes in certain plants.(supposed to be due to root pressure).
9. Matric Potential (Imbibition potential): Refers to the reduction in chemical potential of water due to imbibitions by hydrophilic colloids.
10. Osmosis:The movement of water molecules from a region of their higher concentration to the region of their lower concentration through a semi permeable membrane.
11. Osmotic Pressure (OP):The pressure which would develop in a solution when, the solution is separated from its pure solvent by a semi permeable membrane.
12. Photoactive Stomata: Stomata opening in response to the presence of light.
13. Root pressure:The pressure that develops in the root due to active absorption of water, which brings about pushing of xylem sap.
14. Symplast: Refers to living complex of the plant formed by protoplasts connected through plasmodesmata.
15. Scotoactive Stomata: Stomata opening in dark.
16. Transpiration:The loss of water in the form of water vapour from the aerial parts of the plant.
17.Tensile Strength of water:It is the ability of water column to get stretched without breaking. Tensile strength is due to cohesive force amongst molecules of water and adhesion between water molecules and wall of xylem. Its value is 45-207 atm. (Dixon and Joly, 1894).
18. Water potential:The differences between the free energy of water molecules in pure water and the energy of water molecules in other system.
19. Wilting:Drooping of the leaves and young shoots, when the water loss is more than the uptake of water by the roots.
Additional Information
1. Standard solution: A solution containing known weight of the substance dissolved in a known volume of solvent.
2.Molar solution: One gram molecular weight of a solute dissolved in solvent to make final volume of solution exactly one litre.
3. Molal Solution: One gram molecular weight of a solute dissolved in 1 litre of the solvent.
4. Zero Values: DPD or water potential is zero in a fully turgid cell placed in water. Turgor pressure (TP) or pressure potentialbecomes zero, when a cell is placed in hypertonic solution. Osmotic pressure (OP) cannot become zero in a cell. It is only in case of pure water.
5. Relative Humidity and Stomata: Stomata close down if relative humidity of atmosphere falls below 50%. They remain open at R. H. above 70%.
6. Psychrometer: An instrument used for measuring both relative humidity and transpiration.
7. Tensiometer: Instrument used for measuring soil water tension.
8. Atmometer: An apparatus for demonstrating and measuring pull caused by evaporation of water from a porous pot.
9. Absorption Leg (Kramer, 1973) – It is the short fall of rate of water absorption over the rate of transpiration. It is due to high rate of transpiration during midday, development of water deficit around root hairs and resistance in the movement of water inside the plant.
10. Wilting in Waterlogged Soils: Wilting of plants in waterlogged soils is caused by the development of ethylene precursor by the roots (Bradford and Young, 1981), which on reaching the shoot brings about wilted condition.
11. Potometer: It is an apparatus for measuring the rate of transpiration.
12. Porometer: An apparatus for measuring the relative sizes of stomata.
13. Drought: A dry spell of weather and sustained period of significantly subnormal water or soil moisture supply to the plants.
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