PLANT WATER RELATION

 

Ø Plant Water Relation:-

·         Water is the most important component for the living cell and is responsible for the physiological movement of living organisms.

·         Watermelon has about 92% of water, Herbivorous plant have 10 to 15% water of its weight.

·         Woody plant has little water than the soft plant. Ex. A mature corn plant absorbs about three litter of water while a mustered plant absorb equal to its weight.

·         Terrestrial plant also have huge amount of water but most of them are lost through evaporation called transpiration.

·         Seed also absorb water through a special type of diffusion called imbibation

1.  Water Potential:-

·         Water potential of a cell is the combination of solute potential and pressure potential. i.e. Ψ_w = Ψ_s + Ψ_p

·         Water potential is denoted by Ψ_w  and calculated in the Pascal (Pa)

·         Water molecules have their own kinetic energy because it has random movement in gaseous and liquid form. That means greater the concentration of water have greater their kinetic energy so having greater water potential.

·         The water potential of pure water at standard temperature and pressure is zero.

·         The water potential of pure water is greater than the water potential of solution.

Solute Potential:-

·         When a solute dissolve in a pure water, the concentration of solution decreases so the water potential also decreases i.e. the magnitude of lowering of water potential is called solute potential.

·         It is represented by Ψ_s and is always negative.

·         The water potential = solute potential of a solution at atmospheric pressure.

·         When the pressure greater than atmospheric pressure is applied to a pure water or a solution, increases the water potential.

Pressure Potential:-

·         When water enters into the plant cell the cell becomes turgid i.e. the pressure applied by the protoplasm to the cell wall is called pressure potential.

·         It is represented by Ψ_p  and is usually positive.

 

2.  Osmosis:-

·         The phenomenon of diffusion of water from the higher chemical potential to the lower chemical potential through the semi­­-permeable membrane called osmosis.

·         It is spontaneous process and continuous until equilibrium is reached.

·         The net direction and rate of osmosis depends on the following factors-

1.       The pressure gradient.

2.       The concentration gradient. 

·         There are several experiments occurs regarding the osmosis as

1.       Potato osmometer experiment- we make a cavity in potato and fill them with sugar solution then place the whole system in pure water we will look that the level of sugar solution increased.

2.       Second experiment is that a sucrose solution taken in funnel is separated from the pure water in a beaker through a semi-permeable membrane, after some hours the level of solution in flask increased from previous level.

Osmotic pressure:-

·         The pressure applied to the solution of lower concentration to stop the osmosis before equilibrium called osmotic pressure.

·         Numerically osmotic pressure is equivalent to the osmotic potential but sign is taken opposite to each other i.e. osmotic pressure is taken in positive and osmotic potential is negative.

Reverse osmosis:-

·         The phenomenon in which the pressure greater than osmotic pressure is applied to the solution of lower chemical potential then the water flow stats from the lower chemical potential to the higher chemical potential called reverse osmosis.

3.  Plasmolysis:-

·         Plasmolysis is the phenomenon in which water moves from the cell to the surrounding solution.

·         For Plasmolysis it is necessary that surrounding solution should be hypertonic (more concentrated) in nature.

·         In the plasmolysis the cell membrane and cytoplasm shrinks from the cell wall.

·         Plasmolysis becomes reversible when the cell placed in the hypotonic solution (Less concentrated). In this water diffuses into the cell so the cytoplasm build a pressure against the cell wall called turgor pressure.

·         When the cell placed in isotonic solution (balanced i.e. equilibrium state) then there is no any phenomenon occurs due to equilibrium and the cell said to be flaccid.

4.  Imbibition:-

·         The phenomenon of diffusion where solid substances (colloids) absorb water to increase their volume called imbibition. Ex. Seed germination.

·         . For imbibation of any solid following conditions are important.

1.       It should have affinity for the liquid.

2.       Water potential gradient between the solid and liquid

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