Water Potential and its components
Concept of water potential (Ψw):-
- Osmotic movement of water takes place by a driving force which is the difference between free energies of water on two sides of the semi-permeable membrane.
- Free energy for per mole of non-electrolyte is known as chemical potential. It is denoted by greek letter Psi (Ψ).
- For water, chemical potential is known as water potential and denoted by Ψw. Free energy of water increases with increase in temperature.
Ψw = Ψs + ΨP + Ψm + Ψg
Ψw = Water potential
Ψs = Solute potential
ΨP = Pressure potential
Ψm = Metric potential
Ψg = Gravity potential
Note:- Ψs is always negative.
The total water potential is the sum of four different components:
Metric Potential (Ψm):- The binding of water to surfaces.
Osmotic Potential (Ψs):- Binding to solutes in the water.
Gravity potential (Ψg):- The position of water in a gravitational field.
Pressure potential (ΨP):- Hydrostatic or pneumatic pressure on the water.
But at the cell level Ψm and Ψg are insignificant. Hence –
Ψw = ΨP + Ψs
Ψw = Water potential
ΨP = Pressure potential
Ψs = Solute potential
- Water potential decreases with increase in concentration of solution.
- Water potential of pure water is zero which is maximum.
- In osmosis water moves from high Ψw to low Ψw solution.
Conditions:-
i. Fully plasmolysed cell or Fully flaccid cell:- ΨP = 0
Ψw = Ψs
ii. Fully turgid cell:- ΨP = Ψs
Ψw = 0
Osmotic Adjustment:- The process of lowering of osmotic potential by net solute accumulation in response to water stress, is called osmotic adjustment. It has been considered to be a beneficial drought tolerance mechanism in some crop species.