Water potential, Soil Plant Atmosphere Continuum

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.

Soil Plant Atmosphere Continuum (SPAC):-
>  It is proposed by Huber in 1924.
> All components of field environment i.e. soil, plant and atmosphere, form a physically unified and dynamics networking system in which various water flow processes occur independently. This unified system is called the soil-Plant-Atmosphere-Continuum or SPAC.
 > The fundamental principle of SPAC is water moves from higher total water potential to regions of lower total water potential at a rate depending on the hydraulic resistance of the medium.