The difference between the free energy of water molecules in pure water and the free energy of water molecules present in a solution is called water potential.
This is expressed by Greek symbol ψ(Psi).
The unit of water potential is pascal (Pa). Water potential (ψ) = [Free energy of molecules of pure water] – [Free energy of water molecules of a solution]
According to the modern concept, D.P.D. is expressed as water potential (ψw) , osmotic pressure (O.P.) is ψs , expressed as osmotic potential (ψs) and turgor pressure (T.P.) is expressed as pressure potential (ψp).
Accordingly relation between water potential, osmotic potential and pressure potential is expressed as follows: Water potential = Osmotic potential + Pressure potential
(ψw) = ψs + ψp