Water potential: The difference between the free energy of water molecules in pure water and that of water in any other system (e.g., water in a plant cell or in a solution) is termed as water potential. The concept of water potential was first propounded by Slatyer and Taylor. It is represented by a Greek letter psi (?) and commonly represented as ?w and measured by pressure unit pascal. One megapascal = 10 bars 1 bars = 14.5 lb/n2 = 750 mm Hg = 0.987 atm.