Water potential is defined as the difference between the chemical potential of water at any point in a system and that of pure water under standard conditions. The unit of measurement of water potential is the pascal, Pa (1 megapascal, Mpa = 10 bars). It is represented by greek letter PSi (ψ). Water potential has two components solute potential which is always negative and pressure potential which is usually positive. Therefore, water potential ψ_w is expressed as ψ_w = ψ_s + ψ_p.