Plant Water Relations

Water potential is referred to as the difference between the free energy of water in a cell and of pure water at atmospheric pressure. Water always moves from a region of higher water potential to one of lower water potential. It is represented by the Greek letter y (Psi) and is measured by 'bars', one bar being close to one atmosphere of pressure (1 bar = 0. 987 atmosphere of pressure). At atmospheric pressure the water potential of pure water is 0 (zero). Hence all solutions at atmospheric pressure have lower water potential than water i.e., they have a negative value.

Pure water at atmospheric pressure has a zero water potential. Addition of solutes lower the water potential y of 0.1 M solution containing solutes = -2.3 bars. Since the water solution in soil containing dissolved minerals has a higher potential and the plant cells of roots have a lower water potential, water from the soil moves into the root cells of the plant.

The increase in pressure increases the water potential. If a pressure greater than atmospheric pressure is applied to pure water or a solution, the water potential can be raised from zero to a positive value.