Plant Physiology - Musiyenko M.M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
The concept of water potential

To explain the Movement of Water in living systems, the term water potential (ψ, the Greek letter psi) is used. This term is borrowed from Thermodynamics, where it denotes the Free energy of water in a given system, such as a Cell.

Water potential is a measure of the tendency of water molecules to move from one Location to another. Water molecules, like other molecules, always move down a free energy gradient—from a higher value to a lower one, in accordance with the laws of Entropy. The water potential of a system is the difference between the free energy of that system and that of pure water under standard atmospheric pressure.

Water always moves from a region of higher water potential to one of lower water potential. Under standard atmospheric pressure, the WATER POTENTIAL OF pure water is zero. All solutions, by contrast, have a lower water potential, and therefore its value is negative (-ψ). Since dissolved solutes reduce water potential, a hypotonic solution has a higher water potential than a hypertonic one. During osmosis, water molecules cross a semipermeable membrane into the hypertonic solution until the water potential on both sides of the membrane is equalized.

Thus, the term "water potential" makes it possible to evaluate the tendency of water to move between any two systems—not only from one cell to another, but also, for example, from the soil into a ROOT, from a leaf into the air, or from the soil into the air.