Symbol Ψ. The difference between the chemical potential of the water in a biological system and the chemical potential of pure water at the same temperature and pressure. It is manifested as a force acting on water molecules in a solution separated from pure water by a membrane that is permeable to water molecules only and can be expressed as the sum of the solute potential and the pressure potential:
Water potential is measured in kilopascals (kPa). The water potential of pure water is zero; aqueous solutions of increasing concentration have increasingly negative values. Water tends to move from areas of high (less negative) water potential to areas of low (more negative) water potential. Osmosis in plants is described in terms of water potential. In soils and other extracellular systems, another factor, called matric potential, can contribute significantly to water potential. See also turgor.