Water Potential (Ψ)

Water potential, given the symbol Ψ (the Greek letter psi) and measured in Pascals, is a way of expressing how much potential energy water has in a given system compared with pure water at the same temperature and pressure - in effect, a measure of how freely water is able to move. The concept was introduced by Slatyer and Taylor in 1960 and has since become the standard way plant physiologists describe and predict the direction water will move. Pure water under standard conditions is fixed by definition at a water potential of exactly zero; dissolving any solute in it lowers the kinetic energy of the water molecules and so pushes the water potential below zero, meaning any real solution always has a lower water potential than pure water. Because water always moves from a region of higher water potential toward a region of lower water potential, wherever a water-potential gradient exists between two points in a plant, water flow between them can be predicted just from knowing the sign and size of that gradient. Water potential is formally the sum of two contri …