megapascals
Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water at atmospheric pressure and ambient temperature . Water potential is denoted by the Greek letter psi and is expressed in units of pressure pressure is a form of energy called megapascals MPa . The potential of pure water wpure H2O is, by convenience of definition, designated a value of zero even though pure water contains plenty of potential energy, that energy is ignored . Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to wpure H2O. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:.