(turgor potential) A component of *water potential
, represented by the symbol ψρ. Pressure potentials may be negative, in which case they represent tensions. Tensions arise as a result of transpiration and are caused by resistance of the tissues to water flow. In extreme conditions very low (-150 bar) pressure potentials can develop. Pressure potential gradients are responsible for the upward movement of water in the xylem (see cohesion theory
). Positive pressure potentials are termed hydrostatic pressures.