permanent wilting point A concentration of liquid water in soil at or below which a plant will wilt and no longer recover its turgidity when placed in a water saturated atmosphere. PWP The type of plant and soil are sources of variation for values of this concentration. The physical definition of the wilting point is conventionally measured as the water content at −1,500 kPa (−15 bar) of suction pressure, or negative hydraulic head. wilting point concentration of liquid water in soil