Permanent wilting point
Permanent wilting point or permanent wilting percentage (PWP) is defined as the soil wetness at which a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours. The concept had been introduced in the early 1910s. Lyman Briggs and Shantz (1912) proposed the wilting coefficient, which is defined as the percentage water content of a soil when the plants growing in that soil are first reduced to a wilted condition from which they cannot recover in approximately saturated atmosphere without the addition of water to the soil. See pedotransfer function by Briggs.
Frank Veihmeyer and Arthur Hendrickson from UC Davis found that it is a constant (characteristic) of the soil and is independent of environmental conditions. It is usually taken as the water content at -1500 J kg-1 (Richards and Weaver, 1943). However others have stated that the PWP values under field conditions are not constants for any given soil, but are determined by the integrated effects of plant, soil and atmospheric conditions.
References
Veihmeyer F J and Hendrickson A H 1928 Soil moisture at permanent wilting of plants. Plant Physiol. 3, 355-357.