Permanent wilting point

If the soil water content decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.

The physical definition of the wilting point, symbolically expressed as θpwp or θwp, is said by convention as the water content at −1,500 kPa (−15 bar) of suction pressure, or negative hydraulic head.

Lyman Briggs and Homer LeRoy 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.

[2][3] See pedotransfer function for wilting coefficient by Briggs.

Frank Veihmeyer and Arthur Hendrickson from University of California-Davis found that it is a constant (characteristic) of the soil and is independent of environmental conditions.