It is generally accepted that the first parameter of plant growth affected under conditions of water stress is cell expansion. Cell expansion is dependent upon the parameters in Lockharts Law, E = (P-Y), where Y is the "yield threshold" of the cell wall, a minimum value of turgor (P) required for cell expansion to occur and therefore an important component of cell growth. Leaf cutter psychrometers were used to obtain the yield threshold of expanding and non-expanding leaves of cotton grown under various irrigation regimes. Measurements were made pre-dawn to assure the requirement for measurement of P > Y.