Deficit Irrigation: Arriving at the Crop Water Stress Index Via Gas Exchange Measurements.

Plant gas exchange provides a highly sensitive measure of the degree of drought stress. Canopy temperature (T_c) provides a much easier to acquire indication of crop water deficit that has been used in irrigation scheduling systems, but interpretation of this measurement has proven difficult. Our goal was to test the ability of T_c to quantify the degree of crop water deficit by comparing T_c with simultaneous measurements of cotton leaf-level gas exchange parameters in the field over two growing seasons.  Comparisons between T_c and leaf-level gas exchange were accomplished by measuring T_c diurnally with hand-held infrared thermometers and controlling cuvette leaf temperature (T_L) equal to T_c and then measuring light saturated leaf level net assimilation (A) and stomatal conductance (g).  As plant water deficit became more severe, leaf level gas exchange tended to decline with rising T_L. However, we found that A and g could vary by more than twofold at a given T_L, indicating that T_c was not a particularly robust indicator of the degree of drought stress. Using regression analysis (MAXR, SAS Institute) we found that variables used by the Crop Water Stress Index (CWSI) were better predictors of the degree of drought stress than T_L alone. We conclude that canopy minus air temperature differential (T_c -T_a) either alone or in combination with vapor pressure deficit (VPD) should provide a better predictor of the degree of drought stress than T_c alone.