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Drought

Using Leaf Gas Exchange to Quantify Drought in Cotton Irrigated Based on Canopy Temperature Measurements

^a USDA-ARS, Plant Stress and Water Conservation Laboratory, 302 West I-20, Big Spring, TX, 79720
^b USDA-ARS, Plant Stress and Water Conservation Laboratory, 3810 4th Street, Lubbock, TX 79415
^c USDA-ARS, 1001 Holleman Drive East, College Station, TX 77840

^* Corresponding author (jtbaker{at}lbk.ars.usda.gov)

Received for publication February 24, 2006. Plant gas exchange provides a highly sensitive measure of the degree of drought stress to which a crop is exposed. However, equipment costs and time requirements for gas exchange measurements are major obstacles to the use of gas exchange measurements in real-time irrigation scheduling systems. 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 leaf-level gas exchange parameters, which were viewed as alternative indicators of water deficit. To provide a wide range of plant water deficit conditions for the comparison of T_c with leaf-level gas exchange parameters, cotton (Gossypium hirsutum L.) was subsurface drip irrigated using T_c according to the stress-time index method of irrigation scheduling during 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 leaf level net assimilation (A) and stomatal conductance (g) at a photosynthetically active radiation (PAR) level of 1500 µmol (photons) m^–2 s^–1. In general, 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. Furthermore, we found the leaf minus air temperature differential (T_L – T_a) and vapor pressure deficit calculated based on leaf temperature (VPD_L) were better predictors of the degree of drought stress, as indicated by gas exchange parameters, than T_L alone. Regression of A and g against (T_L – T_a) and VPD_L indicated that the combination of these two variables accounted for >79% of the variability in A and g. We conclude that the term (T_c – T_a) either alone or in combination with VPD should provide a better predictor of the degree of drought stress in cotton than T_c alone.

Abbreviations: A, leaf level net assimilation • g, light saturated stomatal conductance • CWSI, crop water stress index • PAR, photosynthetically active radiation • PFD, photosynthetic photon flux density • SDD, stress degree day • ST, stress time • T_a, air temperature • T_c, canopy temperature • T_L, leaf temperature • VPD_L, vapor pressure deficit calculated based on leaf temperature