Wheat Canopy Temperature: Relation to Plant Water Potential1

Abstract Drought‐induced stomatal closure causes a rise in canopy temperature that can be detected remotely by infrared thermometers. Remote sensing of such a rise could lead to large scale irrigation scheduling and improvements in yield forecasts, provided that the increase in canopy temperature is a reliable indicator of plant water stress. Plant water stress can be measured reliably and simply by determining the plant water potential (Ψ plant ). This experiment was undertaken to demonstrate that crop canopy temperature responds specifically to changes in Ψ plant and therefore can be used for long term crop monitoring by remote sensing techniques. The temperature difference between plant canopy and air (ΔT) was related to Ψ plant , as shown by simultaneous, daily measurements of these parameters at 12 sites on six plots under different irrigation treatments. These data were obtained on clear days at predawn (just before sunrise) and 1400 hours. At predawn, Ψ plant ranged from −2.0 bars in recently irrigated soil, in which the volumetric water content (θ v ) = 0.28, to −16 bars at the wilting point (θ v = 0.15). At 1400 hours, Ψ plant ranged from −15 bars when θ v was 0.28 to −30 bars at the wilting point. At 1400 hours, ΔT was −3.8 C at full plant hydration (Ψ plant = −15 bars). Increasing drought decreased Ψ plant progressively and increased ΔT accordingly: when Ψ plant decreased to −19 bars, ΔT was zero; when Ψ plant was −48 bars, ΔT increased to 4.8 C. In demonstrating the relationship between ΔT and Ψ plant these results support the validity of the temperature difference method for sensing plant response to drought.