Diurnal Changes in Plant Water Potential and Canopy Temperature of Wheat as Affected by Drought¹

Abstract

Abstract The diurnal courses of plant water potential (ψ plant and temperature (canopy‐air) (ΔT) were determined for 4 days for a wet soil (six levels of water content, θ v , corresponding to values of soil water potential (ψ m ) of −0.1 to −0.5 bars) as compared with a dry soil (six values of ψ m , from −1.6 to −18.0 bars), established by withholding irrigations. In wet soil, ψ plant decreased from near zero at predawn to −16 bars from solar noon until 1600 hours, and then returned to near zero shortly after sunset. For this treatment, ΔT increased from an initial negative value (−2 to −9 C, depending on the saturation deficit) to either the least negative or a slightly positive value near 1000 hours, and then it gradually decreased and was negative at 1400 hours in five of the six wet plots. For dry soil, the ψ plant curve reasonably paralleled that for a well watered crop, but was more negative and did not necessarily return to the predawn value after sunset. For dry soil, ΔT increased rapidly after sunrise, peaked near solar noon (rather than at 1000 hours) and decreased only slightly by 1400 hours, and was positive (0.7 to 7.0 C) in all six dry plots. Because typical diurnal curves for ΔT differed in shape depending on soil water content, canopy temperatures of plants on wet and dry soils differed most near 1400 hours. Thus, it is feasible to use ΔT measured at this time to represent the whole day. Such ΔT measurements can lead to routine monitoring capabilities by remote sensing techniques.