Canopy air temperatures and evapotranspiration from irrigated and stressed soybeans

Soil-water stress as it affects the plant-water status and crop production is becoming increasingly important. When soil-water stress is severe, partial closure of the stomates causes a repartitioning of the incident energy, often resulting in increased temperatures. The effect of soil-water stress on air temperatures within a soybean canopy was studied and the increased air temperatures related to decreased evapotranspiration and plant-water stress. One treatment of soybeans was trickle-irrigated when the matric potential at the 15-cm depth was equal to −0.2 bar. Air-temperature profiles were measured in both irrigated and nonirrigated field-grown soybeans, using calibrated thermistors. Evapotranspiration was measured using a portable chamber. Plant-water status was evaluated indirectly through the use of LVDT's (linear variable displacement transducers). The measured stem-diameter changes were related to the air temperature differences in the irrigated and nonirrigated canopies. The data showed as soil-water stress became more severe, canopy air temperatures within nonirrigated soybeans increased above those within the irrigated soybean canopy. The above-canopy minus the within-canopy temperature difference between the irrigated and nonirrigated plots increased during peak radiation with little difference at night. When the plant-wilt symptoms indicated severe stress, evapotranspiration decreased 40–70% when within-canopy air temperatures increased. This increase in the canopy air temperature was related to stem-diameter shrinkage and may be an indirect measure of the plant-water status under field conditions.