Diurnal changes in the instantaneous water use efficiency of a soybean crop

Abstract

Variations in the instantaneous transpirational water use efficiency for a soybean crop were evaluated as affected by time of day, water stress, and vapor pressure deficit (VPD). Canopy transpiration (TR) and photosynthesis (CER) rates were measured hourly during five days of a nine-day soybean field experiment. One treatment was subjected to a drying cycle and one treatment was well-irrigated during the nine-day period. Instantaneous water use efficiency (WUE i) was calculated by dividing canopy CER by TR for each hourly measurement. Soybean canopy WUE i varied with time of day and with water stress. Theoretical considerations result in a predicted inverse relationship between WUE i and VPD with a crop-dependent slope. For well-watered conditions, the WUE i was well-described by this relationship. During water stress conditions, soybean CER was reduced more than TR. As a result, WUE i decreased as water stress progressed and was considerably lower than that of well-watered plants during the last three days of the drying cycle.