Crop evapotranspiration, photosynthesis, and water use efficiency as affected by environmental variables

Field experiments were conducted in 1997 to study influences of water stress on gas exchange of cotton canopy, and in 1998 to compare sunflower (C₃) and sweet corn (C₄) canopies as affected by environmental variables, particularly water vapor pressure deficit (VPD). Leaf gas-exchange characteristics of crops grown in the open field and under elevated CO₂ concentration in growth chamber were measured to obtain some parameters for modeling canopy water-use efficiency (WUE).; Evapotranspiration (ET), CO₂ flux (FCO₂), and energy fluxes over canopies were measured with Bowen ratio/energy balance/CO ₂ gradient systems. Photosynthetically active radiation (PAR), canopy surface temperature, humidity and CO₂ concentration surrounding the canopy, and wind speed were also measured. Canopy conductance (g _cw) was calculated with the Penman-Monteith equation. Soil CO₂ efflux, leaf area index (LAI), leaf water potential, and light interception were measured periodically. On selected days, growth rate of cotton and stem diameter change of sweet corn were also monitored.; Results reveal that FCO₂ and ET of cotton fields were greatly reduced by water stress due to low LAI early in the season, and also by low canopy (apparently stomatal) conductance and limited photosynthetic capacity late in the season. Except for water stressed crops under high VPD weather, diurnal FCO₂ didn't consistently show hysteretic response to PAR. Radiation-use efficiency didn't respond to variations in VPD for well-watered crops. With contrasting stomatal response to humidity between sunflower and sweet corn, g_cw and energy partition, hence ET, of the two crops also responded differently to humidity. Results illustrated different relations, which depend on the response of g_cw to humidity, between WUE and VPD among crops. Scalar and energy fluxes, plant water status, and crop microenvironment all responded rapidly to changes in radiation caused by movements of cloud. Large diurnal fluctuations of CO₂ concentration in the fields were observed on calm days. A simple WUE model, derived originally for single leaves and taking air CO₂ concentration and humidity into account, was directly applied to canopies without explicitly upscaling. Its predictions were remarkably close to the measured values for cotton and sunflower, and reasonable close for sweet corn.