Carbon dioxide and temperature effects on evapotranspiration and water use efficiency of soybean

Rising CO2 and potential global warming will cause changes in evapotranspiration (ET). Our research objective was to determine the impact of CO2 and air temperature on canopy ET, water use efficiency (WUE), foliage temperature, and canopy resistance (Re) of soybean [Glycine max (L.) Merr.]. Plants were grown in sunlit, controlled-environment chambers at cyclic maximum/minimum air temperatures from 28/18degreesC to 44/34degreesC and CO2 of 350 or 700 mumol mol(-1). Maximum ET rate in the early afternoon at 35 d after planting ranged from 7.5 mol m(-2) s(-1) at 28/18degreesC to 19.0 Mol m(-2) s(-1) at 44/34degreesC. Daily ET during the middle of the season ranged from 260 mol H2O m(-2) d(-1)(4.7 mm d(-1)) at 28/18degreesC to 660 mol H2O m(-2) d(-1)(11.9 mm d(-1)) at 44/34degreesC. Mean daily ET was linearly related to mean air temperature (T-air) as: [Mean daily ET = 21.4 X T-air - 306, r(2) = 0.99 (mol H2O m(-2) d(-1)), or mean daily ET = 0.385 X T-air - 5.5 (mm d(-1))]. Doubled CO2 caused a 9% decrease in ET at 28/18degreesC, but CO2 had little effect at 40/30degreesC or 44/34degreesC. Whole-day WUE declined linearly with air temperature, with a slope of -0.150 [(mumol CO2 mmol(-1) H2O) degreesC(-1)]. Changes in ET and WUE were governed by changes in foliage temperature and Re. In conclusion, increases in temperature anticipated by climate change could more than offset decreases of ET that would be caused by increases in CO2.