Carbon Dioxide and Temperature Effects on Evapotranspiration and Water Use Efficiency of Soybean

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