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SOYBEAN

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

^a USDA-ARS, Agron. Dep., Univ. of Florida, Gainesville, FL 32611
^b Agron. Dep., Univ. of Florida, Gainesville, FL 32611
^c Agric. and Biol. Eng. Dep., Univ. of Florida, Gainesville, FL 32611

^* Corresponding author (LAllen{at}gainesville.usda.ufl.edu)

Received for publication November 28, 2001. 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 (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 CO₂ 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 H₂O m^-2 d^-1 (4.7 mm d^-1) at 28/18°C to 660 mol H₂O 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 x T_air - 306, r² = 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 28/18°C, but CO₂ 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 CO₂ mmol^-1 H₂O) °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 CO₂.

Abbreviations: CER, carbon dioxide exchange rate • D, air vapor pressure deficit • DAP, days after planting • EST, eastern standard time • ET, evapotranspiration • LAI, leaf area index • PAR, photosynthetically active radiation • Rc, canopy resistance • SPAR, Soil–Plant–Atmosphere research • TOD, time of day • WUE, water use efficiency

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