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Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of [CO₂] Are Closely Coupled with Decreases in Ecosystem Evapotranspiration^1,2,[W],[OA]

Center for Atmospheric Sciences, Illinois State Water Survey, Champaign, Illinois 61820 (C.J.B., D.R.Q.); Department of Plant Biology (C.J.B., D.R.Q., S.P.L., D.R.O.) and Department of Crop Sciences (S.P.L., D.R.O.), University of Illinois at Urbana-Champaign, Champaign, Illinois 61801; United States Arid-Land Agricultural Research Center, Agricultural Research Service, United States Department of Agriculture, Maricopa, Arizona 85239 (B.A.K.); and Photosynthesis Research Unit, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois 61801 (D.R.O.)

Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO₂ ([CO₂]) have been shown to decrease stomatal conductance (g_s) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease in g_s will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO₂] on g_s at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 µmol CO₂ mol^–1 air) and elevated [CO₂] (approximately 550 µmol mol^–1) using free air CO₂ enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO₂] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with g_s of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in g_s results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO₂]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased g_s of upper canopy leaves at elevated [CO₂] results in decreased transfer of water vapor to the atmosphere.

² This work was supported in part by the Office of Science (Biological and Environmental Research program), U.S. Department of Energy (grant no. DE–FG02–03ER63685 to C.J.B.). SoyFACE was funded by the Illinois Council for Food and Agricultural Research, Archer Daniels Midland Company, Pioneer Hi-Bred International, and the U.S. Department of Agriculture Agricultural Research Service.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Carl J. Bernacchi (bernacch{at}uiuc.edu).

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.089557

^* Corresponding author; e-mail bernacch{at}uiuc.edu; fax 217–244–0220.

Received September 6, 2006; accepted November 14, 2006; published November 17, 2006.

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