Decreases in stomatal conductance of soybean under open-air elevation of [CO2] are closely coupled with decreases in ecosystem evapotranspiration

Abstract

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 (gs) 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 gs will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO2] on gs 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 gs of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in gs 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 gs of upper canopy leaves at elevated [CO 2] results in decreased transfer of water vapor to the atmosphere. © 2006 American Society of Plant Biologists.