Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/14/2006
Publication Date: 1/4/2007
Citation: Bernacchi, C.J., Kimball, B.A., Quarles, D.R., Long, S.P., Ort, D.R. 2007. Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of [CO2] Are Closely Coupled with Decreases in Ecosystem Evapotranspiration. Plant Physiology. 143:134-144. Interpretive Summary: The increasing atmospheric CO2 concentration is likely to cause partial stomatal closure in the leaves of plants growing in the future, and such partial closure would reduce water loss from individual leaves. However, the overall consequences for total plant water requirements are much less certain. Therefore, experiments were conducted during the four growing seasons from 2002-2005 using free-air CO2 enrichment (FACE) at 500 ppm (about 170 ppm above present-day ambient concentrations of 380 ppm) in open-field-grown soybean in which measurements related to water and energy exchange were made. They show that soybean canopy temperatures likely will increase about 0.5'C (0.9'F) at midday and that evapotranspiration will be reduced from 9 to 16% at the elevated concentration of CO2, compared to soybeans grown at today’s ambient CO2 levels. Such increases in canopy temperature may shift the areas for optimal soybean production in the future, while the reductions in evapotranspiration may enable soybean to conserve some soil moisture and grow somewhat further into a drought cycle. This research should benefit soybean growers, as well as the all the human and animal consumers of soybean products.
Technical Abstract: Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentrations of both CO2 ([CO2]) have been shown to decrease stomatal conductance 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 ('E). Since 'E can be influenced both by physiology and physical characteristic of the plant canopy, a response at the leaf level may not be translated to an observable response at the ecosystem level. To test this, soybean (Glycine max) was grown in field conditions under control (ca 375 'mol CO2 mol-1 air) and elevated [CO2] (ca. 550 'mol mol-1) at the SoyFACE research facility in Urbana, IL. Ecosystem 'E was measured from canopy closure to senescence using a residual energy balance approach. This method relies on direct measures of net radiation, sensible and soil heat fluxes to estimate the energy used for the evaporation of water from ecosystem surfaces as well as via the transpiration stream. Elevated [CO2] caused 'E to decrease between 9 and 16% depending on year. The magnitude of the 'E response for soybean mimicked the leaf-level stomatal conductance data collected at SoyFACE for the three years in which the measurements overlapped. The decrease in 'E associated with elevated [CO2] raised the water use efficiencies, which carries important implications for regional climatology. This is particularly true of continental interiors, such as the Midwest Corn Belt, where a large portion of atmospheric humidity during the growing season is provided directly from transpiration.