By the middle of this Century, summer precipitation in the Midwest U.S. is projected to decrease by 5-40%. Concurrently, atmospheric CO2 concentrations are projected to increase to at least 550 ppm. Elevated CO2 stimulates photosynthetic carbon assimilation and reduces stomatal conductance of C3 plants, while drought decreases carbon assimilation and stomatal conductance of plants. Many models of future ecosystem function and food supply assume that elevated atmospheric CO2 will reduce stomatal conductance, crop water use and soil moisture depletion, thereby ameliorating the impacts of drought on photosynthesis and ecosystem productivity. We tested whether this mechanism operates in soybean exposed to ambient (385 ppm) or elevated (585 ppm) atmospheric CO2 in combination with either naturally occurring precipitation (high-H2O) or reduced precipitation (low-H2O) at the soybean free air CO2 enrichment (soyFACE) facility in Champaign, IL. Rainfall was intercepted with retractable awnings to reduce water inputs into low-H2O plots by 79%. Soil volumetric water content was measured every 2-3 days, photosynthetic gas exchange and root distribution were measured on six dates during the 2009 growing season.
Results/Conclusions
Contrary to expectations, elevated CO2 improved soil moisture only in high-H2O plots. Low-H2O treatment decreased photosynthesis of plants in ambient and elevated CO2, but this reduction was greater in plants grown at elevated CO2 (11%) compared to plants grown at ambient CO2 (7.5%). Low-H2O also caused greater depression of stomatal conductance in plants grown at elevated CO2 (22%) compared to plants grown at ambient CO2 (9%). Consequently, low-H2O treatment caused a significant decrease in the ratio of internal to atmospheric [CO2] (Ci/Ca) in elevated CO2 (7%) and a smaller, non-significant decrease in ambient CO2 (2%), suggesting that low-H2O caused greater stomatal limitation to photosynthesis in elevated CO2. We also measured root distribution to a depth of one meter using a minirhizotron camera. Contrary to expectations, plants in ambient CO2 did not increase root length in response to low-H2O. However, plants in elevated CO2 showed large increases in root length at multiple depths in response to low-H2O. We are examining possible links between the response of root length and stomatal conductance to combined elevated CO2 and low-H2O by assaying leaf abscisic acid content and transcript abundance of drought-responsive genes. Our data suggests that models of future food production and ecosystem function are overly-optimistic about the ability of elevated CO2 to ameliorate drought stress.