Free-air CO2 enrichment does not lessen the impact of drought on soybean photosynthesis under field conditions

Background/Question/Methods

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] (C_i/C_a) 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.