Global Warming Can Negate the Expected CO2 Stimulation in Photosynthesis and Productivity for Soybean Grown in the Midwestern United States    

Extensive evidence shows that increasing carbon dioxide concentration (CO2) stimulates, and increasing temperature decreases, both net photosynthetic carbon assimilation (A) and biomass production for C3 plants. However the CO2-induced stimulation in A is projected to increase further with warmer temperature. While the influence of increasing temperature and CO2, independent of each other, on A and biomass production have been widely investigated, the interaction between these two major global changes has not been tested on field-grown crops. Here, the interactive effect of both elevated CO2 (approximately 585 μmol mol(-1)) and temperature (+3.5°C) on soybean (Glycine max) A, biomass, and yield were tested over two growing seasons in the Temperature by Free-Air CO2 Enrichment experiment at the Soybean Free Air CO2 Enrichment facility. Measurements of A, stomatal conductance, and intercellular CO2 were collected along with meteorological, water potential, and growth data. Elevated temperatures caused lower A, which was largely attributed to declines in stomatal conductance and intercellular CO2 and led in turn to lower yields. Increasing both CO2 and temperature stimulated A relative to elevated CO2 alone on only two sampling days during 2009 and on no days in 2011. In 2011, the warmer of the two years, there were no observed increases in yield in the elevated temperature plots regardless of whether CO2 was elevated. All treatments lowered the harvest index for soybean, although the effect of elevated CO2 in 2011 was not statistically significant. These results provide a better understanding of the physiological responses of soybean to future climate change conditions and suggest that the potential is limited for elevated CO2 to mitigate the influence of rising temperatures on photosynthesis, growth, and yields of C3 crops.