Global mean temperatures are expected to increase between 1.1 and 6.4 ^oC as global carbon dioxide concentrations increase from current levels to over ca. 550 ppm by the middle of this century.  Theory and modeling studies indicated that moderate increases in temperature and [CO₂] will work synergistically to enhance leaf photosynthesis, potentially above any increase in respiration that might occur.  However, exactly how the interactions of CO₂ and temperature will affect plant productivity and fitness in open air field conditions is less clear.We grew plants under field conditions at the SoyFACE facility at ambient and elevated CO₂ (385ppm or 585ppm) and in ambient or elevated temperature (+3.5 ^oC) in a factorial experiment to assess the impact of elevating [CO₂] and temperature on carbon assimilation and productivity.  We measured carbon uptake, respiration, the associated mechanistic changes underlying photosynthesis (Vcmax, Jmax, Rd), and the consequent impacts on fitness (seed number), total biomass, and harvest index.

Results/Conclusions

In ambient CO₂ (ca. 385 ppm) increasing temperature by 3.5^oC significantly decreased daily integrated photosynthesis by 6.4 %. This led to a 13% decrease in total biomass translating to 6% decrease in fruit/pod number.  In contrast, in elevated CO₂ (585 ppm) higher temperature increased season averaged daily integrated photosynthesis by 3.9% and final biomass by 5%. Overall, elevating temperature significantly decreased electron transport capacity (Jmax) but had little effect on Vcmax in either ambient or elevated CO₂. Consistent with theory, any negative effects of increased temperature on assimilation and fitness at ambient CO₂ were ameliorated by elevating CO₂ although to a lower extent than predicted theoretically. A combination of stomatal limitations, potentially higher respiration, and photosynthetic acclimation are likely responsible for the muted synergistic responses to higher combined CO2 and temperature.