Biochemical acclimation, stomatal limitation and precipitation patterns underlie decreases in photosynthetic stimulation of soybean (Glycine max) at elevated [CO2] and temperatures under fully open air field conditions

Background/Question/Methods

Growth in elevated atmospheric CO₂ concentration ([CO₂]) predictably stimulates C₃ productivity regardless of the biochemical acclimation of photosynthesis reported for many plants.  When combined with increases in temperature the net effect of photosynthetic acclimation to elevated [CO₂] on plant productivity is poorly resolved.  We assessed the effects of canopy warming (+3.5 ^oC) and fully open air [CO₂] enrichment (+ 200 µmol mol^-1 [CO₂]) on 1) the acclimation of two biochemical parameters that frequently limit photosynthesis (A), the maximum carboxylation capacity of Rubisco (V_c,max) and the maximum potential linear electron flux through photosystem II (J_max), 2) the associated responses of leaf structural and chemical properties related to A, as well as 3) the physical limitations imposed on A, in a factorial experiment for soybean over two growing seasons in a conventionally managed agricultural field in Illinois, USA. 

Results/Conclusions

Acclimation to elevated [CO₂] was consistent over two growing seasons with respect to V_c,max and J_max.  However, elevated temperature had little effect on V_c,max but significantly decreased J_max.  Large seasonal differences in precipitation altered soil moisture availability modulating the complex interaction of elevated temperature and CO₂ on biochemical and structural properties related A.  In 2009, higher Leaf N and soil moisture apparently ameliorated the detrimental effects of temperature on acclimation prior to senescence, but in 2011 low soil moisture coupled with warmer ambient temperatures combined to exacerbate the effect of chronic temperature elevation.  Elevated temperature also reduced the benefit of elevated [CO₂] by eliminating decreases in stomatal limitation at elevated [CO₂], an effect that was more pronounced in a dryer hotter year.  Because photosynthetic parameters are included in models used to predict assimilation from the leaf to the ecosystem, the greater than expected acclimation of key photosynthetic processes reported here has important implications for C₃ photosynthesis beyond that of soybeans. Assessing the magnitude and direction of acclimation is crucial to our understanding of global carbon flux and food security because A modulates the largest exchange of carbon from the atmosphere into ecosystems and is an important determinant of crop yields.