Scaling Photosynthesis Responses To Rising CO2 and Warming From The Leaf To The Canopy For C3 Crops.

Multi-faceted challenges from global climate change and increased demands on agriculture for food, fiber and, increasingly fuel is driving a need to understand how major climate change factors, particularly increasing atmospheric concentrations of CO₂ and rising temperature, will influence photosynthetic carbon assimilation (A). Eight of the ten major crops grown globally utilize the C3 photosynthetic pathway and based on mechanistic understanding of C3 photosynthesis, a synergism exists with rising CO₂ and increasing temperature that is predicted to increase A beyond that of an increase in [CO₂] alone. However, considerable uncertainty surrounds the acclimation response of photosynthesis to global change and, as a result, the influence of physiological adjustments of photosynthesis is currently not represented in leaf, canopy, ecosystem or general circulation models that are used to predict ecosystem-scale responses to global change scenarios. Here, we incorporate into mechanistic leaf and canopy photosynthesis models the acclimation responses of the two key parameters required for modeling C3 photosynthesis, the maximum velocity for carboxylation (V_c,max) and maximum rate of electron transport (J_max), determined from in-field experimentation for soybean, one of the three most widely grown crops. Measurements of V_c,max and J_max from the Soybean Temperature by Free Air CO₂ Enrichment (Soy-T-FACE) experiment in 2009 and 2011 were used to model the response of net carbon uptake to [CO₂] and/or temperature.  The modeling was conducted using the mechanistic leaf photosynthesis model (Farquhar, von Caemmerer, & Berry Model) and the latest generation canopy photosynthesis model with an integrated mechanistic representation of physiology and biophysical components, the Multi-Layer Canopy (MLCan) model.  While the theory behind the interactions of [CO₂] and temperature on photosynthesis are well established, the results from this analysis showed that acclimation resulted in a less than predicted increase in productivity for soybean relative to simulations that neglected acclimation.  While whether these observations extend to other crops depends on a number of factors, it is anticipated that if photosynthesis becomes limited by J_max when grown in elevated [CO₂] the response will be similar to that which we have observed for soybean.