The growth of soybean under free air [CO₂] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity

Down-regulation of light-saturated photosynthesis (A _sat) at elevated atmospheric CO₂ concentration, [CO₂], has been demonstrated for many C₃ species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO₂]. A soybean free air [CO₂] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO₂] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V _c,max) and electron transport through photosystem II (J _max) were determined from the responses of A _sat to intercellular [CO₂] (C _i) throughout two growing seasons. Mesophyll conductance to CO₂ (g _m) was determined from the responses of A _sat and whole chain electron transport (J) to light. Elevated [CO₂] increased A _sat by 15-20% even though there was a small, statistically significant, decrease in V _c,max. This differs from previous studies in that V _c,max/J _max decreased, inferring a shift in resource investment away from Rubisco. This raised the C _i at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in V _c,max was not the result of a change in g _m, which was unchanged by elevated [CO₂]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO₂], most significantly a smaller response of A _sat and an apparent shift in resources away from Rubisco relative to capacity for electron transport.