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.