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,a,) at elevated atmospheric CO2 concentration, [CO2], has been demonstrated for many C-3 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 Asat by 15-20% even though there was a small, statistically significant, decrease in Vc,max. This differs from previous studies in that Vc,(max)/J(max) decreased, inferring a shift in resource investment away from Rubisco. This raised the Ci 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 gm, 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 [CO,,], most significantly a smaller response of A(sat) and an apparent shift in resources away from Rubisco relative to capacity for electron transport.