Abstract
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.
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