The growth of soybean under free air [CO(2)] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity.
Down-regulation of light-saturated photosynthesis (A(sat)) at elevated atmospheric CO(2) concentration, [CO(2)], 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 [CO(2)]. A soybean free air [CO(2)] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO(2)] 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 [CO(2)] (C(i)) throughout two growing seasons. Mesophyll conductance to CO(2) (g(m)) was determined from the responses of A(sat) and whole chain electron transport (J) to light. Elevated [CO(2)] 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 [CO(2)]. 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(2)], most significantly a smaller response of A(sat) and an apparent shift in resources away from Rubisco relative to capacity for electron transport.
Bernacchi
Carl J
CJ
Atmospheric Environment Section, Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820, USA. bernacch@uiuc.edu
Morgan
Patrick B
PB
Ort
Donald R
DR
Long
Stephen P
SP
D002245:Carbon Dioxide / Q000494:pharmacology*; D002940:Circadian Rhythm; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology; D012273:Ribulose-Bisphosphate Carboxylase / Q000502:physiology*; D012621:Seasons; D013025:Glycine max / Q000201:enzymology / Q000254:growth & development / Q000502:physiology*; D013997:Time Factors
D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase