Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2003
Publication Date: 3/1/2004
Citation: Rogers, A., Allen, D.J., Davey, P.A., Morgan, P.B., Ainsworth, E.A., Bernacchi, C.J., Cornic, G., Dermody, O., Heaton, E.A., Mahoney, J., Zhu, X., Delucia, E.H., Ort, D.R., Long, S.P. 2004. Leaf photosynthesis and carbohydrate dynamics of soybeans grown throughout their life-cycle under free-air carbon dioxide enrichment. Plant Cell and Environment. 27:449-458. Interpretive Summary: Carbon dioxide in the world's atmosphere is on the rise. In the last century it has risen 20% and this geologically unprecedented pace will continue resulting in an atmosphere in 2050 containing 50% more CO2 than it did in 1900. While this enrichment of the atmosphere CO2 portends certain hazards of which global warming is the most publicized, it also offers the potential to increase plant production as carbon dioxide is in effect a fertilizer. However, in the case of soybean, the most important oil seed crop globally, the expected enhancements of increasing CO2 are often not realized and the underlying reasons are not adequately understood to allow accurate predictions of future crop production or allow varietal selection for improved productivity in a changing climate. Using technology that enables us to mimic the atmosphere of 2050 under natural field conditions, we found even for a crop lacking constraints that have been considered to be the cause of less than expected response of soybean photosynthesis to CO2 that soybean underperformed expectation. This result suggests that there are yet unrecognized genetic limitations restricting the response of soybean to CO2 and calls for a reassessment of widely held views among crop research scientists concerning what limits response of soybean to CO2 enrichment.
Technical Abstract: A theoretically lower than expected increase in leaf photosynthesis with long-term elevation of CO2 concentration is often attributed to limitations in the capacity of the plant to utilize the additional photosynthate, possible resulting from restrictions in rooting volume, nitrogen supply or genetic constraints. Field grown nitrogen-fixing soybean with indeterminate flowering might therefore be expected to escape these limitations. Soybean was grown from emergence to grain maturity in ambient air (370 umol mol-1 CO2 concentration) and in air enriched with CO2 (552 umol mol-1 CO2 concentration) using Free-Air CO2 Enrichment (FACE) technology. The diurnal courses of leaf CO2 uptake (A) and stomatal conductance (gs) for upper canopy leaves were followed throughout development from the appearance of the first true leaf to the completion of seed filling Across the growing season the daily integrals of leaf photosynthetic CO2 uptake (A) increased by 24.6% in elevated [CO2] and the average mid-day gs decreased by 21.9%. The increase in A' was about half the 44.5% theoretical increase calculated from Rubisco kinetics. There was no evidence that the stimulation of A was affected by time of day, as expected if elevated [CO2] led to a large accumulation of leaf carbohydrates towards the end of the photoperiod. In general, the proportion of assimilated C that accumulated in the leaf as non-structural carbohydrate over the photoperiod was small (<10%) and independent of [CO] treatment. By contrast to A', daily integrals of PSII electron transport measured by modulated chlorophyll fluorescence were not significantly increased by elevated [CO2]. This indicates that A at elevated [CO2] in these field conditions was predominantly ribulose-1,5-bisphosphate (RubP) limited rather than Rubisco-limited.