Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2003
Publication Date: 1/15/2004
Citation: Ainsworth, E.A., Rogers, A., Nelson, R.L., Long, S.P. 2004. Testing the "source-sink" hypothesis of down-regulation of photosynthesis in elevated [CO2] in the field with single gene subsitutions in Glycine max. Agricultural and Forest Meteorology. 122:85-94. Interpretive Summary: CO2 concentration in the atmosphere has increased substantially in the past 100 years and continues to rise. Higher CO2 concentrations could increase production through increased photosynthesis. Previous research has shown that photosynthetic rates often decrease over time even when CO2 concentration remains high but this has not been tested under field conditions. A common explanation for this is that the plant is not adapted to use the extra production from increase rates of photosynthesis and this lack of demand causes the photosynthetic rates to decline. By modifying simple genetic traits in two soybean varieties to change their capacity to utilize more sugar from photosynthesis, we discovered that limiting the demand for sugar did decrease photosynthetic rates when they were tested in elevated CO2 concentrations under field conditions. These same cultivars without detrimental genetic changes were able to maintain higher photosynthetic rates with elevated CO2 concentrations. These results indicate that higher soybean production could be achieved with elevated CO2 concentrations but that this increase can be affected by which variety is grown. These results are of interest to plant physiologists and breeders who are interested in the understanding the effects of increasing CO2 concentrations in the atmosphere on soybean production and finding ways of exploiting that change.
Technical Abstract: Acclimation of photosynthesis to elevated atmospheric carbon dioxide concentration was tested in lines of soybean (Glycine max) that differed by single genes that altered either the capacity to nodulate or growth habit (determinate or indeterminate growth). Both genetic changes provided, within a uniform genetic background, a test of the "source-sink" hypothesis that down-regulation of photosynthesis in elevated carbon dioxide is a result of inability to form sufficient Asinks@ for the additional photosynthate. Plants were grown under ambient and elevated [CO2] (550 mol mol-1) in the field, using Free Air gas Concentration Enrichment (FACE). Mutation of the determinate cultivar, Elf, to an indeterminate form did not result in increased responsiveness to elevated [CO2.] This may reflect a large sink capacity in the selection of determinate cultivars. In elevated [CO2] only the determinate isoline of the indeterminate cultivar (Williams-dt1) and the non-nodulating genotype showed down-regulation of photosynthesis. This resulted from decreases in apparent in vivo Rubisco activity (Vc,max) and maximum rate of electron transport (Jmax). Increase in total non-structural carbohydrate content, which is often correlated with down-regulation of photosynthesis, in Williams-dt1 was 80% greater in elevated [CO2] than in ambient [CO2] controls, compared to 40% in the indeterminate line. The results from mutations of the Williams line are consistent with the hypothesis that genetic capacity for the utilization of photosynthate is critical to the ability of plants to sustain increased photosynthesis when grown at elevated [CO2].