Testing the "source-sink" hypothesis of down-regulation of photosynthesis in elevated [CO₂] in the field with single gene substitutions in Glycine max

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 "sinks" for the additional photosynthate. Plants were grown under ambient and elevated [CO₂] (550μmol 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 (V_c,max) and maximum rate of electron transport (J_max). Increase in total non-structural carbohydrate (TNC) 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].