Abstract
Understanding the basis for intraspecific yield variability may be important in elucidating biological mechanisms that are associated with superior yield performance in response to projected increases in carbon dioxide concentration, [CO&sub2;]. Using a free-air CO&sub2; enrichment (FACE) facility, two rice lines, S63 and W14, which differed consistently in their enhancement of seed yield when grown at elevated [CO&sub2;] in multiple field trials, were examined. To determine if the different cultivar responses were linked to changes in photosynthetic characteristics at elevated [CO&sub2;], spatial and temporal changes in photosynthetic stimulation and the occurrence of down-regulation, or acclimation, in relation to panicle sink development were quantified for the uppermost canopy leaves. Changes in photosynthetic capacity were determined by quantifying changes in the sink:source ratio, leaf nitrogen (N) content, the concentration and mRNA expression of the large Rubisco subunit, and changes in V c,max, the maximum ribulose bisphosphate (RuBP)-saturated rate of carboxylation. For the W14 cultivar, significant reductions in photosynthesis at the elevated, relative to ambient [CO&sub2;], signalling photosynthetic acclimation, were observed following panicle initiation. The observance of photosynthetic acclimation was consistent with significant reductions in N, Rubisco content and expression, and V c,max. In contrast, for the cultivar S63, elevated [CO&sub2;] resulted in increased spikelet number and grain weight, increased sink:source ratios, and continued stimulation of photosynthesis up to grain maturity. Overall, these data suggest that the greater response of the S63 line to elevated [CO&sub2;] may be associated with enhanced carbon sinks relative to sources, and the ability to maintain photosynthetic capacity during grain development.