Leaf respiratory CO2 is 13C-enriched relative to leaf organic components in five species of C3 plants

Here, we compared the carbon isotope ratios of leaf respiratory CO2 (delta(13)C(R)) and leaf organic components (soluble sugar, water soluble fraction, starch, protein and bulk organic matter) in five C-3 plants grown in a glasshouse and inside Biosphere 2. One species, Populus deltoides, was grown under three different CO2 concentrations. The Keeling plot approach was applied to the leaf scale to measure leaf delta(13)C(R) and these results were compared with the delta(13)C of leaf organic components. In all cases, leaf respiratory CO2 was more C-13-enriched than leaf organic components. The amount of C-13 enrichment displayed a significant species-specific pattern, but the effect of CO2 treatment was not significant on P. deltoides. In C-3 plant leaves, C-13-enriched respiratory CO2 appears widespread. Among currently hypothesized mechanisms contributing to this phenomenon, non-statistical carbon isotope distribution within the sugar substrates seems most likely. However, caution should be taken when attempting to predict the delta(13)C of leaf respiratory CO2 at the ecosystem scale by upscaling the relationship between leaf delta(13)C(R) and delta(13)C of leaf organic components.