Leaf respiratory CO is C-enriched relative to leaf organic components in five species of C plants.

•  Here, we compared the carbon isotope ratios of leaf respiratory CO (δ C ) and leaf organic components (soluble sugar, water soluble fraction, starch, protein and bulk organic matter) in five C plants grown in a glasshouse and inside Biosphere 2. One species, Populus deltoides, was grown under three different CO concentrations. •  The Keeling plot approach was applied to the leaf scale to measure leaf δ C and these results were compared with the δ C of leaf organic components. •  In all cases, leaf respiratory CO was more C-enriched than leaf organic components. The amount of C enrichment displayed a significant species-specific pattern, but the effect of CO treatment was not significant on P. deltoides. •  In C plant leaves, C-enriched respiratory CO 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 δ C of leaf respiratory CO at the ecosystem scale by upscaling the relationship between leaf δ C and δ C of leaf organic components.