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 (δ13CR) and leaf organic components (soluble sugar, water soluble fraction, starch, protein and bulk organic matter) in five C3 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 δ13CR and these results were compared with the δ13C of leaf organic components. • In all cases, leaf respiratory CO2 was more 13C-enriched than leaf organic components. The amount of 13C enrichment displayed a significant species-specific pattern, but the effect of CO2 treatment was not significant on P. deltoides. • In C3 plant leaves, 13C-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 δ13C of leaf respiratory CO2 at the ecosystem scale by upscaling the relationship between leaf δ13CR and δ13C of leaf organic components. New Phytologist (2004) 163: 499–505 © New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01153.x [ABSTRACT FROM AUTHOR]