Pronounced differences in diurnal variation of carbon isotope composition of leaf respired CO 2 among functional groups

Summary•  The first broad species survey of diurnal variation in carbon (C) isotope signatures of leaf dark‐respired CO 2 (δ 13 C res ) is presented here and functional differences and diurnal dynamics are linked to fractionation in different respiratory pathways, based on 13 C‐labelling experiments. •  δ 13 C res was analysed with a rapid in‐tube incubation technique in 16 species. •  A large diurnal increase in δ 13 C res (4–8‰) occurred in evergreen, slow‐growing and aromatic species and correlated significantly with cumulative photosynthesis, whereas no variation occurred in herbaceous, fast‐growing plants or temperate trees. The diurnal increase in δ 13 C res declined almost proportionally to reductions in cumulative light and was reduced in growing compared with mature leaves. •  Pyruvate positional labelling provided direct evidence that functional groups differ in C allocation between respiratory pathways owing to different metabolic demands for growth, maintenance and secondary metabolism. Diurnal increase in C flux through pyruvate dehydrogenase (for investment in, for example, isoprene or aromatic compounds) combined with consistently low Krebs cycle activity resulted in pronounced increase in δ 13 C res in evergreen and aromatic species. By contrast, fast growing herbs with high respiratory demand exhibited no diurnal changes since C was fully respired. Hence, diurnal δ 13 C res pattern may provide information for C allocation in plants.