On the metabolic origin of the carbon isotope composition of CO₂ evolved from darkened light-acclimated leaves in Ricinus communis

• The ¹³C isotopic signature (δ¹³C) of CO₂ respired from plants is widely used to assess carbon fluxes and ecosystem functioning. There is, however, a lack of knowledge of the metabolic basis of the δ¹³C value of respired CO₂. • To elucidate the physiological mechanisms driving ¹²C/¹³C fractionation during respiration, the δ¹³C of respired CO ₂ from dark-acclimated leaves during the night, from darkened leaves during the light period, and from stems and roots of Ricinus communis was analysed. The δ¹³C of potential respiratory substrates, the respiratory quotient and the activities of phosphoenolpyruvatecarboxylase (PEPc) and key respiratory enzymes were also measured. • It is shown here that the CO₂ evolved from darkened light-acclimated leaves during the light period is ¹³C-enriched, and that this correlates with malate accumulation in the light and rapid malate decarboxylation just after the onset of darkness. Whilst CO₂ evolved from leaves was generally ¹³C-enriched (but to a lesser extent during the night), CO ₂ evolved from stems and roots was depleted compared with the putative respiratory substrates; the difference was mainly caused by intensive PEPc-catalysed CO₂ refixation in stems and roots. • These results provide a physiological explanation for short-term variations of δ¹³C in CO₂, illustrating the effects of variations of metabolic fluxes through different biochemical pathways.