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