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

The C-13 isotopic signature (delta C-13) 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 delta C-13 value of respired CO2. To elucidate the physiological mechanisms driving C-12/C-13 fractionation during respiration, the delta C-13 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 delta C-13 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 C-13-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 C-13-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 delta C-13 in CO2, illustrating the effects of variations of metabolic fluxes through different biochemical pathways. New Phytologist (2009) 181: 374-386doi: 10.1111/j.1469-8137.2008.02672.x.