Respiratory Carbon Metabolism Following Illumination in Intact French Bean Leaves Using (13)C/(12)C Isotope Labeling

The origin of the carbon atoms in the CO(2) respired by French bean (Phaseolus vulgaris) leaves in the dark has been studied using (13)C/(12)C isotopes as tracers. The stable isotope labeling was achieved through a technical device that uses an open gas-exchange system coupled online to an elemental analyzer and linked to an isotope ratio mass spectrometer. The isotopic analysis of the CO(2) respired in the dark after a light period revealed that the CO(2) was labeled, but the labeling level decreased progressively as the dark period increased. The pattern of disappearance depended on the amount of carbon fixed during the labeling and indicated that there were several pools of respiratory metabolites with distinct turnover rates. We demonstrate that the carbon recently assimilated during photosynthesis accounts for less than 50% of the carbon in the CO(2) lost by dark respiration and that the proportion is not influenced by leaf starvation in darkness before the labeling. Therefore, most of the carbon released by dark respiration after illumination does not come from new photosynthates.