PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Respiratory carbon metabolism following illumination in intact French bean leaves. Salvador Nogués^*,1, Guillaume Tcherkez¹, Gabriel Cornic ¹, Jaleh Ghashghaie¹, ¹ Université de Paris XI, Orsay, France

ABSTRACT- The origin of the carbon atoms in the CO2 respired by French bean leaves in the dark has been studied using 13C/12C isotopes as tracers. The stable isotope labelling was achieved using a new on-line open system composed of a LI-6400 open gas-exchange system directly coupled to an elemental analyser (EA) and to an isotope ratio mass spectrometer (IRMS). In this work we took advantage of the difference in 13C abundance between atmospheric CO2 (d13C=-9.5) and commercially available (12C-enriched) CO2 (d13C=-51.2). The 13C abundance in the CO2 used for labelling is close to that found in nature, thereby allowing us to calculate proportions of new (i.e. recently fixed) carbon in CO2 subsequently respired in the dark. In general, this system also allowed estimations of some leaf metabolic fluxes in vivo. It is found that the CO2 evolved in darkness after illumination was labelled, although not completely. In addition, the labelled carbon abundance in respired CO2 progressively decreased as the dark period increased and finally disappeared. The kinetic of this process depended on the amount of carbon that was fixed during the labelling period and suggested that there were several pools of respiratory metabolites with distinct turnover rates. Interestedly, the carbon recently assimilated by photosynthesis during the labelling accounts for less than 50% of the carbon in the CO2 lost in the dark immediately after the light period. This proportion is not influenced by leaf starvation in darkness before the labelling. It is concluded that most of the carbon feeding dark respiration immediately after illumination does not come from new photosynthates.

KEY WORDS: carbon isotope, bean, respiration, labelling