In vivo respiratory metabolism of illuminated leaves

Day respiration of illuminated C-3 leaves is not well understood and particularly, the metabolic origin of the day respiratory CO2 production is poorly known. This issue was addressed in leaves of French bean ( Phaseolus vulgaris) using C-12/C-13 stable isotope techniques on illuminated leaves fed with C-13-enriched glucose or pyruvate. The (CO2)-C-13 production in light was measured using the deviation of the photosynthetic carbon isotope discrimination induced by the decarboxylation of the C-13-enriched compounds. Using different positional C-13-enrichments, it is shown that the Krebs cycle is reduced by 95% in the light and that the pyruvate dehydrogenase reaction is much less reduced, by 27% or less. Glucose molecules are scarcely metabolized to liberate CO2 in the light, simply suggesting that they can rarely enter glycolysis. Nuclear magnetic resonance analysis confirmed this view; when leaves are fed with C-13-glucose, leaf sucrose and glucose represent nearly 90% of the leaf C-13 content, demonstrating that glucose is mainly directed to sucrose synthesis. Taken together, these data indicate that several metabolic down-regulations (glycolysis, Krebs cycle) accompany the light/dark transition and emphasize the decrease of the Krebs cycle decarboxylations as a metabolic basis of the light-dependent inhibition of mitochondrial respiration.