In Vivo Respiratory Metabolism of Illuminated Leaves
Tcherkez, Guillaume | Cornic, Gabriel | Bligny, Richard | Gout, Elizabeth | Ghashghaie, Jaleh
Day respiration of illuminated C₃ 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 ¹²C/¹³C stable isotope techniques on illuminated leaves fed with ¹³C-enriched glucose or pyruvate. The ¹³CO₂ production in light was measured using the deviation of the photosynthetic carbon isotope discrimination induced by the decarboxylation of the ¹³C-enriched compounds. Using different positional ¹³C-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 ¹³C-glucose, leaf sucrose and glucose represent nearly 90% of the leaf ¹³C 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.
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