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In Vivo Respiratory Metabolism of Illuminated Leaves

Tcherkez, Guillaume, Cornic, Gabriel, Bligny, Richard, Gout, Elizabeth, Ghashghaie, Jaleh
Plant physiology 2005 v.138 no.3 pp. 1596-1606
Phaseolus vulgaris, green beans, C3 plants, leaves, photosynthesis, cell respiration, carbon dioxide, biosynthesis, glucose, pyruvic acid, glycolysis, biochemical pathways, carbon, stable isotopes, light, plant physiology, spectral analysis
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.