In vivo respiratory metabolism of illuminated leaves

13 C-enriched compounds. Using different positional 13 C-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 CO 2 in the light, simply suggesting that they can rarely enter glycolysis. Nuclear magnetic resonance analysis confirmed this view; when leaves are fed with 13 C-glucose, leaf sucrose and glucose represent nearly 90% of the leaf 13 C 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. Illuminated leaves simultaneously assimilate CO 2 through the photosynthetic carbon reduction cycle and lose CO 2 through photorespiration and day respiration. In darkness, leaves no longer assimilate CO 2 via the photosynthetic carbon reduction cycle but produce CO 2 through dark respiration. Although dark respiration is known to involve glycolysis and CO 2 production through pyruvate dehydrogenation and the degradative Krebs cycle (Trethewey and ap Rees, 1994; Plaxton, 1996), the carbon metabolism that is responsible for the CO 2 respiratory release in the light is almost unknown. This is so because the day respiratory CO 2 flux is very low and masked by the photosynthetic carbon fixation and the photorespiratory CO2 production in the light, and is thus difficult to study. Nevertheless, it has been repeatedly shown, using either the Laisk’s (Laisk, 1977) or Kok’s method (Kok, 1948), that the rate of day respiration (Rd) is less than that of dark respiration (Rn; for review, see Atkin et al., 2000) so that light is known to inhibit respiration, with a R d /R n value (usually denoted as m) ranging from 30% to 100% (for a recent study, see Peisker and Apel, 2001). Pioneering gas exchange measurements on mustard suggested that some enzymatic activities are inhibited in the light so that substrates accumulate (Cornic, 1973), explaining the respiratory burst when leaves are darkened: the light enhanced dark respiration. More recently, it has been shown in the unicellular alga Selenastrum minutum that pyruvate kinase (Lin et al., 1989) is inhibited by light. It is also the case of the pyruvate dehydrogenase complex that is partly inactivated by (reversible) phosphorylation in extracts from illuminated leaves (Budde and Randall, 1990;