Metabolic origin of carbon isotope composition of leaf dark-respired CO2 in French bean

The carbon isotope composition (delta(13)C) of CO2 produced in darkness by intact French bean (Phaseolus vulgaris) leaves was investigated for different leaf temperatures and during dark periods of increasing length. The delta(13)C of CO2 linearly decreased when temperature increased, from -19parts per thousand at 10degreesC to -24parts per thousand at 35degreesC. It also progressively decreased from -21parts per thousand to -30parts per thousand when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30degreesC and normal dark period), the evolved CO2 was enriched in C-13 compared with carbohydrates, the most C-13-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO2 was depleted in C-13 even when compared with the composition of total organic matter. In the two types of experiment, the variations of delta(13)C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of delta(13)C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing C-13-enriched CO2 and beta-oxidation of fatty acids producing C-13-depleted CO2 when compared with total organic matter (-27.5parts per thousand). These results are consistent with the assumption that the delta(13)C of dark respired CO2 is determined by the relative contributions of the two major decarboxylation processes that occur in darkness: pyruvate dehydrogenase activity and the Krebs cycle.