Metabolic Origin of Carbon Isotope Composition of Leaf Dark-Respired CO² in French Bean

The carbon isotope composition (δ¹³C) 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 δ¹³C of CO₂ linearly decreased when temperature increased, from -19‰ at 10°C to -24‰ at 35°C. It also progressively decreased from -21‰ to -30‰ when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30°C and normal dark period), the evolved CO₂ was enriched in ¹³C compared with carbohydrates, the most ¹³C-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO₂ was depleted in ¹³C even when compared with the composition of total organic matter. In the two types of experiment, the variations of δ¹³C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of δ¹³C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing ¹³C-enriched CO₂ and β-oxidation of fatty acids producing ¹³C-depleted CO₂ when compared with total organic matter (-27.5‰). These results are consistent with the assumption that the δ¹³C 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.