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

摘要

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.