Metabolic origin of the δ¹³C of respired CO₂ in roots of Phaseolus vulgaris

• Root respiration is a major contributor to soil CO₂ efflux, and thus an important component of ecosystem respiration. But its metabolic origin, in relation to the carbon isotope composition (δ¹³C), remains poorly understood. • Here, ¹³C analysis was conducted on CO₂ and metabolites under typical conditions or under continuous darkness in French bean (Phaseolus vulgaris) roots. ¹³C contents were measured either under natural abundance or following pulse-chase labeling with ¹³C-enriched glucose or pyruvate, using isotope ratio mass spectrometer (IRMS) and nuclear magnetic resonance (NMR) techniques. • In contrast to leaves, no relationship was found between the respiratory quotient and the δ¹³C of respired CO₂, which stayed constant at a low value (c. -27.5‰) under continuous darkness. With labeling experiments, it is shown that such a pattern is explained by the ¹³C-depleting effect of the pentose phosphate pathway; and the involvement of the Krebs cycle fueled by either the glycolytic input or the lipid/protein recycling. The anaplerotic phosphoenolpyruvate carboxylase (PEPc) activity sustained glutamic acid (Glu) synthesis, with no net effect on respired CO₂. • These results indicate that the root δ¹³C signal does not depend on the availability of root respiratory substrates and it is thus plausible that, unless the ¹³C photosynthetic fractionation varies at the leaf level, the root δ¹³C signal hardly changes under a range of natural environmental conditions.