Metabolic Fate of the Carboxyl Groups of Malate and Pyruvate and their Influence on δ(13)C of Leaf-Respired CO(2) during Light Enhanced Dark Respiration

The enhanced CO(2) release of illuminated leaves transferred into darkness, termed “light enhanced dark respiration (LEDR)”, is often associated with an increase in the carbon isotope ratio of the respired CO(2) (δ(13)C(LEDR)). The latter has been hypothesized to result from different respiratory substrates and decarboxylation reactions in various metabolic pathways, which are poorly understood so far. To provide a better insight into the underlying metabolic processes of δ(13)C(LEDR), we fed position-specific (13)C-labeled malate and pyruvate via the xylem stream to leaves of species with high and low δ(13)C(LEDR) values (Halimium halimifolium and Oxalis triangularis, respectively). During respective label application, we determined label-derived leaf (13)CO(2) respiration using laser spectroscopy and the (13)C allocation to metabolic fractions during light–dark transitions. Our results clearly show that both carboxyl groups (C-1 and C-4 position) of malate similarly influence respiration and metabolic fractions in both species, indicating possible isotope randomization of the carboxyl groups of malate by the fumarase reaction. While C-2 position of pyruvate was only weakly respired, the species-specific difference in natural δ(13)C(LEDR) patterns were best reflected by the (13)CO(2) respiration patterns of the C-1 position of pyruvate. Furthermore, (13)C label from malate and pyruvate were mainly allocated to amino and organic acid fractions in both species and only little to sugar and lipid fractions. In summary, our results suggest that respiration of both carboxyl groups of malate (via fumarase) by tricarboxylic acid cycle reactions or by NAD-malic enzyme influences δ(13)C(LEDR). The latter supplies the pyruvate dehydrogenase reaction, which in turn determines natural δ(13)C(LEDR) pattern by releasing the C-1 position of pyruvate.