title: Metabolic Fate of the Carboxyl Groups of Malate and Pyruvate and their Influence on δ(13)C of Leaf-Respired CO2 during Light Enhanced Dark Respiration.
abstract: The enhanced CO2 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 CO2 (δ(13)CLEDR). 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)CLEDR, we fed position-specific (13)C-labeled malate and pyruvate via the xylem stream to leaves of species with high and low δ(13)CLEDR values (Halimium halimifolium and Oxalis triangularis, respectively). During respective label application, we determined label-derived leaf (13)CO2 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)CLEDR patterns were best reflected by the (13)CO2 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)CLEDR. The latter supplies the pyruvate dehydrogenase reaction, which in turn determines natural δ(13)CLEDR pattern by releasing the C-1 position of pyruvate.
authors.affiliation: Laboratory of Atmospheric Chemistry, Paul Scherrer InstituteVilligen, Switzerland; Institute of Agricultural Sciences, ETH ZurichZurich, Switzerland.
authors.forename: Marco M
authors.initials: MM
authors.lastname: Lehmann
authors.affiliation: Ecosystem Physiology, University of Freiburg Freiburg, Germany.
authors.forename: Frederik
authors.initials: F
authors.lastname: Wegener
authors.affiliation: Institute of Agricultural Sciences, ETH Zurich Zurich, Switzerland.
authors.forename: Matti
authors.initials: M
authors.lastname: Barthel
authors.affiliation: Plant Molecular Physiology and Biotechnology Group, Institute of Developmental and Molecular Biology of Plants, Heinrich Heine University and Cluster of Excellence on Plant Sciences (CEPLAS) Düsseldorf, Germany.
authors.forename: Veronica G
authors.initials: VG
authors.lastname: Maurino
authors.affiliation: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute Villigen, Switzerland.
authors.forename: Rolf T W
authors.initials: RT
authors.lastname: Siegwolf
authors.affiliation: Institute of Agricultural Sciences, ETH Zurich Zurich, Switzerland.
authors.forename: Nina
authors.initials: N
authors.lastname: Buchmann
authors.affiliation: Ecosystem Physiology, University of Freiburg Freiburg, Germany.
authors.forename: Christiane
authors.initials: C
authors.lastname: Werner
authors.affiliation: Institute of Agricultural Sciences, ETH Zurich Zurich, Switzerland.
authors.forename: Roland A
authors.initials: RA
authors.lastname: Werner
keywords: LEDR; TCA cycle; fumarase; malic acid; malic enzyme; pyruvic acid; respiration; stable carbon isotopes