AUTHOR=Lehmann Marco M. , Wegener Frederik , Barthel Matti , Maurino Veronica G. , Siegwolf Rolf T. W. , Buchmann Nina , Werner Christiane , Werner Roland A. 

TITLE=Metabolic Fate of the Carboxyl Groups of Malate and Pyruvate and their Influence on δ13C of Leaf-Respired CO2 during Light Enhanced Dark Respiration

JOURNAL=Frontiers in Plant Science

VOLUME=7

YEAR=2016

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2016.00739

DOI=10.3389/fpls.2016.00739

ISSN=1664-462X

ABSTRACT=<p>The enhanced CO<sub>2</sub> 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<sub>2</sub> (δ<sup>13</sup>C<sub>LEDR</sub>). 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<sup>13</sup>C<sub>LEDR</sub>, we fed position-specific <sup>13</sup>C-labeled malate and pyruvate via the xylem stream to leaves of species with high and low δ<sup>13</sup>C<sub>LEDR</sub> values (<italic>Halimium halimifolium</italic> and <italic>Oxalis triangularis</italic>, respectively). During respective label application, we determined label-derived leaf <sup>13</sup>CO<sub>2</sub> respiration using laser spectroscopy and the <sup>13</sup>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<sup>13</sup>C<sub>LEDR</sub> patterns were best reflected by the <sup>13</sup>CO<sub>2</sub> respiration patterns of the C-1 position of pyruvate. Furthermore, <sup>13</sup>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<sup>13</sup>C<sub>LEDR</sub>. The latter supplies the pyruvate dehydrogenase reaction, which in turn determines natural δ<sup>13</sup>C<sub>LEDR</sub> pattern by releasing the C-1 position of pyruvate.</p>