Hydrogen isotopes in assimilates and cellulose, but not in n-alkanes, integrate signals of the plant primary carbon metabolism

Abstract

<p>Recent studies suggest that isotope ratios of the carbon-bound non-exchangeable hydrogen (δ<sup>2</sup>H) in plant cellulose and lipids can indicate changes in the primary carbon and energy metabolism; however, systematic investigations are scarce.</p><p>Here, we studied δ<sup>2</sup>H patterns in two different tobacco (<em>N. sylvestris</em>) model systems, where severe changes in the plant primary metabolism were known: 1) along a nitrogen (N) supply gradient and 2) in a starch-less knockout mutant (<em>pgm</em>). Specifically, we measured δ<sup>2</sup>H of water, bulk soluble sugars, transitory starch, and cellulose in leaves and roots, using a novel hot water vapor equilibration method and TC/EA-IRMS. Besides, we measured δ<sup>2</sup>H values of leaf <em>n</em>-alkanes with GC-IRMS.</p><p>We observed clear δ<sup>2</sup>H differences in sugars and starch along the N gradient and a <sup>2</sup>H-enrichment of both assimilates in <em>pgm</em> compared to a wild type control. The photosynthetic <sup>2</sup>H-fractionation between leaf water and sugars/starch reached a maximum of ca. 100‰ in both model systems and was related to changes in concentrations of primary metabolites (e.g. sugars, starch, organic and amino acids), enzymatic activities, gas-exchange, and growth. The signal of the primary carbon metabolism was also visible in δ<sup>2</sup>H of leaf and root cellulose in both system, but dampened compared to those of sugars and starch. In contrast, the signal was absent in leaf <em>n</em>-alkanes in both systems.</p><p>Our results provide the first direct evidence that changes in the primary leaf carbon metabolism are imprinted on δ<sup>2</sup>H of plant carbohydrates in leaf and roots. The metabolic signal might therefore be reconstructed from plant material of important paleo archives (e.g. tree-ring cellulose, lake sediments) and help to better understand plant-climate interactions. The absence of the signal in δ<sup>2</sup>H of leaf <em>n</em>-alkanes is surprising and suggests a strong difference in metabolic fluxes between carbohydrates and lipids. Yet, this observation may help to further disentangle the processes shaping hydrogen isotopes in plants.</p>