²H-enrichment of cellulose and n-alkanes in heterotrophic plants

Hydrogen (H) isotopes of plant organic compounds are rarely employed in ecological studies. If so, these values are interpreted as being indicative of the plant source and/or leaf water. Recent observations suggest, however, that variations in hydrogen isotope fractionation that occur during the biosynthesis of plant compounds (²H-ε_bio) imprint valuable metabolic information into the hydrogen isotope composition (δ²H values) of plant organic compounds. Here we show a consistent ²H-enrichment of compounds in heterotrophically growing plants across a series of autotrophic/heterotrophic plant pairs. We suggest that this is due to a higher recycling of compounds in the Calvin and tricarboxylic acid cycles in heterotrophic plants that is associated with a more complete exchange of C-bound H with the surrounding ²H-enriched foliar water. Interestingly, we found that ²H-enrichment in heterotrophic plants was larger for carbohydrates than for lipids, with an average ²H-enrichment of 76 ± 9‰ in α-cellulose and 23 ± 23‰ in n-alkanes. We propose that this systematically larger ²H-enrichment for carbohydrates than for lipids is either due to different level of ²H-fractionation associated with heterotrophically produced NADPH, or to the potential uptake of lipids by heterotrophic plants. With the work we present here, we contribute to a better mechanistic understanding of what the biochemical principles are that couple the carbohydrate dynamics of plants to their δ²H values and hope to foster as such the application of H isotopes in plant sciences.