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-εbio) 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.
Keywords: Alkanes; Cellulose; Hydrogen isotopes; Parasitic plants; Plant metabolism.