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 (H-2-epsilon(bio)) imprint valuable metabolic information into the hydrogen isotope composition (H-2 values) of plant organic compounds. Here we show a consistent H-2-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 H-2-enriched foliar water. Interestingly, we found that H-2-enrichment in heterotrophic plants was larger for carbohydrates than for lipids, with an average H-2-enrichment of 76 +/- 9 parts per thousand in -cellulose and 23 +/- 23 parts per thousand in n-alkanes. We propose that this systematically larger H-2-enrichment for carbohydrates than for lipids is either due to different level of H-2-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 H-2 values and hope to foster as such the application of H isotopes in plant sciences.
