Hydrogen isotopic compositions of organic compounds in plants reflect the plant’s carbon metabolism

Abstract:

The main factors controlling δ²H of plant organic compounds are generally assumed to be the plant’s source water and the evaporative deuterium enrichment of leaf water. Hydrogen isotope analyses of plant compounds from sediments or tree rings are therefore mainly applied to assess hydrological conditions at different spatial and temporal scales. However, the biochemical hydrogen isotope fractionation occurring during biosynthesis of plant organic compounds (ε_bio) also accounts for a large part of the variability observed in the δ²H values. Nevertheless, only few studies have directly addressed the physiological basis of this variability and even fewer studies have thus explored possible applications of hydrogen isotope variability in plant organic compounds for plant physiological research. Here we show two datasets indicating that the plant’s carbon metabolism can have a substantial influence on δ²H values of n-alkanes and cellulose. First, we performed a controlled experiment where we forced plants into heterotrophic and autotrophic C-metabolism by growing them under four different light treatments. Second, we assessed the δ²H values of different parasitic heterotrophic plants and their autotrophic host plants. Our two datasets show a systematic shift in ε_bio of up to 80 ‰ depending on the plant’s carbon metabolism (heterotrophic or autotrophic). Differences in n-alkane and cellulose δ²H values in plants with autotrophic vs. heterotrophic metabolisms can be explained by different NADPH pools that are used by the plants to build their compounds either with assimilates that originate directly from photosynthesis or from stored carbohydrates. Our results have significant implications for the calibration and interpretation of geological records. More importantly, as the δ²H values reflect the plant’s carbon metabolism involved during the tissue formation, our findings highlight the potential of δ²H values as new tool for studying plant and ecosystem carbon metabolism.