Biochemical Hydrogen Isotope Fractionation during Lipid Biosynthesis in Higher Plants

Although hydrogen isotopes (δ²H) of leaf wax lipids are increasingly being applied as (paleo-) hydrological proxies, we still do not understand some of the basic processes that shape the δ²H values of these compounds. In general, it is believed that three variables shape the δ²H values of leaf wax lipids: source water δ²H values, evaporative deuterium (²H) enrichment of leaf water and the biosynthetic fractionation (ε_bio) during the synthesis of organic compounds. While the influences of source water δ²H values and leaf water evaporative ²H enrichment have been well documented, very little is known how ε_bio shapes the δ²H values of plant-derived lipids.

I will present the results from recent experiments, where we show that the magnitude of ε_bio, and thus the δ²H value of plant-derived lipids, strongly depends on the carbon (C) metabolism of a plant. Specifically, I will show that plants that rely for their tissue formation on recently assimilated C have δ²H values in their n-alkanes that are up to 60‰ more negative than plants that depend for their tissue formation on stored carbohydrates. Our findings can be explained by the fact that NADPH is the primary source of hydrogen in plant lipids and that the δ²H value of NADPH differs whether NADPH was generated directly in the light reaction of photosynthesis or whether it was generated by processing stored carbohydrates. As such, the δ²H values of plant-derived lipids will directly depend on whether the tissue containing these lipids was synthesized using recent assimilates, e.g. in a C autonomous state or, if it was synthesized from stored or otherwise aquired C sources, e.g. in a not C autonomous state. Given the magnidude of this effect, our results have important implications for interpretation of plant-derived lipid δ²H values when used as (paleo-) hydrological proxies. In addition, our results suggest, that δ²H values of plant-derived lipids could be employed as a new tools to assess the C metabolism in plants.