Biochemical and biophysical drivers of the hydrogen isotopic composition of carbohydrates and acetogenic lipids.

The hydrogen isotopic composition (δH) of plant compounds is increasingly used as a hydroclimatic proxy; however, the interpretation of δH values is hampered by potential coeffecting biochemical and biophysical processes. Here, we studied δH values of water and carbohydrates in leaves and roots, and of leaf -alkanes, in two distinct tobacco () experiments. Large differences in plant performance and biochemistry resulted from (a) soil fertilization with varying nitrogen (N) species ratios and (b) knockout-induced starch deficiency. We observed a strong H-enrichment in sugars and starch with a decreasing performance induced by increasing NO/NH ratios and starch deficiency, as well as from leaves to roots. However, δH values of cellulose and -alkanes were less affected. We show that relative concentrations of sugars and starch, interlinked with leaf gas exchange, shape δH values of carbohydrates. We thus provide insights into drivers of hydrogen isotopic composition of plant compounds and into the mechanistic modeling of plant cellulose δH values.