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

The hydrogen isotopic composition (δ 2 H) of plant compounds is increasingly used as a hydroclimatic proxy; however, the interpretation of δ 2 H values is hampered by potential coeffecting biochemical and biophysical processes. Here, we studied δ 2 H values of water and carbohydrates in leaves and roots, and of leaf n -alkanes, in two distinct tobacco ( Nicotiana sylvestris ) 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 2 H-enrichment in sugars and starch with a decreasing performance induced by increasing NO 3 - /NH 4 + ratios and starch deficiency, as well as from leaves to roots. However, δ 2 H values of cellulose and n -alkanes were less affected. We show that relative concentrations of sugars and starch, interlinked with leaf gas exchange, shape δ 2 H values of carbohydrates. We thus provide insights into drivers of hydrogen isotopic composition of plant compounds and into the mechanistic modeling of plant cellulose δ 2 H values.