Effects of leaf water evaporative ²H‐enrichment and biosynthetic fractionation on leaf wax n‐alkane δ²H values in C3 and C4 grasses

Leaf wax n ‐alkane δ 2 H values carry important information about environmental and ecophysiological processes in plants. However, the physiological and biochemical drivers that shape leaf wax n ‐alkane δ 2 H values are not completely understood. It is particularly unclear why n ‐alkanes in grasses are typically 2 H‐depleted compared with plants from other taxonomic groups such as dicotyledonous plants and why C3 grasses are 2 H‐depleted compared with C4 grasses. To resolve these uncertainties, we quantified the effects of leaf water evaporative 2 H‐enrichment and biosynthetic hydrogen isotope fractionation on n ‐alkane δ 2 H values for a range of C3 and C4 grasses grown in climate‐controlled chambers. We found that only a fraction of leaf water evaporative 2 H‐enrichment is imprinted on the leaf wax n ‐alkane δ 2 H values in grasses. This is interesting, as previous studies have shown in dicotyledonous plants a nearly complete transfer of this 2 H‐enrichment to the n ‐alkane δ 2 H values. We thus infer that the typically observed 2 H‐depletion of n ‐alkanes in grasses (as opposed to dicots) is because only a fraction of the leaf water evaporative 2 H‐enrichment is imprinted on the δ 2 H values. Our experiments also show that differences in n ‐alkane δ 2 H values between C3 and C4 grasses are largely the result of systematic differences in biosynthetic fractionation between these two plant groups, which was on average −198‰ and−159‰ for C3 and C4 grasses, respectively.