Almost no δ O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ O relationship between leaf water and cellulose. We measured δ O values of bulk leaf water (δ O ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε ) of more than 27‰ relative to δ O , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ O and δ O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε = 25.1‰). Interestingly, damping factor p p , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ O but not if modelled directly from δ O of individual carbohydrates. We conclude that δ O is not always a good substitute for δ O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.

Leaf wax n-alkane δ 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 H values are not completely understood. It is particularly unclear why n-alkanes in grasses are typically H-depleted compared with plants from other taxonomic groups such as dicotyledonous plants and why C3 grasses are H-depleted compared with C4 grasses. To resolve these uncertainties, we quantified the effects of leaf water evaporative H-enrichment and biosynthetic hydrogen isotope fractionation on n-alkane δ 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 H-enrichment is imprinted on the leaf wax n-alkane δ H values in grasses. This is interesting, as previous studies have shown in dicotyledonous plants a nearly complete transfer of this H-enrichment to the n-alkane δ H values. We thus infer that the typically observed H-depletion of n-alkanes in grasses (as opposed to dicots) is because only a fraction of the leaf water evaporative H-enrichment is imprinted on the δ H values. Our experiments also show that differences in n-alkane δ 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.

n-Alkanes are long-chained hydrocarbons contained in the cuticle of terrestrial plants. Their hydrogen isotope ratios (δ(2)H) have been used as a proxy for environmental and plant ecophysiological processes. Calibration studies designed to resolve the mechanisms that determine the δ(2)H values of n-alkanes have exclusively focused on n-alkanes derived from leaves. It is, however, unclear in which quantities n-alkanes are also produced by other plant organs such as roots or inflorescences, or whether different plant organs produce distinct n-alkane δ(2)H values. To resolve these open questions, we sampled leaves, sheaths, stems, inflorescences and roots from a total of 15 species of European C3 grasses in an alpine and a temperate grassland in Switzerland. Our data show slightly increased n-alkane concentrations and n-alkane δ(2)H values in the alpine compared to the temperate grassland. More importantly, inflorescences had typically much higher n-alkane concentrations than other organs while roots had very low n-alkane concentrations. Most interestingly, the δ(2)H values of the carbon autonomous plant organs leaves, sheaths and stems were in general depleted compared to the overall mean δ(2)H value of a species, while non-carbon autonomous organs such as roots and inflorescences show δ(2)H values that are higher compared to the overall mean δ(2)H value of a species. We attribute organ-specific δ(2)H values to differences in the H-NADPH biosynthetic origin in different plant organs as a function of their carbon relationships. Finally, we employed simple mass balance calculations to show that leaves are in fact the main source of n-alkanes in the sediment. As such, studies assessing the environmental and physiological drivers of n-alkanes that focus on leaves produce relationships that can be employed to interpret the δ(2)H values of n-alkanes derived from sediments. This is despite the significant differences that we found among the δ(2)H values in the different plant organs. Our study brings new insights into the natural variability of n-alkane δ(2)H values and has implications for the interpretation of n-alkane δ(2)H values in ecological and paleohydrological research.

Our investigation provides new evidence on the timing of cuticular wax synthesis in grass leaves and indicates that the majority of n-alkanes are synthesized during the initial development of the leaf. Our study will improve the interpretation of leaf wax n-alkane δ H values in environmental and geological studies as it suggests that secondary synthesis of leaf wax n-alkanes in grass leaves contributes only slightly to the geological record. Copyright © 2016 John Wiley & Sons, Ltd.