Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ18O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ18O relationship between leaf water and cellulose. We measured δ18O values of bulk leaf water (δ18OLW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ18O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally 18O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (εbio) of more than 27‰ relative to δ18OLW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ18OLW and δ18O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (εbio = 25.1‰). Interestingly, damping factor pexpx, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ18OLW but not if modelled directly from δ18O of individual carbohydrates. We conclude that δ18OLW is not always a good substitute for δ18O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ18O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants. Almost no δ18O data are available for individual leaf carbohydrates, limiting our understanding of oxygen isotope fractionation processes within plants. Here, we show that the leaf water signal can be traced via individual carbohydrates into leaf cellulose, with sucrose generally being 18O‐enriched compared with hexoses and cellulose across grass and tree species. Interestingly, damping factor pexpx responded to relative humidity conditions if modelled from δ18O of bulk leaf water but not if modelled directly from δ18O of individual carbohydrates. Thus, our results are useful for better understanding of (post‐)photosynthetic isotope fractionation processes and for improving models of oxygen isotope fractionation.