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

Almost no δ¹⁸ O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸ O relationship between leaf water and cellulose. We measured δ¹⁸ O values of bulk leaf water (δ¹⁸ O_LW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸ O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸ O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio ) of more than 27‰ relative to δ¹⁸ O_LW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸ O_LW and δ¹⁸ O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio  = 25.1‰). Interestingly, damping factor p_ex p_x , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸ O_LW but not if modelled directly from δ¹⁸ O of individual carbohydrates. We conclude that δ¹⁸ O_LW is not always a good substitute for δ¹⁸ O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ¹⁸ O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.