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

Almost no δ ¹⁸ O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ ¹⁸ O relationship between leaf water and cellulose. We measured δ ¹⁸ O values of bulk leaf water (δ ¹⁸ O _LW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ ¹⁸ 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 δ ¹⁸ O _LW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ ¹⁸ O _LW and δ ¹⁸ 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 δ ¹⁸ O _LW but not if modelled directly from δ ¹⁸ O of individual carbohydrates. We conclude that δ ¹⁸ O _LW is not always a good substitute for δ ¹⁸ O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ ¹⁸ O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.
(© 2017 John Wiley & Sons Ltd.)