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

Abstract

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.

The 18O‐signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms

“…Interestingly, across all growth forms, sucrose was on average the most 18 O‐labelled compound after the fog exposure (Figure b; Table ) but also the most 18 O‐enriched compound at natural isotope abundances. This confirms previous studies measuring δ 18 O values in individual carbohydrates of grass and tree species (Lehmann et al, ; Lehmann et al, ) and shows that the findings can be extended to a wider range of species and growth forms. It also demonstrates that sucrose is more sensitive to isotopic variations in water vapour and leaf water compared with other assimilates.…”

“…In addition, hexoses such as glucose may lose their original leaf water signal faster than sucrose due to isotope exchange processes (Sternberg, DeNiro, & Savidge, ). Oxygen isotopes in aldehyde and ketone groups of hexoses can be exchanged with those in surrounding water (Schmidt, Werner, & Rossmann, ; Werner, ), explaining oxygen isotope fractionations among individual leaf sugars (Lehmann et al, ). It also explains why the isotopic leaf water signal in assimilates is partially obscured by unenriched xylem water before incorporation into structural plant components such as leaf or tree‐ring cellulose (Barbour & Farquhar, ; Roden et al, ).…”

“…The measurement precision was typically <0.4‰ ( SD ) for oxygen and carbon isotopes. No significant oxygen isotope fractionation was observed during sugar purification (Lehmann et al, ; Lehmann, Gamarra, Kahmen, Siegwolf, & Saurer, ).…”

“…δ 18 O values of glucose and sucrose were analysed before and after the 9‐hr labelling event by gas chromatography (GC)/pyrolysis‐IRMS for 38 species of eight different growth forms. An aliquot of sugars per sample (~2 mg DW −1 ) was transferred to a 2 ml reaction vial, frozen, freeze dried, and then methylated (Lehmann et al, ; Lehmann et al, ). Methylated sugars were injected (splitless at 250°C) and separated on a 60 m, 0.25 mm, and 0.25 μm ZB‐SemiVolatiles GC column (Zebron, Phenomenex, Torrance, CA, USA) in a Trace GC Ultra gas chromatograph.…”

The effect of 18O‐labelled water vapour on the oxygen isotope ratio of water and assimilates in plants at high humidity

“…The average measurement precision (SD) for individual sugars was ≤ 0.4‰ for fructose and glucose, ≤ 0.5‰ quercitol and ≤ 0.6‰ for sucrose. No significant oxygen isotope fractionation was observed for bulk and individual sugars (Lehmann et al ., ).…”