Quantifying isotope parameters associated with carbonyl-water oxygen exchange during sucrose translocation in tree phloem.

Stable oxygen isotope ratio of tree-ring α-cellulose (δ 18 O cel ) yields valuable information on many aspects of tree-climate interactions. However, our current understanding of the mechanistic controls on δ 18 O cel is incomplete, with a knowledge gap existent regarding the fractionation effect characterizing carbonyl-water oxygen exchange during sucrose translocation from leaf to phloem. To address this insufficiency, we set up an experimental system integrating a vapor 18 O-labeling feature to manipulate leaf-level isotopic signatures in tree saplings enclosed within whole-canopy gas-exchange cuvettes. We applied this experimental system to three different tree species to determine their respective relationships between 18 O enrichment of sucrose in leaf lamina (Δ 18 O l_suc ) and petiole phloem (Δ 18 O phl_suc ) under environmentally/physiologically stable conditions. Based on the determined Δ 18 O phl_suc -Δ 18 O l_suc relationships, we estimated that on average, at least 25% of the oxygen atoms in sucrose undergo isotopic exchange with water along the leaf-to-phloem translocation path and that the biochemical fractionation factor accounting for such exchange is c. 34‰, markedly higher than the conventionally assumed value of 27‰. Our study represents a significant step toward quantitative elucidation of the oxygen isotope dynamics during sucrose translocation in trees. This has important implications with respect to improving the δ 18 O cel model and its related applications in paleoclimatic and ecophysiological contexts.