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

Abstract

Stable oxygen isotope ratio of tree-ring α-cellulose (δ¹⁸O_cel) yields valuable information on many aspects of tree-climate interactions. However, our current understanding of the mechanistic controls on δ¹⁸O_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 ¹⁸O-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 ¹⁸O enrichment of sucrose in leaf lamina (Δ¹⁸O_{l_suc}) and petiole phloem (Δ¹⁸O_{phl_suc}) under environmentally/physiologically stable conditions. Based on the determined Δ¹⁸O_{phl_suc}-Δ¹⁸O_{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 δ¹⁸O_cel model and its related applications in paleoclimatic and ecophysiological contexts.

Keywords: oxygen exchange; oxygen isotope; phloem loading and transport; sucrose; tree‐ring cellulose.