Does carbonly-water exchange of oxygen occur during phloem loading of sucrose? An exploratory study involving vapor 18O labelling techniques.

Stable oxygen isotope composition of tree ring cellulose (d¹⁸O_cell) has been increasingly used as a tool for reconstruction of environmental conditions (e.g. air temperature, precipitation, relative humidity) and ecophysiological variables (e.g. leaf temperature, stomatal conductance) that prevail during the period of tree growth. The current tree-ring d¹⁸O_cell mechanistic model assumes no oxygen exchange/biochemical fractionation effect during sucrose loading from the source leaf to the phloem, but is has been debated as to whether or not such an assumption is valid. Here, we performed an experimental study to quantify the possible extent of carbonyl-water exchange of oxygen during phloem loading. Towards this goal, we used a custom-made multiple-channel water vapor isotope signal labeling system to expose experimental plants to a range of water vapor d18O compositions under physiologically stable conditions; this in turn allowed the creation of a large gradient in d18O of leaf water, leaf sucrose, and phloem sucrose without disrupting plant physiology. We will present data collected from this experiment to examine the d18O relationships among the various water and sucrose pools, which may provide some insights into the possible isotope effects associated with sucrose translocation from leaf to the phloem.