Xylem, phloem, and transpiration flows in developing sweet cherry fruit

Abstract

Flow in the phloem increased in developing sweet cherry, whereas xylem flow continuously decreased resulting in a fruit water potential that was independent from the tree at fruit maturity. Rain cracking of sweet cherry fruit is associated with water uptake through the skin, but probably also through the vasculature of the pedicel. The aim of this study was to quantify the xylem, phloem, and transpiration contributions to the overall water balance of developing sweet cherry fruit. Using linear variable displacement transducers, changes in fruit diameter were monitored in untreated control fruit, fruit whose pedicels had been steam-girdled, and fruit whose pedicels had been cut through (these fruits were detached, but remained in situ in the canopy). Fruit volume changes with time were inferred from the measured diameter changes. Pedicel xylem and phloem sap-flow rates, and fruit transpiration rates, were inferred from fruit volume changes. Daily phloem flows were low during stage II (phase of pit development), but markedly increased in stage III (phase of cell enlargement in flesh). Xylem flows exceeded phloem flows in stage II, but decreased continuously in stage III to nearly zero at harvest. Transpiration flow essentially mirrored xylem flow in stage II, but exceeded xylem flow in stage III. Transpiration flow was closely related to the water vapour pressure deficit. Phloem flow was linearly related to the increase in fruit dry mass per-unit time. The data reveal a decrease in xylem sap flow throughout stage III resulting in a water-potential isolation of the mature fruit from the tree. Such isolation would prevent uncontrolled osmotic uptake of xylem water by the sugary flesh and a putative backflow in the xylem from fruit to tree.