VASCULAR DEVELOPMENT AND SAP FLOW IN APPLE PEDICELS

Xylem and phloem tissues of the pedicel of apple fruit increase in cross-sectional area throughout development. The increase in phloem is similar in the two cultivars examined (Cox's Orange Pippin and Royal Gala) and reflects a steadily increasing phloem sap flow to the fruit. The increase in xylem tissue is due to a proliferation of nonconducting, structural, components since close examination reveals no increase in the number of vessel elements from just after flowering onwards. The greater number, and the larger diameter, of the vessels in Cox's explains the initially higher xylem conductance found in this cultivar. In vitro measurements of xylem exudation reveal a decline during the growing season in the xylem conductance of both cultivars and an increasing proportion of fruit (particularly in Cox's) in which the xylem comes to be totally nonconducting. This observation is in line with previously reported measurements of xylem sap flow in vivo. The straightforward techniques used in this study offer a feasible alternative to more arduous methods of assessing xylem and phloem sap flows and their balance during growth.