VASCULAR DEVELOPMENT AND SAP FLOW IN APPLE PEDICELS

Xylem and phloem tissues of the pedicel of apple fruit increase in cro ss-sectional area throughout development. The increase in phloem is si milar in the two cultivars examined (Cox's Orange Pippin and Royal Gal a) and reflects a steadily increasing phloem sap flow to the fruit. Th e 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 expla ins the initially higher xylem conductance found in this cultivar. In vitro measurements of xylem exudation reveal a decline during the grow ing season in the xylem conductance of both cultivars and an increasin g proportion of fruit (particularly in Cox's) in which the xylem comes to be totally nonconducting. This observation is in line with previou sly reported measurements of xylem sap flow in vivo. The straightforwa rd techniques used in this study offer a feasible alternative to more arduous methods of assessing xylem and phloem sap flows and their bala nce during growth.