TEMPORAL SEQUENCE OF CELL-WALL DISASSEMBLY IN RAPIDLY RIPENING MELON FRUIT

The Charentais variety of melon (Cucumis melo cv Reticulatus F1 Alpha) was observed to undergo very rapid ripening, with the transition from the preripe to overripe stage occurring within 24 to 48 h. During thi s time, the flesh first softened and then exhibited substantial disint egration, suggesting that Charentais may represent a useful model syst em to examine the temporal sequence of changes in cell wall compositio n that typically take place in softening fruit. The total amount of pe ctin in the cell wall showed little reduction during ripening but its solubility changed substantially. Initial changes in pectin solubility coincided with a loss of galactose from tightly bound pectins, but pr eceded the expression of polygalacturonase (PC) mRNAs, suggesting earl y, PC-independent modification of pectin structure. Depolymerization o f polyuronides occurred predominantly in the later ripening stages, an d after the appearance of PC mRNAs, suggesting the existence of PC-dep endent pectin degradation in later stages. Depolymerization of hemicel luloses was observed throughout ripening, and degradation of a tightly bound xyloglucan fraction was detected at the early onset of softenin g. Thus, metabolism of xyloglucan that may be closely associated with cellulose microfibrils may contribute to the initial stages of fruit s oftening. A model is presented of the temporal sequence of cell wall c hanges during cell wall disassembly in ripening Charentais melon.