Ultrastructural changes in the cell walls of ripening apple and pear fruit.

Ultrastructural changes in the cell walls of "Calville de San Sauveur" apples (Malus sylvestris Mill) and "Spadona" pear (Pyrus communis L.) fruit were followed during ripening. In apple, structural alterations in cell walls became apparent at advanced stages of softening and showed predominantly dissolution of the middle lamella. In pears softening was also associated with the dissolution of the middle lamella, and in addition a gradual disintegration of fibrillar material throughout the cell wall. In fully ripe fruit almost all of the fibrillar arrangement in the cell wall was lost. Application of enzyme solutions containing polygalacturonase and cellulase to tissue discs from firm pear fruit led to ultrastructural changes observed in naturally ripening pears. In apple polygalacturonase alone was sufficient to dissolve the middle lamella region of the cell walls, as was also found to occur in naturally ripening fruit. In both apple and pear the cell wall areas containing plasmodesmata maintained their structural integrity throughout the ripening process. At advanced stages of ripening vesicles appeared in the vicinity of plasmodesmata.