Ultrastructural Changes in the Walls of Ripening Avocados: Transmission, Scanning, and Freeze Fracture Microscopy

Avocado fruit at several documented stages of ripening was prepared for transmission, scanning, and freeze fracture electron microscopy. Changes in the ultrastructural organization of the cell wall were studied by each technique and correlated with changes in the activity of wall-hydrolytic enzymes. Initial wall breakdown apparently involves degradation of pectins in the matrix and in the middle lamella, corresponding to the reported increase in polygalacturonase activity in the tissue. In later stages of ripening, there is a loss of the organization and density of the wall striations accompanied by an increase in fruit softening. The role of cellulase, which becomes highly active during ripening of avocados and several other fruits, is still somewhat questionable. However, both thin sections and freeze fracture replicas of ripening avocados indicate a loss of fibrillar components of the wall during ripening and, therefore, indicate a possible role for cellulase in fruit softening. No correlation between localized wall degradation and the presence of plasmodesmata could be found.