Ultrastructure of the Mesocarp of Mature Avocado Fruit and Changes Associated with Ripening

The mesocarp tissue of ripening avocado fruits was studied by freeze fracture, thin section and scanning electron microscopy. Carbon dioxide and ethylene production by individual fruit were monitored, and samples were analysed at several stages of the ripening process. The tissue is composed primarily of large, isodiametric, lipid-containing parenchyma cells. At maturity these cells contain the normal complement of cell organelles, and all membranes appear intact. When ripening begins, several changes in the ultrastructure occur. The most obvious changes are a loosening and eventual breakdown of the cell wall, and swelling and vesiculation of the rough endoplasmic reticulum. In freeze fracture replicas a significant increase in the number of intramembranous particles in the EF face of the plasmamembrane was observed at the climacteric peak. In post-climacteric, soft fruit the particle density of the EF face of the plasmamembrane decreased to the density observed in the membrane of pre-climacteric cells. All of the organelles and membranes appear whole and intact whether examined by thin section, freeze fracture or scanning electron microscopy. However, the cell walls in post-climacteric fruit have almost completely disappeared. These results indicated that the ripening process per se in avocados does not involve a complete loss of compartmentation nor a breakdown of organelle and membrane integrity. It may, however, lead to these or similar senescence changes as a result of the loss of the cell walls. The variations in particle density of the plasmamembrane during ripening may reflect one or more of several structural, compositional, or functional membrane phenomena, and this aspect of ripening warrants further study.