Development of the banana fruit with special reference to the maturity bronzing disorder

Changes in the epidermal cell size, and morphology and composition of the epicuticular wax were studied in banana fruit Musa (AAA group, Cavendish subgroup) cv. Williams during four growing seasons. Results show that (i) cell division occurs mainly before emergence of the fruit bunch, (ii) there is no visible wax on the peel surface before emergence of the fruit bunch, (iii) within 14-21 days after bunch emergence unstructured ridges are formed on the surface of epidermal cells and these are covered with epicuticular wax, (iv) the cross-sectional area of the epidermal cells changes little during fruit growth, and (v) epidermal cells become· flattened in the radial direction and tangentially-elongated to accommodate the rapid growth of the underlying tissues. Maturity bronzing is characterised by a red-brown lesion which generally occurs on the outer curved region of the fruit. Oxidation products of dopamine are involved in the discoloration of the peel. Cracks in the peel, orientated parallel to the long axis of the fruit, are associated with discoloration. Maturity bronzing symptoms are distinct from those of damage due to banana rust thrips (Chaetanophothrips signipennis (Bagnell)). The occurrence of the maturity bronzing disorder appears to be associated with the inability of the peel to expand sufficiently to accommodate the expansion of the inner fruit. This conclusion is supported by the results of comparative studies of banana cultivars with differing susceptibilities to maturity bronzing. Susceptibility is correlated with several differences in the characteristics of both the epidermal layer and morphology of the epicuticular wax.Treatments were applied in field trials both by spraying solutions onto the fruit surface and by injecting them into the pseudostem. Calcium and growth regulators do not affect fruit growth and have no effect on the incidence of maturity bronzing, Both water stress and the injection procedure increase the severity of maturity bronzing. Stress during the early differentiation/development phase of fruit growth appears to reduce either the rate of, or the duration of, cell division. This reduction in cell division prevents the outer peel layers, such as the cuticle and epidermal layers, from being sufficiently elastic to accommodate the expanding inner tissues as the fruit nears harvest. The most likely control for the disorder is (i) reduce stress during early fruit growth, (ii) change management practices to alleviate the stress on the fruit peel near fruit harvest, and (iii) develop cultivars with reduced susceptibility to the disorder