Changes in the Ultrastructure of the Epicuticular Wax and Postharvest Calcium Uptake in Apples

Structural changes in the cuticle could be partially responsible for the differences in uptake of infiltrated Ca in apple fruit. We examined the relationship between the surface structure of epicuticular wax of ‘Golden Delicious’ apple and Ca uptake by the fruit. Apples were nontreated or pressure infiltrated with distilled water, or with 0.14 or 0.27 mol·L CaCl2 solutions 2 weeks before optimum harvest time, at optimum harvest, or after 2, 4, or 6 months of storage at 0 °C. Examination of the fruit surface with low-temperature scanning electron microscopy revealed that cracks in the epicuticular wax became wider and deeper as storage duration increased. After 6 months of storage, the cracks extended through the cuticle. Uptake of Ca by the infiltrated fruit was greater after 6 months of storage than after shorter storage intervals. These data indicate that as storage duration increased, epicuticular wax cracks became deeper and Ca uptake by the fruit increased. Received for publication 18 Mar. 1998. Accepted for publication 17 Aug. 1998. We express our appreciation to George A. Brown for valuable technical assistance. Use of a company name or product by the U.S. Dept. of Agriculture does not imply approval or recommendation of the product to the exclusion of others that also may be suitable. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. have been investigated (Conway et al., 1991; Glenn and Poovaiah, 1985; Sams and Conway, 1984). Pressure infiltration of fruit with solutions of CaCl2 increases the Ca concentration more effectively than does spraying, dipping, or vacuum infiltration (Conway and Sams, 1983). However, the mechanisms involved in Ca penetration are poorly understood, and an understanding of these pathways could enhance efforts to infiltrate Ca. The plant cuticle forms a continuous nonliving, noncellular lipoidal layer that covers most of the aerial portions of a plant, and serves as a protective barrier between the plant and its environment (Bukovac et al., 1981). It prevents uncontrolled loss of water and hinders attack by pathogens and insects. The surface properties of fruit, particularly those of the cuticle, affect the absorption and distribution of applied chemicals. Little is known, however, about the ultrastructural changes occurring in the apple cuticle during storage. Transmission electron microscopy (TEM) (Chafe and Wardrop, 1973; DeVries, 1968) and scanning electron microscopy (SEM) (Baker and Parsons, 1971; Jeffree, 1986; Jeffree Numerous investigations suggest that many physiological and pathological disorders of apples are related to the calcium (Ca) content of their tissues. An adequate concentration of Ca maintains fruit firmness; lowers the incidence of such disorders as water core, bitter pit, and internal breakdown (Bangerth et al., 1972; Faust and Shear, 1968; Reid and Padfield, 1975); and reduces decay caused by postharvest pathogens (Conway et al., 1991). Because the Ca concentration required to control such disorders is usually higher than can be obtained with fertilization practices, the effects of the direct application of CaCl2 solutions to fruit and the resulting effects on storage quality