Peel russeting is a commonly encountered, economically important surface defect in a number of horticultural crops, including apple (Faust and Shear, 1972a), tomato (Bakker, 1988), banana (Williams et al., 1990), and plum (Michailides, 1991). In most apple cultivars, russeted fruit is downgraded or rejected and used for processing. Anatomically, russeting is caused by a periderm formed in response to damage of epidermal cells, particularly during early fruit development (for review, see Faust and Shear, 1972a). Such damage may result from application of crop protection agents (Creasy and Swartz, 1981; Goffinet and Pearson, 1991), fungal infections (Gildemacher et al., 2004), breaking of epidermal hairs, mechanical injury, frost, or low temperature (Faust and Shear, 1972a). Also, extended periods of surface wetness or high humidity are conducive to russeting (Creasy, 1980; Tukey, 1969).
In our recent study on sweet cherry fruit, we observed formation of microscopic cracks in the cuticular membrane (CM) when fruit surfaces were exposed to water or high humidity (Knoche and Peschel, 2006). These cracks were limited to the CM, did not traverse epidermal or hypodermal cell layers, and were only detectable by microscopy. Formation of microcracks was not related to water uptake, because microcracking occurred also under isotonic conditions where a net water uptake can be excluded as a factor. Also, hydration altered the rheological properties of the isolated sweet cherry CM in vivo, i.e., decreased fracture force and modulus of elasticity, and increased fracture strain, which is consistent with the water-induced microcracking of the CM. Based on these observations and arguments, it may be hypothesized that water also induced microcracking of the apple fruit CM, which, in turn, is followed by formation of a periderm that becomes visible as fruit russeting (Faust and Shear, 1972a). The objective of this study was to establish the effect of surface moisture, i.e., liquid water and water vapor, on formation of microcracks in the CM of apple fruit. We focused on ‘Golden Delicious’ apple because it is an important apple cultivar worldwide, is susceptible to russeting, and is often subject to reduced value resulting from surface defects.
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