Russeting is an important disorder of the fruit surface of many fruit crops including apple (Faust and Shear, 1972a, 1972b). In anatomical terms, a russeted area represents an area of periderm comprising phellogen and phellem. The periderm is thought to form in the hypodermal cell layer of developing apple fruit (Meyer, 1944; Verner, 1938). The phellem appears at the fruit surface after the epidermis and cuticle are shed. The phellem’s suberized cell walls are responsible for the brownish, dull appearance.
Susceptibility to russeting differs widely among commercial apple cultivars (Faust and Shear, 1972a). Thus, the fruits of some cultivars (e.g., Braeburn) remain essentially russet-free under all growing conditions, whereas in other cultivars (e.g., Egremont Russet), the fruit are almost completely russetted and this is seen as an acceptable cultivar characteristic. However, a large number of cultivars (e.g., Elstar) exhibits russeting only under unfavorable growth conditions. As a result, any russeted fruit of this intermediate cultivar group are usually of reduced market value with potentially serious economic consequences for the grower.
The mechanistic basis for russeting is not clear. Fine, cuticular cracks are considered the first detectable symptom of russeting (Faust and Shear, 1972a). These are thought to result from mechanical failure caused by excessive rates of growth strain, probably during early fruit development (Maguire, 1998; Skene, 1982) and/or from extended exposure to surface moisture (Knoche and Grimm, 2008). Russeting can also be a response to some chemical sprays (Sanchez et al., 2001).
Wax affects the mechanical properties of the cuticle (Dominguez et al., 2011; Petracek and Bukovac, 1995) and this, in turn, may affect susceptibility to russeting in various ways. First, wax prevents the release of biaxial elastic strain by converting reversible elastic into irreversible plastic strain (Khanal et al., 2013a). During growth, the fruit surface enlarges and the cuticle is strained. Because wax acts as a filler in the strained polymer network of the cutin matrix (Petracek and Bukovac, 1995), the deposition of wax essentially “fixes” strain (Khanal et al., 2013a). Second, wax decreases the extensibility of cuticles by increasing their stiffness (Khanal et al., 2013a; Petracek and Bukovac, 1995). Third, scanning electron microscopy (SEM) studies suggest that wax deposition occurs in microscopic cracks in the cuticle and this may have a healing effect (Curry, 2008; Roy et al., 1999).
Based on these observations, it seems reasonable to hypothesize that differential susceptibility to russeting could be related to certain mechanical properties of the fruit cuticle and the effects of wax thereon. The aim of this study, therefore, was to measure a number of physical properties of the fruit cuticle and also its key mechanical properties (indexed by the release of biaxial strain and the tensile properties of isolated cuticles) in a large number (22) of apple cultivars selected to represent a maximum range in russet susceptibility.
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