Russeting is an important peel defect in many apple cultivars, and russeted fruit is subject to reduced value (Faust and Shear, 1972a). Apple cultivars differ in their susceptibility to russeting, and fruit in early stages of development is more susceptible than that in later stages (Faust and Shear, 1972a; Simons and Chu, 1978; Skene, 1982). Russeting may be induced by a range of factors, including agrochemicals (Creasy and Swartz, 1981; Sanchez et al., 1992), environmental conditions such as freezing temperatures, extended periods of high humidity and surface wetness (Creasy, 1980; Faust and Shear, 1972b; Simons and Chu, 1978; Tukey, 1969), or microorganisms (Daines et al., 1984; Gildemacher et al., 2004; Heidenreich et al., 1997). Under orchard conditions, russeting is avoided and peel appearance improved in sensitive cultivars by applying gibberellins such as gibberellic acid (GA3) or GA4+7 (Elfving and Allen, 1987; Meador and Taylor, 1987; Steenkamp and Westraad, 1988; Taylor, 1975; Wertheim, 1982). A typical application scheme comprises four treatments with GA4+7 at 10 mg·L−1 applied in 10-d intervals starting at the onset of petal fall (Greene, 1993; Looney et al., 1992).
Microscopic cracks in the cuticular membrane are the first detectable symptoms in russet development (Faust and Shear, 1972a; Hatch, 1975; Simons and Chu, 1978; Verner, 1938). A periderm is subsequently formed as a wound reaction that replaces the primary epidermal tissue on the fruit surface. This periderm tissue causes the brown and rough appearance of a russeted fruit surface (Verner, 1938).
Recent studies from our laboratory established that exposure of the fruit surface to liquid water and water vapor induced microscopic cracks in the CM of mature ‘Golden Delicious’ apple (Knoche and Grimm, 2008). Also, applications of GA3 and GA4+7 enhanced CM deposition in developing tomato (Solanum lycopersicum L.) fruit (Knoche and Peschel, 2007). Therefore, it may be hypothesized that GA4+7 reduces russeting by decreasing the formation of microcracks in the cuticle and that GA4+7-reduced microcracking is related to increased CM deposition and decreased strain of the CM in the course of development.
The objectives of the present study were to investigate whether 1) a decrease in fruit russeting by GA4+7 is paralleled by a decrease in the formation of microcracks in the CM; and 2) a decrease in microcracking by GA4+7 can be accounted for by effects of GA4+7 on fruit growth, cuticle deposition or strain. and the mechanical properties of cuticles isolated from developing apple fruit. We chose ‘Golden Delicious’ apple, because ‘Golden Delicious’ is grown worldwide and is susceptible to russeting.
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