Preharvest apple (Malus ×domestica Borkh.) fruit drop, which occurs before fruit develop optimum red color, maturity, or size, usually causes a serious economic loss. Conversely, picking fruit before adequate maturity may lead to poor storability and poor fresh and processed fruit quality. Early harvest can also result in lower yields and prices because fruit will increase in weight 5% to 7% per week and price is based on larger fruit sizes (Byers and Eno, 2002). Ideally, plant growth regulators should hold fruit on the tree for an additional 3 weeks past optimum harvest date to improve fruit size, color, and crop value by as much as 20% while maintaining fruit quality (Byers and Eno, 2002).
It has been suggested that the balance between plant growth promoters and inhibitors controls fruit abscission and ripening (Addicott, 1982; Brown, 1997; Yuan et al., 2002, 2003). Auxin plays a very important role in the control of fruit abscission (Osborne, 1989). Application of synthetic auxins reduced preharvest fruit drop of apples (Gardner et al., 1940; Marini et al., 1993) and oranges (Gardner et al., 1950; Zur and Goren, 1977). One application of naphthalene acidic acid (NAA) may delay apple fruit drop for ≈10 to 14 d after treatment, and repeated applications of NAA delay fruit abscission more than single applications (Batjer and Moon, 1945; Marini et al., 1993). However, fruit softening is usually increased by two applications of NAA or warm weather after the first application (Smock and Gross, 1947). On the contrary, application of ethephon, an ethylene-releasing compound, effectively promoted mature fruit abscission and ripening in apples (Edgerton and Blanpied, 1970), oranges (Yuan et al., 2002), and cherries (Bukovac et al., 1969), whereas AVG, an inhibitor of ethylene biosynthesis, reduced fruit ethylene production and preharvest fruit drop and delayed fruit ripening in apples (Autio and Bramlage, 1982; Bangerth, 1978; Byers et al., 2005; Greene, 2005; Schupp and Greene, 2004).
1-Methylcyclopropane (1-MCP), an inhibitor of ethylene action, has been used to delay postharvest ripening of climacteric fruit such as apples (Fan et al., 1999; Sisler and Serek, 1997), peaches (Kluge and Jacomino, 2002), avocado (Jeong et al., 2002), and tomatoes (Jeong et al., 2002). 1-MCP, a gas, has been formulated as a powder that releases 1-MCP when mixed with a buffered base and is also widely used in the cut flower industry (Blankenship and Dole, 2003). Recently, a sprayable formulation of 1-MCP became available for use in the field.
The purpose of this investigation was to study the effect of NAA alone or in combination with ethylene biosynthesis inhibitor AVG or ethylene action inhibitor 1-MCP and time of application on control of preharvest fruit drop and maintenance of on-tree fruit quality in ‘Golden Supreme’ and ‘Golden Delicious’ apples.
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