‘Delicious’ is one of the most important apple (Malus ×domestica Borkh.) cultivars grown in the United States. Excessive preharvest apple fruit drop, which occurs before fruit develop optimum red color, maturity, or size, is one of its faults and usually causes a serious economic loss (Byers, 1997). 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).
Naphthaleneacetic acid (NAA), a synthetic auxin, and aminoethoxyvinylglycine (AVG), an inhibitor of ethylene biosynthesis are two compounds that commercial apple growers use to delay preharvest drop of apples since registrations of daminozide (Alar) and 2,4,5-TP were canceled in 1989 and 1986, respectively. One application of 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; Yuan and Carbaugh, 2007). AVG suppresses fruit ethylene production, reduces preharvest fruit drop, and delays fruit ripening in apples (Autio and Bramlage, 1982; Bangerth, 1978; Greene, 2005; Schupp and Greene, 2004; Yuan and Carbaugh, 2007). Our preliminary results showed that the combination of NAA and AVG was more effective in delaying fruit drop than were NAA and AVG alone in ‘Golden Supreme’ and ‘Golden Delicious’ apples while maintaining fruit quality (Yuan and Carbaugh, 2007).
1-Methylcyclopropene (1-MCP), an inhibitor of ethylene action, has been used to delay postharvest ripening of climacteric fruit such as apples (Blankenship and Dole, 2003). Recently, a sprayable formulation of 1-MCP became available for use in the field. Our preliminary results showed that sprayable 1-MCP more effectively delayed fruit drop than did AVG or NAA when they were applied 1 week before commercial harvest in ‘Golden Delicious’ apples (Yuan and Carbaugh, 2007). Elfving et al. (2007) reported that preharvest application of sprayable 1-MCP reduced ‘Delicious’ apple fruit ethylene production and slowed softening during and after storage, but they did not study its effect on preharvest fruit drop in ‘Delicious’ apples.
Apples are typical climacteric fruit characterized by a marked increase in ethylene production and respiration at ripening (Blanpied, 1972; Yuan and Carbaugh, 2007). It has been reported that concomitant with increased fruit ethylene production is increased expression of genes and activity of enzymes associated with cell wall degradation such as polygalacturonase (PG) (Bonghi et al., 2000; Brown, 1997). As a result, the middle lamellae of cells in fruit cortex and fruit abscission zone dissolves, and ultimately, the fruit becomes soft and abscises.
The purposes of this investigation were to: 1) study the effect of NAA and AVG alone or in combination on control of preharvest fruit drop and maintenance of on-tree fruit quality in ‘Delicious’ apples; 2) evaluate the effect of concentration, timing, and frequency of application of sprayable 1-MCP on preharvest fruit drop and fruit quality in ‘Delicious’ apples; and 3) determine the effect of AVG and sprayable 1-MCP on the expression of genes related to ethylene biosynthesis and cell wall degradation in fruit cortex and fruit abscission zones.
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