Sweet cherry production in the Pacific Northwest (PNW) has increased roughly 2-fold over the last decade. Despite high consumer demand for fresh cherries, a short postharvest life and oversupply during ‘Bing’ harvest timing can limit returns paid to orchardists. Subsequently, sweet cherry producers have diversified with early- and late-maturing cultivars to expand the harvest window. In recent years, record sweet cherry crops have incentivized production of late-maturing cherries, which allow time for excessive midseason supplies to diminish. In fact, 61% of all cherry trees planted in Oregon between 1999 and 2005 were late-maturing cultivars (U.S. Department of Agriculture, National Agricultural Statistics Service, 2006); however, recouping higher returns for these cultivars is contingent on exceptional fruit quality at harvest and, particularly, after postharvest cold storage and transportation to export markets.
GA3 has been shown to improve fruit quality of sweet cherries. The most pronounced and consistent effect of GA on sweet cherry fruit is higher FF (Basak et al., 1998; Clayton et al., 2003; Facteau, 1982a; Facteau et al., 1985a; Kappel and MacDonald, 2002, 2007; Looney and Lidster, 1980; Proebsting and Mills, 1973). Cherry fruit size responded positively to GA3 (Facteau, 1982a; Facteau et al., 1985b; Kappel and MacDonald, 2002, 2007), although not all studies have observed a size response (Clayton et al., 2003; Facteau et al., 1985a; Looney and Lidster, 1980). In some cases, the effect of GA3 on fruit size appears to be indirect; attributed to retarded skin color development that grants GA3-treated fruit additional time to mature on the tree relative to untreated fruit (Choi et al., 2002). Skin pigmentation of ‘Lambert’ and ‘Bing’ fruit was significantly delayed in proportion to GA3 rate (Facteau et al., 1985a). The use of GA3 to further delay harvest timing of late-maturing cultivars is compelling; however, few studies have characterized cherry fruit and tree response to GA; of those studied, the emphasis has been on ‘Sweetheart’ (Horvitz et al., 2003; Kappel and MacDonald, 2002, 2007) and ‘Lapins’ (Choi et al., 2002). Although similar conclusions were reached for these two genotypes, we are unaware of any study that has assessed the response of ‘Skeena’ to GA3, a cultivar that has been widely adopted by producers in the PNW. Indeed, response of sweet cherry to preharvest GA3 applications has been shown to be cultivar-dependent (Usenik et al., 2005). Choi et al. (2002) documented an increase in FF and delayed maturation of two late-season genotypes (135-27-17 and ‘Lapins’) treated with GA3 but no effects on the early-maturing varieties ‘Merpet’ and ‘Celeste’.
Commercial application of GA3 occurs near the end of Stage II of fruit development, i.e., pit hardening, although a recent study demonstrated that GA3 efficacy did not depend on fruit development within a 3-week period surrounding pit hardening (Kappel and MacDonald, 2007). Moreover, split applications of GA3 did not improve fruit quality compared with single applications at the same rate for ‘Bing’ and ‘Lambert’ (Facteau et al., 1985a) or ‘Sweetheart’ (Kappel and MacDonald, 2002), implying that timing of treatment application has little effect on fruit response. Few studies have examined incremental rates of GA3 between 10 and 50 ppm (Facteau et al., 1985a; Horvitz et al., 2003; Kappel and MacDonald, 2002). Horvitz et al. (2003) observed a GA3 rate response between 10 and 30 ppm on ‘Sweetheart’ FF; Kappel and MacDonald (2002) did not. Higher rates of GA3 (100 ppm or greater) were associated with arrested floral bud induction (Bradley and Crane, 1960; Facteau et al., 1989; Oliveira and Browning, 1993). Although high rates of GA3 have been investigated as a cropload management strategy for ‘Bing’ the season after application (Lenahan et al., 2006; Proebsting and Mills, 1974), significantly lower return bloom severely reduced crop value when different isomers of GA (GA3 or GA4/7) were applied at 100 and 200 ppm (Lenahan et al., 2006). ‘Bing’, however, is not regarded as a highly productive variety, unlike several of the self-fertile, precocious, and productive late-season cultivars that produce a large proportion of undersized fruit of poor quality during high cropload years (Einhorn et al., 2011). Potentially, different cultivars may respond differently to high rates of GA3.
There is little information available on the influence of GA3 on postharvest fruit quality of sweet cherry (Clayton et al., 2003; Horvitz et al., 2003; Özkaya et al., 2006), especially late-maturing cultivars (Horvitz et al., 2003). ‘Sweetheart’ cherries treated with GA3 at 10 or 30 ppm were significantly firmer and had numerically less stem browning (SB) at the end of cold storage than untreated fruit; the effects were rate-dependent (Horvitz et al., 2003). GA lengthened the storability of ‘Bing’ (Zhang and Whiting, 2011b) and reduced incidence of surface pitting of ‘Lambert’ (Facteau and Rowe, 1979) and ‘Bing’ (Clayton et al., 2003; Drake et al., 1991) in severe pitting years. Surface pitting is the leading cause for product rejection and price adjustments in both domestic and international markets. Pits are defined as irregular, sunken areas on the surface of the fruit (Porritt et al., 1971) caused by mechanical impact or compression during harvest, processing, and transportation (Thompson et al., 1997). Surface pits are typically indiscernible before 1 to 2 weeks of storage in low temperatures. The effect of GA3 on pitting susceptibility of late-maturing cultivars has not been studied yet.
Our objective was to determine the response of fruit quality attributes of late-maturing sweet cherry cultivars to intermediate (10 to 40 ppm) and high (50 to 100 ppm) preharvest rates of GA3 both at harvest and after postharvest storage at low temperature. Additionally, we compared equivalent rates of GA3 applied either as split applications or a single application on ‘Sweetheart’ and ‘Skeena’ fruit quality.
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