Bitter pit is a physiological disorder that has long been associated with low fruit Ca concentrations. The symptoms appear as depressed brown lesions on the skin of the fruit, typically located directly below the peel, but are often found scattered in the cortex near the calyx end (Perring, 1986). Bitter pit develops primarily during storage, but the underlying reasons for bitter pit begin during fruit development in the orchard (Ferguson and Watkins, 1983, 1989; Shear, 1980). Previous research has reported that bitter pit is a consequence of localized Ca deficiencies aggravated by genetic and environmental factors regulating tissue Ca content and cellular Ca distribution, such as alternate dry/wet conditions, excessive vegetative growth, and large fruit size (Bangerth, 1979; de Freitas and Mitcham, 2012; Ferguson and Watkins, 1989; Ho and White, 2005; Saure, 2005; Taylor and Locascio, 2004). Specifically, crop load has been shown to be one of the critical factors regulating the susceptibility to bitter pit (Robinson et al., 2009; Serra et al., 2016). ‘Honeycrisp’ apple is particularly susceptible to bitter pit and, in some years, losses commonly exceed 40% to 50% (Rosenberger et al., 2004).
Tree vigor is an important indicator of tree health and productive balance. Excessive vegetative growth can have a negative effect on fruit quality, yield, and pest control (Greene, 1999; Miller and Tworkoski, 2003). More vigorous tissues can contain higher levels of GA3-like substances (Kato and Ito, 1962; Saure, 2005) that can have a significant impact on tree growth, productivity, and quality. Schumacher et al. (1978) showed that fruit in the most vigorous parts of a tree are more susceptible to bitter pit. Excessive vegetative growth in apple trees can reduce fruit Ca content compared with trees with reduced shoot growth. The total Ca concentration in fruit tissue is usually much less than leaves (Saure, 2005). During early fruit development, vigorous shoots are strong competitors for available Ca (Garman and Mathis, 1956; Greene, 1991; Greene and Lord, 1983; Sharples, 1974; Shear, 1980). Correspondingly, this is also the time when much of the Ca deposition occurs in fruitlets, and excessive vegetative growth during this period may result in less Ca deposition in fruitlets.
Vegetative vigor in apple trees is affected by many separate, but often interacting, factors. These factors can be internal, such as variation in tissue water and carbohydrate balance or differences in plant hormonal balance, or external, such as horticultural management and orchard environment (Rom et al., 1991). Scion cultivar (Atkinson et al., 2003; Cohen and Naor, 2002), rootstock cultivar (Webster, 1995), soil type (Raese, 1995), tree spacing (Bianco et al., 2015), irrigation (Leib et al., 2006), and light quality can all affect shoot vigor. Shoot length can also be manipulated through horticultural management by using plant growth regulators (Evans et al., 1997).
Plant growth regulators that limit shoot length extension are used most often to improve the light environment in the canopy for better fruit color development and quality. P-Ca (Apogee®) is a primary plant bioregulator in pome fruit that inhibits the production of gibberellin (Miller, 2002; Rademacher and Kober, 2003). P-Ca reduces terminal shoot growth by inhibiting 2-oxaglutarate-dependent dioxygenases, which are involved in the formation of growth-specific gibberellins—a group of plant hormones primarily responsible for regulation of shoot elongation in apple trees (Rademacher et al., 2004). Well-timed applications of P-Ca to trees during periods of active shoot growth can significantly inhibit shoot extension. Effective rates for vegetative growth control are typically 63 to 125 mg·L–1, depending on tree vigor. In general, two applications per season are required during early growth immediately after bloom. P-Ca requires about 14 d to slow growth after the first spray application (Cline, 2006).
Because shoot vigor has been linked with Ca concentrations in developing fruit, the objective of this study was to determine whether inhibiting shoot length extension via the application of P-Ca or stimulating shoot length extension via the application of GA3 (ProGibb®) decreases or increases bitter pit incidence, respectively. This work will provide a greater understanding of the association between shoot extension and bitter pit incidence in ‘Honeycrisp’ apple.
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