Blueberry, a fruit crop indigenous to the United States, was valued at over $780 million in 2011 [U.S. Department of Agriculture (USDA), 2012]. Despite its emerging importance, various aspects of blueberry physiology including the process of fruit detachment are not well understood. Enhancing our knowledge of the process of fruit detachment can greatly aid in developing methods to increase the efficiency of mechanical harvesting, which can in turn reduce the costs associated with blueberry production (Austin and Williamson, 1977; Howell et al., 1976; Mainland et al., 1975; Takeda et al., 2008).
Fruit detachment may occur either through the physiological process of abscission or through tearing and physical separation of the fruit from the parent plant. Abscission involves the programmed separation of entire organs in response to plant developmental cues and various types of biotic and abiotic stress. Abscission occurs at specific regions within the plant termed abscission zones (Roberts et al., 2000, 2002; Sexton and Roberts, 1982). The cells in the AZ are morphologically and biochemically distinct before the initiation of the process of separation. The AZs generally consist of one to many layers of small, rounded, densely cytoplasmic cells at the union of the organ destined for detachment and the main body of the plant (Goren, 1993; Roberts et al., 2002; Sexton and Roberts, 1982). The AZ cells generally contain large deposits of starch and have highly branched plasmodesmata (Sexton and Roberts, 1982). These cells also display several ultrastructural changes during differentiation (Goren, 1993; Iwahori and Van Steveninck, 1976; Patterson, 2001; Roberts et al., 2002; Stösser et al., 1969; Webster, 1968). The progression of abscission at the pre-formed AZs involves multiple phases such as a gain in competence to respond to abscission signals, activation of the AZ resulting in the initiation of cell separation in response to the abscission signals, and cell separation followed by formation of a protective layer (Patterson, 2001).
Fruit crops may possess multiple AZs where fruit detachment can occur, often depending on the maturity of the separating organ. For example, in citrus (Citrus sp.) fruit, two AZs are associated with the fruit: 1) the shoot–peduncle AZ (AZ-A); and 2) the peduncle–fruit AZ (AZ-C) (Goren, 1993). Although the AZ-A is active during early fruit development, the AZ-C is the active zone associated with mature fruit detachment (Goren, 1993; Kazokas and Burns, 1998). Similarly, in the sour cherry (Prunus cerasus), two AZs have been described, one at the junction of the pedicel and the peduncle and another at the union of the pedicel and the fruit (Stösser et al., 1969; Wittenbach and Bukovac, 1974). Mature sour cherry fruit abscission occurs primarily at the AZ located at the junction of the pedicel and the fruit (Stösser et al., 1969; Wittenbach and Bukovac, 1974). Also in sweet cherry (Prunus avium), immature fruit detachment occurs at the pedicel–peduncle AZ, whereas mature fruit detachment occurs at the receptacle–fruit AZ (Wittenbach and Bukovac, 1972).
Plant organs can also separate from the plant as a result of mechanical breakage and cell disruption within weak points of attachment in response to physical force instead of abscission. During hand or mechanical harvesting, the fruit may be separated from the plant as a result of cell rupture and breakage at the AZ or at other physically weak points of attachment of the fruit to the plant. For example, mechanical harvesting in sweet cherry can result in mature fruit detachment between the pedicel and the peduncle (Norton et al., 1962), whereas mature fruit normally abscise at the receptacle–fruit AZ (Wittenbach and Bukovac, 1972).
Fruit detachment in blueberry can occur at the point of attachment of the pedicel to the peduncle or at the point of attachment of the pedicel to the berry. Gough and Litke (1980) reported the presence of an AZ at the point of attachment of the pedicel to the berry and indicated that mature fruit detachment in northern highbush blueberry (Vaccinium corymbosum) occurs primarily at this location. This junction is referred to hereafter as the fruit–pedicel junction (FPJ). Hand harvesting and the majority of mechanical harvesting of blueberry cultivars typically result in fruit separation at the FPJ (Howell et al., 1976; Takeda et al., 2008). Recent studies indicate that fruit drop in response to the application of abscission agents such as ethephon (2-chloroethylphosphonic acid) and methyl jasmonate (MeJa) occurs primarily at the point of attachment of the pedicel to the peduncle in different types of blueberries (Malladi et al., 2012). This junction is referred to hereafter as the peduncle–pedicel junction (PPJ). The point of natural detachment of mature fruit is not well understood. Additionally, it is unclear whether fruit separation at these junctions occurs as a result of abscission or as a result of physical tissue disruption.
A better understanding of the point of fruit detachment and the processes mediating it in blueberry is essential to improve the efficiency of mechanical harvesting. Such information can aid in determining the applicability of abscission agents as harvest aids and in the breeding and selection of genotypes better suited for mechanical harvesting. Hence, the main objectives of this study were to determine the point of mature fruit detachment in blueberry and to determine if fruit detachment occurs through the physiological process of abscission or through physical separation. To achieve these objectives, the anatomy of the points of fruit detachment, natural fruit detachment, fruit detachment in response to abscission agents, and fruit detachment in response to mechanical shaking were investigated in rabbiteye blueberry.
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