Fruit size is a commercially valuable trait in many horticultural crops, including rabbiteye blueberry. Sensory evaluations indicate a greater preference among consumers for large-sized blueberry fruit (Donahue et al., 2000; Saftner et al., 2008). Large fruit size is an important trait for selection during the development of new varieties in blueberry breeding programs (NeSmith, 2009). Although considerable variation in fruit size is observed among rabbiteye blueberry genotypes, the basis of this variation is not well understood. Understanding the cellular and molecular basis of such variation is essential to develop tools for enhancing fruit size either through breeding or through the manipulation of fruit growth using horticultural practices. In rabbiteye blueberry, the fleshy mesocarp constitutes the majority of the mature fruit (Edwards et al., 1970). Because growth of the mesocarp tissue is likely mediated by coordinated progression of cell production and cell expansion, these processes may be key factors determining fruit size. Dissecting the relative contribution of these factors is essential to develop a clear understanding of fruit size regulation.
Variation in fruit size is often associated with differences in cell number. Higher cell number is associated with larger fruit size in peach [Prunus persica (Scorza et al., 1991)], olive [Olea europaea (Rapoport et al., 2004)], strawberry [Fragaria ×ananassa (Cheng and Breen, 1992)], melon [Cucumis melo (Higashi et al., 1999)], apple [Malus ×domestica (Denne, 1960)], sweet cherry [Prunus avium (Olmstead et al., 2007)], and tomato [Solanum lycopersicum (Bertin et al., 2003, 2009; Bohner and Bangerth, 1988; Tanksley, 2004)]. Final cell number in the fruit may be determined by: 1) cell production within the ovary before bloom; and/or 2) the rate and duration of cell production during fruit growth. In many fruit crops, cell production before bloom and immediately after pollination/fertilization is the primary factor driving early fruit growth (Gillaspy et al., 1993). In peach and strawberry, cell number in the ovary or receptacle at bloom is an important factor determining final cell number (Cheng and Breen, 1992; Scorza et al., 1991). In fruits such as tomato, japanese pear (Pyrus pyrifolia), sweet cherry, and apple, multiple rounds of cell production during early fruit development regulate the final cell number (Bertin et al., 2009; Goffinet et al., 1995; Olmstead et al., 2007; Zhang et al., 2006). In rabbiteye blueberry, cell production occurs before bloom and during the initial stages of fruit growth (Cano-Medrano and Darnell, 1997; Darnell et al., 1992; Edwards et al., 1970). However, the effect of differences in cell production on variation in final cell number and fruit size among rabbiteye blueberry genotypes has not been determined.
Cell expansion is an important facilitator of organ growth. A majority of fruit growth, especially during the later stages of fruit development, is often achieved through cell expansion (Gillaspy et al., 1993). However, the contribution of cell expansion and final cell size to fruit size variation among genotypes is not clear. In tomato, fruit size and pericarp thickness were correlated with cell size among 20 genotypes (Cheniclet et al., 2005). In apple, cell size contributed to fruit size variation among five genotypes (Harada et al., 2005). Also, alteration in fruit size in a spontaneous apple mutant was associated with enhanced cell expansion (Malladi and Hirst, 2010). In peach, strawberry, japanese pear, and sweet cherry, differences in fruit size among large and small fruit size genotypes were not associated with differences in cell size (Cheng and Breen, 1992; Olmstead et al., 2007; Scorza et al., 1991; Zhang et al., 2006). Additionally, no relationship between fruit weight and cell size was observed among isogenic tomato lines differing in fruit size (Bertin et al., 2009). In rabbiteye blueberry, differences in fruit size between pollinated and parthenocarpic fruit were associated with differences in cell size (Cano-Medrano and Darnell, 1997). However, the relationship between fruit size and cell size among rabbiteye blueberry genotypes has not been determined.
The primary goal of this study was to determine the relationships among fruit size, cell number, and cell area across rabbiteye blueberry genotypes. In addition, the relationships among these parameters in the ovary at bloom and in mature fruit were investigated.
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