Several fresh-market fruit species have textures that range from soft to crisp, including apple (Malus ×domestica), grape (Vitis vinifera), peach (Prunus persica), and sweet cherry (Prunus avium) (Batisse et al., 1996; Ghiani et al., 2011; Sato et al., 2006; Tong et al., 1999). Crisp fruit texture has been described in the southern highbush blueberry cultivars, Bluecrisp, Dolores, Reveille, and Sweetcrisp, and the northern highbush blueberry cultivar Hortblue Poppins (Clark and Finn, 2010; Okie, 1999; Scalzo et al., 2009). In addition, many current selections in the University of Florida SHB breeding program are also considered to have a crisp phenotype (Blaker et al., 2014). Berries with this crisp texture are of particular interest as a result of their enhanced eating quality, prolonged postharvest life, and potential value for mechanical harvesting for fresh-marketed blueberries (Mehra et al., 2013; Padley, 2005; Takeda et al., 2013).
Several cellular components contribute to overall fruit texture, including cell type, size, shape, packing, cell to cell adhesion, extracellular space, and cell wall thickness (Harker et al., 1997). Parenchyma cells are the most numerous type of cells in the flesh of blueberry fruit and have thin, non-lignified cell walls and a large, mostly water-filled vacuole (Harker et al., 1997). The epidermis is composed of specialized parenchyma cells that have thickened primary cell walls and are covered by a cuticle consisting of cutin and associated waxes (Esau, 1977). Collenchyma cells and phloem elements have thickened primary cell walls that provide tensile strength to surrounding tissues. Xylem and sclerenchyma cells such as fibers and sclereids have thick and lignified secondary cell walls that are dead at maturity and give support (Harker et al., 1997).
Cell size varies with different cell types during ripening. Fruit development in blueberry follows a double-sigmoid growth pattern in which the pericarp initially increases in volume (Stage I), then the embryo develops while pericarp growth slows down (Stage II), and ripening occurs in conjunction with a final expansion in pericarp size (Stage III) (Godoy et al., 2008). In rabbiteye blueberry, differences in cell number were apparent in the ovary of different genotypes at bloom (Johnson et al., 2011). Shortly after anthesis, mesocarp cells in the ovary tissue that will develop into blueberry fruit stop dividing and increase only in size as the fruit continues to develop and enlarge (Darnell et al., 1992; Johnson et al., 2011). Johnson et al. (2011) found within-genotype differences in fruit size to be highly correlated with cell number and not cell size, although mesocarp cells increased 18- to 21-fold in area. Cell size is much larger in the mesocarp than in the epidermal and hypodermal layers that together form the epicarp, where cell division occurs over a longer period of time during fruit expansion (Harker et al., 1997). Mann et al. (2005) compared instrumental measurements and sensory evaluations to cell number and size in apple and concluded that fruit with fewer cells per unit area in the apple cortex (mesocarp) were crisper than fruit with more cells per unit area. This may be the result of an increased likelihood for larger cells to burst rather than separate from neighboring cells as is believed to occur in crisp-textured fruit with smaller sized cells and an increased surface area and higher proportion of cell wall material (Harker et al., 1997).
The amount of contact and/or space between neighboring cells is influenced by the shape and packing of cells (Harker et al., 1997). Batisse et al. (1996) observed that crisp-textured sweet cherries had more large intercellular spaces than those with soft texture. The degree to which adjacent cells separate during chewing also has an effect on its perceived texture. In the process of chewing, force is applied to the fruit tissue until it fractures, which can occur by cell separation—as is the case with soft fruit such as banana (Musa sp.)—or by individual cell rupture in crisp fruit such as apple and watermelon (Citrullus lanatus) (Harker et al., 1997). Cell wall strength and cell-to-cell adhesion also contribute to whether cells separate or rupture (Harker et al., 1997).
Blueberry fruit develop from an inferior ovary. The epidermis originates from the floral calyx, and the epidermis and hypodermal layers contain pigmentation from anthocyanins and form the epicarp, commonly referred to as the peel or skin (Gough, 1994). The endocarp is composed of five carpels with 10 locules and five highly lignified placentae to which ≈50 seeds are attached (Gough, 1994). The mesocarp is located between these layers and contains mostly parenchyma cells along with rings of vascular bundles and occasional sclerified stone cells that can be found ≈460 to 920 μm below the epidermis (Gough, 1983). These lignified cells with thick secondary cell walls can occur singly, doubly, or in clusters, and they bind neighboring parenchyma cells, which serve to strengthen their surrounding flesh tissue and may contribute to fruit firmness (Allan-Wojtas et al., 2001; Fava et al., 2006; Gough, 1983). Increased firmness just beneath the epidermal layer where initial rupture of the berry takes place could contribute to the crisp texture observed in some blueberry cultivars. Results of a trained sensory panel that evaluated the texture of several SHB genotypes ranging from soft to crisp also suggested that the crisp texture may be related to the blueberry peel or skin (Blaker et al., 2014). Genotypes receiving high sensory scores for crisp texture by the panel were often also rated for having a high level of skin toughness. Together these findings suggest that crisp texture is likely associated with differences in or near the epidermal layer of the berry. The objective of this study was to perform a histological analysis of cell type, area, and structure of the outermost cell layers of standard and crisp-textured fruit from nine SHB genotypes.
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