Production of fresh market blackberries in the United States has expanded greatly compared with most of the world, especially in California along with new commercial production in Georgia, North Carolina, Arkansas, and Texas (Clark and Finn, 2014). This expansion is due to several factors, including an increase in demand and resulting consumption in the United States and Europe (Clark and Finn, 2014). In 2005, Europe had 7692 ha of blackberries in commercial production, and Serbia was the leading country with 69% of the area in Europe (Strik et al., 2007). Serbia continues as one of the top blackberry producers in the world with more than 5000 ha planted (Clark and Finn, 2014), although this production is primarily for processing. Mexico leads the world with over 6500 ha in production, almost all production for the fresh market and export (Clark and Finn, 2014).
The blackberry industry is in expansion and reasons for this phenomenon are several, but include that blackberry is a new crop in many areas of the world; new blackberry cultivars ship better allowing for extension of the harvest and maturity season; fruit quality is improved in newer cultivars; blackberry fruits have high levels of anthocyanins and antioxidants providing for increasing appeal to consumers; and longer-season production allows for year-round marketing enhancing grower, packer, and processor profitability (Clark and Finn, 2014; Clark et al., 2007).
The quality of fruits for the fresh market is determined largely by how the genotype responds to storage and handling from the day of harvest until purchased and eaten by the consumer (Finn and Clark, 2012). Blackberries have been considered as one of the most difficult fruits to ship due to postharvest softening and leakage during handling (Clark, 2005). Blackberry fruit firmness was once suggested to be an intractable trait, meaning that it is a difficult character to improve in breeding. Firmness varies with cultivar, ripeness stage, and storage duration (Clark, 2005; Perkins-Veazie et al., 1996).
Generally in fruits, firmness is related to modifications of the polysaccharide components of the primary cell wall and middle lamella during fruit ripening, resulting in a weaker fruit structure at the end of the ripening process (Brummell, 2006; Winkler et al., 2015). Alterations in the bonding between polymers along with degradation of polysaccharides can cause an increase in cell separation, softening, and swelling of the cell wall (Brummell, 2006). These alterations, combined with changes in cell turgor, cause fruit softening and textural changes in fruits (Brummell, 2006). Blackberries increase their pectin solubility activity during ripening (Brummell, 2006). In apples (Malus ×domestica Borkh.), ripening is accompanied by decreasing strength of the middle lamella, resulting in reduced intercellular adhesion and increased cell separation under stress (Atkinson et al., 2012). A suppression of the gene that promotes polygalacturonase (PG) activity resulted in firmer ‘Royal Gala’ apples due to different distribution of pectins along with increased integrity of the middle lamella (Atkinson et al., 2012). Also, in the PG-suppressed genotypes, cells in the hypodermal layers of the fruit below the cuticle remained densely packed (Atkinson et al., 2012).
An important postharvest disorder affecting blackberry fruit destined for the fresh market is color reversion (also called reddening or red drupelet; Clark and Finn, 2011). Affected drupelets of blackberry fruits turn red, often during cold storage or when exposed at room temperature after being in cold storage (Clark and Finn, 2011; Finn and Clark, 2012). In a mature fruit cell, ≈90% of the volume is occupied by the vacuole, a cell organelle that is dynamic and multifunctional and provides the primary site of macromolecule storage and turnover (Fontes et al., 2011). The vacuole accumulates sugars, aromas, flavors, ions, and water; all these compounds are transported across the tonoplast (vacuole membrane) by a specific transporter protein (Fontes et al., 2011).
Retention of black color can be selected for in breeding, but it cannot be determined in the field. Therefore, postharvest evaluations must be done to verify if resistance to reversion is present (Clark and Finn, 2011). The UA blackberry breeding program, with a focus on postharvest quality of fruits for successful postharvest storage for the fresh market, began postharvest evaluations in 2008 to characterize postharvest traits of advanced breeding selections (Clark and Perkins-Veazie, 2011).
Crispy fruit with high firmness were first observed in the UA blackberry breeding program several years ago on a floricane-fruiting, thorny selection. Since that time, this texture trait has been advanced to improved selections with the aim to transfer the crispiness into improved seedlings and resulting selections with increased yield, fruit size, fruit flavor, and primocane fruiting. Two of these, A-2453 and A-2454, are believed to hold the most promise for use in breeding for this trait. These crispy genotypes show improved postharvest performance compared with previously released cultivars from the breeding program. They maintain the high firmness observed in the field and after storage, and also show reduced color reversion (drupelets developing red color) after 7 d of cold storage (J.R. Clark, unpublished data).
The objectives of this study focused on characterization of crispy and noncrispy genotypes including compression and penetration forces in different tissues of the fruit. Also, analysis of cell structure of drupelet and receptacle tissue of crispy and noncrispy genotypes was conducted to reveal physical aspects contributing to the unique texture.
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