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Carbohydrate availability is a key factor determining fruit growth in apple (Malus ×domestica) and other fruits. However, the molecular mechanisms regulating fleshy fruit growth in response to changes in carbohydrate availability are not well understood. In this study, carbohydrate availability was manipulated by reducing fruit load, and its effects on apple fruit growth, cell production and expansion, and the expression of genes associated with these processes was investigated. Reduction of fruit load during early fruit development led to a rapid increase in early fruit growth. The increase in fruit growth was associated with a transient increase in sorbitol and fructose concentrations and altered expression of sorbitol dehydrogenase and sucrose synthase genes. Increase in early fruit growth was mediated primarily by an increase in cell production. The aintegumenta gene, MdANT1, an AP2-domain transcription factor associated with the regulation of cell production and fruit growth, displayed an increase in expression by up to 5-fold during early fruit development in response to the reduction in fruit load. Additionally, multiple cell cycle genes positively associated with cell production such as the cyclins, MdCYCA2;1, MdCYCA2;3, MdCYCB1;1, and MdCYCB2;2, and the B-type cyclin-dependent kinases, MdCDKB1;1, MdCDKB1;2, and MdCDKB2;2, displayed higher expression by up to 5-fold under reduced fruit load conditions during early fruit growth. These data indicate that carbohydrate availability affects the expression of key transcription factors and cell proliferation genes, thereby regulating cell production during early fruit growth. Several genes associated with cell expansion such as the expansins, and cobra and cobra-like genes, also displayed altered expression in response to the reduction in fruit load. The expression of three expansin genes was higher under reduced fruit load conditions at maturity, a stage at which a minor increase in cell size was apparent. Together, data from this study indicate that fruit load reduction induces changes in carbohydrate availability and metabolism, which in turn affect cell production-related mechanisms, thereby enhancing early fruit growth.

Two abscission agents, ethephon and methyl jasmonate, were investigated in five studies to determine their potential for increasing fruit detachment during harvest in rabbiteye (Vaccinium ashei Reade) and southern highbush (hybrids based largely on Vaccinium corymbosum L. and Vaccinium darrowi Camp.) blueberry. In the first study with a rabbiteye blueberry genotype, T-451, ethephon applications up to 1000 mg·L⁻¹ did not affect fruit drop but reduced fruit detachment force (FDF) by up to 21%. In the second study with two southern highbush blueberry genotypes, ethephon (up to 1500 mg·L⁻¹) and methyl jasmonate (MeJa; up to 10 mm) applications resulted in significant fruit drop in ‘Star’ but neither of the growth regulators affected the fruit detachment characteristics of ‘Farthing’. In a third study with rabbiteye blueberry genotypes, MeJa applications of 10, 20, and 30 mm displayed an increasing linear trend in fruit drop in ‘Climax’ and linear and quadratic trends in fruit drop in ‘Powderblue’. In a fourth study with ‘Powderblue’, MeJa (20 mm) and ethephon (1000 mg·L⁻¹) applications resulted in rapid and significant fruit drop. The fruit drop induced by MeJa in this study was attenuated by the coapplication of aminoethoxyvinylglycine (AVG), an ethylene biosynthesis inhibitor, suggesting that MeJa induced fruit detachment partly through its effects on ethylene biosynthesis. In another study with the southern highbush blueberry genotype, O’Neal, MeJa applications (20 mm) induced significant fruit drop but ethephon (1000 mg·L⁻¹) applications did not affect fruit detachment. Overall, MeJa applications (20 mm or greater) generally induced rapid and extensive fruit abscission, often within 1 day after treatment. Applications of MeJa resulted in leaf yellowing and necrosis of leaf tips and margins, especially at high rates of application (20 mm or greater). Ethephon applications resulted in the abscission of mature and immature berries. Both ethephon and MeJa applications resulted in the detachment of the pedicel along with the fruit. Together, these data suggest that although ethephon and MeJa have the potential to be used as harvest aids in blueberry, the rates of application require further optimization to minimize potential phytotoxicity. Additionally, effective de-stemming of the berries may be essential if these compounds are to be used as harvest aids.

Fruit size is a valuable commercial trait in blueberry. The cellular basis of variation in fruit size among rabbiteye blueberry (Vaccinium ashei) genotypes was investigated. Twenty genotypes, including cultivars and advanced selections from the University of Georgia blueberry breeding program, were analyzed. Among the 20 genotypes, fruit weight and fruit diameter varied by over threefold and 1.6-fold, respectively. Regression analysis indicated a linear relationship between fruit weight and fruit diameter (R² = 0.97, P < 0.001), suggesting that fruit diameter is a good predictor of fruit weight. Among the 20 genotypes, mesocarp cell number and cell area varied by almost 2.5-fold and 1.5-fold, respectively. Although fruit diameter and cell number were significantly related (R² = 0.79, P < 0.001), no relationship could be established between fruit diameter and cell area. These data indicate that variation in fruit size among rabbiteye blueberry genotypes is primarily facilitated by variation in cell number. Two small and two large fruit size genotypes were further analyzed. Differences in cell number among some of these genotypes were apparent at bloom suggesting that cell production before bloom is an important mechanism contributing to variation in final cell number. Differences in final cell number among other genotypes were manifested during fruit development, indicating that cell production during fruit development was also instrumental in determining variation in final cell number. This study suggests that fruit size variation in rabbiteye blueberry genotypes is determined by mechanisms that regulate cell production before bloom and during fruit development.