Analysis of Cellular Basis of Fruit Size Variation in Rabbiteye Blueberries (Vaccinium ashei)

Fruit size is a valuable commercial trait in crops such as blueberry. In spite of its importance, little is known regarding the mechanisms involved in regulating fruit size. In this study, we investigated the basis of variation in fruit size across rabbiteye blueberry genotypes. Twenty rabbiteye blueberry cultivars and advanced selections from the University of Georgia blueberry breeding program were used in this study. At harvest, fruit weight, fruit diameter, cell number and cell cross sectional area were determined in the above genotypes. Fruit weight varied over a 3-fold range from 0.9 g to 2.8 g while fruit diameter, at the widest part of the fruit, ranged from 11.5 mm to 18.8 mm. Varieties such as ‘Brightwell’ and ‘Powderblue’ constituted the lower range of fruit weight and fruit diameter, while advanced selections such as T-959, T-960 and T-965 constituted the upper range. Fruit weight and fruit diameter were significantly related (r² = 0.97; P < 0.001) indicating that fruit diameter is a good predictor of fruit weight in rabbiteye blueberries. Cell number and cell area were determined by sectioning fruits using a vibratome followed by image analysis using ImageJ (NIH). While little variation in cell cross sectional area was observed, cell number varied by almost 2.5-fold among the 20 genotypes. Cell number was strongly related to fruit diameter (r² = 0.77; P < 0.001) while no significant relationship between cell cross sectional area and final fruit diameter was observed. Additionally, cell number and cell cross sectional area were not significantly related. These data indicate that variation in fruit size across rabbiteye blueberry genotypes is largely facilitated by variation in cell number. To determine if difference in cell number between large fruit-size and small fruit-size genotypes was due to differential cell production during fruit growth, cell number at bloom and at harvest were measured in ‘Powderblue,’ a small fruit-size variety, and T-959, a large fruit-size selection. Cell number at bloom in ‘Powderblue’ was not significantly different from that in T-959. In ‘Powderblue’ cell number at harvest was not significantly different from that at bloom, but in T-959, cell number increased by ~1.75-fold during fruit development. These data indicate that larger fruit size in rabbiteye blueberries is facilitated by increased cell production during fruit development.