Stone Cell Frequency and Cell Size Variation of Crisp and Soft Textured Fruits from Nine Southern Highbush Blueberry Cultivars

The microstructure of southern highbush (Vaccinium corymbosum L.) blueberry fruit with crisp and soft texture was investigated by light microscopy.  The frequency of stone cells and variation in epidermal and hypodermal cell size was measured.  Stone cells, which are sclerified cells having a thick and lignified secondary cell wall, are known to vary in number between cultivars of northern highbush and rabbiteye blueberry species, and may contribute to fruit firmness.  Cell size can also contribute to differences in fruit firmness.  Fruit from nine cultivars determined by sensory and instrumental analysis to vary in fruit texture were harvested at two developmental stages:  green and mature blue.  Tissue was fixed in FAA, trimmed to a 3 mm equatorial thickness, dehydrated using a graded solvent series, infiltrated and embedded in paraffin, sectioned to 12 µm, and stained with Safranin O and Aniline Blue for light microscopy.  Stone cells within ca. 1200 µm of the epidermis were counted and cell height and width was measured in the epidermal layer and 3 layers beneath the epidermis of the fruit.  The average number of stone cells in a single berry ranged from 0 to 95 between cultivars.  Significant differences in the number of stone cells just below the epidermal layer and in cell size within the exocarp were detected between cultivars, but did not correspond to sensory and instrumental measures of fruit firmness, crispness, or skin toughness.  These results suggest that cell size and increased stone cell formation beneath the fruit skin does not have a significant affect on berry texture, and that firmness differences across blueberry cultivars may be better explained by other factors affecting fruit texture such as cell wall composition/disassembly, cell membrane integrity, and/or turgor pressure.