Effect of Fruit Set Levels and Contact between Berries on Fruit Quality in ‘Jinok’ and ‘Campbell Early’ Grapes

This study was carried out to compare fruit quality outcomes according to berry set levels, and to analyze the relationships among epicuticular wax, skin cell development, texture characteristics, and cracking sensitivity due to contact with neighboring berries under a condition of excessive-berry set per cluster of ‘Jinok’ and ‘Campbell Early’ grapes. A berry thinning treatment of both cultivars reduced the numbers of berries per cluster to 45, 60, 75, and 90 at 20 days after full bloom. With regard to the fruit quality characteristics, the number of berries, the berry density, and the cluster weight all showed increases with higher berry set levels, but the 75 and 90 berry thinning levels showed lower soluble solid contents and higher acidity than the 45 and 60 berry treatments. Significant berry cracking occurred in the 75 and 90 berry treatments in both cultivars, though its occurrence was significantly lower in the ‘Jinok’ than the ‘Campbell Early’ grape. A ‘platelet’ shape of epicuticular wax was observed on the non-contact side of the berries, but the contact sides of neighboring berries were poorly developed. The epidermal cells (outer and sub-epidermal) of the contact sides of neighboring berries were pressed down by internal and external growth pressure more than they were in normal berries (non-contact), showing a long oval shape with a long length and width. On the contact sides of neighboring berries of the ‘Jinok’ and ‘Campbell Early’ grapes, berry hardness was reduced by 23.9% and 36.5%, respectively, compared to non-contact berries.