Exocarp Characteristics of Berries and the Effect of the Berry Density on the Fruit Quality and Occurrence of Cracking in ‘Hongju Seedless’ Grapes

‘Hongju Seedless’ is a seedless table grape variety conventionally treated with plant growth regulators to achieve berry enlargement. However, improved cultivation methods are required due to the increased occurrence of fruit cracking. This study was conducted to identify the optimal berry density by comparing the fruit quality and occurrence of cracking at different berry densities in the ‘Hongju Seedless’ grape variety. Additionally, the relationship between the anatomical characteristics of the berry exocarp and cracking was assessed. The number of berries per cluster was thinned to 35, 45, 55, and 65 berries at 20 days after full bloom, and the berry densities based on the cluster length were 1.7, 2.2, 2.7, and 3.1 berries·cm^-1, respectively. Regarding the fruit quality, an increase in the berry density was correlated with an increased cluster weight, a greater number of berries, higher titratable acidity, a greater soluble solids content and a decreased Hunter a value and tended to delay ripening. The occurrence of cracking was increased as the density of the berries increased. Moreover, the cracking of stylar-end parts of the berries was 95.0% and 74.1% in 2021 and 2022, respectively, greater than the stem-end parts (5.0% and 22.2%) and the equatorial parts (0% and 3.6%). The berry skin thickness was greater in the stylar-end parts (102.0 μm) than in the stem-end parts (77.9 μm) and the equatorial parts (89.7 μm) of the berry at harvest period. Similar trends were also observed in the number of sub-epidermal cells per 1mm². On the other hand, the pericarp hardness was lower in the stylar-end part (774.1 g) than in the stem-end part (847.6 g) and the equatorial part (947.8 g) of the berry. The stigma-stain located in the stylar-end area of the berry was larger and had a depressed center in ‘Hongju Seedless’ compared to ‘Shine Muscat’ grapes, which was presumed to be related to berry hardness and the occurrence of cracking. Based on these findings, the optimal berry density per cluster for quality improvement should be adjusted to 1.7 to 2.2 berries·cm^-1 (35 to 45 berries per cluster).