Relationships among apple fruit abscission, source strength, and cultivar

Fruit thinning is an essential part of the commercial production of quality apples. A portion of the excessive fruitlets is abscised, or shed, physiologically as a self-regulatory mechanism; however, the degree of abscission varies depending on the environmental conditions and cultivars. The number of abscised fruitlets within a cluster is influenced by the number of leaves in the cluster. In some cultivars, the fewer leaves there are in a cluster, the larger the number of fruitlets undergo abscission is. Conversely, the growth rate of a fruit cluster is marginally influenced by the number of leaves in the cluster and appears constant for each cultivar. The constant growth rates of fruit clusters indicate the source strength, which supports the growth of individual fruitlets (sink organ) on the tree. The cultivar differences in the percentage of fruitlet abscission were highly correlated with the source strength of the cultivars, indicating that the rate of abscised fruitlets was determined by the source strength of the cultivars. The number of days between full bloom and diminishing growth of abscising fruitlets was also correlated with the source strength. Therefore, a cultivar with low source strength is prone to abscising many fruitlets at an early date compared with one with high source strength. The source strength of a cultivar, therefore, can be a useful index for selecting cultivars characterized by easy and early abscission.