Corymb hierarchy modulates physiological fruit drop and chemical thinning response in apple (Malus domestica L. Borkh)

Chemical thinning of apple is often highly variable because of complex interactions among fruit load, tree physiology, climate and thinning intensity. Corymb hierarchy is a key physiological driver of selective abscission, but it has never been quantified and incorporated into predictive models. In this study, we propose a hierarchy index (Ih) derived from the size distribution of fruitlets within the corymb and evaluate its ability to explain and predict partial fruit drop across cultivars, geographical areas and thinning strategies. Using a large, multi-site dataset, linear and logistic models were developed by integrating Ih with thinning dosage, cultivar, geographical area and fruit number per corymb. The hierarchy index consistently emerged as a strong predictor of fruit drop, exceeding the effect of thinning dosage alone and remaining robust across genetic and environmental contexts. A significant interaction between Ih and dosage indicated that hierarchy-driven differences are most relevant at low to moderate thinning intensities and progressively diminish as thinning strength increases. Cultivar and geographical area significantly modulated baseline fruit drop levels, while the effect of Ih was stable across these factors. Analyses focused on Brevis applications showed that climatic predisposition affected the probability of extreme thinning responses, emphasizing the importance of dosage context and dataset composition. Overall, the Ih-based framework provides a physiologically grounded, parsimonious and transferable approach for predicting apple fruit drop. Its limited data requirements and additive structure make it particularly suitable for integration into decision support systems aimed at improving thinning reliability and reducing the risk of under- or overthinning.