Abstract Chemical thinning is an important practise in apple production, yet current thinning agents face limitations in environmental safety and efficacy under variable conditions. This study investigates the thinning potential and physiological effects of two food-grade emulsifiers, polysorbate 20 (P20) and polysorbate 80 (P80), compared to a standard ethephon-based as well as a metamitron-based thinner on ‘Gala’ apple trees. Fruitlet drop, sugar content and phenolic profiles were monitored in relation to fruitlet position and developmental stage. Both P20 and P80 induced significant fruitlet abscission, with P20 showing a comparable thinning effect to commercial thinners. The P20 and P80 treatments altered the natural cluster hierarchy, and fruitlets with reduced growth (reduced diameter and weight) showed increased abscission, indicating lower sink strength. Sucrose content was significantly reduced in P20 treated king fruitlets, which correlated with increased fruit drop. Fructose and sorbitol content varied over time, but not between treatments, while glucose levels remained stable. Phenolic profiling revealed a significant increase in dihydrochalcones and anthocyanins after treatment with P20, suggesting a stress-induced response. These results support the carbohydrate theory of fruitlet abscission and emphasise the potential of polysorbates as alternative late-acting thinning agents with additional insights into the physiological mechanisms behind fruit drop. Further research is needed to clarify the exact mode of action of polysorbates in apple fruitlet thinning.
Hillmayr et al. (Thu,) studied this question.