Crop Load Management of Plums Using 1-Aminocyclopropane Carboxylic Acid (1-ACC)

Abstract

Excessive fruit set in European (Prunus domestica L.) and Japanese (Prunus salicina L.) plum trees necessitates thinning to optimize crop load, enhance fruit quality, and reduce manual hand thinning. This study evaluated the efficacy of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (1-ACC) as a chemical thinning agent for the European plum cultivar ‘Vibrant’ and the Japanese plum cultivar ‘Early Golden’. Treatments were applied at full bloom (FB) and at the fruitlet stage (8-10 mm diameter) using 1-ACC concentrations of 150, 300, and 450 mg·L⁻¹. Horticultural measurements included crop load, yield, fruit size distribution, fruit quality, phytotoxic effects, and crop value. Applications of 1-ACC effectively reduced crop load, with FB applications producing greater thinning responses than fruitlet-stage applications. Increasing application rates resulted in decreased yield and crop value, while average fruit weight increased. No hand thinning was required in both years of the study due to poor pollination in the ‘Early Golden’ orchard and atypical fruit drop unrelated to 1-ACC treatments following early fruit development in the ‘Vibrant’ orchard; therefore, commercial thinning was not required. Thinning responses varied by cultivar and application timing, underscoring the importance of cultivar-specific thinning strategies. Ethylene evolution following 1-ACC application supported its role in fruit abscission, with ethylene peaks observed within 24 h and differing in intensity by application timing. These results indicate that 1-ACC is a viable chemical thinning agent for plums, although optimization of application timing and concentration, particularly in seasons with heavier fruit set, is required to balance thinning efficacy with economic returns.