Integrated application of ACC and AVG mitigates pre-harvest fruit drop and enhances red coloration in apple

Abstract Ethylene, a key plant hormone, regulates various fruit ripening traits, including pre-harvest fruit drop (PFD) and color development. In commercially valuable apple cultivars like ‘Honeycrisp,’ excessive fruit drop before the commercial harvest date poses a significant challenge, leading to substantial crop losses. While ethylene inhibitors such as aminoethoxyvinylglycine (AVG) effectively reduce PFD, they can also hinder fruit coloration. To address this trade-off, we evaluated various plant growth regulators (PGRs), such as Accede (1-aminocyclopropane-1-carboxylic acid (ACC)-10%), Motivate (ethephon-21.7%), and ReTain (AVG-15%), in ‘Honeycrisp’ apples over two consecutive years (2023–2024). Our study found that ACC, alone or combined with AVG, promoted chlorophyll degradation and intensified red coloration at the normal harvest date and two weeks after the normal harvest date (WAH). ACC, ethephon, and AVG + ACC increased anthocyanin accumulation on apple fruit peel in both growing seasons. For fruit drop control, AVG and its combination with ACC significantly reduced PFD compared to the untreated control. Notably, AVG + ACC was the most effective treatment in reducing PFD by 27.05–46.30% while simultaneously enhancing red color. The anthocyanin content of AVG + ACC-treated apples was significantly greater than that of the control, correlating with the upregulation of anthocyanin biosynthetic genes such as MdCHS , MdCHI and MdDFR . No significant differences were observed in other fruit quality traits, e.g. fruit firmness, brix, and starch index, between AVG + ACC-treated and untreated fruits at any time point. Our results indicate that the AVG + ACC combination is a promising strategy for mitigating pre-harvest fruit drop while improving fruit color in apples.