Assessing the impacts of aminoethoxyvinylglycine and 1-methylcyclopropene on fruit drop, cracking, quality, and related transcript accumulation in ‘Ambrosia’ and ‘Fuji’ apples during on-the-tree ripening

Fruit drop, cracking, and advanced ripening prior to fruit harvest can promote significant losses in important apple cultivars such as 'Ambrosia' and 'Fuji' grown in the mid-Atlantic. These losses result from environmental factors, delays in harvest due to the lack of red skin color development, and cultivar-specific characteristics, among others. Aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene (1-MCP) are ethylene-inhibiting plant growth regulators (PGRs) that can alter preharvest fruit drop, cracking, maturity, and quality by impeding ethylene biosynthesis and perception, respectively. However, there is a knowledge gap on understanding the impacts of specific timings and dosages of such PGR treatments on 'Ambrosia' and 'Fuji' under mid-Atlantic environmental conditions. The objective of this work was to characterize and compare the impacts of specific rates of preharvest ethylene-inhibiting PGRs on fruit drop and cracking, ethylene production, physicochemical properties, skin color, and associated gene expression in 'Ambrosia' and 'Fuji' apples during on-the-tree ripening. Multivariate statistical methods for identifying meaningful correlations among the measured variables were applied. Our results show that both full-rate AVG (130 mg a.i. L^-1) and 1-MCP (150 mg a.i. L^-1) significantly reduced preharvest fruit cracking compared with control fruits in 'Ambrosia', but not in 'Fuji'. Furthermore, among all treatments, full-rate AVG (130 mg a.i. L^-1) treatment displayed the lowest ethylene production and gene expression of ethylene- related genes. 1-MCP (150 mg a.i. L^-1) and half-rate AVG (65 mg a.i. L^-1) treatments presented a reduction in the ethylene production and gene expression of ethylene-related genes when compared to control fruits. Regarding color, apple skin blush and anthocyanin-related gene expression were the lowest in full-rate AVG (130 mg a.i. L^-1)-treated fruits, explaining why these fruits met the 50% red blush coverage 1 week later than all other treatments in both cultivars. Correlations amongst the assessed features were also identified. These results suggest a cultivar-specific effectiveness under mid-Atlantic conditions and specifies a framework for the use of ethylene-inhibiting PGRs under mid-Atlantic environmental conditions.