Ethephon Alters the Physiology of Young Apple Fruits

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Methods are needed to quickly assess the efficacy of chemical thinning treatments, so, if needed, additional treatments can be applied. When fruit diameter averaged 19 mm, `Smoothee' trees were sprayed with Ethrel at the rate of 3.7 ml formulation/L to induce abscission of most fruits on the tree. The diameter of 10 fruit/tree were measured daily and fruits were sampled 2, 4, and 6 days after treatment (DAT) to evaluate fruit development. Fruit on nontreated trees increased in diameter at the rate of about 0.7 to 0.9 mm/day. Fruit growth on treated trees was normal for 1 day, slowed to 0.5 mm for 2 days, and grew <0.1 mm/day thereafter. Fruit water potential was less negative for treated trees 2 DAT. Fresh and dry mass of treated fruit ceased increasing after 4 DAT. Percent dry mass of control fruit declined on each sampling date, but did not decline for treated fruit after 4 DAT. Cellulase was active in the abscission zone 4 DAT for treated fruit, but not control fruit. Compared to control fruit 6 DAT, treated fruit had elevated respiration and less starch accumulation. Fruits began to abscise 6 DAT. Following Ethrel application. The first indication that a fruit is destined to abscise are reduced fruit growth and changes in water potential 2 DAT.