1-Aminocyclopropane-1-Carboxylic Acid (ACC) Increases Ethylene Evolution and Fruit Abscission in Developing Apple Spurs (Malus x domestica Borkh.)

Thinning reduces biennial bearing in apple. ACC (1-aminocyclopropane-1-carboxylic acid, a precursor of ethylene) is a recent thinning agent. The objective was to identify factors affecting ethylene evolution and fruit abscission following ACC application. Long term, time-course studies reveal that an ACC spray (200 mg l-1) applied 21 days after full bloom (DAFB) induces ethylene evolution which peaks about 1 d after application. From 32 days after application onwards, ethylene evolution did not differ from the un-treated control. ACC induced fruit abscission from 16 days after application onwards. Abscission ceased about 21 days after application. By this time, 27% of the fruit remained on the tree, compared to 61% in the un-treated control. Ethylene evolution per spur peaked, when ACC was applied between 16 and 47 DAFB (equiv. to fruit diameters from 8 to 32 mm). Later ACC applications had little effect on ethylene evolution. Ethylene evolution following ACC application was localized. Selective application to a fruit resulted in increased ethylene evolution by the fruit, but not by leaves and vice versa. ACC effects on ethylene evolution and thinning were similar across six apple cultivars. Ethylene evolution increased linearly with increasing ACC concentration. Most of the ethylene released from spurs was accounted for by ethylene from fruits. Less ethylene was released from leaves. Thinning occurred for ACC concentrations of 200 to 400 mg l-1, lower concentrations were ineffective. Across all experiments, the percentage of fruit remaining on the spur was negatively related to ethylene evolution. Our study shows that ACC (200 to 400 mg l-1), applied when fruit are 6 to 20 mm diameter, induces ethylene evolution and abscission.