Benzylaminopurine Application on Two Different Apple Cultivars (Malus domestica) Displays New and Unexpected Fruitlet Abscission Features

It has been previously shown that abscission of apple fruitlets is preceded by an increase in ethylene evolution and in the amount of transcripts for 1-aminocyclopropane-1-carboxylate oxidase (ACO), an enzyme catalysing the final step in ethylene biosynthesis. These events are concomitant with shedding induction and chemical thinning. There are several thinners but their mode of action and efficacy is poorly understood. One of them is benzylaminopurine (BA), a cytokinin believed to act by enhancing vegetative activity and stressing the competition between shoots and fruitlets, thus leading to fruitlet shedding. Nevertheless, the specific mechanism of action of BA and the variable effect depending on apple cultivar (easy or difficult to thin) are poorly understood.