Benzylaminopurine application on two different apple cultivars (Malus domestica) displays new and unexpected fruitlet abscission features

Background and Aims 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-amin ocyclopropane-1-carboxyl ate 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. Methods Abscission, the amount of MMdACOI transcripts and other parameters were followed in immature apple fruits during the period of physiological drop. The cultivars studied were 'Golden Delicious' and the 'spur' type 'Red Delicious'. BA was used as a thinning agent and was sprayed 14 d after petal fall (DAPF). Fruitlets were divided into central (C) and lateral (L) fruitlet populations. Key Results Fruitlet size was significantly different between C and L fruitlets but it did not differ much between the populations within the same cultivar. C fruitlets were characterized by basal ethylene evolution while L fruitlets displayed an increase in hormone biosynthesis during abscission induction. Cluster composition evaluated by the L/C ratio differed in the two varieties, being almost unchanged throughout abscission induction in 'Golden Delicious' and progressively decreasing in 'Red Delicious'. Shoot growth activity evaluated at the end of the season indicated a possible connection with both the ongoing abscission and BA application. MDACOI transcripts were mainly detected in L fruitlets and the accumulation was related to total abscission in 'Golden Delicious', while in 'Red Delicious' expression was observed in both C and L fruitlets. Conclusions BA probably exerts its thinning effect through vegetative growth. In the 'spur' type 'Red Delicious' the chemical is ineffective, probably due to a limited action on shoot growth due to genetic characteristics. The amount of MdACOI transcripts in seeds is a good indicator of abscission.