title: Benzylaminopurine application on two different apple cultivars (Malus domestica) displays new and unexpected fruitlet abscission features.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  Abscission, the amount of MdACO1 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.
  
  
  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. MDACO1 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 MdACO1 transcripts in seeds is a good indicator of abscission.
authors.affiliation: Department of Environmental Agronomy and Crop Science, University of Padova, Viale dell'Università 16, 35020 Legnaro (Padova), Italy. vdalcin@ufl.edu
authors.forename: Valeriano
authors.initials: V
authors.lastname: Dal Cin
authors.forename: Andrea
authors.initials: A
authors.lastname: Boschetti
authors.forename: Alberto
authors.initials: A
authors.lastname: Dorigoni
authors.forename: Angelo
authors.initials: A
authors.lastname: Ramina
mesh_terms: D000594:Amino Acid Oxidoreductases / Q000235:genetics; D001593:Benzyl Compounds / Q000494:pharmacology*; D015152:Blotting, Northern; D005638:Fruit / Q000187:drug effects / Q000235:genetics / Q000254:growth & development; D020869:Gene Expression Profiling; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D027845:Malus / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development*; D011687:Purines / Q000494:pharmacology*
chemical_list: D001593:Benzyl Compounds; D011687:Purines; D000594:Amino Acid Oxidoreductases; C059460:1-aminocyclopropane-1-carboxylic acid oxidase; C480551:benzylaminopurine