Benzyladenine as a chemical thinner for 'McIntosh' apples. II. Effects of benzyladenine, bourse shoot tip removal, and leaf number on fruit retention

Experiments were conducted to evaluate the effects of BA, removal of bourse shoot tips including only folded leaves and growing point, and different n umbers of leaves per fruit on fruit retention and fruit development in 'Mor e-Spur McIntosh'/Malling 7 (M,7) apple trees [Malus sylvestris (L,) Mill va r. domestica (Borkh.) Mansf,], Removal of the bourse shoot tip increased fr uit retention, whereas BA thinned fruit regardless of whether shoot tips we re removed or not, There was no interaction between BA application and shoo t tipping. BA thinned fruit only when one leaf per fruit was on a girdled s mall fruiting branch, but not when leaf number per fruit was two or greater . Fruit weight and soluble solids concentration increased dramatically with increasing leaf number per fruit. BA reduced fruit growth rate when <16 le aves per fruit were present on the girdled branches between 3 and 7 days af ter treatment, but it did not affect fruit growth rate when 32 leaves per f ruit were on the girdled branches. Increasing leaf number also increased vi able seed number per fruit while decreasing the number of aborted seeds, bu t it had no effect on the number of total seeds per fruit. BA reduced the n umber of viable seeds per fruit only when the number of leaves per fruit wa s less than four, Results suggest that BA thins apple fruit mainly by reduc ing carbohydrates available to developing fruitlets, Chemical name used: N- (phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)].