Benzyladenine as a chemical thinner for 'McIntosh' apples. I. Fruit thinning effect1s and associated relationships with photosynthesis, assimilate translocation, and nonstructural carbohydrates

BA was applied at 50 or 100 mg.L-1 to 'More-Spur McIntosh'/Malling 7 (M.7) apple trees [Malus sylvestris (L.) Mill var. domestica (Borkh,) Mansf.] at the 10 mm stage of fruit development. BA thinned fruit and increased fruit size. There were two distinguishable peaks of fruit abscission during 'June drop'. BA accentuated the naturally occurring waves of fruit abscission, a nd enhanced translocation of C-14-sorbitol from leaves to fruit when applie d directly to the fruit, but not when applied directly to the leaves. Net p hotosynthesis was decreased and dark respiration was increased when tempera ture following BA application was high (30 degrees C), whereas there was no effect when temperature was lower (20 degrees C), Total nonstructural carb ohydrates, total soluble sugars, and starch in the leaves decreased dramati cally over the 12- or 13-day observation period, regardless of BA treatment . These carbohydrate concentrations in the leaves were lowered further by B A application. Abscising fruit, based on specific reddening of the pedicel, had higher carbohydrate levels than persisting fruit, regardless of BA app lication. We conclude that BA thins fruit, at least in part, by increasing dark respiration and decreasing net photosynthesis. Chemical name used: N-( phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)].