Physiological Investigations of Chemical Thinner Efficacy in Apples

Chemical fruit thinning of apples is a challenging task; it is influenced by many factors including weather conditions, tree health status, etc. As these factors vary year by year thinner efficacy also varies from one year to another. The aims of this study were: 1) to investigate the physiological background of thinner efficacy in apples using commercially available and test compounds; and 2) to measure how carbohydrate balance of trees affects fruit sensitivity to thinner applications. ‘Buckeye Gala’ trees on B.9 rootstock in an experimental orchard located at Wooster, OH, were sprayed at 10–12 mm fruitlet size with either of the following compounds: 100 ppm 6-benzyladenine (6-BA), 600 ppm carbaryl, 10 ppm of two different formulations of 1-naphthaleneacetic acid (NAA), 200 ppm metamitron, 200 ppm 1-aminocyclopropane carboxylic acid (ACC), 300 ppm S-abscisic acid (S-ABA), and water (control). Fruit set, cell division and cell size, relative chlorophyll content, chlorophyll fluorescence, photosynthesis, ethylene evolution, and fruit quality were measured. MaluSim Carbohydrate model was used to estimate daily carbohydrate balance of trees. Fruitlet response to thinners, i.e., fruit drop pattern, could be well correlated with the carbon balance of trees. According to the model calculations, during the thinner application period carbohydrate balance was positive. Sensitivity of fruitlets to thinners was therefore significantly reduced. ACC and metamitron had the greatest efficacy in causing fruit abscission. The greatest reduction in photosynthesis was measured for metamitron, followed by ABA, NAA, and ACC. Only metamitron caused damage to PSII efficiency or chloroplast ultrastructure, evident as leaf yellowing and reduced relative chlorophyll content of shoot leaves. Neither cell division nor cell enlargement of the fruits nor fruit quality was affected by thinner application; all showed similar patterns over the season in all treatments.