The Growth of Apple Central Fruits as Affected by Thinning with NAA, BA and Naphthenic Acids

Abstract

Plant growth regulators such as α‑naphthylacetic acid (NAA) or 6‑benzyladenine (BA) are commonly used for thinning apple fruits. NAs exhibits an auxin- and gibberellin-type biological activity, stimulate the uptake and translocation of auxins in plant tissue, as well as IAA synthesis and act as surfactants. It was assumed that NAs applied at the stadium of early fruit growth might cause fruit abscission in apple, similarly to NAA. The goal of the research was to test the presumed thinning effect of naphthenic acids on apple cultivars ‘Golden Delicious’ and ‘Red Delicious’. Three treatments were applied on both apple cultivars including 16.5 μL L⁻¹ NAA, 200 μL L⁻¹ BA and 2.63 mg L⁻¹ potassium salts of NAs. Cortical cell size and the number of cell layers were measured in order to determine the contribution of cell size and cell division in early fruit growth, as affected by chemical thinning treatments. The effects of plant growth regulators NAA, BA and NAs on fruit set, fruit size and the anatomy of the fruit cortex highly depend on varietal differences. In ‘Golden Delicious’ fruit set reduction by chemical thinning increases both the number of cell layers and cell size of apple fruit, while in ‘Red Delicious’ NAA inhibits fruit growth, while BA is ineffective. Cell growth promoting activity of NAA and NAs, accompanied by the decrease in fruit set and reduced competition among fruits within the canopy, results in a significant fruit weight increase at harvest. The research confirms the auxinic properties of NAs, and their assumed thinning activity.