NAA (α-naphthylacetic acid) thins apple fruits inconsistently regarding fruit set and weight and may cause small or misshapen fruits occurrence. Considering hormonal activity, surfactant properties and the impact of naphthenic acids (NAs) on the metabolism of auxins in plant tissue, it is assumed that thinning response and fruit size could be improved by adding NAs to NAA for chemical thinning of apple. The purpose of the research was to evaluate thinning efficacy of the mixture of NAA and NAs in two commercially grown apple cultivars, ‘Golden Delicious’ and ‘Fuji’, known to respond differently to the application of NAA. The following treatments were applied during a three-year period, from 2010 to 2012: 10 mg∙L^-1 NAA, 10 mg∙L^-1 NAA + 1.56 mg∙L^-1 NAs, 20 mg∙L^-1 NAA, 20 mg∙L^-1 NAA + 3.12 mg∙L^-1 NAs and an untreated control. The addition of NAs to NAA did not alter the effectiveness of NAA in reducing fruit set, while it significantly increased fruit weight compared to NAA applied alone in two of three years of the experiment. NAs modulated the effect of NAA on fruit weight of apple cultivar ‘Fuji’ by retarding fruit growth and causing a higher number of small fruits to occur compared to NAA applied alone.

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