Streszczenie. The objective of this research was to determine the effects of chemical thinning with α-naphthaleneacetic acid (NAA) and nitrogen (N) fertilization, as well as their interaction in relation to fruit set and physical properties of apple cv. Golden Delicious Reinders®. Nitrogen rates used were 30 and 60 kg·ha⁻¹ N applied at the pink bud stage. NAA for fruit thinning was applied at the rates of 10, 12 and 14 μl·l⁻¹. Fruit set was increased by increasing the amount of N applied while the efficacy of NAA for fruit thinning was not linearly correlated to the concentration of the chemical. The total number of fruits per tree was inconsistently reduced in NAA treatments at 30 and 60 kg·ha⁻¹ N, while total yield was reduced by thinning at 60 kg·ha⁻¹ N, meaning that apple trees are more responsive to NAA application at higher N supply. At sufficient N supply, fruits responded more intensively to thinning with NAA by increasing their growth. Thinning with NAA caused a significant decrease in flesh firmness only in fruits from unfertilized plot.

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