Evaluating the Impact of Copper Concentrations on Apple Variety Performance: Insights from a Callus Media System

Copper (Cu)-based fungicides are widely used in agriculture, but excessive Cu accumulation in soils can pose risks to crop performance, particularly in perennial fruit crops like apples. This study evaluated the physiological responses of three apple varieties—Braeburn, Fuji, and Cripps Pink—under Cu concentrations ranging from 0 to 20 mg Cu kg⁻¹ using a callus media system. Key traits including the callus growth, fresh and dry weight, copper accumulation, tolerance index (TI), and bioaccumulation factor (BCF) were assessed. The results revealed that high Cu concentrations significantly inhibited the callus growth and biomass. Braeburn’s growth was notably reduced to 0.27 times the control at 15 mg Cu kg−1, and Fuji showed no growth at concentrations above 10 mg Cu kg−1. Cripps Pink displayed greater tolerance, maintaining growth even at 20 mg Cu kg−1. Copper accumulation peaked in Fuji at 4200 mg kg−1 at 20 mg Cu kg−1, while Cripps Pink’s BCF peaked at 120 under 15 mg Cu kg−1. The TI for Cripps Pink consistently exceeded 100% across all Cu levels, contrasting with Fuji’s TI, which dropped significantly at 15 mg Cu kg−1. These findings suggest that maintaining soil Cu levels below critical thresholds is essential to preserve varietal performance and inform Cu management practices in apple cultivation.