Effect of Cytokinins on Enzyme Activities, FRAP Antioxidant Capacity and Total Phenolic Content in Apple Tissue Cultures of Two Cultivars

Commonly used cytokinins in apple tissue cultures include 6-benzylaminopurine riboside (BAR), thidiazuron (TDZ), and meta-topolin (TOP). In this study, shoots of two distinct apple cultivars (‘Húsvéti rozmaring’ and ‘McIntosh’) were cultured on standard Murashige and Skoog medium supplemented with 4.5 µM of either BAR, TDZ, or TOP, alongside a cytokinin-free control medium. Since comprehensive comparative studies on these specific cytokinins and apple cultivars have not yet been conducted, we evaluated their individual effects on critical enzyme activities, including acid and alkaline phosphatase, β-glycerophosphatase, β-NAD diphosphatase, peroxidase (POD), and polyphenol oxidase (PPO), as well as total phenolic content (TPC) and ferric reducing antioxidant power (FRAP). Substantial changes in both enzymatic activity and antioxidant profiles were clearly detected across both cultivars after any cytokinin treatments in shoots. The highest acid phosphatase (183.0% ± 8.72) and β-glycerophosphatase (191.9% ± 11.95) activities, as well as the highest FRAP (122.9% ± 8.16), were observed in the TOP-treated HR plants. The highest alkaline phosphatase (144.3% ± 5.51) and polyphenol oxidase (204.3% ± 5.07) activities were measured in the BAR-treated HR plants. The highest β-NAD diphosphatase (148.5% ± 5.29) activity was determined in the TDZ-treated HR plants, while the highest peroxidase activity was measured in the control HR and McI plants. The highest TPC (119.7% ± 2.03) was determined in the BAR-treated McI plants. Distinct and notable variations were observed not only among the effects of individual cytokinins, but also between the two apple scions examined. In conclusion, TDZ exhibited consistent stress-like effects, such as the inhibition of shoot and root elongation, whereas TOP effectively maintained the overall physiological balance in the in vitro apple shoot cultures, keeping the photosynthetic pigment content similar to that of the control.