Modulation of Plant-Derived Bioactive Phenolic Compounds by Cytokinins in Hypericum amblysepalum Shoot Cultures

Cytokinins are key plant growth regulators that play an important role not only in morphogenesis but also in the regulation of plant-derived bioactive compound production in plant tissue culture systems. In this study, the effects of two cytokinins, 6-benzylaminopurine (BAP) and zeatin (ZEA), applied at four concentrations (0.1, 0.25, 0.5, and 1.0 mg L−1), were compared in terms of biomass production and phenolic compound accumulation in shoot cultures of Hypericum amblysepalum Hochst. Both cytokinins significantly enhanced plant growth compared to the control, with the highest dry weight (26.5 ± 8.6 mg DW) and shoot number (11.2 ± 4.4 shoots per explant) recorded in cultures supplemented with 0.1 mg L−1 BAP. In contrast, ZEA was more effective in stimulating the accumulation of secondary metabolites. LC–MS/MS analysis revealed that 0.1 mg L−1 ZEA markedly increased the accumulation of major plant-derived bioactive phenolic compounds, including hypericin, pseudohypericin, chlorogenic acid, rutin, luteolin, hesperidin, luteolin-7-glucoside, and hyperoside, compared to the control. Consistent with these findings, total phenolic content (TPC) and total flavonoid content (TFC) were significantly higher in ZEA-treated cultures, which also exhibited stronger DPPH radical scavenging activity, indicating enhanced antioxidant potential. Overall, these results demonstrate that BAP is more suitable for biomass enhancement, whereas ZEA is more effective in improving the production of bioactive phenolic compounds and antioxidant capacity in H. amblysepalum shoot cultures, highlighting their potential as a sustainable source of valuable plant-derived bioactive metabolites for pharmaceutical applications.