Mitigating copper toxicity in Carthamus tinctorius L. through 6-benzylaminopurine during seed germination and vegetative growth

Cytokinins are essential regulators of plant growth and development and are actively involved in plant responses to both biotic and abiotic stresses. In the present study, the role of 6-Benzylaminopurine (BAP) in modulating growth performance and copper (Cu) tolerance in Carthamus tinctorius L. was examined. Treatments consisted of four CuSO4 levels and three BAP levels in a factorial design. High Cu levels significantly reduced seed germination parameters, growth characteristics, photosynthetic pigments, root and shoot protein content, and peroxidase (POX) activity, while increasing soluble carbohydrates, polyphenol oxidase (PPO) activity, Cu accumulation, total phenolic content, anthocyanin levels, and the relative expression of SOD and PAL genes. BAP application markedly improved seed germination and growth under Cu stress, likely by modulating biochemical, physiological, and molecular mechanisms. Treatment with 350 µM BAP under severe Cu toxicity (0.016 g kg− 1) enhanced Chl a (78.30%), Chl b (160%), total chlorophyll (166%), carotenoids (100%), soluble carbohydrates (116%), root protein (106%), shoot protein (59.23%), root POX (112%), shoot POX (229%), root PPO (97%), shoot PPO (65.38%), and the relative expression of PAL (43.05%) and SOD (5.51%). Conversely, BAP reduced total phenolic content (41.50%) and anthocyanin content (31.42%) in the shoots. Overall, the results demonstrate that BAP effectively mitigates Cu-induced toxicity in C. tinctorius by enhancing antioxidant activity and regulating defense-related gene expression, thereby improving plant tolerance to heavy metal stress.