Exogenous Application of Cytokinins Confers Copper Stress Tolerance in Ricinus communis L. Seedlings

As an essential element, copper (Cu) is involved in various metabolic processes in plants. However, this metal becomes a potential stress factor when the concentration reaches the threshold level. A comparative analysis was conducted to investigate the potential role of two cytokinins [kinetin (KIN) and 6-benzylaminopurine (BAP)] in Cu stress alleviation in Ricinus communis seedlings by assessing the metal bioaccumulation, antioxidation mechanisms, anatomical changes as well as analysis of various essential elements and bioactive compounds in the cotyledonary leaves. Application of KIN and BAP regulated the Cu toxicity in castor seedlings via modulation of metal uptake and antioxidation mechanisms. The increase in antioxidant enzyme activities during Cu stress was further increased upon exposure to cytokinin treatments, and the enhancement reached up to 25 folds for CAT, 16 folds for POD, and 8 folds for SOD over the control, which helps the plant to alleviate the toxic effects of Cu stress. The FTIR analysis of the cotyledonary leaves revealed that the functional groups associated with cell wall materials contributed to the Cu sequestration in the cell wall, supported by the xylem wall thickening as observed in the SEM analysis. The GCMS analysis revealed that most of the secondary metabolites identified were phenolic and flavonoid compounds with antioxidant properties, which help the castor seedlings withstand the Cu stress, and the presence of these compounds was more prominent in cytokinin-treated seedlings. Findings revealed that BAP was more effective than KIN to withstand the Cu stress effects in castor seedlings.