Foliar Application of Thidiazuron Alleviates Cd Toxicity by Modulating Stress Enzyme Activities and Stimulating Pigment Biosynthesis in Cajanus cajan (L.)

Plant growth regulators such as thidiazuron (TDZ) can enhance the detoxification capacity of plants against cadmium stress, which continues accumulating in soil and poses a significant threat to plant physio-biochemical processes. However, the potential of TDZ to mitigate Cd stress in plants remains poorly understood and documented. C. cajan is the most important grain legume crop in semi-arid and tropical regions, but less resistant to metal stress. This study investigates the potential of foliar-applied TDZ to mitigate the effect of Cd on mineral element accumulation, growth, and physio-biochemical traits of C. cajan. Seedlings of Cajanus cajan were exposed to three rates of Cd treatment (10, 20, and 50 mg Cd/kg soil) and two levels of thidiazuron (−TDZ and + TDZ). TDZ was supplied at a rate of 100 µmol/L per pot. Foliar application of 100 µmol/L of TDZ was introduced 3 weeks after transplanting and when the plant was appropriately established. Cadmium significantly decreases biomass, height, root nodulation, and photosynthetic pigments. Cadmium triggered PPO, CAT, SOD, and GPx activities in the leaves, but downregulates proline. Cd at 20 and 50 mg Cd/kg significantly increases root SOD and GPx activities. Foliar application TDZ improves the height, reduces ROS molecule (H₂O₂) accumulation in roots and leaves, and stimulates pigment biosynthesis. Foliar application TDZ showed dose- and organ-specific mitigation of Cd stress. TDZ upregulates the activity of CAT and PPO at higher Cd doses but suppressed SOD activities in the root and GPx in both the root and leaf. Additionally, TDZ improved root K, Mn, Fe, Cu, and Zn contents at all Cd doses but increased root Na content only at 20 and 50 mg/kg Cd. Overall, these findings offer new insight into the potential interplay of multiple mechanisms of thidiazuron to abate Cd stress at higher doses in C. cajan.