Enhancing Cd and Pb tolerance of Robinia pseudoacacia (black locust) by regulating antioxidant defense system, macroelement uptake and microstructure

Abstract Woody plants had received considerable attention for the phytoremediation of heavy metal-contaminated soil. This study aimed to investigate the changes in antioxidant enzyme activity, macroelement uptake, and microstructure of the woody plant Robinia pseudoacacia (black locust) for the phytoremediation of cadmium (Cd) and lead (Pb) co-contaminated soil based on dynamic sampling. The results showed that black locust demonstrated strong tolerance in Cd and Pb co-contaminated soil. After 30–120 d of cultivation, the activities of superoxide dismutase (SOD), peroxidase (POD), and the macroelement (K and Ca) content in plant leaves significantly declined in response to Cd and Pb. However, after 160 d of cultivation, the antioxidant enzyme activities, chlorophyll, sulfhydryl, and soluble protein contents, as well as Ca and Mg content in plant leaves were returned to normal levels under the 40 mg·kg−1 Cd and 1000 mg·kg−1 Pb contaminated soil (CdPb3). Meanwhile, K content in plant leaves under the CdPb3 treatment was significantly (P < 0.05) increased by 68.9% compared to the control. Cd and Pb was primarily accumulated in black locust roots. Scanning electron microscope (SEM) analysis indicated that the sieve tubes in the roots and stems of plant might block the transport of Cd and Pb. Transmission electron microscope (TEM) analysis indicated that the number and volume of osmiophilic particles in plant leaves were increased and the cell walls were thickened in response to Cd and Pb stress. Path analysis further indicated that the growth of plant was related to macroelements uptake and physiological change (photosynthesis, antioxidant enzyme activity, and chelation). Thus, black locust could effectively regulate the antioxidant defense system, macroelement absorption, and microstructure to enhance plant tolerance to Cd and Pb stress. Moreover, black locust could maintain the normal urease, acid phosphatase, and sucrase activities in the Cd and Pb co-contaminated soil. These findings suggested that black locust could be considered as a useful woody plant for the phytostabilization in Cd and Pb-contaminated soil.