The Tolerance, Absorption, and Transport Characteristics of Macleaya cordata in Relation to Lead, Zinc, Cadmium, and Copper under Hydroponic Conditions
Heavy metal pollution has potential hazards to plant, animal, and human health, and phytoremediation is recognized as a safe and efficient technique for the revegetation of heavy-metal-polluted soil. Macleaya cordata was found in heavily tailing areas with fast growth rates, large biomass, and huge taproots. In our study, the seedlings of M. cordata were exposed to cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) in a Hoagland solution. After 20 days, the tolerance index as well as the content and distribution of Cd, Pb, Cu, and Zn in roots, stems, and leaves were determined. The results showed M. cordata had higher tolerance to Pb and Zn than to Cd and Cu under hydroponic culture conditions. Pb and Cu mainly accumulated in the roots, and the translocation efficiency to the shoots was very low, while about three-quarters of Zn concentrations in the plants were accumulated in the shoots; even the Cd content per shoot of M. cordata exceeded some Cd hyperaccumulators. In the present study, the metal ions in the roots or leaves of M. cordata were firstly determined in situ using dithizone staining, and the degree of root-tip staining was consistent with the amountof the total metal content in the roots. The addition of Zn or Cu in the Pb treatment solution increased the Pb content in the stems and leaves of M. cordata, while the addition of Zn or Cu in the Cd treatment solution had the opposite effect. Pb or Cd in the compound treatment decreased the Zn content in all parts of M. cordata. Our results suggest that Pb can be transported above ground via some special pathways in M. cordata. The different absorption and transport mechanisms of M. cordata in relation to Cd, Zn, Cu, and Pb can be important for the plant to be applied for the remediation of compound-polluted soil or water.