Formation of iron plaque on roots of Iris pseudacorus and its consequence for cadmium immobilization is impacted by zinc concentration

The impact of iron plaque (IP) on bioavailability of heavy metals to plants has been well documented, but the role of zinc (Zn) in modulating the associated processes remains elusive. We took Iris pseudacorus used in wetland for remediating Cd-contaminated water as an example and systematically studied the combined influence of Cd and Zn concentration on formation of IP and its consequence for immobilization and plant uptake of Cd. The experiment was conducted in hydroponic culture and in each treatment, we measured the physiological traits, activity of antioxidant enzymes (SOD, POD, CAT), mass of the IP, as well as the Cd content in both plant tissues and IP. The results showed that increasing Cd concentration resulted in a steady reduction in IP while the impact of zinc on IP was complicated and appeared to be coupled with Cd. When the Cd concentration was low (0.5 mg L-1 measured as CdCl2 2·5H2O) increasing Zn concentration reduced IP, while when the Cd concentration was increased to 5 mg L-1 increasing zinc concentration led to an increase in IP mass first followed by a decline after Zn concentration exceeded 100 mg L-1 (measured as ZnSO4·7H2O). The change in IP as affected by Zn had a strong consequence for immobilization and plant uptake of Cd. When Cd concentration was low, the IP was comparatively abundant and hence adsorbed most Cd. In contrast, when Cd concentration was high, the IP reduced and the amount of Cd taken up by plant roots and translocated to shoots and leaves increased. Both Cd immobilization and its plant uptake were modulated by Zn concentration. At low Cd concentration the combined Cd immobilized and taken up by plant peaked when the Zn concentration was 50 mg L-1, while at high Cd concentration the combined Cd reached maxima when theZn concentration was 100 mg L-1. The activity of the antioxidant enzymes changed significantly with Zn rather than with Cd. Regardless of Cd concentration, the activity of all three antioxidant enzymes increased first with zinc concentration before declining when the Zn concentration exceeded approximately 100 mg L-1 in all treatments, comparable with the change in immobilization and plant uptake of Cd as the Zn concentration increased. SEM analysis did prove the formation and variation of IP on the root surface of Iris pseudacorus in different treatments. We also found that the plant developed a survival strategy by scarifying its leaves with high Cd content. The results presented in this paper has wide implications as it revealed that care needs to be taken in applying Zn to enhance Cd immobilization and its plant uptake as exceeding the optimal application rate might reduce remediating efficiency rather than increase it.