Root porosity, radial oxygen loss and iron plaque on roots of wetland plants in relation to zinc tolerance and accumulation

Background and aims Wetland plants have been widely used in constructed wetlands for the clean-up of metalcontaminated waters. This study investigated the relationship between rate of radial oxygen loss (ROL), root porosity, Zn uptake and tolerance, Fe plaque formation in wetland plants. Methods A hydroponic experiment and a pot trial with Zn-contaminated soil were conducted to apply different Zn level treatments to various emergent wetland plants. Results Significant differences were found between plants in their root porosities, rates of ROL, Zn uptake and Zn tolerance indices in the hydroponic experiment, and concentrations of Fe and Mn on roots and in the rhizosphere in the pot trial. There were significant positive correlations between root porosities, ROL rates, Zn tolerance, Zn, Fe and Mn concentrations on roots and in the rhizosphere. Wetland plants with higher root porosities and ROL tended to have more Fe plaque, higher Zn concentrations on roots and in their rhizospheres, and were more tolerant of Zn toxicity. Conclusions Our results suggest that ROL and root porosity play very important roles in Fe plaque formation, Zn uptake and tolerance, and are useful criteria for selecting wetland plants for the phytoremediation of Zn-contaminated waters and soils/sediments.