Abstract
The phytoremediation of metals, with an increasing development recently, is a cost-effective green technology based on the use of metal-accumulating plants to remove toxic metals. Many aquatic plants are reported to bioconcentrate heavy metals in natural water areas as well as upon exposure to wastewater. A further understanding of metal uptake and entry by aquatic plants can enhance the performance of metal removal from polluted water. Coontail (Ceratophyllum demersum L.) is a submerged, floating rootless macrophyte, which can accumulate heavy metals. In this study, entry of copper ions into coontail plants was investigated using chlorophyll fluorometer and ICP-MS. From the chlorophyll fluorescence and ROS detection experiments, it was found that leaf is the major site attacked by copper ions. Besides, chlorophyll fluorescence image indicated that the fluorescence signal decayed starting from the apical to the basal part of leaves. The result of ICP-MS analysis confirmed that the highest copper accumulation was in the leaf, especially the tip part, while the copper content in stem remained unchanged after Cu2+ treatment. We also found that both ATPase inhibitor and protein synthesis inhibitor could alleviate the inhibition of photosynthesis and decrease copper uptake. Moreover, microscopic observation revealed that excess copper could loose binding between cell walls at a zone closed to the base of leaves, causing defoliation. We thus suggest that copper ions might be taken up by binding to the ATPase located in the apical part of leaf. However, there seems to be a barrier at the leaf base that prevents translocation of copper ions to stem. The impediment could decrease the interaction between cell walls, followed by leaf detachment. It may represent a protective response of the plants to prevent further uptake of copper ions.