Iron and Mn plaques on the surface of roots of wetland plants
湿地植物根表的铁锰氧化物膜

Ferrous and other reducing forms of metals or metalloids become phytotoxic due to their increasing solubility in waterlogged soils. The roots of many wetland plant species are able to release oxygen and / or oxidants into the rhizosphere, resulting in the oxidation of ferrous to ferric iron and the precipitation of iron oxide or hydroxides on the root surface in the form of a plaque. The nature of plaque formation could reflect the biochemical conditions in the rhizosphere, which may be a function of wetland plant species and / or the properties of soil and solution chemistry. Iron plaque is commonly formed on the surfaces of roots of wetland plants, including Oryza sativa, Typha latifolia and Phragmites communis. It may be amorphous or crystalline, in the forms of ferric hydroxides, goethite and lepidocrocite. Since iron (hydro) oxides possess some functional groups that may react with some cations and anions in soils, they have high adsorption capacity to some elements. Therefore, Fe plaque deposited on the root surface of wetland plants also provides a reactive substrate for metal (loid) sequestration-Mn, Zn, Pb, Cu, P and As. Acting as an entrance for nutrients and pollutants into wetland plants, iron plaque may be a barrier to take up heavy metals or a reservior to enhance uptake of nutrient elements depending on the amounts of plaque formed on the root surface. However, the mechanisms and degree of iron plaque influencing on metal (loid) uptake by plants are not. yet to be resolved. The role of iron plaque on the surface of roots is important for us to develop a practical approach to reducing heavy metal uptake or regulating the nutrition of wetland plants It is therefore necessary to investigate the ecological functions and uptake mechanism of iron plaque. This paper provides a conclusive review on the following subjects: (1) Iron and Mn plaques were formed on the surface of wetland plants roots. It is necessary for aquatic plants to form plaque that there are enough Fe and Mn amounts in the substrates and an oxidizing situation in the environment; (2)The composition and properties of plaques; (3)The accumulation of heavy metal on the iron and Mn plaques; (4)The effect of plaques on uptake of nutrients by wetland plants; (5)The effect of iron and Mn plaques on uptake of pollutants (focus on heavy metal) by wetland plants; (6) It is better to understand the mechanisms of metal-(loid) sequestration in the plaques to use X-ray absorption spectroscopy and X-ray fluorescence microtomography. Moreover, The analytical methods to measure amounts of Fe and Mn plaques should be improved.