Progress in research of iron plaque on root surface of wetland plants

Field: Environmental Science (Physical Sciences)

Waterlogged soils are frequently anaerobic,with low partial pressure of oxygen. To acclimate to an anaerobic condition when submerged in water,wetland plants must allow aerenchyma to transfer oxygen from aerial parts to the rhizosphere,which is termed radial oxygen loss. Radial oxygen loss also causes the oxidation of ferrous to ferric and the precipitation of oxides or hydroxides on the root surface. This material is termed iron Iron plaque is a mixture of crystalline and amorphous oxide or hydroxide,and is mainly composed of ferric hydroxides,goethite and lepidocrocite. Given the high adsorption capacity of functional groups on oxides or hydroxides,iron plaque can influence the chemical behavior and bioavailability of heavy metals and nutrient elements in the rhizosphere. In recent years a number of studies have been conducted on the formation of plaque and uptake of heavy metals and nutrient elements in wetland plants. In this paper,progress in research on plaque was analyzed and summarized from four aspects,i.e.,the formation of plaque and factors influencing its development,the nutritional effect of plaque on plants,the ecological and environmental effects of plaque on heavy metals,and techniques for characterization of plaque. The formation of plaque is controlled by a number of biotic and abiotic factors,such as Fe2+concentration,the oxidizing capability of the root system,and plant species. Iron plaque is an enriched reservoir of plant nutrient elements,but some results indicate that this function is related to the amount of plaque. Many studies have been conducted on the relationship between plaque formation and uptake of heavy metals in wetland plants,but the results reported among the studies are inconsistent. Some studies have indicated that the formation of plaque acts as a physicalbarrierto uptakeof heavy metals through immobilization and co- precipitation,whereas other studies have reported no difference in the presence or absence of plaque. Furthermore,iron plaque may promote the uptake of heavy metals. In addition,some results indicate that the inhibitory and promotive actions of plaque on uptake of heavy metals were related to the amount of plaque on the root surface. Thus our understanding of the control of plaque formation and the mechanism of action of plaque on heavy metals uptake remains incomplete at present. The following suggestions for future research are put forward:( 1) expand the fields of research,such as lake wetlands;( 2) consider the influence of environmental factors on dynamic changes in processes involving plaque;( 3) pay close attention to the effect of plaque on the physiology and morphology of plants; and( 4) use advanced characterization techniques to analyze the mechanism of action of plaque on heavy metals. For example,ion selective electrodes could be used to measure root oxygen release,and gas chromatography-mass spectrometry used for characterization and quantification of root exudates. In situ techniques should be employed to elucidate the mechanism of action of plaque on heavy metals. In particular,X-ray absorption fine structure technology has high potential for clarification of the molecular mechanism of action of plaque.