Study On The Formation Of Iron Plaque On The Root Surfaces Of Wetland Plants From Poyang Lake And Its Translocation Mechanism On Lead

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 iron to ferric iron and the precipitation of iron oxides or hydroxides on the root surface. This material is termed “iron plaque”. Iron plaque is a mixture of crystalline and amorphous iron oxide or hydroxide, and is mainly composed of ferric hydroxides, goethite and lepidocrocite. Given the high adsorption capacity of functional groups on iron oxides or hydroxides, iron plaque can influence the chemical behavior and bioavailability of heavy metals and nutrient elements in the rhizosphere. Iron plaque is the portal of the pollutants into the plant body, and it plays an important role in pollutants reducing. In order to explore the effect of iron plaque on the roots of Lake Wetland plants on heavy metals uptake, Poyang Lake wetland was taken as research object. Solution culture experiment and pot experiment were conducted to investigate the information of iron plaque on the root surfaces of wetland plants from Poyang Lake, and the effects of iron plaque on translocation of Pb in soil- Carex cinerascens Kukenth. System, using Scanning Electron Microscopy(SEM) and Energy Dispersive X-Ray Spectrometry(EDX).(1) The amount of iron plaque on the root surfaces of different wetland plants from Poyang Lake showed significant difference. It was the order: Grass family > Cyperaceae > Polygonaceae > Composite family. Even belong to the same family of the wetland plants, the amount of iron plaque also existed difference, Carex cinerascens Kukenth. and Heleocharis belonged to Cyperaceae and had the same habitats, the amount of iron plaque on the root surfaces of Carex cinerascens Kukenth. was 6 times than that of Heleocharis. Habitats and season affected the amount of iron plaque on the root surfaces of wetland plants, the amount of iron plaque on the root surfaces of wetland plants in Nanjishan wetland which has high Fe concentration than that of other. The amount of iron plaque on the root surfaces of wetland plants collected on November was higher than that on April.(2) The formation of iron plaque was controlled by Fe2+ concentration in environment and the oxidizing capability of the root system. The effects of Fe2+ concentration, plant species, culture times, pH, variation of water level, root exudates on iron plaque were investigated by hydroponics condition and soil-sand pot experiment. Results showed that the amount of iron plaque increased as the Fe2+ concentration and culture times increased. Root exudates concentration was negatively correlated with the content of iron plaque. The amount of iron plaque first increased and then decreased with the increase of water level, the amount of iron plaque reached max when water level was 2-3cm and pH=5.5~6.(3) The amount of iron plaque on the root surfaces of Carex cinerascens Kukenth. increased and then decreased with increasing culture time. The generation rate of iron plaque was the largest in the first week, and treaded balance in the third week. But in the fourth week, the generation rate of iron plaque was negative, and the amount of iron plaque began to decline. The root activity of Carex cinerascens Kukenth. increased at first and then decreased with increasing culture time, the low concentration iron was contributed to promote the root activity. Fe0/Fed of iron plaque was 0.0667 to 0.2265, amorphous iron accounted for 6% to 18% of the total iron oxide, and most of iron oxide was crystalline. Fe0/Fed of iron plaque increased at first and then decreased with the extension of culture time. At the beginning of fifth week, the iron plaque began aging.(4) The physico-chemical properties of iron plaque were determined using Scanning Electron Microscopy(SEM) and Energy Dispersive X-Ray Spectrometry(EDX). The analysis of SEM-EDX showed that iron plaque was present as an amorphous coating which followed the contours of the cells. The coating was not uniform and plaque deposits were absent from large areas of the root surface. The plaque at 0 mg Fe·L-1 treatment appeared loose, and the plaque at 100 mg Fe·L-1 treatment was dense. The plaque formed on the root surfaces of Carex cinerascens Kukenth. had a very specific chemical signature and was composed of C, O, K, Si, Al, Fe, together with lesser amounts of Ca and Cl. The weight percentage of Fe was 40.35% in plaqued root and 4.92% in unplaqued root. The weight percentage of Fe was 18.78% in plaqued root without Fe addition, and the weight percentage of Fe was 38.29% in plaqued root at 100 mg Fe·L-1 treatment. This showed that the amount of iron plaque increased with Fe additions. The analysis of XRD showed that iron plaque was α-Fe2O3.(5) A hydroponic experiment was conducted to investigate whether iron plaque could affect the uptake and translocation of lead by two dominant wetland plants, Carex cinerascens Kukenth. and Phalaris arundinacea Linn., growing in Poyang Lake, Jiangxi Province, China. After iron plaque on plant roots was induced by growing plants in solution containing 0, 30, 60, 100, 150, or 210 mg·L-1 Fe2+ for 7 days, plants were transferred to nutrient solution containing 10 mg·L-1 Pb2+ for 5 days. The amount of iron plaque formed on the root surfaces of Carex cinerascens Kukenth. was slightly higher than that of Phalaris arundinacea Linn. A significant positive correlation was observed between the amount of iron plaque and Fe2+ concentration in the solution for both the species. The Pb concentrations and percentage distribution in different matrices were in the order iron plaque > root > shoot. We observed that 50 – 60% of the total Pb accumulated in iron plaque and nearly 90% of the total Pb accumulated in iron plaque and roots. Iron plaque on the root surface increases sequestration of Pb in the root and inhibits the translocation Pb from root to shoot under hydroponic conditions to some extent.(6) Carex cinerascens Kukenth. from Poyang Lake wetland was taken as a case study. A pot experiment was conducted to investigate the effect of iron plaque on Pb uptake by and translocation in Carex cinerascens Kukenth. grown under open-air conditions. The Fe concentrations increased significantly with increasing Fe supply regardless of Pb additions. The most beneficial condition for the formation of iron plaque was 500 mg·kg-1 of Fe addition, which was under the condition that the concentration of Pb was from 0 mg·kg-1 to 100 mg·kg-1. The presence of iron plaque on the root surfaces of Carex cinerascens Kukenth. increased the concentrations of Pb adsorbed by iron plaque. Compared with 0 mg·kg-1 Fe treatment, the percentage of Pb in roots increased by 14.52% at 500 mg·kg-1 Fe treatment, and the distribution coefficient(DC) of Pb in Carex cinerascens Kukenth. was significantly decreased from 0.39 to 0.13. It was found that translocation factor(TF) root increased with Fe additions, but translocation factor(TF) shoot decreased with Fe additions. Our results suggested that iron plaque could promote the translocation of Pb from soil to roots to some extent, it not only played a role to reduce heavy metals pollution of Poyang Lake wetland, and prevented migratory birds from indirectly adsorbing heavy metals by the way of "soil- aquatic organism-migratory birds" to a certain extent as well.