| Soil is an important carrier for crops,rice is one of the main crops for human survival.However,with the development of economy,more and more paddy fields are polluted by heavy metals,Cd is one of the main heavy metals.Iron and manganese plaque is a layer of iron and manganese oxide attached to the root surface of wetland plants.Due to the loose structure and abundant functional groups on the surface,it have strong ability of chelating heavy metals and storing nutrients.Iron and manganese oxidizing bacteria is a kind of bacteria that can oxidize Fe(II),Mn(II),or aggregate their oxides.The strain Providencia SP.LLDRA6 involved in this study has been proved to have high manganese oxidation capacity through a large number of previous studies.In this study,we investigated the iron oxidation capacity of Providencia sp.LLDRA6,and discussed its ability mediating iron and manganese plaque formation and its blocking effect on Cd transport in rice roots.Firstly,the growth characteristics of bacteria Providencia sp.LLDRA6 on Fe(II)were analyzed,and the iron oxidation ability of the strain LLDRA6 was verified by o-phenoline spectrophotometry and ferrous sulfide gradient tube method.The results showed that the optimum Fe(II)concentration of LLDRA6 was 5 m M,the optimum growth temperature was 25℃,and the optimum growth p H was 8.The strain LLDRA6 cultured by FSD gradient tube method formed a layer of red-brown iron oxide at a distance of 2-3 cm from the bottom of the tube.The results of o-phenanthroline spectrophotometry showed that the oxidation rate of Fe(II)in LLDRA6 treatment group was higher than that in the control group,which confirmed that LLDRA6 equiped with Fe(II)oxidation capacity.Secondly,the effect of LLDRA6 on the formation of iron and manganese plaque in roots was preliminarily investigated through hydroponic experiments of two rice varieties,Huazhan and TP309.It was found that the root surface of rice treated with LLDRA6,Fe(II)and Mn(II)showed a clear rust red color compared with other treatment groups.The DCB-Fe content in rice roots treated with LLDRA6 and Fe(II)was higher than that in other treatment groups.In TP309,the content of DCB-Mn in rice root treated by LLDRA6 and Mn(II)was higher than that in other treatment groups.LLDRA6 and Fe(II)treatment significantly reduced Cd content in overground parts of rice in both cultivars compared with CK control group.Therefore,it can be preliminarily concluded that LLDRA6 has the ability to promote the formation of iron and manganese plaque,and can block the absorption and transport of Cd in rice.Finally,the results of hydroponic experiment were further verified by pot experiment and field experiment of rice Huazhan and TP309.The root surface of potted Huachan rice was characterized by SEM-EDS,results showed that a large amount of massive precipitation was formed on the root surface of the rice treated with LLDRA6 and Fe(II),and the content of Fe and O was high in the deposition area,which could be identified as iron oxide plaque.In pot experiment,the Cd content in grains of the two rice varieties in each treatment group was lower than that in the controul group,and the Cd content in grains of LLDRA6 and Fe(II)treatment group of Huazhan variety was the lowest.The Cd content in grains treated with LLDRA6 and Fe(II)was the lowest among all the treatments of the two rice varieties in the field experiment,reaching the requirement of less than 0.2 mg/kg of Cd residue in grains stipulated by national standard GB2762-2012.These results indicated that the iron and manganese plaque formed by LLDRA6 strain on the surface of rice root had a significant barrier effect on Cd uptake by rice. |
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