Characteristics And Response Mechanism Of Uranium And Cadmium Enrichment In Plants

The safe and effective disposal of radioactive wastes and wastewater is necessary so as to isolate them from the biosphere effectively for the sustainable development of uranium mining and metallurgy.Uranium tailings in southern china pose a serious potential threat to the environment due to the well-developed water system and the migration of nuclides with surface(underground)water.In recent years,phytoremediation is economic and environmentally protective and thus has been widely concerned by researchers in the treatment of heavy metal pollution in soil and water.On the basis of sufficient investigation and related data analysis,the Typha angustifolia l.,Rohdea japonica,Acorus calamus L,which have strong growth and high biomass tolerance to uranium were selected.The effects of uranium(U)and compound stress of U and cadmium(Cd)on plant antioxidant system,accumulation and tolerance mechanism of plant on U,and regulation of plant absorption and accumulation of U by chelating citric acid(CA)and EDTA were studied by pot experiment.The physiological mechanism of plant response to uranium and cadmium stress and resistance to heavy metal toxicity were investigated.The main research results are as follows:(1)Bioaccumulation of uranium in stems,leaves and roots of Rohdea japonica,Typha angustifolia l.,Acorus calamus L increased with the increase of uranium stress concentration.When the concentration of U was 20 mg/L,the bioaccumulation of U reached the maximum,and the enrichment of U in roots were 887±3.03 mg/kg,1234±3.95mg/kg and 2802.6±3.92mg/kg respectively.The highest content of enriched U in stems and leaves of Rohdea japonica was 239.45±1.39mg/kg.As a result of the increase of U concentrations,the root bioaccumulation coefficient(BCF)and the translocation factors(TF)turned to decrease gradually,and,so does the ability for U-transfer from the roots to the above-ground parts of the plant.Compared with Typha angustifolia l.and Acorus calamus L,Rohdea japonica had the highest transfer coefficient and stronger ability to transfer uranium to stem and leaf when uranium concentration was higher than 1 mg/L.The absorption and accumulation characteristics of Rohdea japonica under Cd(?)and U(?)stress were studied through hydroponic experiments.The results indicated that under the stress of the same cadmium concentration,the enrichment coefficient of root was higher than that of stem and leaf,and the cadmium accumulation of root reached the maximum value of 482.75±2.86 mg/kg.Furthermore,as a result of exposure to 20 mg/L Cd(II)and 20 mg/L U(VI)of Rohdea japonica,the uranium accumulation in the roots tended to reach its highest limit of(898.48±2.53)mg/kg over the stems and shoots of(277.27±2.52)mg/kg of dry weight.Thus,it could be concluded that the Rohdea japonica enjoys great potentiality to tolerate and accumulate the U and Cd toxicity.Transmission electron microscopy revealed that the chloroplasts and mitochondria of Rohdea japonica cells were damaged under 40mg/L cadmium stress.Under the stress of lmg/L uranium,the presence of uranium element was detected in the leaves,and the damage to the leaves was not obvious.Transmission electron microscope results showed that the chloroplasts,nucleus and mitochondrial membrane system of the leaves of Rohdea japonica were damaged by radiation under the treatment of 10mg/L uranium concentration.X-ray energy spectrum analysis showed that uranium mainly distributed in the cell wall and vacuole.(2)The significance of the variations of antioxidant enzyme activity and antioxidant content,the physiological structure response of three plants under different uranium concentrations were explored.The study results show that,the contents of chlorophyll a,chlorophyll b,carotenoids,soluble protein content reached maximum at low uranium concentration(0.1mg/L)in Rohdea japonica and Typha angustifolia l,and the contents of chlorophyll a,chlorophyll b,carotenoids and soluble protein reached maximum with 5 mg/L of U stress in Acorus calamus L.The contents of malondialdehyde(MDA),reduced glutathione(GSH)and oxidized glutathione(GSSG)in roots and leaves of three species of plants increased with the increase of U concentration.The activities of peroxidase(POD)and glutathione reductase(GR)in Rohdea japonica,Typha angustifolia l.and Acorus calamus L showed an increasing-decreasing trend.The activities of superoxide dismutase(SOD)and catalase(CAT)in Typha angustifolia l.and Acorus calamus L showed an upward-downward trend,while the activities of SOD and CAT in Rohdea japonica did not change significantly under high U stress.The increase of uranium concentration promoted plant to increase the antioxidant enzyme activity and ascorbic acid-GSH(AsA-GSH)circulation to resist oxidative stress.Under the single cadmium stress(0?40mg/L),with the increase of cadmium concentration,the photosynthetic pigment content,soluble protein content,POD activity and GR activity of Rohdea japonica increased first and then decreased;the MDA content,SOD activity and CAT activity of Rohdea japonica increased slowly.The physiological responses and tolerance characteristics of Rohdea japonica under Cd(?)and U(?)stress were studied by hydroponic experiment.The result shows that:with the increase of the U-concentrations at the same amount of Cd(II)content,the malondialdehyde content would like to reveal a remarkably increasing trend in the shoots and roots,while the cell inclusion parameters of the antioxidant enzymes POD and GR,the photosynthetic pigments content and the soluble protein content,showed a first ascending and then descending trend in Rohdea japonica.In addition,the SOD and the CAT content increased slowly in the shoots and the roots.Under the stress of uranium(20mg/L)and cadmium(40mg/L),the leaves of Rohdea japonica slightly lose green,which shows that the antioxidant system and osmotic regulation of Rohdea japonica can reduce the damage of high concentration of uranium and cadmium to plant cells.(3)The experiment results of chelating agent(CA)and EDTA(0?20mmol/L)for the remediation of uranium-containing wastewater by Rohdea japonica indicate that:activity of Rohdea japonica photosynthetic pigments,soluble protein content,POD,CAT,SOD and GR contents rose firstly then dropped with CA(EDTA)concentration increased,MDA content was increasing.Both concentration of 5 mmol/L citric acid and EDTA promoted the growth of Rohdea japonica,while 20 mmol/L citric acid and EDTA inhibited the growth of Rohdea japonica.The effect of chelating agent(0?20mmol/L)on the enrichment of uranium in Rohdea japonica had shown that:With the increase of concentration of CA and EDTA,U enrichments in Rohdea japonica first increased and then descended,meanwhile,the root bioconcentration factor decreased gradually and the translocation factor gradually decreased.Uranium enrichments in Rohdea japonica were tends to reveal a remarkably increasing trend,but all presented that accumulation in roots were high than ground parts.The results show when the concentration of citric acid and EDTA was 5 mmol/L,the concentration of uranium in stems,leaves and roots reached the peak value.CA was better than EDTA in enrichment of uranium.The simulation results by Visual MINTEQ software showed that when citric acid was added at pH=6.5,the main form of uranium in solution is UO2-Citrate-(over 99%).When EDTA was added at pH=6.5,the uranium in solution mainly exists in the form of(UO2)2OHEDTA-,and is greatly affected by the concentration of EDTA.These two forms of uranium are usually difficult to fix in the roots of Rohdea japonica,but the transfer of uranium from the roots to the stems and leaves is more efficient.