| With the development of industrial production and urban modernization,the problem of heavy metal pollution in farmland soils is becoming more and more serious in China,which has a great risk to the safe production of crops and residents’ health in our country.According to the survey of the Ministry of Environmental Protection and the Ministry of Land and Resources,major heavy metal pollutants in farmland soil are cadmium,nickel,copper,arsenic,mercury and lead.Heavy metal pollution prevention and control have became the difficult and hot topic of international research owing to the characteristics of accumulation,concealment,long-term,huge dangers and irreversibility of heavy metal pollution.As a new green biotechnology,phytoremediation has many advantages of low cost,high benefit and environmental friendliness,and has wide application prospect in polluted environment ecological restoration.Sedum alfredii Hance is a kind of good potential material for the restoration of heavy metal contaminated plants which has large biomass and grows quickly,and has been gradually applied to the practice work of phytoremediation of heavy metal contaminated soil in China.Previous studies have shown that Sedum alfredii Hance can’t only accumulate heavy metal zinc and cadmium,but also has a strong tolerance and accumulation capacity to lead and copper.However,it has been still unknown if Sedum alfredii Hance is able to tolerate and accumulate high amounts of nickel so far.It is helpful to figure out the characteristics of the tolerance and accumulation of nickel in Sedum alfredii Hance,a kind of Zn/Cd hyperaccumulator,contributing to practical application of heavy metal compound contaminated soil containing cadmium and nickel.Based on this study,Zn/Cd hyperaccumulating ecotype Sedum alfredii Hance and non-hyperaccumulating ecotype were used as materials.It was studied by means of solution culture,microscopic fluorescence imaging,Transmission Electron Microscope,synchrotron radiation micro-X-ray fluorescence(μ-XRF),ICP-MS and so on,studying the mechanisms of tolerance and accumulation of nickel in Zn/Cd hyperaccumulating ecotype Sedum alfredii Hance systematically.The main results include two aspects:(1)The tolerance of roots to high nickel stress in Zn/Cd hyperaccumulating ecotype Sedum alfredii Hance:The experiment was carried out by microscopic fluorescence imaging,transmission electron microscopy,μ-XRF,ICP-MS and so on in hydroponics,analyzing the tolerance differences in nickel stress between the hyperaccumulating ecotype(HE)and non-hyperaccumulating ecotype(NHE)Sedum alfredii Hance.First,it was showed a certain degree of loss of plasma membrane integrity,lipid peroxidation damage and excessive accumulation of reactive oxygen species(hydrogen peroxide and superoxide anion)by measuring the difference of loss of plasma membrane integrity,lipid peroxidation,reactive oxygen species(ROS),cell ultrastructure,and the effects of different concentrations(0-200 μM)and different time(0-72 h)under the nickel stress of two ecotypes.However,the degrees of loss of plasma membrane integrity,lipid peroxidation and the accumulation of reactive oxygen free radicals in the hyperaccumulating ecotype of Sedum alfredii Hance were significantly lower than those of non-hyper-accumulating ecotypes in root tips after treated with high nickel concentration(100 μM)for 72h.The results of ultrastructural analysis of root tip cells confirmed that the ultrastructure of root tip cells was significantly destroyed after 72 h of high concentration of nickel in the non-hyperaccumulating ecotype(NHE),but there is no significant difference in the hyperaccumulating ecotype(HE).It can be seen that the root system of the hyperaccumulating ecotype has stronger tolerance to high nickel stress.The accumulation of nickel in the roots of the hyperaccumulating ecotype was significantly lower than that in the non-hyperaccumulating ecotype,which is analyzed by ICP-MS under the same nickel concentration and time.It is presumed that the absorption rate of nickel is lower or the loading capacity of nickel is stronger than that of non-hyperaccumulating ecotype.The results confirmed that the loading rate of root xylem was higher than that of non-hyperaccumulating ecotype,especially under high nickel stress,so as to promote the transfer of nickel to the shoots rapidly,reducing the amount of root nickel accumulation by analyzing sap of xylem.In addition,it was found that the distributions of nickel and superoxide anion in the two kinds of ecotypes’ rootswere significantly different by synchrotron radiation μ-XRF and fluorescence microscopic analysis.After the non-hyperaccumulating ecotype was treated with 100μM Ni for 24 h,Nickel and superoxide anions accumulated in the vascular cylinder,but this phenomenon was not found in the hyperaccumulating ecotype Sedum alfredii Hance,which further confirms that Ni passes through efficient xylem loading system to the ground rapidly in the hyperaccumulating ecotype Sedum alfredii Hance,reducing the oxidative stress of excessive accumulation of nickel in the root.In summary,Zn/Cd hyperaccumulating ecotypes of Sedum alfredii Hance were more tolerant to high-nickel stress compared with the non-hyperaccumulating ecotype,mainly due to the fact that the xylem of roots had a more efficient loading system for nickel uptake and accumulation,promoting nickel transportation to the ground to reduce the excessive accumulation of nickel in the root cells caused by oxidative stress,so as to maintain the normal growth of roots and physiological functions.(2)The accumulation capacity and localization characteristics of nickel in the Zn/Cd hyperaccumulating ecotypes of Sedum alfredii Hance.Two kinds of ecotypes were used to analyze the effects of different nickel concentration(0-200 μM)on growth status,nickel contents and distribution characteristics in hydroponics.The results showed that there were different levels of toxic symptoms after long periods of treatment,but the non-hyperaccumulating ecotype was more serious,and the biomass decreased sharply after 28 days of Ni≥25μM treatment,while Zn/Cd hyperaccumulating ecotypes of Sedum alfredii Hance at Ni ≤50μM concentration were normally grow,which confirmed the latter has a stronger tolerance to nickel stress.Furthermore,the results showed that there was no significant difference in the total absorption of nickel between the two ecotypes,but the transport rate of nickel to the shoot was higher than that of the non-hyperaccumulating ecotype in the case of high nickel stress.The content of nickel in the shoots was significantly higher than that in the latter.The content of nickel in the stem was 753±44 mg kg-1,but that in the root was relatively low.However,No matter what kind of ecotype Sedum alfredii Hance,the content of nickel in the roots was highest,the second in stems and the lowest in leaves.By means of synchrotron radiation μ-XRF analysis,it was found that the nickel in the stems and leaves of the two ecotypes was mainly distributed in the vascular tissue and the second in epidermis.After 28 days of treatment,with the increase of the concentration of nickel,the content of nickel in the vascular bundles of the hyperaccumulating ecotypes of Sedum alfredii Hance increased rapidly and largely,but the content of nickel in the other parts of the stem increased little.A similar phenomenon occurred,although the nickel increase in the vascular bundle is relatively low.Leaves of two ecotype of Sedum alfredii Hance were only able to detect nickel signals in the veins and epidermis under high nickel treatment.Although the results above confirmed that the Zn/Cd hyperaccumulating ecotypes of Sedum alfredii Hance have strong tolerance to nickel stress,it also shows that the accumulation of nickel in the shoots is not very prominent,mainly owing to that although the ability of hyperaccumulating ecotypes of Sedum alfredii Hance to transport Ni from the roots to the shoots is significantly higher than that of non-hyperaccumulating ecotypes,thus reducing the accumulation of root nickel to resist the toxic effects of long-term nickel stress,but most of nickel is compartmentalized in xylem tissue,then cannot be transferred to other cells of stems and leaves,which largely limits the accumulation ability of nickel in shoots. |
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