Study On The Effect Mechanism Of Exogenous Addition Of Sulfur And Selenium On Arsenic Accumulation And Toxicity In Wheat

Arsenic（As）is a heavy metal that is highly toxic to living organisms and can cause serious health risks.Crops are a major source of As contamination in human food,and limiting As content in edible parts of crops is an effective strategy to reduce the hazards of As in food chain.Sulfur（S）is an essential element for plant growth and development,and plays a vital role in reducing the toxicity of heavy metals.Selenium（Se）can also alleviate heavy metal stress in plants to a certain extent.However,the effects and mechanisms of S and Se on the uptake and growth of As of wheat were not clear,so it is of great theoretical and practical significance to study the mechanism of As tolerance in wheat under the action of exogenous S and Se.In this experiment,As-stressed wheat was used as the experimental object.By simulating the arsenic-polluted environment,four concentrations of exogenous S,including 0 mg/kg（S0）,100 mg/kg（S1）,150 mg/kg（S2）and 200 mg/kg（S3）,were applied to the arsenic-contaminated soil in the form of sodium sulfate（Na₂SO₄）,and four concentrations of exogenous Se,including 0 mg/kg（Se0）,1 mg/kg（Se1）,2mg/kg（Se2）and 2.5 mg/kg（Se3）in the form of sodium selenite（Na₂Se O₃）,were applied to the arsenic-contaminated soil,and the interaction between each level S and Se was completely designed to carry out pot experiments（S0Se0 is considered as a control group and is denoted as CK）.By measuring the content and distribution of As in the root,stem and leaf subcellular fraction of wheat,the uptake and distribution of As in wheat under the action of S and Se were analyzed.By measuring the activities of antioxidant enzymes such as superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）in the roots,stems and leaves of wheat,and the content of antioxidant substances such as non-protein sulfhydryl groups（NPT）,glutathione（GSH）and plant chelates（PCs）,the detoxification mechanism of wheat under the action of S and Se was explored.By measuring the physiological and biochemical characteristics of wheat,such as plant height,stem fresh weight,leaf fresh weight,malondialdehyde（MDA）content,chlorophyll content,etc.,the effects of S and Se on arsenic-stressed wheat growth were analyzed,and the As content of wheat grain was measured,and the effects of S and Se on the accumulation of As in edible parts of wheat were discussed.The main findings are as follows:（1）In terms of subcellular distribution of wheat,under different treatment conditions,the fixation of cell wall is the primary coping mechanism in response to As stress,and the application of S and Se can promote the transfer of As to cell fluid components,reduce the As content of metabolically active organelle components,and alleviate the poisoning of As to wheat.The proportion of AS content in cell wall and cell fluid components to the total As content of subcellular was high,and the average percentage of As content in organelle components was the lowest,and cell wall and cell fluid were the key parts of wheat plant roots,stems and leaves to store arsenic.（2）In terms of response to antioxidant system,the addition of exogenous S and Se reduced the activity of SOD,POD and CAT in wheat,which could slow down the sharp increase of the activity and stress of antioxidant enzyme activity in wheat,and prevent the excessive activation of antioxidant enzymes from consuming a lot of energy.The addition of exogenous S and Se promotes the synthesis of sulfhydryl substances NPT,GSH or PCs in wheat,which can be chelated with As into cell fluid sequestration to reduce the content of free As,thereby reducing the stress of As on wheat,combined with the reduction of antioxidant enzyme activity,the detoxification of As by S and Se is not by activating the antioxidant enzyme defense mechanism,but mainly by promoting the synthesis of sulfhydryl substances to reduce the toxicity of As to wheat.（3）In terms of wheat growth,S application significantly alleviated the oxidative stress of wheat,promoted the synthesis of chlorophyll,and reduced the content of MDA.The supply of Se also played a certain role in promoting the growth of arsenic-stressed wheat,and promoted the increase of chlorophyll content.Combined sulfur-selenium application can effectively alleviate the toxic effect of As on wheat plants and improve wheat growth.Under the same conditions of S levels,the application of Se resulted in a significant increase in chlorophyll in wheat.When the sulfur application rate was S1,Se1,Se2 and Se3 were added,and the chlorophyll content of wheat increased by 26%,94%and 71%,respectively,compared with the S1 treatment.When the sulfur application level was S2,the chlorophyll content of wheat increased by 3%,54%and28%,respectively,compared with S2 treatment.When the sulfur application rate was S3,the chlorophyll content of wheat increased with the increase of selenium application,and the chlorophyll content was the highest under Se3 treatment,which was about 2.89times that of the S3 treatment group and 5.02 times that of the CK group,respectively.（5）In terms of the accumulation of As in wheat grain,under the condition of no Se,the application of S and Se reduced the content of As in the grain,which was related to the fact that S and Se could promote the transfer of As to cell fluid.S can participate in the synthesis of S-containing antioxidants in wheat,which forms plant chelating peptides with As（III）,reducing the bioavailability of As,thereby reducing the transport of As in wheat and reducing the content of As in grains.Se can promote the synthesis of GSH by increasing the activity of glutathione peroxidase（GSH-Px）,thereby effectively inhibiting the accumulation of As in the grain.In summary,the main mechanism of S and Se detoxification of As of wheat is the formation of antioxidant substances NPT,GSH and PCs,which to sequester As ions in the cell fluid components,thereby reducing the As content in the metabolically active organelle components,alleviating the toxicity of As,and reducing the accumulation of As in grain.