Regulation Effect Of Selenium And Iron Plaque On The Absorption Of Mercury In Rice

Selenium?Se?is an essential micronutrient for the human body.Ingestion of appropriate amounts of Se is of great importance to enhance human immunity and prevent cancer.Considering rice are the staple food for humans,increasing Se content in rice is an effective way to improve Se nutrition balance and health from the source.Mercury?Hg?is a highly toxic heavy metal contaminant that can easily be enriched in rice,which in turn can affect human health.Studies have shown that Se has an antagonistic effect on rice Hg uptake.Rice roots in flooded environments form red-brown iron oxides called"iron plaque".The iron plaque has a strong enrichment effect on non-metallic Se and Hg.Therefore,the interaction between Se and the iron plaque on the root surface may regulate the absorption and transportation of Hg by rice,and thus affect the Hg content of rice.The main conclusions of this thesis are as follows:?1?In order to investigate the effects of iron plaque on the absorption and transportation of Se on rice root,a hydroponics experiment was used to first pretreat rice seedlings with Se?supplied from Na₂SeO₃?at 10?mol L^-1,and then the Fe²⁺?FeSO₄?solutions containing 0,25,50,and 100mg L^-1 was added to the above culture solution for 48 h,which were labeled as Fe0,Fe25,Fe50,and Fe100,respectively.The results showed that there was no significant difference in the content of Fe?DCB-Fe?in the DCB extract?i.e.,the amount of iron plaque?of two varieties of rice seedlings.However,with the increasing of Fe²⁺concentration,the content of Se?DCB-Se?in the DCB extract of two rice varieties increased gradually,and the DCB-Fe was positively correlated with the DCB-Se content.Compared with the Fe0 sample,the content of Se in the roots of two varieties of rice seedlings treated with Fe25,Fe50,Fe100 were reduced by 5%,17%,23%?NJ35?and 6%,17%,22%?ZX?,respectively,but the content of Se in the shoots were increased by 17%,8%,3%?NJ35?,and 18%,9%,3%?ZX?,respectively.The content of Se in roots and shoots of NJ35cultivars was higher than that of ZX cultivars under the same Fe²⁺concentration treatment.After the formation of iron plaque(Fe²⁺?25 mg L^-1),DCB-Se of NJ35 and ZX cultivars is 72.5%-84.6%and 71.1%-83.5%of the total Se content in rice seedlings,but the percentage of both Se in rice roots and shoots gradually decreases.In summary,there is a positive correlation between DCB-Fe and DCB-Se content,thus increased the content of Se in the shoot of rice seedlings.The amount of iron plaque and the concentration of DCB-Se in different varieties of rice roots are differences.?2?In order to explore the influence of phosphorus?P?deficiency on the uptake and transport of methylmercury?MeHg?by iron palque,a hydroponics experiment was used to study three different varieties of rice with phosphorus?+P?treatment and non-phosphorus?-P?treatment.The samples were then transferred to a medium containing 0.2 mg L^-1 MeHgCl.The results showed that the content of Fe?DCB-Fe?in the DCB extract of three varieties of rice was significantly higher than that of+P under-P treatment,and the DCB-Fe content of NJ35 and TY196 varieties was 2.04and 2.24 times that of+P treatment,respectively.The DCB-MeHg concentrations?the concentration of MeHg in DCB extract?of NJ35,TY196,and ZX with-P treatment was 2.6,2.2and 1.8 times higher than that with+P treatment,the Fe concentration in both roots and shoots of three varieties with-P treatment were higher by 13.2%-59.3%and 12.5%-41.4%than that with+P treatment.The content of MeHg in the shoots of the three varieties with-P treatment decreased significantly,and the MeHg content in roots and shoots of different cultivars was significantly different.The TF_root?the transfer coefficient of MeHg from the iron plaque to the root?of NJ35,TY196 and ZX with-P treatment were lower by 61.4%,54%,and 46%than that with+P treatment,respectively,and the content of MeHg in the roots of three varieties with-P treatment is lower by3.7%-9.2%than that with+P treatment and the content of MeHg in shoots is lower by 7.0%-7.6%.However,the content of MeHg in DCB had increased by 10.8%-16.9%,and the variability of TY196 was the largest,and that of ZX varieties was the least.The content of DCB-MeHg and the content of DCB-Fe were linear relationship.In summary,-P treatment significantly increased the number of iorn palque and enriched more MeHg,which in turn impedes the transport of MeHg to the shoots of rice;the number of iron plaque formed on roots of different rice varieties and the absorption and transportation of MeHg are all significantly different.The absorption and transportation of MeHg in grain low-yield rice?TY196 and NJ35?was lower than that of grain high-yield rice?ZX?.?3?In order to investigate the effects of iron palque on the absorption and transportation of Hg in rice under Se pretreatment,a hydroponics experiment was first conducted on rice seedlings Se0and Se0.5(0.5 mg L^-1 Se by Na₂SeO₃).Then,the above Se medium was added with Fe²⁺?FeSO₄?solution containing 0,25,50,and 100 mg L^-1 to continue to culture 24 h,respectively,and finally transferred to a culture solution containing 0.3 mg L^-1 of HgCl₂.The results showed that with the same Se treatment,with the increase of Fe²⁺concentration,the proportion of Fe in the DCB extract of rice seedlings?57.3%-71.5%?was significantly higher than the proportion of Fe?8.6%-17.5%?in the shoot of rice seedlings and the proportion of Fe?19.9%-25.9%?in the root of rice seedlings.Compared with Se0 treatment,Se0.5 treatment reduced the content of the shoots and the DCB extracts.After the formation of iron palque(Fe²⁺?25 mg L^-1)and the same Fe²⁺concentration,compared with Se0 treatment,the content of Hg in the roots?0.1%-6.1%?and shoots?22.0%-28.6%?of rice seedlings was decreased by Se0.5 treatment,but the content of Hg in the DCB extract?DCB-Hg?increased by 2%-13.6%.The proportion of Hg to total Hg in rice roots and shoots decreased gradually after Se0.5 treatment,but the proportion of Hg in the DCB extract increased gradually.When treated with Se0 or Se0.5,there was a positive correlation between the content of DCB-Fe and the content of DCB-Hg,while the slope coefficient of Se0.5?7.269?was significantly higher than that of Se0?4.347?.In summary,the iron plaque can enrich a large amount of Hg,by increaseing the content of DCB-Hg and reduceing the partition coefficient of Hg in rice,Se would increase the holding capacity of the iron plaque to Hg and relieve the Hg stress to the rice.?4?In order to explore the physiological mechanism of Se in relieving the Hg stress of rice,a0.3 mg L^-1 HgCl₂ treatment of rice was conducted using a hydroponics experiment and the concentrations of 0,1,2,3 and 4?mol L^-1 of Se?supplied from Na₂SeO₃?concentration medium were respectively added to each treatment for 7 days.The results showed that Se could increase the biomass of roots and shoots of rice under Hg stress.With the increase of Se concentration,the contents of chlorophyll a and chlorophyll b of rice leaves increased first and then decreased under Hg stress.Se could significantly reduce the cell membrane permeability and leaf MDA content of rice under Hg stress.With the increase of Se concentration,the activities of POD,SOD,CAT and APX in leaves and roots of rice increased first and then decreased.Se could increase the activities of GlyI and GlyII in leaves and roots of rice.Se can significantly reduce the content of proline in leaves under Hg stress,but it has no significant effect on the content of proline in roots,while the content of soluble sugar in leaves increases with the increasing of Se concentration.In summary,Se can alleviate the effects of Hg stress on the content of chlorophyll a and chlorophyll b in rice leaves,Se can reduce electrolyte leakage rate and MDA content in rice leaves under Hg stress,thereby reducing Hg damage to leaf cell membrane structure and function.Low concentration Se(?2.0?mol L^-1)can increase the activities of POD,SOD,CAT,APX,GlyI and Gly II in rice,and then increase the antioxidant capacity of rice seedlings,Se can significantly reduce the content of proline in leaves and increase soluble sugar content,in turn to enhance the tolerance of rice seedlings to Hg stress.