Effects Of Genotypes And Edaphic Factors On Mercury And Methylmercury Accumulation In Rice (Oryza Sativa L.)

Mercury (Hg) is a toxic element, and is now considered a key global pollutant. Incertain environmental conditions (e.g. paddy field), Hg was methylated tomethylmercury (MeHg) which is more toxic (neurotoxin) than the inorganic form.Nowadays, more and more paddy fields have been contaminated by Hg due toanthropogenic Hg emission, resulting elevated Hg levels in rice and subsequently inhumans foods posing human health threat. Recent studies have demonstrated that riceseed has the highest ability to accumulate MeHg compared to other tissues. There istherefore urgency in management and utilization these paddy fields contaminated byHg, to develop mitigation measures to reduce total mercury (THg) and MeHg in ricegrain and to ensure food safety. However, cost-effective methods are not available toremediate large areas of Hg-polluted paddy fields. Prevoius studies reported that therewere significant differences in THg and MeHg uptake and tolerance among ricegenotypes grown in soils added with Hg under greenhouse conditions. The resultssuggest that it is possible to screen rice genotypes to ensure rice safety inHg-contaminated fields. However, currently the genetic controls of THg and MeHg accumulation in rice remain largely unknown. In this study, field and greenhouseexperiments have been conducted to investigate the genotype variation and stablity ofrice in THg and MeHg accumulation, and to explore the mechanisms of this variationfrom the aspects of radial oxygen loss (ROL) and iron plaque. The effects of edaphicfactors in THg and MeHg accumulation in rice genotyes were also investested inpresented study. The major results and conclusions are as follows:Firstly, THg concentration in soils of paddy fields and rice grains collected from13mines sites and3control sites across Guizhou, Hunan and Guangdong Provinces wereinvestigated. The results showed that (1) the paddy fields in the13mines sites havebeen seriously contaminated by Hg, up to136.5mg kg^-1in paddy soils at Tongren(TR) Hg mine site in Guizhou Province;(2) the rice was also polluted by Hg, meanTHg concentrations of rice grain in Guizhou, Hunan and Guangdong were28.6ng g^-1,20.5ng g^-1and5.98ng g^-1, respectively. The highest concentration of THg in ricegrain was54.4ng g^-1(Danzhan Hg mine of Guizhou) which is exceed the permissablelimit (20ng g^-1) of the Maximum Levels of Contaminants in Foods of China (MLCFGB2762-2005).Secondly, twenty-six rice genotypes were cultivated in three paddy fieldscontaminated Hg across Guizhou (Wanshan Hg mine), Hunan (Xinhuang Hg mine)and Guangdong (Fankou Pb/Zn mine) Provinces. Results showed that there wassignificant genotype variation (p <0.01) for grain THg and MeHg at the three fieldsites. The mean THg and MeHg concentrations of rice grain in Wanshan, Xinhuangand Fankou were25.8ng g^-1,28.6ng g^-1and14.5ng g^-1;9.4ng g^-1,11.4ng g^-1and1.2ng g^-1, respectively. At Wanshan and Xinhuang field sites (paddy soil THgexceeded40mg kg^-1in the two sites), there were6genotypes (Zhonghua11,Ribenqing, Yueguang, Laohuangdao, Nanfengnuo and Suyunuo) with the lower grainTHg which were under the permissable limit of MLCF. At Fankou site, concentrationsof THg in grains of19genotypes were lower than the permissable limit of MLCF. Forthe26genotypes at the three sites there were highly significant correlation betweenthe concentrations of THg between Wanshan and Xinhuang (r=0.81, p <0.0001), Wanshan and Fankou (r=0.64, p <0.001), Fankou and Xinhuang (r=0.76, p <0.0001). Moreover, there were also significant positive correlations (p <0.05⁰.01)between grain yields at the three field sites. A significant positive correlation betweenthe concentrations of MeHg between Wanshan and Xinhuang (r=0.67, p <0.001).Two rice genotypes (Ribenqing and Nanfengnuo) were identified with a combinationof low grain THg and MeHg.Thirdly, the paddy fields in Fankou Pb/Zn mining area were co-contaminated by acocktail of mixed toxic heavy metals (Hg, Cd and Pb). This study indentified ricegenotypes with both low Cd and Pb accumulation under Cd-and Pb-contaminatedfield conditions, and the interactions of toxic elements Cd and Pb with micronutrientelements iron (Fe), zinc (Zn), manganese (Mn) and nickel (Ni) were also studied.Among32rice genotypes tested, there were significant differences (p <0.001) inconcentrations of6elements of brown rice. Significant decreases in concentrations ofFe and Mn were detected with increasing Cd concentrations and a significantelevation in Fe, Mn and Ni with increasing Pb concentrations. A similar result wasalso shown by Cd and Ni. Three genotypes were identified with a combination of lowbrown rice Cd and Pb, high micronutrient and grain yield (Wufengyou2168, Tianyou196and Guinongzhan).Fourthly, four rice genotypes were cultivated in11soils contaminated by Hgcollected from different paddy sites (10Hg mine and1Pb/Zn mine across threeprovinces). The pot experiment was conducted under greenhouse condition toinvestigate effect of genotypes and edaphic factors on THg and MeHg accumulationin rice. Results indicated that genotypes as well as locations (soil sources) had asignificant effect on grain THg and MeHg. Moreover, there was also a significantgenotype by location interaction for rice THg and MeHg. There were significantnegative correlations between rice THg and concentrations of total organic carbon(TOC), Fe and Ni in soil; a significant positive correlation was observed between riceTHg and soil total potassium (TK). There was a negative correlation between riceMeHg and soil pH. Finally, a pot trial was conducted using soil-sand combination rhizosbag systemwith four rice genotypes, to study dynamic variation of radial oxygen loss (ROL) andiron plaque on root surface, and THg/MeHg uptake in six rice growing stages(tillering, elongation stage, booting, flowering, filling stage and mature stage). Resultsshowed that there were significant differences (p <0.01) in rates of ROL amongdifferent rice genotypes, and among different growing stages. There were alsosignificant variations for degree of iron plaque formation on root surface amongdifferent genotypes and different growing stages. The mean rates of ROL in the fourgenotypes (Tianyou196, Huayouguangkangzhan, Nanjing35and Zixiangnuo) were5.92mmol O2kg^-1root d. w. h^-1,5.27mmol O2kg^-1root d. w. h^-1,8.48mmol O2kg^-1root d. w. h^-1and3.4mmol O2kg^-1root d. w. h^-1in six rice growing stages,respectively. The mean concentrations of Fe on root surface (DCB-Fe) of the fourgenotypes were20474mg kg^-1,16979mg kg^-1,24224mg kg^-1and14952mg kg^-1,respectively. Straw THg/MeHg and root MeHg were gradually decline in rice growingperiod. However, root THg was gradually increased from tillering to flowering.There was a significant negative correlation (p <0.05⁰.0001) between rate of ROLand rice THg, similarly, a significant negative correlation (p <0.05⁰.0001) betweeniron concentration on root surface and rice THg was also found. However, there wereno significant correlations between rice MeHg and rate of ROL, iron concentration onroot surface.In summary, present results showed that there were significant genotype variationsfor rice THg and MeHg concentrations under both field and greenhouse conditions.Some rice genotypes accumulated lower THg and MeHg in grains (under thepermissable limit of food safety) even if they were cultivated the paddy fieldsseriously contaminated by Hg. Moreover, rice genotypes tended to stabilize in THgaccumulation under different field conditions, and in MeHg uptake in similarenvironment conditions. Ribenqin and Nanfengnuo were potential useful genotypeswith simultaneously low grain THg and MeHg. Similarly, Wufengyou2168, Tianyou196and Guinongzhan were identified with a combination of low grain mixed toxic heavy metals (Cd and Pb). The degree of soil Hg-contamination is an important factorfor THg and MeHg accumulation in rice. Some edaphic factors (e.g., TOC, Fe, Ni andK) had significant effects on rice THg, and soil pH on rice MeHg accumulation. Theresults from rhizobag trial suggest that ROL-iron palque combination play animportant effect on THg accumulation in rice. Booting is a key stage for Hg(especially MeHg) accumulation in rice growing period, imply that some measures(e.g. water management) could be carry out to mitigate THg and MeHg accumulationin rice at this stage. As a whole, present results indicate that selection of rice genotypeand control of edaphic factor were effective ways in reducing THg and MeHgaccumulation in rice, this may have potential to be applied in Hg-contaminatedregions.