| Arsenic is widely known as a human carcinogen,and is ranked first among all hazardous elements by the Agency for Toxic Substances and Disease Registry(ATSDR)in terms of adverse human health effects.Chronic As exposure can cause serious health effects including skin,bladder,and lung cancer.Arsenic-contaminated drinking water and crops are the two main sources of human intake of As.Wheat is one of the most important crops all over the world.However,As contamination and accumulation in wheat grain is still not known,also,little is known about the difference in As accumulation among different wheat cultivars.Therefore,in the present study,we focused on the diffenerce in As accumulation by different wheat cultivars,the possible mechanisms of this difference were also been studied.In the present research,field studies were conducted to investigate As contamination in agricultural soils and wheat crops during coal mining in Huaibei,China.Hydroponic and pot experiments were also conducted to studies on the mechanisms of different arsenic accumulation by different wheat cultivars.The major results are listed as follows:1.Results from field studies showed that coal mining can cause increase in arsenic(As)concentrations in wheat seedlings,straws,and grains.Arsenic concentrations in wheat plants were significantly correlated with the distance from the coal mine(p<0.01),but the same trends were not observed for Cu and Zn.The hazard index(HI)of aggregate risk through consumption of the wheat grains was 2.3-2.4 for rural inhabitants and 1.4-1.5 for urban inhabitants in Huaibei,China.The average intake of inorganic As for rural inhabitants in Huaibei was above 10 ?g per day.The results of the health risk analysis indicated that As contamination around the coal mine in Huaibei poses a potential health risk to the local inhabitants through consumption of wheat grains,particularly for rural populations.2.Roots growth of wheat seedlings decreased significantly with increasing concentrations of arsenate(As(V))in nutrient solution.Arsenic speciation in wheat plant was mainly exists as arsenite(As(?)),and the other part was As(V).Phosphate(P),sulfur(S)and silicon(Si)have different effects on As accumulation and toxicity to wheat seedlings under 15 ?mol L-1 As(V)exposure for 72 h.Phosphate and higher concentrations of Si(960 ?mol L-1)alleviated As toxicity of wheat seedlings by decreasing As uptake.High P(120 ?mol L-1)can significantly increased relative relative As(?)efflux((As(?)efflux as as a percentage of As(V)uptake),while Si did not affect relative As(?)efflux by wheat roots.There is no significant effect of S on root morphology,total As(?)uptake and As(?)efflux,whereas relative As(?)efflux was reduced significantly in high S(960 ?mol L-1)concentration.3.Arsenate tolerance and As accumulation in roots and shoots of wheat seedlings were significantly(p<0.001)varied over 57 wheat cultivars after 72 h exposure to 10 ?mol L-1 As(?).Arsenate tolerance of wheat seedlings was positively correlated with As(?)uptake(p<0.05),roots As concentrations(p<0.001),but negatively correlated(p<0.05)with TFs and relative As(III)efflux.No significant correlation between As(?)efflux and As(?)tolerance was found(p = 0.442).56-83%of total As taken up by roots was extruded to nutrient solution.Root As concentrations was positively correlated with As(?)uptake(not significant,p = 0.100),negatively correlated(p<0.001)with relative As(?)efflux,whereas not significantly correlated(p = 0.773)with As(?)efflux.These results indicated that the difference in As(?)tolerance is the main reason for the difference in arsenic accumulation of diffemet wheat cultivars.Arsenate-tolerant wheat cultivars have much higher capacity of root As accumulation,thus have lower relative As(?)efflux.4.Arsenate uptake kinetics were conducted for four wheat cultivars of different arsenic accumulation.Mianmai45(MM45)showed high affinity to As(?)than other cultivars.No significant difference were found in the Michaelis constant(Km)between other three cultivars.Roots As concentration of Huaimai29(HM29)and Qingfengl(QF1)were higher than MM45 and Fanmai8(FM8).However,roots As concentration were significantly higher in MM45 and FM8 than in QF1 and HM29 after wheat seedlings exposed to 10 ?mol L-1 As(?)for 72 h.Arsenate uptake kinetics in four wheat cultivars were significantly(p<0.05)affected by pH and P.There was no significant difference between MM45 and HM29 in the capacity of As(?)reduction in roots.Speciation of As in roots revealed that the predominant species present was As(?)in wheat plants after 0.5 h or 72 h exposure to 10 ?mol L-1 As(?),accounting for 96.9%-100%and 96.8%-99%of the total As in wheat roots of MM45 and HM29,respectively.Only Cys(cysteine),GSH(glutathione),and PC2(phytochelatins)were detected in wheat plants with no traces of PC3 and PC4.Thiols concentrations of Cys,GSH and PC2 in wheat roots and shoots were significantly higher(p<0.05)in MM45 than in HM29 under 10 ?mol L-1 As(?)exposure or not.Compared to the control,GSH and PC2 were significant induced in roots of wheat seedlings after exposed to 10 ?mol L-1 As(?)for 72 h,especially for MM45.But GSH and PC2 concentrations in wheat roots and shoots were not significantly affected by As(?)exposure for 0.5 h,except for PC2 concentration in HM29 roots was significantly increased(P<0.05).The trend of the difference in roots As concentration was same with difference in roots PC2 concentration between MM45 and HM29.BSO(L-buthionine sulphoximine,a specific inhibitor of ?-glutamylcysteine synthetase.The BSO treatment completely suppressed the synthesis of GSH and PCs of wheat seedlings.BSO decreased As accumulation in the roots,with As concentration decreased by 83%-90%.However,BSO increased As accumulation in the shoots,with As concentration were increased to 1.7-to 2.5-fold.Wheat roots and shoots As concentrations were significantly higher in MM45 than in HM29 under 10 ?mol L-1 As(?).But the differences were not significant in the BSO treatment.These results indicated that arsenate uptake kinetics by wheat roots and As(?)reduction in wheat roots were not the reason for the difference in roots As accumulation of different wheat cultivars.Non-protin thiol compounds in wheat seedlings play the key role for the difference in root As accumulation and As(?)tolerance of different wheat cultivars.Cultivar with the higher concentrations of thiol compounds produced prossesses higher As-tolerant ability and higher As accumulation capacity.So,the relative relative As(?)efflux and the rate of As translocation from root to shoot were lower in those arsenate-tolerant wheat cultivars.5.Pot experiments were conducted to investigate the difference in arsenic(As),cadmium(Cd),and phosphorus(P)uptake,accumulation,and translocation among 12 wheat cultivars and their relationships with each other in soil "naturally" contaminated with both As and Cd.As and Cd concentrations in wheat grain,straw,and root differed significantly(p<0.05)among the 12 wheat cultivars.The grain As concentration was not correlated with straw and root As,or the total As content in plants,but was significantly(p<0.05)correlated with As translocation factors(TFs),i.e.,TFs(Grain/Root)and TFs(Gran,straw).The grain Cd concentration was positively correlated with the total Cd content and TFs(Grain/Straw).Both As and Cd in wheat grains were correlated with P accumulation and transport.A significant positive correlation was observed between root As and Cd,but no significant relationship was detected between grain As and Cd concentrations.Compared with As,Cd was more easily transported to the wheat grain,and the rachis played a key role in ensuring this difference.Grain As concentration was positively correlated with husk(p<0.01)and rachis(p<0.05),but it was not for Cd.The wheat grain As concentration was mainly related to the translocation of As from the root and shoot to the grain.The grain Cd concentration not only depended on the wheat plant uptake,but also on the ability of Cd to be retranslocated from the shoot to grain. |
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