Arsenic Uptake,chemical Speciation And Toxicity In Rice As Influenced By Cultivars And Agronomic Practices

Consumption of rice constitutes a major exposure route of inorganic arsenic(As),a class-one carcinogen.Typically,arsenite(As(?))and dimethylarsinic acid(DMA)are the predominant As species in rice grain,with the former being much more toxic than the latter.Rice plants are not able to methylate inorganic As to produce various forms of organic As.The methylated As species in rice grain is mainly derived from soil.As(?)is the dominate As species in soil solution under anaerobic conditions.Continuous flooding promotes the formation of anaerobic conditions,increases the mobility and bioavailability of As,accelerates the microbial As methylation process,and enhances the uptake of methylated As by rice.Compared with inorganic As,methylated As can be more effectively transported into the above-ground tissues of rice plants,and finally accumulate into rice grain.Various environmental factors can influence the uptake and transportation of As in rice.However,there is little information about varietal difference in the absorption,transportation and accumulation of different As species in rice.This study aims to understand the responses of various rice cultivars to As toxicity and determine the tolerance of rice cultivars to different As species.In addition,the effects of agronomic practices on As accumulation and speciation in rice grain were investigated.The study included three parts:(1)The difference of absorption and transportation of As(?)and DMA among eight rice cultivars differing in the sensitivity to the As-related straight-head disease;(2)The toxicity of As(?)or DMA to different rice cultivars and the relationship with the sensitivity to the straight-head disease;(3)The environmental factors that affect the accumulation and distribution of As in rice grain.The main results are summarized as follows:1)Pot experiments were conducted to investigate the absorption and accumulation of As(?)and DMA by eight rice cultivars.The results showed that compared with the control,the addition of As(?)or DMA had a significant effect on the As concentration in rice straw and grain.The addition of As(?)or DMA significantly increased the concentration and the proportion of methylated As species,in rice grain.Under As(?)treatment,there was a linear correlation between the methylated As concentration and total As concentration.Furthermore,inorganic As concentration increased with the increment of total As.Under DMA treatment,the methylated As increased much more steeply with increasing total As concentration compared to the As(?)treatment.However,there was no significant effect of DMA addition on the inorganic As concentration in the grain.There were no correlation between total As,inorganic As and organic As concentration in rice grain and Straight-head rate.Suggested that the accumulation of As has noting to do with the occurrence of straighthead disease.2)The toxicity of As species to different rice varieties and the tolerance of rice cultivars to As species were also studied.Eight different straighthead susceptible cultivars were grown in a hydroponic experiment with different As concentration gradients.The results showed that the sensitivity to As(?)toxicity was closely related to the susceptibility to the straighthead disease,while there was no correlation between the latter and DMA toxicity.The difference in As(??tolerance between rice cultivars was related to their As detoxification capacity by non-protein thiol(NPT).As(?)tolerant cultivars produced higher concentrations of NPT,which can detoxify As(?)via the formation of As(?)-thiol complexes.The difference in DMA tolerance among rice cultivars was related to the DMA transport and accumulation in rice shoots.Cultivars with a higher DMA transport efficiency from the roots to the leaves were more sensitive to DMA toxicity.3)Two rice varieties(Priscilla and Mars)with different capacities of As accumulation in the grain were further selected to study the different behaviors of As(?)and DMA in rice,in terms of absorption,transportation and accumulation in a hydroponic experiment.The results showed there was no significant difference in terms of root As absorption by the two cultivars,while the leaves of Priscilla absorbed As more efficiently than Mars.In a soil pot experiment,the additions of As(?)and DMA significantly increased the As concentrations,especially the organic As concentration,in the Priscilla grain much more than the Mars grain.Application of organic manure is an important agronomic practice in the field.Priscilla and Mars were chosen to study the effect of As(?),organic manure and the combination of these two factors on As accumulation and speciation in straw and grain.The As concentration in the straw of Mars significantly increased by the additions of the As(?)or manure,while little effect was observed for grain As concentration.In contrast,all treatments significantly increased the grain As concentration in Priscilla but had little effect on straw As concentration.All treatments also had a significant effect on the As species in rice grain,and significantly increased the methylated As concentration and the proportion of methylated As in total As.In addition,the arsM copy number in soil was significantly increased by the additions of As(?)and organic manure.4)Field experiments were conducted to investigate the impact of agronomic practices on As accumulation and speciation in rice grain over four cropping seasons.The results indicated that paddy water management and straw incorporation had significant effects on As accumulation and speciation in rice grain.Compared with intermittently flooding,continuous flooding increased the grain As concentration,especially methylated As,suggesting that intermittently flooding was a effective way to reduce the grain As concentration.However,the applications of nitrogen fertilizer,biochar,or manure had little effect on grain As concentration.The impact of paddy water management and straw incorporation on methylated As concentration was higher than that on inorganic As.Continuous flooding significantly increased the abundance of arsM gene in soil,suggesting that flooding could enhance microbial As methylation in soil and consequently the uptake of methylated As by rice.There was significant nonlinear positive correlation between the arsM copy number in soil and methylated As concentration in rice grain.In summary,there were differences with respect to the absorption,transportation and accumulation of different As species among rice cultivars.Microbial As methylation in soil played an important role in accumulation of As species in rice grain.The susceptibility to the straighthead disease among different rice cultivars was closely related to the sensitivity to As(?)toxicity,but not to DMA toxicity.Continuous flooding in paddy field significantly increased As accumulation and played an important role in As speciation in rice grain.This study provided theoretical basis and practical guidelines for controlling As accumulation and speciation in rice grain.