| Mercury is a highly toxic,non-essential chemical for organisms.It has been listed by many international organizations as a priority pollutant due to its strong neurotoxicity.Lately,global atmospheric mercury emissions have remained high,and soil mercury pollution are found frequently.The latest mercury emissions assessment report shows that anthropogenic mercury emissions in the past century have caused global atmospheric mercury concentration to increase by 300% to 500%.In 2010,atmospheric mercury emissions in Asia accounted for nearly 50% of the total global emissions in that year,and mainly produced in East and Southeast Asia.However,mercury released into the atmosphere eventually entered the soil through dry and wet deposition,causing soil pollution.Methylmercury is the most toxic mercury compound,and its bioaccumulation and biomagnification effects pose a potential threat to human health.Studies have confirmed that paddy rice fields,which are flooded intermittently,are important sites for methylmercury production.Rice feeding has also become one main way for methylmercury exposure in mercury-contaminated areas.Therefore,as rice is one staple food crop in China and Southeast Asia,its safety needs to be secured from methylmercury pollution.Roots are the main vegetative organs for plant uptake,translocation and storage of nutrients,and are also important ways for soil contaminants to enter plants.Root growth,morphogenesis and ecological stoichiometry affect plant uptake of water and nutrients,as well as regulation of the absorption and translocation of pollutants such as metals and organic matter.The root morphology and ecological stoichiometry of plants are mainly affected by variety and environmental conditions.Therefore,this paper selects rice as experimental material,based on the indoor simulation experiment,under the condition of hydroponic culture,by altering the environment condition(phosphorus,sulfur level in the root zone,environmental CO2 concentration),the root morphology and ecological stoichiometry of rice seedlings are induced to change.Methylmercury exposure experiments were performed on pretreated rice with different genotypes and different environments.By studying the differences of rice roots,the effects of genotypes and environmental conditions on the ability of rice to absorb and transport methylmercury were analyzed,and the regulation of roots in the absorption of methylmercury in rice was revealed,which provided a new perspective for regulating rice absorption of pollutants.The main findings are as follows:1.Two different genotypes of rice,one indica rice ‘Eryou' and one japonica rice ‘Tiyou',were selected.The roots of the two rice varieties were significantly different.The root length,root surface area,root average diameter and root volume of indica rice ‘Eryou' were significantly higher than that of japonica rice ‘Tiyou'(P<0.05).Under the same condition of methylmercury stress,the absorption and translocation of methylmercury in japonica rice ‘Tiyou' was significantly lower than that of indica rice ‘Eryou'(P<0.05),and the tolerance of japonica rice to methylmercury was better.2.The phosphorus and sulfur conditions in the root zone affected the root morphology,mainly showed that the biomass of roots decreased significantly and the root porosity decreased significantly upon phosphorus deficiency,phosphorus-enriched and sulfur-enriched treatments(P<0.05).The total length of root,root surface area,root average diameter,root volume and the number of root tips have a trend to increase under the phosphorus deficiency and sulfur-enriched treatment.The sulfur concentration in the root decreased,while the root thickening of rice under sulfur-rich treatment was obvious.Phosphorus deficiency and enriched treatments inhibited,while sulfur-enriched treatment increased the absorption of methylmercury by roots.All three treatments promoted the transport capacity of methylmercury.The total amount of methylmercury in roots of rice was significantly positively correlated with the average diameter of roots(R=0.6635,P<0.01),and the two showed a quadratic function relationship.The methylmercury transport coefficient of rice was significantly negatively correlated with root porosity(R=-0.7435,P<0.01).The content of methylmercury in shoot/root was significantly positively correlated with the sulfur content of the corresponding part(R=0.4840,P<0.05;R=0.5296,P<0.05).Also there was a significant negative correlation between carbon to nitrogen ratio of root and the methylmercury content in the shoots(R=-0.5774,P<0.01).3.After the increase of environmental CO2 concentration,the total root length,root surface area and root volume of rice seedlings increased significantly(P<0.05),and the root biomass and root porosity also slightly increased.Under the same treatment,the methylmercury content in the shoot of ‘Eryou' was significantly higher than that of ‘Tiyou'(P<0.05).The methylmercury content in the root of ‘Eryou' decreased first and then increased with the increase of CO2 concentration.The methylmercury content in the shoot of ‘Eryou' was stable,and the transport capacity was firstly increased and then decreased.The methylmercury content in the root of ‘Tiyou' increased with the increase of CO2 concentration,and the methylmercury transport capacity is shown to be reduced.The total amount of methylmercury in rice shoots was significantly positively correlated with root surface area and root volume(R=0.8039,P<0.01;R=0.8399,P<0.01),and both showed a quadratic function relationship.There was a significant negative correlation between the methylmercury transport coefficient and the aboveground carbon-nitrogen ratio(R=-0.7668,P<0.01),and a significant positive correlation with the root sulfur content(R=0.5728,P<0.05).4.By comparing the kinetic characteristics of methylmercury absorption by rice root pre-treated in different phosphorus concentrations of root zones,it was found that the root morphology had an effect on the methylmercury absorption kinetics of rice.Phosphorus deficiency and phosphorus-enriched treatment promoted the growth of rice roots,but the sulfur content in roots and shoots decreased,and its kinetic characteristics showed an increase in affinity and a decrease in absorption potential compared with the control of normal phosphorus treatment.The above results confirmed that the root morphology and ecological stoichiometry of rice were affected by variety and environmental conditions,and the roots had a certain regulatory effect on the absorption and translocation of methylmercury.The results of the study may have some value on the selection of rice varieties in the mercury-contaminated areas to ensure the food security. |
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