Study On The Critical Value Of Toxicity Of Mercury Stress To Rice

Mercury is a typical global heavy metal pollutant,which has the characteristics of non-degradability,easy migration and bioaccumulation.At present,the soil environmental quality standard in China is only evaluated from the pollution degree of soil mercury,and has not been refined to different kinds of crops.Rice is one of the main food crops.Mercury in rice can affect its normal growth and can be enriched into grains to cause harm to the public through the food chain.Therefore,the study on the critical value of rice toxicity of soil mercury is of great significance to the establishment of soil environmental quality standards.In this paper,hydroponic and soil culture experiments were conducted to study the toxic effects of different concentrations of mercury on rice and its critical value,in order to provide a reference basis for ecological risk assessment of heavy metal mercury.The main conclusions are as follows:（1）The biomass,mercury content in root and aboveground part,transport coefficient,enrichment coefficient and tolerance coefficient of different rice varieties under mercury treatment were studied and analyzed.It was found that there were significant differences in biomass and aboveground mercury content among 7 rice genotypes.Cluster analysis showed that Yongyou 4949 and Zhengyou 538 were low biomass rice varieties,others were high biomass rice varieties,and the aboveground high mercury accumulation rice varieties were Chuankangyou 727,Qiannuoyou 88,Zhengyou 538 and low mercury accumulation rice varieties were Longliangyou 1308,Taixiangyou 5318,Yongyou 4949 and Jingliangyou 1988.Combined with other index characteristics,the rice varieties Yongyou 4949 with low mercury accumulation and Chuankangyou 727 with high mercury accumulation were selected.（2）The results of toxicity test at seedling stage in hydroponic culture showed that there were significant differences in apparent symptoms and toxicity sensitivity between low mercury accumulation rice variety Yongyou 4949 and high mercury accumulation Chuankangyou 727 under mercury stress.Its growth indexes（plant height,aboveground fresh weight,aboveground dry weight,chlorophyll）increased at first and then decreased with the increase of mercury concentration,and the effect of mercury stress on the growth of Yongyou 4949 was greater than that of Chuankangyou727.Antioxidant enzyme tests showed that the activities of three enzymes in the two rice varieties were basically higher than those in the control.The stimulating activity of POD increased with the increase of mercury stress concentration,while CAT and SOD increased at first and then decreased with the increase of mercury stress concentration.（3）The results of dose-effect model fitting showed that there was a significant correlation between aboveground biomass（plant height,aboveground fresh weight,aboveground dry weight）and mercury concentration（P<0.05）,among which the aboveground fresh weight of rice was the most sensitive to mercury stress.Therefore,the fresh weight of rice aboveground EC₂₀（mercury concentration 0.21mg/kg,2.31mg/kg）was taken as the mercury toxicity critical value of Yongyou 4949 and Chuankangyou 727.It can be seen from EC₂₀that Yongyou 4949 is more sensitive to mercury than Chuankangyou 727.（4）The pot experiment of paddy soil culture showed that there was no significant change in plant height,stem dry weight and leaf dry weight of Yongyou 4949 and Chuankangyou 727 rice under soil mercury treatment,but under 6 mg/kg mercury treatment,the grain yield of Yongyou 4949 and Chuankangyou 727 decreased significantly by 9.33%and 8.09%,respectively.Mercury contents in stems,leaves and husks of the two kinds of rice changed little,while mercury contents in brown rice and roots increased sharply under 4～6 mg/kg mercury concentration treatment.The trend of mercury accumulation coefficient（BCF）in rice organs was root>brown rice>leaf>husk>stem,and the transport coefficient from root to brown rice（TF）was the highest in rice organs.（5）The content of mercury in rice organs of Yongyou 4949 and Chuankangyou 727 was significantly positively correlated with total mercury and available mercury in potted soil,and the content of mercury in brown rice had the best correlation with total mercury and available mercury in soil.Based on the national food mercury pollutant limit index（0.02 mg/kg）,the corresponding soil total mercury safety critical value0.33 mg/kg and effective mercury safety critical value 0.24μg/kg were calculated.The calculated critical values of soil mercury safety are all less than the national soil environmental quality standard paddy field screening value（0.6 mg/kg）.