Characteristics Of Transport And Accumulation Of Mercury In Different Rice Cultivates And At Various Growing Stage

Rice is the staple grain crop for half of the world’s population China is the country with largest rice production and consumption.Therefore,it is especially important to ensure rice security in our country.The Wanshan mercury（Hg）mining area in Guizhou is the largest Hg contaminated area in the world,and the research on the remediation and management of Hg contaminated environment in this area has been played widely attention both at home and abroad.To screen high and low accumulated Hg rice cultivars is of great significance for the remediation and management of Hg polluted paddy ecosystem and the safety production of rice in Wanshan Hg mining area.In this study,we investigated the concentrations,transport factors and biological concentration factors of Hg and methylmercury（Me Hg）in different rice cultivars and various growth periods of rice in order to screen low and high accumulated Hg rice cultivars.41 cultivars of rice and upland rice were experimentally cultivated in the paddy fields of Wanshan Hg mining area,and the biomass of tissues,concentrations of Hg and Me Hg in different rice cultivars and growth stage were determined.The biological concentration factor（BCF）and transport factor（TF）of Hg and Me Hg were calculated by the data of contents of Hg and Me Hg and biomass of rice tissues.This study has scientific value to reveal the molecular biological mechanism of Hg accumulation differences among different rice cultivars,and could provide important basic data for further accurate screening of low accumulation Hg and Me Hg rice cultivars,and has some application value in the remediation and management of Hg pollution in agricultural fields in mercury mining areas and food security production.The main results and understanding obtained from the study are as follows:（1）The concentrations of total mercury（THg）and Me Hg in rice tissues varied among cultivars.The average value and range of THg as well as Me Hg in seeds was 63.1±20.44μg/kg（34.4-117μg/kg）,and 22.5±7.19μg/kg（11.5-35.0μg/kg）,respectively.The difference of THg concentration between rice and upland rice cultivars was not significant,but that of Me Hg was significant.Generally,THg concentration was following order:root>leaf>stem>seed,N₄>N₃>N₂>N₁,IN₄>IN₃>IN₁>IN₂,LS₄>LS₃>LS₂>LS₁;L₄>L₃>L₂>L₁,Me Hg content:seed>root>leaf≈stem,and the proportion of Me Hg account for THg in seeds was much greater than that in other tissues,in the range of 21.17-58.70%with an average value being over 37%.（2）The BCFs of THg and Me Hg in various tissues were different among rice and upland rice cultivars.BCFs of THg was much greater in roots than in other tissues,followed by leaves,and smallest in stem and seed.In contrast,BCFs of Me Hg was different with THg,and BCFs of Me Hg in all tissues were much greater than THg,and most of them exceeded 1.BCFs of Me Hg in seeds were largest,up to 9.65 in rice cultivars,and up to 10.87 in upland rice,while That of roots were second,and in stems and leaves were lowest.BCFs of THg in nodes and internodes were generally following order:N₄>N₃>N₂>N₁,IN₄>IN₃>IN₂>IN₁.The BCFs of Me Hg in all rice tissues were much more than that of THg.（3）The transport factor（TF）of different cultivars was different,and TFs of THg and Me Hg were much higher from root to shoot than from root to seed.The TF of Hg between in different nodes and internodes showed the trend as following:TF_N1-IN1>TF_N2-IN2>TF_N3-IN3>TF_N4-IN4.In rice cultivars:TF_N1-IN1 of THg ranged from 0.28 to 0.90,TF_N1-IN1 of Me Hg ranged from 0.11 to 1.44;TF_N1-L1 of THg ranged from 3.64 to 10.28,TF_N1-L1 of Me Hg ranged from 0.14 to 6.21,and in upland rice cultivars:TF_N1-L1of THg ranged from 2.34 to The TF_N1-L1 of Me Hg ranged from 0.37 to 4.81.（4）The rice cultivars with high accumulated THg and Me Hg were Guang eight You 199 and Taiyou 187,which THg and Me Hg contents were101μg/kg and 34.3μg/kg,117μg/kg and 29.1μg/kg,respectively;the rice cultivars with low accumulated THg and Me Hg were Hengfeng You Huazhan and Chuanzhongyou 3607,which THg and Me Hg contents were38.4μg/kg and 11.5μg/kg,35.1μg/kg and 12.6μg/kg,respectively;the upland rice varities with high accumulated THg and Me Hg were Lvhan 1#and Lanrendao 1#,which THg and Me Hg were were 113μg/kg and 32.8μg/kg,88.9μg/kg and 34.4μg/kg respectively;the upland rice cultivars with low accumulated THg and Me Hg were Zhenghan 9#and Gaoshan 1#,which THg and Me Hg contents were 49.2μg/kg and 11.4μg/kg,48.1μg/kg and 11.4μg/kg,respectively.（5）Aboveground nodes,internodes,and leaf tissues play important roles in the transport and accumulation of mercury in rice.Nodes N1 and N2 which are near to the rice seeds have the roles of storage and redistribution of Hg in the process of Hg transport in aboveground tissues,while nodes N3 and N4 which are near to the roots have the roles of restricting and blocking Hg translocation toward upper tissues.（6）The mechanisms of uptake,transport and enrichment of Me Hg in soil-rice system are different from those of inorganic Hg.Most of the Me Hg in soil is transported and enriched in rice seed,and the retention and interception effect of Me Hg in roots and nodes were not as obvious as that of inorganic Hg,and the uptake and transport of Me Hg in rice is obviously stronger than inorganic Hg.（7）The Hg contents,BCFs and TFs of various rice tissues were distinctive at different growing stage.The THg and Me Hg contents in rice seed were higher in late maturity satge than in early period,and the BCFs and TFs of Hg of different tissues at different stage were inconsistent and changed,with abnormally high value on some dates.Hg enrichment in rice seed was significantly positively correlated with Hg in tissues underneath panicle,while it was not or weakly correlated with roots and tissues close to the roots.