Effects Of Different Environmental Factors On Rice Root Growth And Hg Accumulation Under Mercury Stress

Rice has the capability of accumulating Hg to a certain extent.Nowadays,more and more paddy fields have been contaminated by Hg due to anthropogenic Hg emission,resulting in elevated Hg levels in rice and subsequently in human foods,posing great threat to human health.Therefore,it is urgent to improve the management and utilization these paddy fields contaminated by Hg in order to reduce Hg concentration in rice grain,so as to ensure food safety.However,there are still no cost-effective methods available to remediate Hg-polluted paddy fields.Previous studies reported that there were significant differences in patterns of Hg absorption among different rice species,which indicates that by proper choice of rice,it is possible to reduce the Hg concentration in rice grains.Root is the primary organ of assimilation in rice.The anatomy of root not only affects the absorption of water and nutrient,but also make a great impact on the transportation of pollutants including heavy metal.When the surrounding environment changes,the roots of the rice would adjust its growth and change its anatomy,thus alter the absorption behavior.Therefore,the present study chose two rice species(O.sativa L.cv.Lvhan No.1 and O.sativa L.cv.Wuyunjing No.21)with different root anatomies as the objects,using indoor cultivation and statistical analysis as main methods,and tried to explore the effects of different physiological and toxic conditions(including nutrient supply,iron plaque formation and Cd stress)on the rice root growth under Hg stress.In addition,the Hg accumulation patterns in roots and shoots in these conditions are investigated.The poisonous effect and mechanism are discussed,so as to provide some theoretical basis and practical reference for the preservation and protection of the agriculture ecosystem polluted by Hg.The main findings are as follows:1.Compared with O.sativa L.cv.Lvhan No.1,O.sativa L.cv.Wuyunjing No.21 has thicker and longer roots,with larger root surface area and root volume.Under Hg stress(0.5 mg/L Hg(?)and 1 mg/L Hg(?)),there's no significant variation in Wuyunjing No.21's roots,which means it has a higher stability of Hg.Lvhan No.l is very sensitive to Hg stress,manifesting higher root-shoot ratio in 0.5 mg/L Hg(?)cultured solution and less root tips in 1 mg/L Hg(?)cultured solution.2.Compared with the group under Hg stress(0.5 mg/L Hg(?)),the addition of Cd(0.5 mg/L Hg(?)+ 0.5 mg/L Cd(?))can reduce the Lvhan No.l's root-shoot ratio,and the concentrations of Hg in root and shoot both decrease(root Hg concentration reduces from 63.35 mg/kg to 30.74 mg/kg,shoot Hg concentration reduces from 12.94 mg/kg to 10.89 mg/kg).The translocation factor of Hg,however,get a rise from 20.41%to 35.41%.3.In the condition of Hg stress(0.5 mg/L Hg(?)),cutting down the N supply can significantly lower Wuyunjing No.21's root diameter and root volume,with a decrease from 160.42 mg/kg to 106.9 mg/kg of Hg concentration in the roots.Nevertheless,the Hg concentration goes up from 11.6 mg/kg to 15.33 mg/kg,and the translocation factor rises from 7.23%to 14.34%.4.For the rice species provided in the test,we find that iron plaque formation can improve the rice root growth under the condition of normal fertilization,increasing the root length,root surface area,No.of root tips,root biomass and shoot biomass significantly.Nonetheless,while under Hg stress(0.5 mg/L Hg(?)),the iron plaque formation poses little eflfects on all the root indicators(including root length,root diameter,root surface area,root volume,number of root tips,biomass and root-shoot ratio),but can reduce the transport of Hg from root to shoot effectively(the translocation factor descend from 20.42%to 16.33%in Lvhan No.l,and from 7.23%to 3,05%in Wuyunjing No.21).5.Our tests find that iron plaque formation could still improve rice roots growth under Hg stress while the nutrients are not sufficient.While with less N supply under Hg stress,the iron plague formation can boost the root length of Lvhan No.l,with significant increase in the biomass of root and shoot.Moreover,the root Hg concentration goes down from 228.61 mg/kg to 215.54 mg/kg and shoot Hg concentration goes down from 9.58 mg/kg to 5 mg/kg,the translocation factor of Hg decrease from 4.19 to 2.32.When it comes to Wuyunjing No.21 at the same condition,iron plaque induction can raise the rice's root length,root surface area,number of root tips and shoot biomass,causing the root Hg concentration ascend from 106.9 mg/kg to 450.93 mg/kg.The shoot Hg concentration,however,decreases from 15.33 mg/kg to 8.37 mg/kg,and the translocation factor reduces tremendously from 14.34 to 1.86.The research indicates that O.sativa L.cv.Wuyunjing No.21 is more fitted to grow in the field contaminated by Hg,for its root growth is more stable,no matter in the variation of nutrition condition or under the stress of heavy metal.On the other hand,due to the swift responses of roots,Lvhan No.1 could be utilized as the indicator of Hg pollution.In the whole process of planting rice,the nutrients in the fertilizer should be adjusted so as to control the risk of Hg pollution posed on food safety.