Physiological Mechanism Of High And Low Mercury Accumulation Of Robinia Pseudoacacia And Rhizobia Symbionts Responding To Mercury Stress

Mercury is a highly toxic and bioaccumulative pollutant,which will not only destroy the balance of the ecological environment,but also cause great harm to human body.As human activities have significantly increased the amount of mercury emissions,environmental mercury pollution has become increasingly serious and widespread worldwide,and remediation is imminent.Due to the shortcomings of traditional physical and chemical remediation methods,it has become an urgent problem to explore low-cost and eco-friendly mercury pollution treatment technology.Robinia pseudoacacia can grow rapidly and has multi-dimensional resistance.As a reforestation,it can improve the physical and chemical properties of the soil in the polluted mining area,and can accumulate and absorb heavy metals in the soil.It has strong adaptability on the abandoned land in the mining area The effect of plant-microorganism combined remediation technology on contaminated soil is better than that of phytoremediation or microbial remediation alone.Robinia pseudoacacia has great potential in remediation of heavy metals in soil due to its advantages of combining with rhizobia.Through screening the species of Robinia pseudoacacia and rhizobia,this experiment explored the effects of Robinia pseudoacacia-rhizobia symbiosis under mercury stress on the growth,photosynthetic parameters,mercury absorption,membrane lipids,and antioxidant system of Robinia pseudoacacia.The specific research results are as follows:（1）The aboveground biomass and mercury content of 11 Robinia pseudoacacia varieties were statistically analyzed under the treatment of 0.3 mg·kg^-1and 30 mg·kg^-1mercury.The results showed that the aboveground biomass and mercury and cadmium contents of Robinia pseudoacacia from different provenances were significantly different due to different genotypes and metabolic processes.Through cluster analysis,taking the aboveground biomass and mercury content as screening indicators,two varieties with large differences in mercury content were initially selected from the middle biomass category,among which Robinia pseudoacacia of Shaanxi provenance was the one with high mercury accumulation(The aboveground biomass under high mercury stress:0.34±0.05 g,aboveground mercury concentration:126.94±10.11 ug·kg^-1),and Robinia pseudoacacia of Henan provenance was the one with low mercury accumulation(The aboveground biomass under high mercury stress:0.31±0.05 g,aboveground mercury concentration:68.31±11.93ug·kg^-1).（2）The growth of five rhizobium strains under 1 mg·kg^-1mercury treatment was analyzed.The OD600 value was used as the screening index to screen two varieties with large differences in biomass under mercury stress,of which QD（M.huakuii,from Qingdao,Shandong Province）was mercury-tolerant rhizobium and YH（M.amorphae,from Yanghe,Jiangsu Province）was mercury-sensitive rhizobium.（3）Under the condition of 0.3 mg·kg^-1and 3 mg·kg^-1mercury treatment,Robinia pseudoacacia was cultured in water.The mercury accumulation of Shaanxi provenances was higher than that of Henan varieties,and the mercury absorption rate of the two kinds of Robinia pseudoacacia was relatively fast at the early stage of treatment（D1 and D2）,and the content changed slowly at the later stage,and was relatively saturated;Under 0.3 mg·kg^-1mercury stress,the underground mercury content of Robinia pseudoacacia in Shaanxi and Henan decreased at the later stage of treatment.（4）Mercury stress can promote the biomass accumulation and increase the photosynthetic rate of Robinia pseudoacacia leaves.The chlorophyll content of Robinia pseudoacacia in Henan decreased gradually with the increase of mercury stress,while that of Robinia pseudoacacia in Shaanxi was higher under low mercury stress.The biomass accumulation,chlorophyll content and leaf photosynthetic rate of the two species of black locusts inoculated with rhizobia increased,and the promotion of tolerant rhizobia was more significant.On the whole,the chlorophyll content and photosynthetic capacity of Robinia pseudoacacia of Shaanxi provenance are higher than that of Robinia pseudoacacia of Henan provenance,which indicates that Robinia pseudoacacia of Shaanxi provenance has better growth and photosynthetic conditions than Robinia pseudoacacia of Henan provenance under long-term mercury stress,and has better mercury remediation potential.（5）Under mercury stress,the contents of MDA and H₂O₂in leaves of Robinia pseudoacacia from Henan and Shaanxi provenances increased,the cells were damaged,and the membrane damage was aggravated.The antioxidant enzyme activity of Robinia pseudoacacia leaves in Shaanxi increased with the increase of mercury stress,and the activity was higher under 0.3 and 3 mg·kg^-1mercury stress.The activity of antioxidant enzymes and the content of antioxidant substances of Robinia pseudoacacia in Henan were higher under lower mercury stress(0 and 0.3 mg·kg^-1).The ROS content of Robinia pseudoacacia leaves in Henan is higher than that of Robinia pseudoacacia leaves in Shaanxi,and the antioxidant substances are lower as a whole.After inoculation with rhizobia,the antioxidant capacity of Robinia pseudoacacia increased,which was manifested in the reduction of membrane damage,the increase of antioxidant content and the enhancement of antioxidant enzyme activity.In this study,high and low mercury accumulation robinia pseudoacacia and mercury tolerant and sensitive rhizobia were screened.Later,the physiological indexes of robinia pseudoacacia and rhizobia inoculated under mercury stress were studied.It was found that the Shaanxi provenological robinia pseudoacacia had relatively high tolerance to mercury,and its ability to maintain cell osmotic balance and antioxidant was also relatively strong.Although inoculation with rhizobium could enhance the tolerance of mercury to Shaanxi Robinia pseudoacacia,the accumulation of mercury in the ground part of Shaanxi Robinia pseudoacacia decreased,but was still significantly higher than that of Henan Robinia pseudoacacia.The results indicated that the inoculation of rhizobium could enhance the tolerance type of robinia pseudoacacia,and the Shaanxi provenological robinia pseudoacacia had better adaptability to mercury-contaminated soil,could absorb and enrich more mercury,and had high remediation potential of mercury-contaminated soil,so it could be used as a good phytoremediation material for mercury-contaminated soil control.