Effects Of Salt Stress On The Longhuang Variety Soybean-rhizobia Symbiosis System And Its Microbial Community

The problem of soil salinization has seriously restricted the development of agriculture,especially in the northwest region of the main soybean planting areas in our country,where the area of soil salinization accounts for more than 10%of the arable land area.Therefore,in order to improve the yield and quality of soybeans in the northwestern region,it is necessary to breed new varieties of salt-tolerant soybeans.Longhuang 1,2 and3 cultivated by Gansu Academy of Agricultural Sciences are new soybean varieties suitable for planting in Northwest China.Their physical and chemical characteristics such as stress resistance,stable yield and wide adaptability have been affirmed in extensive experiments.However,its mechanism of symbiosis with rhizobia in nitrogen fixation and its differential mechanism in response to salt stress need to be further studied.Therefore,we used Zhonghuang 13 as a control,through the isolation and identification of rhizobia,the screening of salt-tolerant varieties after rhizobia back-infection,the selected salt-tolerant Longhuang soybean were then planted in substrates and in situ soil pots to study the growth phenotype,symbiotic nitrogen fixation,antioxidant defense,soil physicochemical and microbial diversity of salt-tolerant Longhuang soybeans.To reveal the physiological response of the Longhuang soybean-rhizobia symbiotic system under salt stress and its effect on the microbial community.The main results are as follows:(1)Through the isolation and sequencing of the strains,we found that the rhizobia in the soybean nodules of the three Longhuang varieties were the same Sinorhizobium fredii strain with certain salt tolerance.(2)By measuring the plant height,dry weight and photosynthetic indexes of Zhonghuang 13,Longhuang1,2 and 3 soybean under salt stress,then the membership function value of the salt tolerance index of each phenotype was calculated by the method of fuzzy mathematical membership function,and the most salt-resistant soybean variety was selected Longhuang 3.(3)Longhuang 3 and Zhonghuang 13 were planted in the substrate.Through the observation of their growth phenotypes,symbiotic nitrogen fixation ability and antioxidant properties at different growth stages,it was found that inoculation with rhizobia could significantly alleviate the two varieties’sensitivity to high-salt environment.Compared with Zhonghuang 13,Longhuang 3 can respond positively to salt stress in the P1 period and promote the synthesis of early leghemoglobin,thereby accelerating the growth of nodules,expanding the nitrogen fixation area,and improving the nitrogen fixation of nodules.There was no significant difference between the two breeds in the P2 and P3periods.In addition,the symbiosis of the salt-tolerant variety Longhuang 3 with soybean rhizobia can specifically improve the antioxidant defense ability by inhibiting lipid membrane peroxidation,reducing the production of malondialdehyde(MDA),and inoculating rhizobia can improve the activity of antioxidant enzymes,but with Zhonghuang13 There was no significant difference in antioxidant enzyme activity between the two groups.(4)The in situ soil planting experiment found that the specific difference in salt tolerance between Longhuang 3 and Zhonghuang 13 was mainly manifested in the flowering period.The distribution of N and P nutrients required for vegetative growth and reproductive growth of the two varieties was different.Longhuang 3 accumulates a large amount of Na~+in the root system,prevents the transport of Na~+to the shoot,and maintains the Na~+/K~+balance in the root system by absorbing a large amount of K~+,thereby improving the salt tolerance.Longhuang 3 in P1 period under salt stress can promote the availability of soil phosphorus by increasing the activity of alkaline phosphatase and the content of microbial phosphorus.After inoculation with rhizobia,there were bacterial groups significantly related to soil water-soluble K~+and available phosphorus(Olsen-p)at the P1 stage,namely Ellin6067,Gemmatimonas,MND1,Ideonella,Azotobacter,Noviherbaspirillum,Ram Libacter,Gaiella,and Arthrobacter.Therefore,it is speculated that the Longhuang 3-rhizobia symbiotic system can improve the availability of nutrients such as K and P in the salt-stressed soil by synergizing with the rhizosphere growth-promoting bacteria,thereby improving its tolerance to salt.In summary,after inoculation with rhizobia,Longhuang 3 soybean cultivar can enhance the symbiotic nitrogen fixation ability of root nodules,inhibit the upward transport of Na~+in the root system,reduce lipid film peroxidation,enhance the availability of soil phosphorus,and enhance the composition of soil microbial community.The availability of soil nutrients in turn promotes the salt tolerance of plants and ensures the growth and yield of Longhuang 3 under salt stress.This study comprehensively revealed the salt-tolerant mechanism of the symbiosis between the salt-tolerant variety Longhuang 3 and rhizobia from the perspectives of plant physiological response,soybean-rhizobia symbiotic interaction,rhizosphere and soil microecology.It provides more theoretical basis and scientific basis for the further large-scale promotion and application of Longhuang 3soybean in the northwest region with serious soil salinization.