Effects Of Endophytic Nitrogen Fixing Bacteria A02 On Cassava Growth And Nitrogen Absorption And Utilization

Cassava has become an important resource in China’s biomass energy industry.Guangxi Zhuang Autonomous Region ranks first in the cassava planting area and fresh tuber yield in the country.Cassava has a long growth cycle and needs a large amount of nitrogen fertilizer to provide nutrition for its growth and development.In order to realize the environmental goal of"clear waters and lush mountains",it is an effective way to strengthen biological nitrogen fixation to reduce nitrogen fertilizer application and pollution.Therefore,it is of great significance to study the biological nitrogen fixation for cassava production.The research group isolated and identified an endophytic nitrogen-fixing bacterium A02 from the main cassava varieties in Guangxi,and identified the bacterium as Curtobacterium sp.,but the absorption law of cassava nitrogen（N）by endophytic nitrogen-fixing bacteria infected by the bacterium has not been clearly defined so far.In this study,pot experiments were conducted to compare the effects of A02 strain on the growth of cassava seedlings,and the ¹⁵N isotope dilution method was used to determine the percentage and amount of biological nitrogen fixation.The effect of A02 strain on nitrogen metabolism in cassava seedlings was preliminarily analyzed in order to explore the physiological basis of A02 strain promoting cassava growth from the perspective of nitrogen absorption and transformation and to provide a reference for nitrogen reduction and synergistic production of cassava.The main results are as follows:（1）Inoculation of A02 strain can effectively promote the growth of cassava seedlings,especially the growth of the root system.The plant height increased by 15.24%compared with the control group（low nitrogen treatment）,and the dry weight of leaves and roots increased significantly by 25.29%and 47.4%respectively compared with the control group（low nitrogen treatment）.（2）Inoculation of A02 strain can increase the nitrogen content and nitrogen accumulation of cassava seedlings.The nitrogen content of leaves,stems,and roots of cassava seedlings increases by 21.4%,58.7%,and 22.45%respectively compared with the control group（low nitrogen treatment）,and the nitrogen accumulation of leaves,stems,and roots increases by 75.03%,124.38%,and62.5%respectively compared with the control group（low nitrogen treatment）.The highest nitrogenase activity is 120.3nmol/（m L·h）in leaves;The percentage of nitrogen fixation in leaves,stems and roots of cassava seedlings is 13.44%,12.44%,and 3.98%,respectively.Leaves are the main parts for colonization and nitrogen fixation of the A02 strain.The biological nitrogen fixation of the whole cassava plant reaches 120.13 mg/plant;The proportion of nitrogen accumulation from air,fertilizer and soil is 29.86%,12.73%and 57.41%respectively.（3）Inoculation of A02 strain promotes the accumulation of NO₃^--N and NH₄⁺-N contents in cassava seedlings.The NO₃^--N contents in leaves and roots increase by 28.87%and 30.76%compared with the control group（low nitrogen treatment）,and the NH₄⁺-N contents in leaves increase by 29.22%compared with the control group（low nitrogen treatment）.There is a significant correlation between NH₄⁺-N and NO₃^--N in roots,which may be related to the assimilation of NO₃^--N in roots.There is no significant correlation between NH₄⁺-N and NO₃^--N levels in leaves,which may be due to NH₃produced by biological nitrogen fixation in leaves breaking the balance between NH₄⁺-N and NO₃^--N.（4）There are two pathways of ammonia assimilation in cassava,namely GS/GOGAT pathway,and GDH pathway.NH₄⁺generates Glu through these two pathways.The results of correlation analysis of NO₃^--N,NH₄⁺-N contents and activities of four key nitrogen assimilation enzymes show that NH₄⁺-N contents in leaves of cassava seedlings are significantly positively correlated with GS and GOGAT activities,indicating that NH₄⁺-N produced by NR assimilation in leaves and NH₄⁺-N produced by biological nitrogen fixation are mainly assimilated into glutamic acid through GS/GOGAT cycle.The NO₃^--N level in the root of cassava seedlings is significantly positively correlated with NR and GDH,indicating that NO₃^--N absorbed by the root is reduced to ammonia through NR and further assimilated to Glu through GDH.Inoculation of A02 strain can still maintain high NR,GS,GOGAT,and GDH enzyme activities in cassava seedlings under low nitrogen,thus promoting nitrogen transformation and utilization.