Effects Of Different Nitrogen Levels On Growth,nodulation And Nitrogen Fixation Of Medicago Truncatula

In recent years,with the national strategic development of "returning farmland to grass" and "changing grain to feed",the forage feed industry has developed rapidly,and the market is pursuing high quality and high yield forage grass.Among them,alfalfa is very popular because of a variety of production advantages and superior feed nutritional value.The growth and development,nutrient composition and metabolic biological activities of alfalfa are all related to nitrogen,but at present,for the effectiveness of exogenous nitrogen input,studies on plant growth and development,photosynthesis,nutritional characteristics and symbiotic nitrogen fixation are mostly focused on saturated nitrogen stress,but there are few studies on the effects of low nitrogen on alfalfa growth characteristics and nodulation nitrogen fixation.Some studies have reported that under the low nitrogen level,the application of low nitrogen in the early stage will cause the effect of "priming nitrogen",that is,supplying available nitrogen fertilizer at the early growth stage,promoting plant early development and nodule formation,and promoting nitrogen fixation.However,how much concentration of nitrogen application causes "priming nitrogen",how does "priming nitrogen" affect plant growth and nodulation and nitrogen fixation,and what is the relationship between them? It is a blank subject put forward in production practice.In this study,the model plants(Medicago truncatula)A17 and R108 were inoculated with Sinorhizobiummeliloti 1021.The growth of alfalfa under different nitrogen application treatments was observed and recorded with plant growth characters,photosynthetic indexes,nutritional quality and nodulation nitrogen fixation indexes as screening indexes.To explore the effects of exogenous nitrogen at different concentrations of low nitrogen on plant growth,nodulation and nitrogen fixation,in order to provide theoretical basis for water and nitrogen management of alfalfa in legume production system.The following conclusions are drawn:1.The plant height,aboveground and underground biomass,and net Photosynthetic efficiency of Medicago truncatula at the nitrogen level of 5 mmol/L increased with the increase of nitrogen level.After the nitrogen level of 2 mmol/L,plant height,root length,chlorophyll content,aboveground and underground biomass,and photosynthetic characteristics will fluctuate significantly,forming a two-stage growth curve of high before and low after.This indicates that nitrogen may have a significant promoting effect on plant growth and photosynthesis before 2 mmol/L,reaching its peak at 2 mmol/L.2.The nutritional quality of the alfalfa plant of Medicago truncatula was significantly increased within a nitrogen level of 5 mmol/L.As the nitrogen level increased,crude protein,crude ash,calcium,and phosphorus increased,while washing fiber and sugar gradually decreased.Moreover,the content of crude fiber,sugar,crude ash,calcium,and phosphorus fluctuated significantly at a nitrogen concentration of 2 mmol/L,and the rate or curve of change changed significantly.This indicates that nitrogen may have a significant promoting effect on plant nutritional quality before 2 mmol/L,and the nutritional quality of alfalfa can be improved faster at 2 mmol/L nitrogen levels.3.Within a nitrogen level of 5 mmol/L,with the gradual increase of nitrogen level,the number of nodules in Medicago truncatula significantly decreased,the nitrogen content of plants gradually increased,and the nitrogenase activity showed an overall significant upward trend of decrease after 2 mmol/L.This result indicates that within the range of 5 mmol/L,nitrogen leads to an increase in nitrogen fixation efficiency and a decrease in nodulation ability of alfalfa.Before 2 mmol/L,it will have a "explosive nitrogen" promoting effect on plant nitrogen fixation ability,greatly stimulating nitrogenase activity.In summary,low nitrogen will promote the growth and photosynthetic capacity of Medicago truncatula,improve the nutritional quality of Medicago truncatula,promote plant nitrogen fixation ability,and enhance plant production performance.Moreover,at 2 mmol/L,it will have the maximum "explosive nitrogen" promoting effect on alfalfa,which can be used for reference in field management to promote accelerated growth of alfalfa and rational utilization of water and fertilizer resources.