| Nitrogen, the crucial element for the survival of the biology, only can be utilized by the propagation in the compound form instead of molecular form. However,some microorganism can use the nitrogen directly which account for 79% of the atmosphere, called biological nitrogen fixation, which means the deoxidize procession from nitrogen into N2 with the nitrogenase of the microorganism. Many countries are doing a great deal of research work on that not only because of the important role in the nitrogen circulation among nature but also the potential application value. Early study object of the biological nitrogen fixation bacterium is the Rhizobium which related to the nitrogen fixation of the Leguminosae. In 1970'D?bereiner found the associative nitrogen-fixing bacterium in the gramineous plants and improved the potential ability to fix nitrogen of that kind of plant. As the strict host specification, Rhizobium can only operate in the gramineous plants. Thus, taking full use of the nitrogen-fixing potential of the gramineous plants is the main route to explore new-type fertilizer containing nitrogen fixation bacterium and assure the sustainable development of agriculture. In this study, we collected samples of the wheat handled with techniques of enrichment, isolation and purification et al. Five strains with higher nitrogenase activity were isolated and primary work about taxonomic status,configuration and physiological characteristic has been done. At the same time, strain H08 with highest nitrogenase activity was mutagenized by ultraviolet light. Then the positive mutant strains were selected to make farther inoculation effect test. The result is as follow. Screening and identification: Thirty-one strains were isolated from wheat rhizoesphere samples collect with nitrogen deficiency culture medium and microorganism purification culture technique. Then 5 strains were screened according to the higher nitrogenase activity in the liquid culture state. Strain H08 had the highest nitrogenase activity. The 72hour ethylene production is 100.765nmol/ml·min ,higher than others ,the same as the bacteria concentration. This strain came into logarithmic phase after 15h culture in the improved D?bereiner nitrogen deficiency liquid culture medium, continuing growth about 60h in the Plateau phase, then came into decline phase at 72h. All the 5 strains belong to the Gram-negative bacterium, short bacilliform or oval, the average diameter of the colonies is about 4.0mm ,the diameter of the bacterium less than0.5μm oxidase negative, catalase positive , with capsule , no spore, lack of the mobile capability. According to the analysis of the physiological, chemical characteristic and the description of the Bergey's Manual of Determinative Bacteriology, which was preliminary identified as Beijerinckia.sp. The mutagenesis through the ultraviolet light and mutant screening: Strain H08 irradiated with 15W ultraviolet light, 20cm away, different livability were detected according to the irradiation time. Then the best irradiation time was selected. Selected the survival strains which colony diameter were bigger than the average level and then the nitrogenase activity were detected with gas chromatography. Result: After 20-60seconds irradiation , the livability began to decline, so the best time was 60s.Six strains were selected from the survival colonies and inoculated in the improved D?bereiner nitrogen deficiency liquid culture medium .Result: After 72hours culture ,three mutant strains of the 6 ones had different boost degree of the nitrogenase activity than before. Strain H086 was the dominant one which was 138.726 nmol/ml·min, he same as the concentration. Inoculation efficacy of Strain H086:Wheat and corn were inoculated with Strain H086 respectively and the results show that both Strain H08 and Strain H086 improved the growth of the plants.
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