Assefa Fassil (PhD)Temesgen Diriba2018-07-132023-11-082018-07-132023-11-082017-06http://etd.aau.edu.et/handle/123456789/8520Soybean [Glycine max (L) Merr.] is a nutritious crop used as food, feed and a raw material for manufacturing various products. Soybean improves soil fertility due to its association with symbiotic bacterial groups known as Bradyrhizobium, Rhizobium/Sinorhizobium and Agrobacterium species. It is also associated with diverse plant growth promoting rhizobacteria (PGPR) that enhance its health, growth and productivity. Soybean is widely grown in the lowlands regions of Ethiopia with average yield of about 2.0 tons ha-1 compared to 2.70 tons ha-1 of world average. The low yield of soybean in the country is predominantly attributed to low soil fertility associated with the absence of effective indigenous rhizobia that nodulate and fix enough nitrogen to the host. Attempts to inoculate the crop with exotic rhizobia showed inconsistent and unsatisfactory results that necessitated the search for effective local rhizobia adapted to ecological conditions of the country. To this end, 140 soil samples were collected from various sites of Ethiopia to screen for symbiotically effective soybean rhizobia, and plant growth promoting rhizobacteria (PGPR). The rhzobial isolates were trapped, authenticated and tested for their symbiotic effectiveness using three soybean varieties (Cheri, Ethio-Yugoslavia and Jalele) under greenhouse conditions. The PGPR were screened in vitro for their multiple plant growth promoting traits and potential ecological adaptations. The diversity of the selected rhizobia and PGPR was studied using their phenotypic (numerical taxonomy), and genotypic characters via sequence analysis of 16S rRNA (and some other genes of rhizobia). The most effective rhizobia and the most versatile PGP Achromobacter were inoculated on a soybean cultivar (Jalele) to evaluate their effect on nodulation, growth and yield of the crop against a standard soybean inoculant Bradyrhizobium japonicum SBTAL379 under field conditions. The result showed that only 18 soil samples (13%) induced nodulation on the host variety from which 21 bacterial isolates were authenticated as soybean rhizobia. The isolates were equally distributed into fast growing (11) and slow growing (10), and grouped under the genus Rhizobium and Bradyrhizobium, respectively as classified previously. Based on genetic characters, a fast growing isolate (SNB 41) was identified as Rhizobium/Agrobacterium sp. whereas three slow growing isolates (SNB57B, SNB70 and SNB120A) were identified as Bradyrhizobium spp. Likewise, the representative PGPR isolates were also classified into seven genera; six under Proteobacteria (Gram negative): Achromobacter, Acinetobacter, Enterobacter, Microbacterium, Pseudomonas and Stenotrophomonas; and one under the Firmicutes (Gram positive): Bacillus. The isolates under the genera Pseudomonas and Stenotrophomonas were the most diverse group among the PGPR. With regard to their plausible ecological adaptations tested under in vitro, the fast growing soybean rhizobia were more tolerant to pesticides, higher temperature and higher NaCl concentrations and more versatile to utilize different carbon and nitrogen sources than the slow growing isolates which were better in their inherent antibiotic resistance (IAR). The majority of the rhizobacteria were grown at 40oC, 4% NaCl and showed multiple antibiotic and heavy metal resistance. Some of the soybean rhizobia and rhizobacteria also demonstrated multiple PGP traits (2 to 9). The data also showed the overall better performance of gram negative rhizobacteria and fast growing rhizobia in terms of the number of PGP traits and tolerated stresses. The nodulation and symbiotic effectiveness tests of the rhizobia showed that SNB57B, SNB120A, SNB120C, SNB125A, SNB125B and SNB140 nodulated all the three soybean varieties with prolific nodulation (54-173 nodules plant-1; 1.76-2.33 mg of nodule dry weight plant-1) and shoot dry weight (1.10-2.27 g plant-1) showing highly effective symbiosis (80-100%) in relation to the nitrogen-fertilized control plants under greenhouse experiment. The isolates showed similar pattern of relatively high nodulation parameters and symbiotic performance on Jalele and Cheri varieties compared to the Ethio-Yugoslavia variety. The findings also showed co-inoculation of rhizobia and the PGP Achromobacterium significantly increased more growth and yield parameters of soybean at Dembi station of Debrezeit Agricultural Research Center (DDARC) field site with low population of indigenous soybean rhizobia and where maximum nodule number (168 plant-1) and dry matter (1.96 g plant-1), shoot dry matter (25 g plant-1) and total nitrogen (4 %), number of pods (114 plant-1) and seeds (214 plant-1) and grain yield (4.01 tons ha-1) were recorded. There were highly significant (p≤0.05) effects of the rhizobial isolates on most growth, nodulation and yield parameters. Indigenous soybean rhizobia performed much better than the exotic Bradyrhizobium japonicum SBTAL379 and control treatments under greenhouse and field conditions so that they can be further validated and recommended as inoculant (together with the PGP bacterium) to improve growth and productivity of the crop in the country. Key words: soybean, rhizobia, PGPR, diversity, nodulation, yieldenSoybeanRhizobiaPGPRDiversityNodulationYieldThesis