| dc.description.abstract | Soybeans [Glycine max Merr.] and bambara groundnuts (Vigna subterranea L.Verdc) are nutritionally important legume crops capable of forming nitrogen (N) fixing symbiosis with a distinct group of soil bacteria, generally known as rhizobium. Their seeds are rich in essential • o acids and can improve human nutrition in low resourced regions such as Lake Victoria basin in Western Kenya. Despite their potential benefits of increased soil fertility and improved food security, the crops are currently under-cultivated. In addition, there is limited information on the genetic diversity and symbiotic efficiency of root nodule bacteria which associate with the two legumes in the region. The objectives of this study were to genetically characterize rhizobium bacteria which freely nodulate with soybeans and bambara groundnuts in four soil types of Lake Victoria basin and to examine their influence in nodulation and overall crop productivity. Soil samples were collected from farmers' fields with no history of artificial inoculation at Port Victoria, arungu, Kisumu and Kendu bay and analyzed for chemical status. The soils were used to examine gronomic performance of two soybean varieties ('specific' Safari and 'promiscuous' SB19) and two bambara groundnut landraces (BUSB and KAKC) and also to trap resident rhizobia. Pure isolates were morphologically characterized on Yeast Extract Mannitol Agar (YEMA) with Congo red or Bromothymol blue. Sixty four bambara groundnut and forty two soybean isolates were evaluated using 16S rRNA, nifH, nodC, recA and atpD gene markers. Glasshouse inoculation experiments were conducted using eighteen isolates from bambara groundnuts, sixteen isolates from soybeans and two reference strains;Bradyrhizobium japonicum USDAl 10 and Bradyrhizobium sp. KFR259 to compare • nodulation status and symbiotic efficiency. Field izoculation experiments with five isolates from both legumes, one reference strain USDAl10 and un-inoculated +N (with inorganic N fertilizer) and -N (without inorganic N fertilizer) controls were conducted at KALRO Oyani and KALRO Kibos to determine their effect on nodulation and plant growth, Glasshouse experiments were arranged in Randomized Complete Block Design (RCBD) with four replicates while field experiments were arranged in Split plot design with RCBD. Data collected included days to germination, days to first flower, number of nodules, root and shoot dry weights and total plant biomass. Agronomic data was subjected to Analysis of Variance (ANOVA) where significant, means were separated by LSDs%- Soils majorly varied in pH (4.10 - 6.3), available phosphorus (5.0 - 23.0 meq), exchangeable cations and these variables influenced plant agronomic performance. Soils from Karungu and Port Victoria cultivated with landrace BUSB and variety SB 19 resulted in faster emergence, greater number of nodules and better plant growth. There was marked heterogeneity in the morphological and 16S rRNA gene analysis of rhizobial • isolates from bambara groundnut which were identified asRhizobium • sp., Agrobacterium sp., Burkholderia sp. and Bradyrhizobium sp. at above 97% sequence identity values. Soybean Variety B 19 had a wider diversity of symbiotic partners including Bradyrhizobium sp., Rhizobium sp. and Agrobacterium sp., unlike Variety Safari which was more specific to Bradyrhizobium sp. Soil chemical factors, particularly soil pH, available P and sodium (Na) had a significant impact on isolate distribution across the study sites. Inoculation experiments showed that soybeans and ambara groundnuts have greater symbiotic preference toBradyrhizobium isolates. Lateral transfer of Sym plasmids may have conferred nodulation and N2-fixation ability to one Agrobacterium sp. AMK.bay 1 ). Two Burkholderia strains were remarkably symbiotically effective on bambara groundnuts. This study has increased understanding the diversity of rhizobial associates of soybeans and bambara groundnuts. Expectedly, the findings herein provide opportunities for selection of effective symbionts of soybeans and bambara groundnuts for inoculum production that be used to improve soil fertility, better crop yields and alleviate food insecurity in the region. | en_US |