ABSTRACT
Global crop and energy production are fast dwindling inversely to population growth. Common bean (Phaseolus vulgaris L.) yield has reduced worldwide due to soil infertility which can be reversed through application of chemical fertilizers. The chemical fertilizers used to ameliorate nitrogen, phoshorus and potassium are expensive and cause both deleterious physico‒chemical modification of soil and water mass eutrophication. The common bean has ability to fix atmospheric nitrogen symbiotically with rhizobia but only a few effective strains have been recovered from African soils, most of them ineffective under field conditions. Prospecting from local pool of strains trapped by wild native species like sesbania can increase the number of elite inoculant production strains for both species in agroforestry systems practiced under diverse soil and eco-climatic conditions. The specific objectives of this study were: to assess the phenotypic and genotypic characteristics of rhizobia from root nodules of East Africa and Namibia Sesbania spp. (here after referred to as sesbania) using morpho-cultural characteristics and PCR‒RFLP methods; to assess the nitrogen fixation potential of sesbania isolates on S. sesban using growth parameters; and to determine the infectiveness and symbiotic effectiveness of sesbania rhizobia on common beans. Experiments were carried out at the Kenya Forestry Research Institute, Muguga‒Nairobi, Kenya. Morpho-cultural techniques were used to characterize and cluster 128 presumptive sesbania rhizobia collected from Kenya, Uganda, Tanzania and Namibia. The diverse growth characteristics of rhizobia on YEMA media, intrinsic antibiotic resistance and salt tolerance were used to select for 79 sesbania isolates that were later subjected to fingerprinting assays using PCR‒RFLP of the 16S rDNA in comparison with 17 reference strains. The presumptive sesbania rhizobial isolates were used to inoculate S. sesban and common bean under glasshouse controlled conditions to test for their infectiveness and symbiotic effectiveness. Reference inoculants strains KFR647 and BA37 for S. sesban and bean respectively, were included in the test. Uninoculated positive control (70 ppm N as KNO3) and a negative control (0 ppm N) were included for strain effectiveness comparison and to check for contamination. The sesbania rhizobia were grouped into nine morphotypes and various ribotypes per site. The rhizobia varied in their infectiveness and symbiotic effectiveness on S. sesban and the common bean resulting in three categories viz: (1) highly effective (2) effective and (3) ineffective. The mean shoot dry weight, nodule number and nodule dry weight were all significantly different (p < 0.001). The shoot N content range was 0.16‒5.66 mg plant-1 and 0.34‒3.08 mg plant-1 for S. sesban and common beans at 8 and 4 weeks of growth respectively. Based on shoot dry weight due to inoculation, rhizobial isolate KFR402 was preferred as a common inoculant production strain for both common beans and S. sesban. However, data in the present study shows that the highest shoot dry weight was obtained with strain MASS133 (S. sesban) inoculated on Rose coco bean variety (0.87 g plant-1 ) and MASS172 (S. sesban) on S. sesban (1.06 g plant-1 ). Rhizobia recovered from sesbania grown in East Africa and Namibia are phenotypically and genetically diverse. The isolates exhibit great variations in effectiveness and nitrogen fixation efficiency on S. sesban and common beans (variety: Rose coco). Prospecting for elite rhizobia inoculant strains should be prioritized and tested for effectiveness on both S. sesban and common bean grown in diverse edaphic and agro-ecological conditions under agroforestry systems.