Abundance, Genetic Diversity And Symbiotic Efficiency Of Cowpea (Vigna Unguiculata L.) Rhizobia In Soils Of South Western Kenya

Abstract

Cowpea is an important food crop in Kenya, but its production is limited by biotic and abiotic constraints. One of the most important abiotic constraints to crop production in South Western Kenya (SWK) is soil acidity, which may limit symbiotic efficiency of indigenous strains of rhizobia and also availability of nutrient elements such as phosphorous. In order to understand the influence of soil ecological conditions on symbiotic performance of cowpea rhizobia, a study was done in soils of SWK and selected reference regions with the following objectives: 1) to characterize the genetic diversity of native cowpea nodulating rhizobia; 2) to determine the abundance and symbiotic efficiency of native cowpea rhizobia; 3) to determine the effects of rhizobia inoculation on nodulation, growth, yield, nitrogen fixation and nodule occupancy of cowpea at two sites in SWK; 4) to determine the effects of phosphatic fertilizer and liming on symbiotic efficiency of native cowpea rhizobia under acid conditions at two sites in SWK. Genetic diversity of cowpea rhizobia was determined through sequence analyses of 16S rRNA and recA genes (objective 1). Abundance of rhizobia was determined through the most probable number (MPN) plant infection technique in germination pouches, symbiotic efficiency of rhizobia was determined through pot experiments in greenhouse using soil samples from seven agro-ecological zones in SWK and two reference regions (Machakos and Kilifi) (objective 2). In objective 3, a field experiment was conducted where four cowpea varieties (KVU 27-1, K80, M66 and Ngor- a landrace) were each inoculated with Bradyrhizobium sp. strain USDA 3456 and subjected to three N fertilizer levels (0 kg N ha-1, 20 kg N ha-1 and 40 kg N ha-1), using randomized complete block design in a 4x4 factorial arrangement. In objective 4, cowpea varieties (KVU 27-1, M66 and Ngor) were each treated with lime (4 t CaO ha-1 or 0 t CaO ha-1) and subjected to three P fertilizer levels (0 kg P ha-1, xvii 25 kg P ha-1 and 50 kg P ha-1) in a randomized complete block design with a 2 x 3 x 3 factorial arrangement. Data collected included: species diversity of cowpea rhizobia and endophytic bacteria, rhizobial population in soils, nodule numbers and dry weights, leaf area index, shoot dry weight, N-fixed, tissue concentration and uptake of N and P, tissue protein content and grain yield. There is wide genetic diversity of native cowpea rhizobia and endophytic plant growth promoting bacteria in the study area. Cowpea is predominantly nodulated by rhizobial species in the genus Rhizobium in soils of the geographic regions covered by this study. Abundance of rhizobia ranged from 0 – 1.0 x 105 cells g-1 soil, higher rhizobial population was recorded in soils with pH close to 7. Soils that had higher levels of organic carbon, total N and exchangeable aluminium were characterized by lower abundance of rhizobial cells. Rhizobial inoculation had no significant (P≤.05) effects on nodulation, N-fixed, growth and grain yield of cowpea in two acidic soils of SWK. Two nodule endophytes (Bacillus megaterium and Bacillus aryabhattai) were the main cowpea nodule occupants in the two soils of SWK. Lime application was not beneficial to cowpea plants at moderately acidic soils (pH of 5.6). Phosphorous fertilizer enhanced nodulation, growth and cowpea N uptake in acidic soils of south western Kenya. In conclusion, there is wide genetic diversity of symbiotic and endophytic bacteria in the study regions, which may be utilised in future as bio-inoculants to promote cowpea production. Low pH, high levels of N and Al3+ in soils depress abundance of rhizobial cells. Cowpea plants do not respond to rhizobial inoculation in acidic soils of SWK, but phosphorous enhance symbiotic efficiency of cowpea in these soils. Key words: Cowpea, diversity, nitrogen, phosphorous, rhizobia.