Phenotyping Groundnut Rhizobia Native to Phosphorus Deficient Soils in Western Kenya

Abstract

Most soils in western Kenya are characterized by high acidity level and phosphorus (P) deficiency, which affect nodulation and nitrogen fixation of groundnut (Arachis hypogaea L.). This study aimed at characterising rhizobia capable of nodulating groundnut in P deficient soils in Western Kenya. Sixty four isolates out of the 68 were confirmed to be rhizobia due to their ability to nodulate groundnut. Ninety six percent of the isolates exhibited semi-globose to globose colony shape on yeast extract mannitol agar (YEMA). Groundnut was nodulated by both fast and slow growing rhizobia isolates with 81% being fast growers. Fifty one isolates representing 75% produced acid on YEMA medium supplemented with bromothymol blue (BTB). The isolates varied in their response to pH with 39 and 61 growing at pH 4.0 and 5.5, respectively. All the isolates grew at pH 7.0 and 8.5. YEMA medium containing glucose, sucrose, starch and citrate had 64, 61, 56 and 5 isolates growing, respectively. Sixty four isolates exhibited clear zone of solubilization on medium containing dicalcium phosphate as source of inorganic phosphate. Solubilization index (SI) varied from 1.1 to 6.8. Fast-growing rhizobia isolates N01, B02, I06, Q01, F05, C02, E01, Q03, I01 and B01 recorded the highest solubilization index of 3.8, 4.5, 4.6, 4.6, 4.7, 5.0, 5.1, 6.1, 6.1 and 6.8, respectively. Groundnut rhizobia showed variation in their potential to solubilize inorganic phosphate and effectively nodulate the host. The most promising isolates from this study would be used as bio-fertilizer upon further validation in the greenhouse and field.