Morphological and Genetic Diversity of Rhizobia Nodulating Cowpea (Vigna unguiculataL.) from Agricultural Soils of Lower Eastern Kenya

Limited nitrogen (N) content in the soil is a major challenge to sustainable and high crop production in many developing countries. The nitrogen fixing symbiosis of legumes with rhizobia plays an important role in supplying sufficient N for legumes and subsequent nonleguminous crops. To identify rhizobia strains which are suitable for bioinoculant production, characterization of rhizobia is a prerequisite. The objective of this study was to assess the morphological and genetic diversity of rhizobia that nodulates cowpea in agricultural soils of lower eastern Kenya. Twenty-eight rhizobia isolates were recovered from soil samples collected from farmers’ fields in Machakos, Makueni, and Kitui counties in lower eastern Kenya and characterized based on morphological characteristics. Thirteen representative isolates were selected and characterized using BOX repetitive element PCR fingerprinting. Based on the dendrogram generated from morphological characteristics, the test isolates were distributed into two major clusters at a similarity of 75%. Phylogenetic tree, based on BOX repetitive element PCR, grouped the isolates into two clusters at 90% similarity level. The clustering of the isolates did not show a relationship to the origin of soil samples, although the isolates were genetically diverse. This study is a prerequisite to the selection of suitable cowpea rhizobia to develop bioinoculants for sustainable crop production in Kenya.

FUNCTIONING OF SYMBIOTIC SYSTEMS OF COWPEA – NODULE BACTERIA

The nodule bacteria were isolated from the nodules of cowpea. On the basis of phenotypical properties these rhizobia were referred to slow growing bacteria of Bradyrhizobium genus. Interaction features of cowpea with the nodule bacteria of cowpea (Bradyrhizobium sp. (Vigna)) and soybean (Bradyrhizobium japonicum) on nitrogen-free substrate and in soil culture were studied. It was established that the cowpea rhizobia strains possess high specificity to the host plant, promote symbiotic nitrogen fixation activity in 1,8-2,6 times and increase plants aboveground mass yield in 1,4-3,4 times, in comparison with control. The significant positive influence of the active soybean microsymbiont Bradyrhizobium japonicum 46 on the growth and development of cowpea was shown.