Nodulation Potential and Phenotypic Diversity of Rhizobia Nodulating Cowpea Isolated from Major Growing Areas of Ethiopia

Rhizobia are specific to a particular legume and need to be characterized by different methods. In Ethiopia, very few efforts have been made to evaluate the phenotypic characteristics and diversity of cowpea nodulating rhizobia. This study aimed to assess the nodulation potential and phenotypic diversity of rhizobia nodulating cowpea isolated from major growing areas of Ethiopia. Thirty soil samples were collected from different cowpea growing areas of Ethiopia of which 28 (93.3%) samples induced nodulation in the greenhouse. A total of 28 isolates obtained from nodulated plants were evaluated using presumptive tests and subjected to different phenotypic characteristics. On YEMA containing CR, the isolates produced colony colors that were whitish to pale pink. All isolates showed no growth on peptone glucose agar and the isolates were capable to grow on ketolactose medium but did not turn the media color to yellow. All the isolates were also confirmed to be gram-negative, rod-shaped, and non-spore-forming. The colony texture study revealed that 78.57% of the isolates had large mucoid colony whereas 21.43% of the isolates exhibited large watery colony texture with diameters ranging between 1 and 5mm. Depending on the colony color on YEMA containing bromothymol blue, 22 (78.57%) isolates were slow-growing whereas 6 (21.43%) isolates were fast-growing rhizobia. The cluster analysis based on phenotypic characteristics revealed a grouping of the isolates into two clusters at a similarity level of 75% and six further sub-clusters at a similarity level of about 91% indicating that there are variations and diversity among the isolates. Overall, this study revealed great variations among the cowpea nodulating rhizobial isolates and suggests that the soils of Ethiopia harbor populations of phenotypically diverse isolates. A further test to confirm these findings using genetic characterization and inclusion of reference strain is highly recommended.