PHENOTYPIC DIVERSITY, SYMBIOTIC EFFECTIVENESS AND NATIVE POPULATION OF RHIZOBIA NODULATING COWPEA (Vigna unguiculata (L.) Walp.) ISOLATED FROM MAJOR GROWING AREAS OF ETHIOPIA

Abstract

Investigating the native rhizobia nodulating legumes play an important role in understanding the infectiveness and effectiveness of the background rhizobia and development of effective rhizobia to boost the productivity of a particular legume. This study was initiated to investigate the phenotypic diversity, symbiotic effectiveness and native population of rhizobia nodulating cowpea isolated from major growing areas of Ethiopia. A total of 28 isolates were isolated from root nodules of cowpea and characterized phenotypically for colony texture, transparency, color, diameter, mucus production and tested for different eco-physiological and physiological characteristics. All isolates were presumptively tested and verified to be root nodule bacteria, gram negative, rod shaped and non-spore forming. The colony morphology on YEMA medium revealed that 78.57% of the isolates had large mucoid (LM) colony texture whereas 21.43% of the isolates exhibited large watery (LW) texture. Based on the color around the colony on YEMA-BTB, 22 (78.57%) isolates were slow growers whereas 6 (21.43%) isolates were fast growers. It was also found that all tested isolates grew within temperature range between 200C and 350C, the pH values between 6.0-8.5 and NaCl concentration level ranging from 0.1-1%. The result showed that all isolates grew well on medium supplemented with different carbon and nitrogen sources. The numerical analysis based on phenotypic, eco-physiological and physiological characteristics revealed five major clusters at 90% similarity level showing a great diversity and did not correlate with their geographical origin. The infectiveness and effectiveness tests on sand culture under greenhouse condition showed that all isolates were able to nodulate cowpea Bole variety and inoculation of isolates resulted in highly significant (P<0.0001) effect on nodule number, nodule dry weight and shoot dry weight per plant. Based on the relative shoot dry matter accumulation of inoculated plants with nitrogen-fertilized control, 10 isolates (35.72%) were highly effective, 16 isolates (57.14%) were effective and two isolates (7.14%) were poorly effective. Among all isolates, HUCR-3, HUCR-5, HUCR-7, HUCR-11, HUCR-15 and HUCR-25 showed better nutrient utilization, ecophysiological tolerances and high symbiotic effectiveness which make them the candidate isolates to be used for further study and inoculant preparation. The enumeration of native rhizobia revealed that the size of rhizobia in the study area varied ranging between 3.1x104 and 1.0x107 rhizobia cells g -1 of soil. The result generally showed that there is wide range of phenotypic diversity of rhizobia nodulating cowpea in Ethiopia soils. The study areas harbored high number of rhizobia nodulating cowpea which are symbiotically effective under greenhouse condition. Further tests to confirm these findings using genetic characterization, field conditions experiment, an inclusion of reference strains and using different cowpea genotypes are still needed and recommended.