Symbiotic and phenotypic diversity of Rhizobium leguminosarum bv. viciae from Northern Gondar, Ethiopia

Abstract

Rhizobia that nodulate cool season legumes are widely spread in the Mediterranean and highland agro ecosystems. Faba bean is one of these important crops that represent the major protein source of human nutrition, cash crop for farmers and component of the farming systems. This study is aimed at identifying rhizobia isolates capable of guarantying efficient nitrogen derived from biological nitrogen fixation. Due to this, a total of twenty-one isolates of Rhizobium were isolated from as many sampling sites of northern Gondar using plant infection method. The isolates were characterized morphologically and physiologically and tested on sand and soils to evaluate their symbiotic effectiveness. Studies on symbiotic effectiveness on sand culture indicated that, the strains showed shoot dry matter ranging from 0.4 (AUFR127) to 2.3 g/plant (AUFR124), with negative control (0.2 g/plant) and positive control of 2.4 g/plant. Eighty percent of the isolates were found to be effective and have very effective nitrogen fixers based on their shoot dry matter accumulation (50 to 100%) in relation to the nitrogen fertilized control plants. Culturally, almost all of them displayed large colonies with diameters of 2 to 4.5 mm, generation time of 1.9 to 4.3 h and showed characteristics of fast growing rhizobia. With a few exceptions, isolates grew at temperatures of 15 and 35°C and were found to be sensitive to salt, except AUFR118 that grew up to 5% NaCl. The isolates also grew on a wide range of moderate acidity and alkalinity (pH5.5-9).With the exception of gluconate, citrate and tartarate, almost all isolates grew on all carbohydrates. The pattern of intrinsic antibiotic resistance showed that, almost all of them were tolerant to chloramphenicol, nalidixic acid and erythromycin. Isolates AUFR118, AUFR128 and AUFR132 were found to be resistant to almost all tested antibiotics. These isolates were also included in the effective and very effective symbiotic groups. The trial of the 5 selected effective isolates on two soil types showed that, they performed well without any significant difference on their respective soils. Interestingly, the isolates were found to nodulate the host on the highly acidic soil (pH 4.8) that failed to be nodulated by the indigenous rhizobia. Generally, the present work shows the physiological and symbiotic diversity of the isolates in the traditional agricultural areas of the region and the potential of these rhizobia to be used as effective commercial inoculants in areas where the indigenous rhizobia fail to do so. This will help to reclaim acidic soils for faba bean production.

Key words: Faba bean, Northern Gondar, Rhizobium, phenotypic diversity, symbiotic effectiveness