Morphophysiological and Symbiotic Characteristics of Rhizobia Nodulating Faba Bean (Vicia faba L.) from Bale, Ethiopia

Abstract

Faba bean among legumes is the most important pulse crop in Ethiopia,and like other legumes  can form symbiotic association with soil bacteria generally known as rhizobia. With the help of this association, it satisfies its nitrogen need to sustain life. Accordingly, the present study focused on isolation and selection of symbiotically effective rhizobia from unaddressed areas of Ethiopia. Thirty faba bean nodulating isolates trapped from soil collected from five districts (Agarfa,Dinsho,Gasera,Goba and Sinana) of Bale zone using plant infection method. The isolates were characterized for their cultural,nutritional,ecophysiological and symbiotic effectiveness.Almost all the  isolates were large and mucoid,and all were acid producers and attained colony size that ranged 1 to 4mm  with mean generation time ranging from 1.26 to 5 h and showed characteristics of fast growing rhizobia .Except for few isolates that tolerated extreme temperatures of 4 and 45^oC, many grew in temperature ranges of 15 to 35^oC,  and were salt and acid tolerant, and relatively tolerated many of antibiotics except tetracycline. with respect to nutritional versatility, they grew on wide ranges of Carbon and Nitrogen sources. Pot experiments were conducted for parameters including; nodules number, nodule dry weight, shoot dry weight and total nitrogen were significantly improved by inoculation (p< 0.05). In cases of nitrogen accumulation, sixteen of isolates perform relatively better than the positive control and more than 76% of the isolates performed better than the negative control, in cases of shoot dry weight accumulation most of (83%) of the isolates are moderate to highly effective. Four test isolates (FBD5, FBS2, FBS5, FBA5) were found to be  highly effective with their homologous host, faba bean  which excelled the reference strain and were observed for better performance than all other isolates and the negative control. Isolate FBD5 which is recovered from soil of Dinsho is more and significantly accumulated higher dry weight than 70ppm KNO3 treated positive control and the reference strain. Taken together, our results suggest the presence of morphophysiologically  diverse and symbiotically effective rhizobia nodulating faba bean growing in Ethiopia. In order to elucidate their exact taxonomic position, further characterization is needed based on molecular markers. Furthermore,their symbiotic performance should be tested under field condition in order to develop them into inoculants for faba bean production in Ethiopia and beyond.

Keywords: Bale, Biological Nitrogen Fixation, Faba Bean, Rhizobium leguminosarum, Symbiotic effectiveness