GROWTH-CHARACTERISTICS AND SYMBIOTIC EFFICIENCY OF RHIZOBIA ISOLATED FROM COWPEA NODULES OF THE NORTHEAST REGION OF BRAZIL

In Brazil, the gee-environmental conditions of the north-east region e xert an effect on the rhizobia diversity. A large collection of rhizob ia isolates were obtained from nodules of cowpea planted in different soils from this region of Brazil. Cowpea was selected as the scavenger for this study of rhizobia diversity because it is a known broad host -range leguminous plant and it is also widespread in the region. A tot al of 536 nodule isolates were obtained and later arranged in 78 diffe rent groups based on colony morphology and growth. Growth in culture m edia of isolate representatives of these groups revealed that 27 of th em showed fast growth characteristics although the plant is characteri stically nodulated by strains described as Bradyrhizobium spp. Further more, the analysis of the isolates from each region showed an increase d proportion of fast-growing rhizobia when going from the coast to the semi-arid region. Fifty-five of the rhizobia representatives from the Brazilian north-east region induced Hup(+) phenotype in cowpea nodule s indicating the predominance of this phenotype. They were also inocul ated into soybean, since this legume plant is nodulated by the slow-gr owing rhizobia species B. japonicum and B. elkanii. Twenty-five of the group representatives were not capable of nodulating soybean. Inocula tion with some of them promoted the formation of pseudo-nodule structu res which despite not contributing significantly to the nitrogen conte nt of the plant are an indication that signal exchange between plant a nd rhizobia is occurring. The remaining isolate representatives were c apable of nodulating soybean. The nodulation of soybean by the isolate s obtained in soils of the semi-arid region of Brazil is quite remarka ble since this crop has never been cultivated in the region before. Da ta on host-regulated uptake hydrogenase phenotype (Hup-hr) is discusse d. (C) 1997 Elsevier Science Ltd.