Symbiotic properties of sinorhizobia isolated from Acacia and Prosopis nodules in Sudan and Senegal

The host specificity, infection process and effectiveness of nodules produced by several African sinorhizobial strains on different Acacia and Prosopis species (Leguminosae, Mimosoideae) were studied. Sinorhizobium arboris strain HAMBI 1552^T, S. kostiense strains HAMBI 1489^T and HAMBI 1493, S. saheli strain HAMBI 1496 and S. terangae bv. acaciae strain ORS 1058 induced nitrogen fixing nodules on seedlings of the following African or Latin American species (marked with ^*): A. angustissima^*, A. mellifera, A. nilotica, A. oerfota (synonym A. nubica), A. senegal, A. seyal, A. sieberiana, A. tortilis subsp. raddiana, P. chilensis^*, P. cineraria, P. juliflora and P. pallida^*. All strains increased plant yield significantly compared with uninoculated seedlings watered with nitrogen-free medium, but none appeared to be superior. The sinorhizobial strains were unable to effectively nodulate Sesbania rostrata (Papilionoideae).All roots had hairs, but particularly in the case of Acacia spp. they were often sparse. After inoculation root hairs were deformed and, in general, infection in Acacia spp. occurred through short root hairs and in Prosopis spp. through longer ones. After entry, the rhizobia filled infection pockets in the root hair, which later expanded into sac-like structures. When infection threads occurred, they usually started from sac-like structures. Elongation and ramification of the nodules indicated that Acacia spp. and Prosopis spp. have indeterminate nodules. A persistent apical meristem, which is the characteristic feature of the indeterminate nodule type, was much clearer in Prosopis spp. than in Acacia spp. Sinorhizobial strains formed tumour-like structures with undifferentiated cell tissue on the Australian acacia A. holosericea and ineffective, nodule-like structures on the African P. africana.