Physiological diversity of rhizobia nodulating promiscuous soyabean in Zimbabwean soils

Rhizobial isolates were obtained from nodules of promiscuous soyabean varieties Hernon 147 and Magoye and specific Roan grown in a range of Zimbabwean soils. A total of 129 isolates authenticated as true rhizobia were characterized using growth rate, elasticity, colour, size, colony shape, acid/alkali production on YEM and tolerance to low and high pH, elevated temperature and salt concentration. Isolates separated into 2 major clusters at a similarity level (%Ssm) of 66%. Cluster I contained isolates forming dry colonies (77%) which separated into 9 groups and Cluster 11 contained those forming the wet colonies (23%) with 4 groups. Acid and salt tolerance patterns did not differ among the two main clusters (the dry and the wet colony types). More isolates forming wet colonies (47%) survived at 40 degrees C than those forming dry colonies (13%). Salt, temperature and acid pH tolerance were not related to geographic origin of the isolates. The promiscuous soyabean variety Magoye nodulated with the widest range of rhizobia (12 groups) followed by Hernon 147,(11 groups) and then Roan (9 groups). Guruve soils had the most diverse range of isolates belonging to 12 groups followed by those from Chiweshe (9 groups) and then those from Chikomba (8 groups). Our results indicate that soyabean is nodulated by a wide range of indigenous rhizobia in African soils.