PHENOTYPIC CHARACTERISTICS AND COMPOSITION OF RHIZOBIA ASSOCIATED WITH WOODY LEGUMES GROWING IN DIVERSE KENYAN CONDITIONS

Over 480 rhizobia were isolated from root nodules of woody legume and herbaceous trap host species grown in soils collected from 12 differen t Kenyan sites. The isolates were differentiated by growth and morphol ogical characteristics, intrinsic antibiotic resistance (IAR) and salt (NaCl) tolerance levels (STL) when grown on yeast mannitol mineral sa lts agar and broth media. The bulk of the isolates (91%) were watery, milky-translucent and curdled milk types with moderate to copious extr acellular polysaccharide (EPS). The rest were creamy or white opaque w ith little to moderate EPS production. Overall, they showed a wide ran ge of growth rates: very fast-growing (mean generation time 1.6-2.5 h) , fast-growing (2.8-4.8 h), intermediate between fast- and slow-growin g (5.6-5.7 h) and slow- and very slow-growing (6.4-8.8 h). The isolate s were tentatively grouped into Rhizobium spp., to include very fast, fast and intermediate (acid-producing) types; and Bradyrhizobium spp., to include very slow, slow and intermediate (alkali-producing) types. Bradyrhizobium spp. were more sensitive to antibiotics (40 mu g mL(-1 )) than Rhizobium spp., contrary to the general opinion which indicate s that they are normally resistant. Cluster analysis based on sensitiv ity responses of IAR and STL could not distinguish Rhizobium spp. from Bradyrhizobium spp., neither was there any association by site nor ho st of isolation except for those isolates trapped with Phaseolus vulga ris at Kibwezi. Our data demonstrated a high diversity of tropical rhi zobia associated with trees.