Numerical Taxonomic Study of Legume Nodulating Bacteria

Abstract

Forty-five isolates of both indigenous and imported rhizobia (from different cross-inoculation groups) were characterized using 114 different taxonomic traits. Cross-inoculation experiments showed a wide range in the degree of host X isolate efficiency. This was reflected by variations in number of nodules, shoot dry weight and nitrogen content produced by the same host when inoculated by different isolates of rhizobia. Characterization of the isolates showed that, all of the tested isolates were Gram-negative, rods and microaerophilic. They do not absorb Congo red and unable to grow on glucose peptone agar media. The isolates showed a very wide range of variations in all other biochemical tests investigated. Some of the isolates were able to tolerate high temperatures and most of them showed poor growth at 10°C. All isolates were able to grow on Yeast Extract Mannitol Agar media containing high NaCl concentration. The isolates 10a and 10b (Cyamopsis tetragonoloba) were able to tolerate an extreme NaCl concentration of 150 g/L. All isolates showed considerable growth on neutral and alkaline media. However, Isolates 1b (Medicago sativa), 5d (Phaseolus vulgaris), 7b, 7c, 7d (Vigna unguiculata), 8a, 8b (Cajanus cajan), and 14b (Acacia Senegal) didn’t grow at pH 10. Isolate 1a (Medicago sativa) have shown from well to very well growth at all pH values. Different isolates showed varying degrees of antibiotic resistance against different antibiotics tested. Isolate 1d (Medicago sativa) proved absolute resistance against ampicillin (25 _6;g), nitrofurantion (200 _6;g), streptomycine (25 _6;g) and tetracycline (25 _6;g). The isolate 15b (Sesbania sp.) moderately tolerated tetracycline (25 _6;g) and showed a better tolerance to cortimoxazole (25 _6;g), whereas it showed sensitivity against the other antibiotics. Isolate 3d (Vicia faba) showed medium tolerance to gentamycine (10 _6;g). However, it showed high sensitivity against the other antibiotics. Isolate 1d (Medicago sativa), 6d (Glycine max), 9b (Lablab purpureus), 10c (Cyamopsis tetragonoloba), 15a (Sesbania sp.) showed resistance against all antibiotics tested in the study. The DNA extracted from different isolates showed resemblances between some of them. These are: 1a and 1d (Cyamopsis tetragonoloba); 2 (Trigonella phoenum-graecum) and 5a (Phaseolus vulgaris); 9a (Lablab purpureus) and 15b (Sesbania sp.); and finally 10a (Cyamopsis tetragonoloba), 11d (Arachis hypogaea) and 12d (Cicer arietinum). The 45 isolates were separated into 6 groups according to their indices and their standard deviations. The isolates within each cluster group expressed distinct features that could differentiate them from other groups, these features were not obviously correlated with the infectiveness or effectiveness of the isolates with a particular legume host. This situation suggests that cross- inoculation grouping has a very weak taxonomical value. Group (A) included isolate 11b (Arachis hypogaea) which belongs to the cowpea group. Group (B) belongs to pea group and it included isolate 3a (Vicia faba). Group(C) consisted of three isolates (11a, 11c, and 11d) (Arachis hypogaea). Group (D) included 12 isolates and group (E) included 26 isolates belong to different cross inoculation groups. Group (F) included two isolates (5a) (Phaseolus vulgaris) and (9a) (Lablab purpureus