Host range nodulation and adaptation in frenchbean rhizobia.

The host range nodulation efficiency of four genetically marked frenchbean rhizobial strains (HURR-3, Raj-2, Raj-5 and Raj-6) was studied with five legume hosts namely, frenchbean (Phageolus vulgaris L.), pigeonpea [Cajanus cajan (L.) Millsp.], mungbean [Vigna radiata (L.) Wilezek.], urdbean [Vigna mungo (L.) Hepper.] and soybean [Glycine max (L.) Merril.]. Except soybean and pigeonpea, all other legume hosts were nodulated by two or more frenchbean rhizobial strains tested. Rhizobia were isolated from nodules produced by strains, HURR-3 and Raj-5, on main (frenchbean) and different (mungbean and urdbean) hosts. There was marked improvement in host range nodulation and nitrogen fixation efficiency of rhizobial strains, HURR-3 and Raj-5. after their isolation from chance nodules on different hosts. This is clearly evident from the ability of such isolates to form nodules on pigeonpea besides mungbean and urdbean, and higher nodulation in all the above three different hosts. The phage-susceptibility pattern and intrinsic antibiotic resistance (used as markers) of the two strains did not change after their passage through different hosts. The results indicate that frenchbean rhizobia had undergone some modification in symbiotic behaviour to adapt to wide host range during their passage through different (alternate?) hosts.