Host specificity and characterization of fast-growing rhizobia from Macroptilium atropurpureum cv. Siratro in Pakistan

The growth characteristics, intrinsic antibiotic resistance pattern, utilization of different C sources and symbiotic characteristics of 18 fast-growing rhizobial strains isolated from root nodules of Macroptilium atropurpureum (Siratro) were studied. Colonies on agar at 2–3 days were large, gummy, translucent and spreading (2–4 mm dia) and were acid producing. These strains did not utilize sodium citrate as a sole source of C and were sensitive to low concentrations of chloramphenicol (25 μg ml−1), tetracycline (8 μg ml−1) and streptomycin (12 μg ml−1). The host specificity and effectiveness of these strains on different legume hosts revealed that they are different from other fast-growing strains. All strains failed to nodulate Glycine max (soybean cv. William, Lee and Peking). Sixteen of the isolates nodulated Leucaena leucocephala (ipil-ipil) and all 18 nodulated Phaseolus vulgaris (common bean). The fast-growing strains showed varying degrees of effectiveness in N2 fixation on M. atropurpureum, Vigna mungo (blackgram) and V. unguiculata (cowpea). The isolates formed ineffective symbioses on two genotypes of V. radiata (mungbean).