de Sá, N. M. H.; Marriel, I. E.; Franco, A. A. Variability in Individual Nodule Activity of Single Strains of <i>Rhizobium etli< i> and R. <i>tropici in< i> Symbiosis With <i>Phaseolus vulgaris< I>. PAB 1994, 29, 1897-1904.