Azospirillum effects on susceptibility to Rhizobium nodulation and on nitrogen fixation of several forage legumes

Azospirillum brasilense Cd cell concentration of 10⁵-10⁷ colony-forming units (cfu)/L applied 24 h before Rhizobium (10⁶ cfu/mL), increased nodule formation in the non root hair zone, more than twofold, in pouch-grown Medicago polymorpha and Macroptilium atropurpureum seedlings, compared with Rhizobium alone. The increase in nodule formation in pouch-grown Trifolium alexandrinum following preinoculation with Azospirillum was 20%. The percentage of nodulated seedlings rose from 0 to 25% when Medicago polymorpha was preinoculated with Azospirillum followed by the application of 10 cfu/mL Rhizobium meliloti, a level which by itself was not sufficient to initiate nodule formation. Acetylene reduction activity in Medicago polymorpha and Macroptilium atropurpureum seedlings after inoculation with Azospirillum-Rhizobium was markedly increased. A possible reason for the increased susceptibility to Rhizobium infection may be that Azospirillum stimulates the formation of a larger number of epidermal cells that differentiate into infectable root hairs.