The introduction of new rhizobia inocula into soil containing
native Rhizobium populations frequently results in only a small
proportion of nodules containing the introduced strain. This is
due to competition with ineffective indigenous strains and
other rhizospheric bacteria (McLoughlin et al., 1985). The
nodulation process involves signal exchange between the host
and the bacterium. Plant growth and nodulation by rhizobia
are promoted by certain rhizobacteria (Bai et al., 2002; Dahsti
et al., 1998; Rao & Pal, 2003). Pigeonpea (Cajanus cajan (L)
Milsp.) is grown extensively under rain-fed conditions in the
semiarid tropics. The potential of introduced Rhizobium
strains to improve its productivity is often not realized because
of competition from the native rhizobial population. This
results in ineffective nodulation in this legume. We carried
out experiments in order to assess the associative effects of
certain rhizobacteria isolated from the pigeonpea rhizosphere
and Rhizobium sp. on growth, nodulation and N2 fixation by
the host.