Bean (Phaseolus vulgaris L. cv. Derby) seeds, supplied by Ferry-
Morse Seed Company, CA, USA were used. The competence
of the selected 10 isolates to colonize the rhizosphere of
beans was determined in two stages. An indicator root
colonization plate assay (Kortemaa et al., 1994) was carried
out in vitro to determine whether the root exudates of beans
could support the growth of each isolate. Promising isolates
from this assay were subjected to a rhizosphere competence
assay using the non-sterile sand tube method described by
Ahmad and Baker (1987) using rifampicin resistant mutants
as described by Misaghi and Donndelinger (1990). After 3
weeks, in the study involving the sand tube method, the
length of the harvested roots were measured, and only the
first 14 cm of roots were retained for further studies and the
balance of roots discarded. These were then cut aseptically
into 2 cm segments and sequentially numbered from the
seed (Ahmad and Baker, 1987). The root segments and the
associated rhizosphere soil were studied separately to
determine the root colonization frequencies and population
densities of the NSA in the rhizosphere soils using OMYEA
plates (Kortemaa et al., 1994). A rhizosphere or root segment
was considered to be colonized when the microbe was
detected on the plates either on a root segment or in a
corresponding rhizosphere soil sample or both after 10 days
of incubation at 28 8C. The experiments were repeated twice
with 10 replicates in each.