A METHOD FOR ASSESSING THE ROOT-COLONIZING CAPACITY OF BACTERIA ON MAIZE

A closed test tube assay was developed for measuring the root colonization capacity of bacteria in raw soil-sand. Bacteria were coated onto seeds at inoculum levels of 103-109 colony-forming units (cfu) per seed, and root-colonizing bacterial strains successfully grew along the emerging radicle. Quantification of specific bacterial populations on roots was highly reproducible between experiments with the assay. Of 54 strains tested, 24 colonized roots at populations of 104-106 cfu/g root. All root-colonizing strains were Gram negative. Two fluorescent pseudomonad strains that failed to colonize maize roots in raw soil did colonize in autoclaved soil. Captan seed treatment and temperature of incubation (14.degree. and 24.degree. C) had no substantial effect on subsequent root colonization. Root population densities determined in the sand-soil assay were comparable with those determined with plants grown in soils under greenhouse conditions.