Relationship between bacterial seed inoculum density and rhizosphere colonization of spring wheat

A field experiment was established with a rhizosphere-competent Xanthomonas maltophilia isolate used as a wheat seed inoculant at six different inoculum rates. The isolate used was resistant to erythromycin and streptomycin which facilitated field monitoring. Inoculum densities ranged from log 10 3.6 to log 10 7.4 cfu (colony forming units) per seed. Roots were assayed four times before anthesis to determine the number of introduced bacteria colonizing roots for each inoculum rate treatment. No significant differences in root-colonization values were observed among the inoculum rates at any plant growth stage. A subsequent experiment utilized the X. maltophilia isolate plus a rhizosphere-competent Pseudomonas fluorescens isolate resistant to erythromycin and rifampicin. Although significant differences in root-colonization values were observed for X. maltophilia for the first root assay, these differences in root-colonization values were not maintained for the subsequent root assays. Overall, initial seed inoculum densities differed by at least log 10 3.2 cfu per seed but the differences in root-colonization densities averaged only log 10 0.7 cfu mg −1 (dry weight) roots plus rhizosphere soil. The results demonstrated that bacterial root-colonization densities were largely independent of the initial inoculum densities on the seed.