Soil water flow affects the rhizosphere distribution of a seed-borne biological control agent, Pseudomonas fluorescens

Abstract Four-day-old wheat seedling roots were inoculated at specific sites with 3.8 × 10 5 colony forming units of Pseudomonas fluorescens strain 2–79 Rn 10 resistant to rifampin and naladixic acid. Seedlings inoculated at the root apex were transplanted into chambers containing non-sterile soil and harvested 5 h, 2, 4 and 7 days later. Root segments were excised and processed for dilution plating on a selective medium to determine the population distribution of the marked strain in relation to root elongation. Strain 2–79 RN 10 was recovered from a localized area near the point of inoculation but was no longer associated with the root apex by day 4. In a subsequent experiment, roots were inoculated approximately 4 cm above the root tip. Water applied to the soil surface and allowed to drain through the soil profile was found to affect the rhizosphere distribution of 2–79 Rn 10 . The marked strain was recovered 3.5 cm below the point of inoculation after 24 h when water had been added, compared to 0.5 cm below the inoculation site in the absence of added water.