Enhanced Colonization of Pea Taproots by a Fluorescent Pseudomonad Biocontrol Agent by Water Infiltration into Soil

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Abstract
Root colonization by an introduced strain of Pseudomonas fluorescens was examined to determine the importance of the root apex in passive transport and to quantify the effect of infiltrating water on distribution of the bacterium. Pea seeds coated with strain PRA25rif of P. fluorescens were sown in in columns containing a sandy field soil at soil-water matric potentials of .sbd.1, .sbd.6, or .sbd.10 kPa. After 7 days, the largest population density of the bacterium was found on roots at .sbd.1 kPa, but the bacterium was detected on only 5% of root segments 4-5 cm below the seed, approximately 8 cm above the root apex. At .sbd.6 and .sbd.10 kPa, the bacterium could not be detected on roots beyond 3 cm from the seed, more than 16 cm from the root apex. Addition of 27.2 and 54.4 mm of water to the top of the columns 4 days after planting increased the depth from which PRA25rif was recovered. The bacterium was detected on root segments at least 9-10 cm from the seed 24 hr after water was applied. Transport of the bacterium on the root apex apparently was limited to a short period after seed germination, but the bacterium was carried long distances by percolating water.