COLONIZATION OF PEA (PISUM-SATIVUM L) TAPROOTS BY PSEUDOMONAS-FLUORESCENS - EFFECT OF SOIL-TEMPERATURE AND BACTERIAL MOTILITY

Root colonization by introduced strains of Pseudomonas fluorescens was examined to determine if temperature and bacterial motility affect th e distribution of bacteria along the taproot of peas. Seeds coated wit h strain PRA25rif were sown in soil held at a constant soil-water matr ic potential of -6 kPa and temperatures of 16, 20, 24 and 28-degrees-C . After 7 days, 1 cm segments of the epicotyl (0-1 cm above the seed) and taproot (0-5 cm below the seed) and the seed were assayed for bact erial populations by dilution plating. A significant (P < 0.01) linear decrease in the density of PRA25rif populations with increasing tempe rature was observed for the epicotyl, seed and taproot segment 0-1 cm below the seed, but not for the taproot segment 1-2 cm below the seed (P = 0.68). Of the temperatures tested, the temperature optimal for ro ot colonization was 16-degrees-C, whereas root growth was most rapid a t 24-degrees-C. The bacterium was never recovered from taproot segment s > 3 cm below the seed, indicating that transport of this strain with the growing root, even when roots grew slowly, was not a major means of dispersal in the pea rhizosphere. The effect of bacterial motility on root colonization was assessed with P. fluorescens strains WCS374(T n5) (motile) and LWM74-30 (non-flagellated, non-motile mutant) at 24-d egrees-C with both a constant soil-water matric potential of -6 kPa an d with 27.2 mm of water added 4 days after planting. Seven days after planting the epicotyl, seed and 1 cm taproot segments 0-18 cm below th e seed were assayed as above for bacterial populations. Neither the mo tile nor the non-motile strain was recovered > 3 cm below the seed und er static conditions. However, both strains were recovered 14-16 cm be low the seed 3 days after water was added, indicating that passive mov ement with water flow, and not motility, played a large role in bacter ial dispersal in this system.