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 the distribution of bacteria along the taproot of peas. Seeds coated with strain PRA25rif were sown in soil held at a constant soil-water matric 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 bacterial populations by dilution plating. A significant (P < 0.01) linear decrease in the density of PRA25rif populations with increasing temperature 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 root colonization was 16-degrees-C, whereas root growth was most rapid at 24-degrees-C. The bacterium was never recovered from taproot segments > 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(Tn5) (motile) and LWM74-30 (non-flagellated, non-motile mutant) at 24-degrees-C with both a constant soil-water matric potential of -6 kPa and 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 the seed were assayed as above for bacterial populations. Neither the motile nor the non-motile strain was recovered > 3 cm below the seed under static conditions. However, both strains were recovered 14-16 cm below the seed 3 days after water was added, indicating that passive movement with water flow, and not motility, played a large role in bacterial dispersal in this system.