Jh. Bowers et Jl. Parke, COLONIZATION OF PEA (PISUM-SATIVUM L) TAPROOTS BY PSEUDOMONAS-FLUORESCENS - EFFECT OF SOIL-TEMPERATURE AND BACTERIAL MOTILITY, Soil biology & biochemistry, 25(12), 1993, pp. 1693-1701
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.