Soil strength and rate of root elongation alter the accumulation of Pseudomonas spp. and other bacteria in the rhizosphere of wheat

Results from a controlled environment system and the field showed that slow root elongation rate was associated with accumulation of Pseudomonas spp. in the rhizosphere; fast root elongation avoided accumulation. In the controlled environment system, total bacteria and bacteria belonging to the genus Pseudomonas were quantified along wheat ( Triticum aestivum L. cv. Janz) seminal roots elongating at rates of 2.4 or 0.8 cm d(-1) in loose and compacted field soil, respectively. Although total numbers of bacteria were similar for both rates of elongation, more Pseudomonas spp. accumulated on the slow-growing roots and their numbers were greatest 0.5-1 cm from the root tips. A reduced rate of root elongation in compacted soil accelerated the differentiation of root hairs, branch roots and adhesion of rhizosheath soil. Elongation rate and distance between the root tip and the zone of root hair development were positively correlated (r= 0.9), providing a morphological indicator of root elongation rate in the field. Slow-growing roots from the field had 20 times more Pseudomonas spp. per unit root length than fast-growing field roots, while total bacteria were 8-fold higher; differences were greatest 0 - 1 cm from the tips. These results may explain how soil structure and Pseudomonas spp. interact in conservation farming. Rapid root elongation is identified as a desirable trait for avoiding accumulations of bacteria.