Among the factors which may affect colonization of roots by soil bacteria is that of rhizosphere oxygen partial pressure (pO2). The oxygen concentration in the root zone influences both microbes and roots. Roots exposed to low pO2, as might occur during flooding and waterlogging of the soil, become more leaky and loss of soluble carbon increases. To determine whether periods of low pO2 increased root colonization by a genetically altered pseudomonad we inoculated 3- to 4-week-old maize plants, grown in soil and transferred to a hydroponic system or grown in fritted clay, with Pseudomonas putida PH6(L 1019)(lacZY+) following exposure of the roots to air or cylinder N2. Numbers of heterotrophs and the marked pseudomonad were determined by dilution plating. Low pO2 generally increased the numbers of bacteria associated with roots exposed to the treatments in solution or in undisturbed flitted clay rooting medium. Under low pO2 in a hydroponic system, roots of intact maize plants tended also to have higher soluble organic C and hexose (anthrone-detectable sugars) than roots exposed to air. The effect of low pO2 was most pronounced in the fritted clay where low pO2 favored colonization by the marked strain; numbers were 3- to 96-fold greater than those on roots flushed with air but accounted for only 0.06-0.61% of the total population. Roots exposed to low pO2 tended to accumulate more C. Results suggest that in the fritted clay, the pseudomonad was able to exploit the increased C supply and to achieve greater numbers on roots exposed to low pO2, whereas the dilution of carbon released from roots in the hydroponic apparatus did not allow for the same magnitude of increase on roots.
- Low pO