EFFECT EF LAW PO(2) ON COLONIZATION OF MAIZE ROOTS BY A GENETICALLY ALTERED PSEUDOMONAS-PUTIDA [PH 6(L1019)]

Among the factors which may affect colonization of roots by soil bacte ria is that of rhizosphere oxygen partial pressure (pO(2)). The oxygen concentration in the root zone influences both microbes and roots. Ro ots exposed to low pO(2), as might occur during flooding and waterlogg ing of the soil, become more leaky and loss of soluble carbon increase s. To determine whether periods of low pO(2) increased root colonizati on 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(L1019)(lacZY+) fol lowing exposure of the roots to air or cylinder N-2 Numbers of heterot rophs and the marked pseudomonad were determined by dilution plating. Low pO(2) generally increased the numbers of bacteria associated with roots exposed to the treatments in solution or in undisturbed fritted clay rooting medium. Under low pO(2) in a hydroponic system, roots of intact maize plants tended also to have higher soluble organic C and h exose (anthrone-detectable sugars) than roots exposed to air. The effe ct of low pO(2) was most pronounced in the fritted clay where low pO(2 ) favored colonization by the marked strain; numbers were 3- to 96-fol d greater than those on roots flushed with air but accounted for only 0.06-0.61% of the total population. Roots exposed to low pO(2) tended to accumulate more C. Results suggest that in the fritted clay, the ps eudomonad was able to exploit the increased C supply and to achieve gr eater numbers on roots exposed to low pO(2), whereas the dilution of c arbon released from roots in the hydroponic apparatus did not allow fo r the same magnitude of increase on roots.