REDUCTION OF BACTERIAL-GROWTH BY A VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS IN THE RHIZOSPHERE OF CUCUMBER (CUCUMIS-SATIVUS L)

Cucumber was grown in a partially sterilized sand-soil mixture with th e vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum or left uninoculated. Fresh soil extract was places in polyvinyl chlorid e tubes without propagules of mycorrhizal fungi. Root tips and root se gments with adhering soil, bulk soil, and soil from unplanted tubes we re sampled after 4 weeks. Samples were labelled with [H-3]-thymidine a nd bacteria in different size classes were measured after staining by acridine orange. The presence of VAM decreased the rate of bacterial D NA synthesis, decreased the bacterial biomass, and changed the spatial pattern of bacterial growth compared to non-mycorrhizal cucumbers. Th e [H-3]-thymidine incorporation was significantly higher on root tips in the top of tubes, and on root segments and bulk soil in the center of tubes on non-mycorrhizal plants compared to mycorrhizal plants. At the bottom of the tubes, the [H-3]-thymidine incorporation was signifi cantly higher on root tips of mycorrhizal plants. Correspondingly, the bacterial biovolumes of rods with dimension 0.28-0.40 x 1.1-1.6 mum, from the bulk soil in the center of tubes and from root segments in th e center and top of tubes, and of cocci with a diameter of 0.55-0.78 m um in the bulk soil in the center of tubes, were significantly reduced by VAM fungi. The extremely high bacterial biomass (1-7 mg C g-1 dry weight soil) was significant reduced by mycorrhizal colonization on ro ot segments and in bulk soil. The incorporation of [H-3]-thymidine was around one order of magnitude lower compared to other rhizosphere mea surements, probably because pseudomonads that did not incorporate [H-3 ]-thymidine dominated the bacterial population. The VAM probably decre ased the amount of plant root-derived organic matter available for bac terial growth, and increased bacterial spatial variability by competit ion. Thus VAM plants seem to be better adapted to compete with the sap rophytic soil microflora for common nutrients, e.g., N and P, compared to non-mycorrhizal plants.