Alterations in plant growth and in root hormone levels of lodgepole pines inoculated with rhizobacteria

The presence of other soil microorganisms might influence the ability of rhizobacterial inoculants to promote plant growth either by reducing contact between the inoculant and the plant root or by interfering with the mechanism(s) involved in rhizobacterially mediated growth promotion. We conducted the following experiments to determine whether reductions in the extent of growth promotion of lodgepole pine mediated by Paenibacillus polymyxa occur in the presence of a forest soil isolate (Pseudomonas fluorescens M20) and whether changes in plant growth promotion mediated by P. polymyxa (i) are related to changes in P. polymyxa density in the rhizosphere or (ii) result from alterations in root hormone levels. The extent of plant growth, P. polymyxa rhizosphere density, and root hormone concentrations were determined for lodgepole pine treated with (i) a single growth-promoting rhizobacterial strain (P. polymyxa L6 or Pw-2) or (ii) a combination of bacteria: strain L6 + strain M20 or strain Pw-2 + strain M20. There was no difference in the growth of pines inoculated with strain L6 and those inoculated with strain L6 + strain M20. However, seedlings inoculated with strain Pw-2 had more lateral roots and greater root mass at 12 weeks after inoculation than plants inoculated with strain Pw-2 + strain M20. The extent of growth promotion mediated by P. polymyxa L6 and Pw-2 in each treatment was not correlated to the average population density of each strain in the rhizosphere. Bacterial species-specific effects were observed in root hormone levels: indole-3-acetic acid concentration was elevated in roots inoculated with P. polymyxa L6 or Pw-2, while dihydrozeatin riboside concentration was elevated in roots inoculated with P. fluorescens M20.