Rhizosphere colonization and seedling growth promotion of lodgepole pine by Bacillus polymyxa

Abstract

Lodgepole pine seed was inoculated with Bacillus polymyxa isolate L6-16R, a strain marked with antibiotic resistance to 100 mg rifamycin per litre. Inoculation resulted in statistically significant seedling biomass increases after 8 weeks of pine growth. The L6-16R rhizosphere population declined by an order of magnitude per month, from ca. 10⁶ cfu/g dry weight of pine root tissue 4 weeks past inoculation to ca. 10⁴ cfu/g of root tissue 12 weeks past inoculation. Seed-lings from three of four pine provenances had mean shoot and root dry weight increases of up to 35% due to seed inoculation, but shoot growth of the fourth provenance was inhibited. The size of the L6-16R rhizosphere population was not correlated to the magnitude of the shoot growth response 4 weeks after inoculation, but it showed a strong correlation with shoot growth 8 weeks after inoculation. These results indicate that the promotion of pine seedling growth may be a function of the size of the bacterial rhizosphere population that develops 4 weeks after inoculation, and that B. polymyxa may promote growth of lodgepole pine collected from many, but not all provenances. Key words: Bacillus polymyxa, lodgepole pine, growth promotion, rhizosphere colonization.