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Colonization of Winter Wheat Roots by Inhibitory Rhizobacteria¹

ABSTRACT

Colonization of the winter wheat (Triticum aestivum L.) rhizoplane by root-inhibiting nonfluorescent pseudomonads was investigated in laboratory and greenhouse studies with genetically marked (rifampicin resistant) isolates. Colonization was similar at root zone temperatures of 5, 10, and 15°C but was dependent on soil type, with higher populations occurring in the soil with lower organic matter and microbial biomass. Rhizoplane populations of test strains were over tenfold higher on roots in pasteurized soil than in non-pasteurized soil or vermiculite. Deleterious effects on growth of winter wheat were related to the extent of rhizoplane colonization by the bacteria; approximately 10⁶ colony forming units (cfu) mg^–1 root were required for significant inhibition of root growth. The shoots and roots of winter wheat were significantly shorter on plants inoculated with the inhibitory bacteria in pasteurized or methyl bromide fumigated soil as well as in nonpasteurized Ritzville soil (Calciorthidic Haploxerolls). The difference in early stages of plant growth, especially shoots, between bacterized (seed treated with antibiotic-resistant inhibitory bacteria) and nonbacterized winter wheat increased with time. The antibiotic-resistant bacteria maintained a high population on the roots during this period. The bacteria were observed to extensively colonize winter wheat roots (rhizobacteria) and are considered a potential constraint on winter wheat yield in the Pacific Northwest.

¹ Contribution from Agric. Res. Serv., USDA, in cooperation with the College of Agric. Res. Center, Washington State Univ., Pullman, WA 99164. Scientific Paper no. 6966.

² Formerly Graduate Student, Dep. Agron. and Soils, Washington State Univ. and Research Associate, USDA-ARS now Microbiologist, Battelle Pacific Northwest Laboratories, Richland, WA 99352; and Microbiologist, USDA-ARS, Pullman, WA 99164.

Received for publication December 24, 1984. Accepted for publication March 26, 1985.