Sulfur-oxidizing bacteria as plant growth promoting rhizobacteria for canola

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Abstract
Canola (Brassica napus) has a high sulfur requirement during vegetative growth and exhibits symptoms of sulfur deficiency when cropped on Saskatchewan soils low in plant available sulfur. Elemental sulfur (S0) is frequently used as a fertilizer to alleviate this deficiency. The potential of sulfur-oxidizing microorganisms to enhance the growth of canola in S0 fertilized soils was assessed. Sulfur-oxidizing bacteria and fungi were isolated from the rhizosphere and rhizoplane of canola grown in four different Saskatchewan soils under growth chamber conditions. Of 273 bacterial isolates, 245 (89.7%) oxidized S0 to thiosulfate or tetrathionate in vitro, and 133 (48.7%) oxidized S0 to sulfate; 70 fungal isolates oxidized S0 to sulfate. Eighteen bacterial isolates demonstrating the highest in vitro sulfur oxidation were tested as seed inoculants under growth chamber conditions, with S0 as sulfur source. Fourteen isolates increased canola leaf size measured at the bud stage of growth, and seven isolates increased root and pod dry weights at maturity. Three of the 14 isolates were also able to stimulate canola leaf area in the presence of plant available sulfate. The shoot material from canola inoculated with two of these isolates contained more iron, sulfur, and magnesium than uninoculated canola. Three of the 14 isolates inhibited the growth of the canola fungal pathogens, Rhizoctonia solani AG2-1, R. solani AG4, and Leptosphaeria maculans "Leroy." Another isolate was antagonistic towards both R. solani strains and another inhibited the growth of R. solani AG2-1 and L. maculans "Leroy." Thus some sulfur-oxidizing isolates appear to stimulate canola growth due to the enhancement of mineral nutrient uptake, whereas in other cases antibiosis towards canola pathogens may also be involved. Key words: elemental sulfur, oxidation, canola, rhizosphere, plant growth promoting rhizobacteria.

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