Exploiting The Adaptive Acidtolerance Of Commercial Legume Inoculants Destined For Acidic Soils

M.E. Dunne1, G.W. O'Hara1, G.K. Bullard2, M.J. Dilworth1

'Ctr for Rhizobium Studies, Murdoch U, Western Australia

2Biocare Technology PTY LTD, Somersby, NSW, Australia

Legume inoculants for acidic soils are usually grown and stored in peat under non-acid conditions. These bacteria show an adaptive acid-tolerance response (ATR); cultures grown under moderately acidic conditions withstand exposure to highly acidic conditions much better than those grown under non-acid conditions.

Rhizobium leguminosarum bv. viciae (WSM1455) grown at pH 7.0 and then transferred to acidic defined medium of pH 3.5 displayed a decimal reduction time (D) of 10 h. When grown at pH 4.8 and exposed to pH 3.5, the D value was 16 h. R. leguminosarum bv. trifolii (WSM409) behaved similarly; the corresponding D values were 7.2 h and 9 h when grown at pH 7.0 and 5.5. Sinorhizobium meliloti strains also displayed an ATR; with D values of 10 min (pH 7) and 25 min (pH 5.5) for WSM419, and 5.8 h (pH 7) and 7.9 h (pH 5.5) for WSM688.

These strains acidify the commercial culture medium, the pH fell from 7.2 to 6.1 for both R. leguminosarum strains and to pH 5.7 (WSM419) and 5.8 (WSM688). Cultures of WSM409 inoculated into gamma-irradiated limed peat increased 100-fold while the pH of the peat remained neutral. Further studies are necessary to assess the field benefits of inoculants grown in acidic peat.

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