PERFORMANCE OF Rhizobium meliloti STRAINS SELECTED FOR LOW-pH TOLERANCE

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Abstract
Twelve strains of Rhizobium meliloti were examined for their ability to nodulate alfalfa (Medicago sativa) and fix nitrogen under acid soil conditions. The strains were separated into a low-pH-sensitive group and a low-pH-tolerant group on the basis of plant dry matter yields in greenhouse experiments. The dry matter yields of alfalfa inoculated with the low-pH-tolerant strains were 5.9, 10.0 and 1.3 times greater than the yields with the low-pH-sensitive strains at soil pH 5.0, 5.5 and 6.0, respectively. Yields of alfalfa at soil pH 6.7 were the same for both groups. Four strains were selected from the low-pH-tolerant group for further evaluation. Plant yield, nitrogen fixation, specific nitrogenase activity, nodule number and nodule weight were measured for each strain in relation to soil pH, inoculum level and alfalfa cultivar. Soil pH, inoculum level and cultivar had significant effects on most of the characteristics measured. There were no significant differences in yield and nitrogen fixation within the four strains, and only minor differences in nodule number, nodule weight and specific nitrogenase activity. The low-pH-tolerant strains were competitive with indigenous strains and effective on a range of alfalfa cultivars. Inoculation rates at 105 rhizobia per seed were required to obtain maximum effect from the applied inoculum. R. meliloti strains selected for low-pH tolerance provide the potential for substantially increasing alfalfa yields on moderately acid soils.