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Acid Tolerance of Rhizobia in Culture and in Symbiosis with Cowpea¹

ABSTRACT

In greenhouse trials, 21 strains of slow-growing rhizobia were tested for symbiotic effectiveness and ability to nodulate three varieties of cowpea, Vigna unguiculata L. Walp., in two Ultisol subsoil samples, each adjusted to pH 4.6 and pH 6.0–6.2.

The results confirmed that cowpea rhizobia contain a large and perhaps continuous variation in symbiotic tolerance of soil acidity. Some strains combined tolerance with high effectiveness.

Laboratory testing, based on ability of rhizobia to grow in acid (pH 4.5) liquid media containing Al (50 µM), successfully identified about 65% of the strains that proved symbiotically sensitive in the greenhouse trials. No highly tolerant effective strain was misidentified as sensitive. Similar simple rapid growth tests, appropriately modified, might prove useful for screening other groups of rhizobia.

¹ Contribution from Dept. of Land, Air and Water Resources, Univ. of Calif. Davis, CA 95616. Supported by grants from the U.S. Agency for Int. Dev. and N.S.F.

² Post Graduate Research Scientist, Professor, and Graduate Research Assistant, respectively.

Received for publication November 13, 1978. Accepted for publication March 19, 1979.