Acid Tolerance of Rhizobia in Culture and in Symbiosis with Cowpea

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. Additional Index Words: nodule function, nodulation, infection, tropical legumes. Keyser, H. H., D. N. Munns, and J. S. Hohenberg. 1979. Acid tolerance of rhizobia in culture and in symbiosis with cowpea. Soil Sci. Soc. Am. J. 43:719-722. R HIZOBIAL STRAINS are known to differ in ability to grow in acid soils and culture media (Munns, 1977, 1978). More important are differences in "symbiotic acid tolerance," ability to sustain satisfactory growth of the dinitrogen-dependent host plant despite acid-related stress in the soil. Amongst strains of rhizobia tested on Pisum sativum, Lie (1971) observed variation in performance at pH 4.6 that was unrelated to effectiveness at neutral pH. Similar variation in symbiotic tolerance of acidity, independent of effectiveness at favorable pH, appeared in trials with 40 strains of rhizobia of the cowpea group tested on soil-grown plants of two varieties of Vigna radiata, mung (Munns, et al-., 1979). Some strains possessed the valuable combination of high effectiveness and high symbiotic tolerance of soil acidity. This paper describes similar greenhouse screening of cowpea rhizobia for symbiotic acid tolerance, with different acid soils and a different host species, Vigna unguiculata L. Walp., cowpea. The main purpose was to test a rapid laboratory method of prescreening rhizobia for tolerance of soil acidity, potentially valuable for reducing the number of strains going to more cumbersome greenhouse screening. We supposed that many symbiotically acid-sensitive strains fail in acid soil simply because they cannot grow and colonize the host rhizosphere, let alone complete later acid-sensitive steps in symbiotic establishment. We also supposed that Al toxicity would be a significant factor, since cowpea can nodulate in very acid soils. Given these suppositions, symbiotic failure of many strains in acid soil should be '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. Received 13 Nov. 1978. Approved 19 Mar. 1979. 'Post Graduate Research Scientist, Professor, and Graduate Research Assistant, respectively. predictable from their inability to grow in a defined medium at, pH 4.5 with 50 AM Al (Keyser and Munns, 1979x). To be successful, this laboratory screening should eliminate a large proportion of symbiotically sensitive strains but few of the strains that remain highly effective at low pH. MATERIALS AND METHODS