An Agar Plate Method for Rapid Screening of Rhizobium for Tolerance to Acid-Aluminum Stress

An agar plate method has been developed for the rapid, preliminary screening of large numbers of rhizobia for tolerance to acidity and aluminum (acid-Al). Media containing pH indicators are used, and distinguish true tolerance from cases where growth depends on a previous pH shift. Strains which were highly tolerant or sensitive to acid Al on agar behaved similarly in defined liquid media. Strains that formed dry, pinpoint colonies were more sensitive to acid-AI than those that formed large, gummy colonies. Additional Index Words. acidity, aluminum, cowpea rhizobia, pH indicators, Rhizobium japonicum. Ayanaba, A., S. Asanuma, and D.N. Munns. 1983. An agar plate method for rapid screening of Rhizobium for tolerance to acid-aluminum stress. Soil Sci. Soc. Am. J. 47:256-258. METHODS have been developed recently for the preselection of Rhizobium strains tolerant of the soil acidity factors: aluminum, phosphate, and acidity (Date and Halliday, 1978; Keyser and Munns, 1979) and so of possible value as inoculant strains in the tropics. In these studies test strains are added to the stress media, incubated, and then examined for turbidity. In more refined tests, incubated cultures are monitored for cell multiplication prior to detectable pH changes. Screening for tolerance of acidity and Al can be vitiated by shifts in pH, which rhizobia sometimes induce depending on the buffering and particular composition of the medium (Date and Halliday, 1978). In agar media, local neutralization at the point of inoculation could become specially important, especially with large inocula. We report on an agar plate method that permits rapid screening of large numbers of rhizobia for tolerance to ' Contribution from the International Institute of Tropical Agriculture (IITA), P.M.B. 5320, Ibadan, Nigeria, and the Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616. IITA Jour. Paper Ser. no. 217. Supported in part by a grant from the United Nations Development Programme (UNDP). Received 7 June 1982. Approved 21 Sept. 1982. 2 Soil Microbiologists, IITA, Ibadan, Nigeria, and Professor of Soil Science, Univ. of California, Davis. an acid-A1 stress; and the application of this method to the screening of 224 rhizobia belonging to Rhizobium japonicum and Rhizobium sp. MATERIALS AND METHODS