Tolerance of Rhizobia to Acidity, Aluminum, and Phosphate

Low levels of phosphorus and high levels of aluminum are important soil acidity factors for the growth of higher plants; however, very little is known about their effects on the soil rhizobia. The present study was conducted to determine the relative effects of acidity, P, and Al on rhizobia. Tolerance of low pH (4.5), low P (510 μM), and high AI (50 μM) was assessed for 10 strains of cowpea rhizobia by detailed growth studies in defined liquid media. Tolerances to these factors were determined for 65 strains of cowpea rhizobia and Rhizobium japonicum by a rapid method based on attainment of turbidity from a small inoculum. Strains varied in response. Low P (as compared with 1,000μM) limited total attainable population density to 5 X 10 cells/ml, and slowed the growth of some strains. Acidity generally increased lag time or slowed growth of most strains, and stopped growth of about 50% of them. Tolerance of acidity did not necessarily entail tolerance of Al. Aluminum (50 μM) increased the lag time or slowed growth of almost all strains tolerant of low pH. It virtually stopped growth of 40% of the strains. With our system the rhizobia had to make 1,000-fold growth in the stress media before they could significantly raise pH and precipitate Al. A valid rapid screening can be based on ability to attain visible turbidity in culture under acid or Al stress, so long as initial density is small («10 cells/ml). The cowpea rhizobia tended to have more tolerance to Al than R. japonicum and overall Al was a more severe stress than low pH or low P. Additional Index Words: acidity, phosphorus, aluminum, rhizobia, cowpea miscellany, Rhizobium japonicum. Munns, D. N., and H. H. Keyser. 1979. Tolerance of rhizobia to acidity, aluminum, and phosphate. Soil Sci. Soc. Am. J. 43: 519-523. EFFECTS of ACIDITY on species of Rhizobium are well documented. Pure culture studies (Graham and Parker, 1964) have shown the critical low pH range for growth is from about 4.0 to 6.0, with the slower growing R. japonicum, R. lupini, and cowpea miscellany being in general more acid tolerant than the others, and R. meliloti being the most acid sensitive. These 1 Contribution of the Department of land, Air and Water Res., University of Calif. Davis. Partly supported by grants from Agency for International Development (NiFrAL Project) and NSF/Rann. Received 21 July 1978. Approved 6 Feb. 1979. 2 Postgraduate Research Scientist and Professor of Soil Science, respectively. Senior author is currently with Cell Culture & Nitrogen Fixation Lab., USDA-SEA-AR, BARC-W, Beltsville, Md 20705. 51 9 observations on relative tolerance of the species agree with studies in acid soils (Damirgi et al., 1967; Jensen 1969). Within each species, important strain-to-strain variation has been demonstrated (Graham and Parker, 1964; Munns, 1965a). Besides low pH per se, acid mineral soils have low levels of phosphorus and high levels of aluminum (Kamprath, 1973; Pearson, 1975). There has been little research on the effects of these two soil factors on rhizobia. An early study of effects of P on rhizobia showed positive growth response to P additions in soil (Truesdell, 1917). Kamata (1962) related the ability to nodulate P-deficient soybeans with the rhizobial strains relative response to P in culture media. Werner and Berghauser (1976) demonstrated that three strains of Rhizobium were better than two other common bacteria at taking up P from very low concentrations in solution. Recent studies (Rerkasem, 1977; de Carvalho, 1978) have shown that some rhizobia can indeed survive high Al concentrations at low pH in both solution media and soil. However, there is still insufficient evidence to establish Al-tolerance in rhizobia as distinct from tolerance of low pH. There are no data concerning effects of Al on rhizobial growth rate. The objectives of this investigation were (i) to determine the effects of low P and high Al on the survival and growth rate of some rhizobia at low pH, (ii) to examine the relationship between acid-tolerance and Al-tolerance, and (iii) to rate the probable importance of the three stresses (acidity, P, and Al) according to their inhibitory effects on rhizobial growth. MATERIALS AND METHODS