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Tolerance of Rhizobia to Acidity, Aluminum, and Phosphate¹

ABSTRACT

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 (5–10 µM), and high Al (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 1,000 µM) limited total attainable population density to 5 x 10⁷ 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 Alstress, so long as initial density is small (<<10⁵ 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.

¹ Contribution of the Department of Land, Air and Water Res., University of Calif. Davis. Partly supported by grants from Agency for International Development (NifTAL Project) and NSF/Rann.

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.

Received for publication July 21, 1978. Accepted for publication February 6, 1979.

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