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The Correlation Between Extracellular Polysaccharide Production and Acid Tolerance in Rhizobium¹

ABSTRACT

Rhizobial isolates from six host genera (Cicer, Phaseolus, Leucaena, Lens, Melilotus and Trifolium) were screened for ability to colonize successively more acidic agar plates. Sufficient isolates from Cicer and Phaseolus were screened to show that relative tolerance to acidic conditions was normally distributed. Extracellular polysaccharide (EPS) was isolated by ethanol precipitation from 10-d-old solution cultures of the most and least acid tolerant of these strains. Acid-tolerant isolates from Cicer, Phaseolus, Leucaena and Melilotus produced significantly more (P < 0.001) EPS than did those which were acid sensitive. The differences between isolates from Trifolium were significant at the P < 0.05 level. No significant difference in EPS production was found among the 8 isolates from Lens. The correlation between EPS production and acid tolerance was tested further with EPS isolated from 20 strains of Rhizobium phaseoli. The quantity of EPS produced varied from 18 to 75 mg 100 mL culture, was normally distributed, and was positively correlated with acid tolerance (P < 0.01), albeit with an r² value of only 0.331. It is postulated that EPS could modify the microenvironment of the rhizobia and so decrease the stresses induced by an acid soil.

¹ Partially supported by a grant from the U.S. Agency for International Development and subcontract from Univ. of Hawaii/NifTAL project.

² Former graduate student and professor, respectively; Dep. of Land, Air and Water Resources, Univ. of California, Davis.

Received for publication April 18, 1984. Accepted for publication July 6, 1984.