Effects of Rhizobial Extracellular Polysaccharide on pH and Aluminum Activity1

A positive correlation has been demonstrated between the ability of Rhizobium strains to colonize acidic agar surfaces and their production of extracellular polysaccharide (EPS) in solution culture at pH 7.0. This set of experiments examined the effects of EPS on solution pH levels and Al activity, two common growth-limiting factors in acid soils. Samples of EPS from each of 20 previously acidscreened Rhizobium phaseoli isolates were titrated under controlled conditions and a buffering capacity was calculated. In addition, EPS from all strains were compared for ability to chelate aluminum by sodium saturation followed by equilibration against an aluminumphosphate solid phase. Total aluminum of the digested samples was measured by atomic adsorption spectrophotometry. The buffering ability of the EPS varied considerably between strains but was not correlated with the acid tolerance of the strain from which each was isolated. Similarly, while there were significant differences in the ability of the EPS to chelate aluminum, these were not correlated with acid tolerance. Acid tolerance did not correlate with EPS production weighted for either buffering capacity of the EPS or its ability to chelate Al. Thus, while the amount of ethanolprecipitatable EPS from different rhizobial strains varied significantly in these two capacities, rhizobial EPS does not appear to confer acid tolerance on a strain by means of buffering pH or aluminum levels. Other possible causes of the observed relationship between acid tolerance and EPS production should be examined. These could include a role in increasing solution phosphate levels and/or the necessity of increased EPS for binding to plant lectins in an acidic