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Role of Extracellular Polysaccharide Production and Clays in the Desiccation Tolerance of Cowpea Bradyrhizobia¹

ABSTRACT

A study was conducted to determine the roles of extracellular poly-saccharide and clays in the desiccation tolerance of cowpea bradyrhizobia. Seven nonmucoid strains of cowpea Bradyrhizobium were more resistant to drying in a sandy soil than five mucoid strains. A significant correlation was not observed (r = 0.26) between the amount of extracellular polysaccharide produced by these strains and their desiccation tolerance in a sandy soil. Investigation of two strains showed that they survived better in two sandy soils subjected to slow than to rapid drying, but the rate of drying had little effect in quartz sand. The percentage survival of the two strains in both soils was less at an inoculum density of 10⁹ than at 10⁶ cells/g of soil, but the reverse was true in quartz sand. The effect of desiccation on the number of survivors was similar if the drying was carried out with sterile or nonsterile soil. At the lower inoculation density, survival of the two strains in quartz sand increased if the clay fractions from the soils were added. The clay fractions consisted primarily of kaolinite. When the clay percentage of Ibadan soil was reduced by adding sand, the survival of the two strains was similar to that of Maradi sand. Thus, survival was affected by the amount but not the type of clay. The data suggest that inoculation density and the length of drying are important in the survival of cowpea bradyrhizobia undergoing desiccation in soil, and biotic factors and extracellular polysaccharide production are unimportant. Clay is important because it affects drying time.

¹ Contribution from the Dep. of Agron., Cornell Univ., Ithaca, NY 14853. Supported by a grant from the United Nations Development Programme.

² Assistant Professor, Dep. of Agron., Univ. of Georgia, Athens, GA 30602, and Professor of Soil Science, Dep. of Agron., Cornell Univ., Ithaca, NY 14853, respectively.

Received for publication November 29, 1985.

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