Adsorption of Slow- and Fast-Growing Rhizobia to Soybean and Cowpea Roots

Abstract

Roots of soybean and cowpea were immersed in suspensions containing 104 Rhizobium cells/ml of a N-free solution. After 30-120 min the roots were rinsed, and the distal 2 cm segments excised and homogenized. Portions of the homogenates then were plated on a yeast-extract mannitol medium for bacterial cell counts. The adsorption capacities of 4 slow-growing rhizobia and a fast-growing R. meliloti strain varied considerably. Adsorption was independent of plant species and of the abilities of the Rhizobium strains to infect and nodulate. R. lupini 96B9 had the greatest absorption capacity, and Rhizobium sp. 3G4b16 the least. Rhizobium sp. 229, R. japonicum 138 and R. meliloti 102F51 were intermediate, except on cowpea, where the adsorption of strain 102F51 was similar to that of strain 3G4b16. The initial adsorption rates of bacteria cultured in synthetic media and in the presence of soybean roots were about the same. Addition of soybean lectin to the bacterial inoculum failed to influence initial adsorption rates. Both treatments reduced the numbers of bacteria that bound after incubation with roots for 120 min. The relationship between the logarithm of the number of strain 138 cells bound per soybean root segment and the logarithm of the density of bacteria in the inoculum was linear over 5 orders of magnitude. Binding of strain 138 to soybean roots was greatest at room temperature (27.degree. C) and substantially at both 4.degree. and 37.degree. C. Although R. lupini 96B9 strongly rejected a model hydrophobic plastic surface, there were no simple correlations between bacterial binding to model hydrophobic and hydrophilic plastic surfaces and bacterial adsorption to roots.