Counting Rhizobium sp. (Phaseolus) by a plant-infection method using a small-seeded cultivar of Phaseolus silvestre L. grown enclosed in test tubes

Abstract We report laboratory experiments that tested the suitability of a wild-type small-seeded cultivar of Phaseolus silvestre L., Morelos 628, for counting Rhizobium sp. ( Phaseolus ) by the most probable number method (MPN). There was good agreement between MPN estimates of numbers in pure cultures using Morelos 628 grown in test tubes and a larger commercial variety of P. vulgaris L. grown in Leonard jars, and with colony counts on yeast-mannitol agar (YMA) plates. Morelos 628 did not nodulate in bacteria-free conditions when inoculated with unrelated strains of root nodule bacteria. After 28 days, MPN estimates of R. sp. ( Phaseolus ) were not significantly different from counts made after 3 days on YMA, at either low or high cell concentrations in pure culture or in unsterile soil. Nodulation of Morelos 628 was at a maximum after 21 days.