Use of wild soybean ( Glycine ussuriensis Regel and Maack) as a test plant in dilution—nodulation frequency tests for counting Rhizobium japonicum

Abstract

In cross-inoculation tests with Glycine ussuriensis Regel and Maack and soybean, Glycine max (L.) Merrill, it was found that the species were very similar symbiotically. This finding is consistent with cytogenetic evidence that taxonomically G. ussuriensis and G. max are one species. A dilution-nodulation frequency technique (plant-infection test) is described whereby G. ussuriensis, a small-seeded species, is grown in cotton-wool-plugged specimen tubes as a test plant for identification and enumeration of Rhizobium japonicum. In a pure culture comparison with direct counting (agar plate counts), the plant-infection test gave an accurate estimate of numbers provided that a plant nutrient agar substrate was used for growing the test plants. This indicated that the probability of a single rhizobial cell initiating nodulation of the test plant was close to P = 1. With a vermiculite substrate, the plant-infection test was less reliable. Tables are presented which permit calculation of most probable numbers of nodule bacteria from the distribution of positive (nodulated) test plants in plant-infection tests based on 5- and 10-fold dilution series.