Nodulation of Phaseolus vulgaris L. as affected by Rhizobium phaseoli growth phase

Abstract

The influence of culture age and of growth rate on the nodulation ability of strain F 45 str. Rhizobium phaseoli was studied. Roots of Phaseolus vulgaris L., grown in pouches, were infected with rhizobial suspensions (about 1 × 105 cells/root) taken from different batch cultures at different growth phases. After 24 h the free bacterial population was inhibited by adding tetracycline to the rooting medium. Nodules were counted 15–20 days after inoculation. More nodulation was obtained with rhizobia from early, mid, or late exponential phase than from lag or stationary phases. Differences in nodulation obtained had no correlation to the root attachment capacity of the cells nor to the rhizobial binding to Phaseolus vulgaris L. seed lectin. Bacterial attachment to bean roots was maximal with stationary phase bacteria, while lectin binding reached its maximal value with early exponential phase rhizobia, being very low with mid exponential phase cells. These results suggested that the difference in nodulation achieved with Rhizobium phaseoli at different growth phases could be caused by a step of the infection process not related to early (1 h) microbial attachment to roots nor to bacterial binding to Phaseolus vulgaris L. lectin.