INFLUENCE OF THE HERBICIDE PHOSPHINOTHRICIN ON GROWTH AND NODULATION CAPACITY OF RHIZOBIUM-MELILOTI

Rhizobium meliloti proved to be sensitive to low concentrations of the herbicide phosphinothricintripeptide (PTT) and its active ingredient phosphinothricin (PT), which was formerly assumed to be nontoxic for m ost of the bacteria analysed. Growth was more strongly reduced in ster ile synthetic media and less reduced in sterile soil; in unsterile soi l only a transient growth reduction was detectable. Sensitivity was al so observed in five out of eight other species analysed. In all sensit ive species tested, spontaneous resistances to PT occurred. Under ster ile conditions, PTT and PT reduced rhizobial nodulation rates of PT-re sistant alfalfa plants drastically; however, nitrogen fixation in the few nodules that arose was unaffected. Because of the small number of nodules, the overall fixation rate was strongly diminished. In unsteri le soil, nodulation and nitrogen fixation rates were not changed, poss ibly because of the rapid degradation of PTT and PT in the soil. Using a herbicide as model substance it could be demonstrated that the sens itivity of R. meliloti to chemical additives in the soil can be detect ed by analysing its growth rate, and that even a weak impact carl infl uence its nodulation capacity. R. meliloti has proven to be a fast, ea sy and sensitive detection system for bacteriostatic components presen t in the soil.