Effects of herbicides Gramoxone, Diuron and Totacol® on growth and nodulation of three strains of Rhizobium meliloti

We have studied the influence of three herbicides: Gramoxone, Diuron and Totacol®, on the growth of three strains of Rhizobium meliloti and on many nodulation parameters which seem to be affected by the presence of herbicides in culture medium. Toxicity of the assayed herbicides as a function of growth varied, but not considerably, among the three strains assayed were: TA, M-79 and 870. All of them proved to be more sensitive to Gramoxone than to Diuron or Totacol®. Recommended field dose (10 ppm) did not affect exponential growth rate; concentrations of at least 300 ppm were necessary to inhibit growth. However, the pretreatment with only 10 ppm of any of the herbicides assayed, caused a decrease in the in vitro infectivity, represented by a lower number of nodules per plant and nodulation percentage. In the same way, plant length and crop or harvest weight were less than control values. Nitrogenase activity evaluated by an acetylene-reduction assay (ARA) was unaffected by pretreatment with 10 ppm, but seriously reduced by 50 ppm of any of the three herbicides. We suggest that these herbicides could affect molecular interaction processes which lead to the recognition, binding and penetration of rhizobia in leguminous roots. We did not observe in the Totacol® (mixture of 10% Paraquat and 30% Diuron) a synergistic toxic effect on symbiosis. Therefore, if this mixture allows us to lower the field dose because of its high phytotoxicity, we could consider it as a good alternative to other herbicides applied to leguminous crops.