Biotoxic effects of pesticides on symbiotic properties of Rhizobial sps

The effects of 15 commonly used pesticides on nodulation, nitrogenase enzyme activity, and sym plasmids in three rhizobial species were investigated. The plants used were pigeon pea, lentil, and alfalfa, all of which possess symbiotic plasmids Rhizobium sp. The pesticides investigated were monocrotophos, acephate, dichlorvos, DDT, Danitol, Sumicidin, Blitox, captan, captfol dithianon, hexaconazole, Zineb, butachlor, simazine, and oxyflorfen. The minimum inhibitory concentration (MIC) of the pesticides was determined, and then plant assays for nitrogen fixation and for the detection of plasmids were carried out. The MIC of the pesticides ranged between 700-25,600 ppm. Of the 15 pesticides, 13 of them showed a 55-100% loss of both nodulation for pigeon pea, as well as nitrogenase activity. Zineb and DDT showed a maximum of 16% loss at the highest concentration tested. For lentil, fungicides and herbicides showed a 20-40 and a 35-75% loss, respectively. In alfalfa, reduced efficiency amounted to 20-100% in nodulation and nitrogenase activity. All cultures that had lost the symbiotic properties also exhibited plasmid loss.