Herbicide effects on the growth and nodulation potential of Rhizobium trifolii with Trifolium subterraneum L

A study was made of the effect of the herbicides 2,4-D, amitrole, atrazine, chlorsulfuron, diclofop-methyl, diquat, glyphosate, paraquat and trifluralin on the nodulation of sub-clover (Trifolium subterraneum L. 'Clare'), the growth of R. trifolii TA1 in liquid nutrient medium and the ability of herbicide-treated inoculum to successfully nodulate sub-clover plants. As concentrations of amitrole, diclofop-methyl and glyphosate in the rooting environment increased from 0 to 20 mg ai L⁻¹, nodulation decreased linearly. The other herbicides at these concentrations caused more severe decreases in nodulation. Growth of R. trifolii TA1 in nutrient broth was significantly retarded by all concentrations of diquat, 2 mg ai L⁻¹ of paraquat, 10 mg ai L⁻¹ of glyphosate and 2 mg ai L⁻¹ of chlorsulfuron. Other herbicides did not suppress rhizobial growth. Inoculation with TA1 that had been grown in the presence of amitrole, atrazine or glyphosate and then washed free of the herbicide decreased nodulation of sub-clover, indicating that these herbicides may physiologically influence the nodulating potential of certain strains of Rhizobium. The remaining herbicides showed no indications of this effect.