Influence of herbicides on Rhizobium growth and its symbiosis with pigeonpea

The present study was undertaken to assess the effect of herbicides imazethapyr, paraquat and pendimethalin on in vitro growth of Rhizobium in terms of optical density and on symbiotic and growth parameters under field conditions. Pendimethalin at 1980 and 396 ppm did not affect the growth of Rhizobium and was at par with that observed in control medium, however, imazethapyr significantly inhibited Rhizobium growth at both concentrations. Paraquat at 255 ppm showed marginal increase in Rhizobium growth, however, at 1272 ppm it totally inhibited the growth. Under field conditions, nodulation efficiency viz., number and dry weight of nodules was not affected in case of pendimethalin @ 0.45 kg/ha or 0.75 kg/ha and was at par with two hand weeding treatments. This is in corroboration with the response recorded under in vitro conditions. The treatment of combination of herbicides too did not largely affect symbiotic parameters. Herbicide treatments comprising of pendimethalin followed by paraquat 6 or 8 weeks after sowing (WAS) offered grain yield at par to that obtained by two hand weedings treatment