Root/Rhizome Exudation of Nicosulfuron from Treated Johnsongrass (Sorghum halepense) and Possible Implications for Corn (Zea mays)

Abstract

Experiments were conducted to evaluate the occurrence and significance of release of herbicide through subterranean parts of nicosulfuron-treated johnsongrass. In a bioassay, the rooting medium of johnsongrass treated foliarly with 50 or 100 μg nicosulfuron plant−1was inhibitory to the radicle elongation of sorghum and corn indicating the increased toxicity of the rooting medium of nicosulfuron-treated johnsongrass. The study with14C-nicosulfuron indicated a basipetal translocation of foliarly applied nicosulfuron in johnsongrass to its roots/rhizomes and also into the rooting medium. By 30 DAT, around 23% of the14C-label absorbed by johnsongrass was found exuded into the rooting medium. Radiochromatogram scans of thin layer chromatography plates of rooting medium indicated unmetabolized nicosulfuron as the major14C-labeled compound (56%). The study also revealed a subsequent uptake of exuded14C by corn roots sharing the medium. On the whole, the amount of14C-label recovered from untreated corn amounted to 4.3% of the total applied to johnsongrass. In another experiment, the presence of nicosulfuron in the rooting medium was detrimental to corn growth. Reductions in corn growth occurred at concentrations of 10−8M nicosulfuron or greater in the rooting medium. The sensitivity of corn to root uptake was attributed to greater accumulation of nicosulfuron at a faster rate in the growing parts.