Uptake, translocation, and metabolism of [14C]pretilachlor in fenclorim-safened and unsafened rice seedlings

The influence of the safener fenclorim and of the synergist tridiphane on the uptake, translocation, and metabolism of the herbicide pretilachlor by 15-day-old rice seedlings was studied under greenhouse conditions. Water sown rice (Oryza sativa L., var. "Lemont") seedlings were grown hydroponically in a nutrient solution containing 10 micromole of [14C]pretilachlor and harvested following 6, 24, and 48 hr of exposure. Fenclorim and tridiphane were also used at 10 micromole in the nutrient solution when needed. The uptake of root-applied pretilachlor by control or fenclorim-treated rice seedlings increased steadily with time. Fenclorim and tridiphane significantly reduced the root uptake of pretilachlor by rice seedlings at 24 and 48 hr after application of [14C]pretilachlor. TLC analysis of water-soluble extracts from rice roots and shoots revealed five major metabolites of pretilachlor. Fenclorim enhanced the metabolism of pretilachlor via conjugation to reduced glutathione (GSH) at all time intervals. This safener-induced enhancement of the glutathione-mediated metabolism of pretilachlor was coupled with a concomitant decrease in the amount of radioactivity corresponding to the parent herbicide and was more pronounced in root extracts than in shoot extracts. Tridiphane slightly reduced the formation of the glutathione conjugate of pretilachlor in both roots and shoots of treated rice seedlings only at the 48-hr time period. The effect of fenclorim on the nonenzymatic formation of the glutathione conjugate of pretilachlor was also studied. Fenclorim used over a range of concentrations (1 to 160 micromole) increased the reactivity of pretilachlor with GSH in a concentration-dependent manner. The results of the present study show that the protection afforded by fenclorim to rice appears to result from a fenclorim-mediated reduction in the root uptake of pretilachlor and an enhancement of the metabolism of pretilachlor via enzymatic or nonenzymatic conjugation with GSH.