The basis of imazethapyr tolerance in cowpea (Vigna sinensis)

Greenhouse and laboratory experiments were conducted to determine the basis of imazethapyr tolerance in cowpea. Cowpea required more than 700 g ha^-1 imazethapyr to reduce shoot growth 50% while corn required 70 g ha^{- 1} to reduce growth 50% in greenhouse experiments. Extractable acetohydroxy acid synthase activity from cowpea 24 h after foliar imazethapyr treatment was only 20% of that of untreated cowpea but increased to control levels 72 h after treatment (HAT). The unifoliate leaves of cowpea absorbed 25% of the ¹⁴C-imazethapyr applied 72 HAT. Movement of ¹⁴C-imazethapyr out of the treated zone increased gradually to approximately 47% of the absorbed ¹⁴C 168 HAT. Parent imazethapyr decreased from 41 to 27% of the recovered ¹⁴C between 4 and 24 HAT, and continued to decrease to 14% by 168 HAT. Polar metabolites increased from 27 to 55% of the recovered ¹⁴C from 24 to 168 HAT. The polar metabolites of imazethapyr included hydroxyimazethapyr and conjugates of hydroxyimazethapyr. Tolerance of cowpea to imazethapyr is based on the plants ability to metabolize the herbicide to nontoxic metabolites.