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 70 0 g ha(-1) imazethapyr to reduce shoot growth 50% while corn required 70 g ha(-1) to reduce growth 50% in greenhouse experiments. Extractabl e acetohydroxy acid synthase activity from cowpea 24 h after foliar im azethapyr treatment was only 20% of that of untreated cowpea but incre ased to control levels 72 h after treatment (HAT). The unifoliate leav es of cowpea absorbed 25% of the C-14-imazethapyr applied 72 HAT. Move ment of C-14-imazethapyr out of the treated zone increased gradually t o approximately 47% of the absorbed C-14 168 HAT. Parent imazethapyr d ecreased from 41 to 27% of the recovered C-14 between 4 and 24 HAT,and continued to decrease to 14% by 168 HAT. Polar metabolites increased from 27 to 55% of the recovered C-14 from 24 to 168 HAT. The polar met abolites 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.