MODE OF ACTION OF NAPHTHALIC ANHYDRIDE AS A SAFENER FOR THE HERBICIDEAC-263222 IN MAIZE

In hydroponic experiments, seed-dressing with the herbicide safener 1, 8-naphthalic anhydride (NA), significantly enhanced the tolerance of m aize, (Zea mays L., cv. Monarque) to the imidazolinone herbicide, AC 2 63222, -methyl-5-oxo-2-imidazolin-2-yl]-5-methylnicotinic acid). Uptak e, distribution and metabolism studies where [C-14]AC 263222 was appli ed through the roots of hydroponically grown maize plants showed that NA treatment reduced the translocation of radiolabel from root to shoo t tissue and accelerated the degradation of this herbicide to a hydrox ylated metabolite. Reductions in the lipophilicity and, therefore, mob ility of this compound following hydroxylation may account for NA-indu ced retention of radiolabel in the root system. Hydroxylation of AC 26 3222 suggested that NA may stimulate the activity of enzymes involved in oxidative herbicide metabolism, such as the cytochrome P-450 mono-o xygenases. In agreement with this theory, the cytochrome P-450 inhibit or, 1-aminobenzotriazole (ABT), synergized AC 263222 activity and inhi bited its hyroxylation in vivo. NA seed-dressing enhanced the total cy tochrome P-450 and b(5) content of microsomes prepared from etiolated maize shoots. Isolated microsomes catalyzed AC 263222 hydroxylation in vitro. This activity possessed the characteristics of a cytochrome P- 450 mono-oxygenase, being NADPH-dependent and susceptible to inhibitio n by ABT. Activity was stimulated four-fold following NA seed treatmen t. Differential NA enhancement of AC 263222 hydroxylase and the cytoch rome P-450-dependent cinnamic acid-4-hydroxylase (CA4H) activity, sugg ested that separate P-450 isozymes were responsible for each activity. These results indicate that the protective effects of NA result from enhancement of AC 263222 hydroxylation and concomitant reduction in he rbicide translocation. This may be attributed to the stimulation of a microsomal cytochrome P-450 system. (C) 1998 SCI.