Naphthalic Anhydride Induces Imazethapyr Metabolism and Cytochrome P-450 Activity in Maize

Naphthalic anhydride is a seed-applied herbicide safener which reduces the toxicity of imazethapyr, an imidazolinone herbicide, to maize (Zea mays L.). Protection of maize from imazethapyr was dependent on the amount of naphthalic anhydride applied to the seed. Metabolism of imazethapyr by maize roots and shoots was increased by exposure of the roots to a solution containing naphthalic anhydride. Increased imazethapyr metabolism due to naphthalic anhydride treatment of roots was observed within 4 h following safener exposure. Nitrogen, carbon monoxide and tetcyclacis inhibited imazethapyr metabolism in maize coleoptiles grown from naphthalic anhydride treated seed. This suggests that imazethapyr is metabolized by a cytochrome P-450 monooxygenase. Naphthalic anhydride application to maize seed increased the level of cytochrome P-450 in the seedling shoots. Microsomes isolated from coleoptiles grown from naphthalic anhydride treated seed, but not untreated seed, converted bentazon to hydroxy-bentazon but did not metabolize imazethapyr. Protection of maize from imazethapyr damage by naphthalic anhydride is due to a safener-induced higher rate of imazethapyr metabolism associated with elevated cytochrome P-450 levels. However, this was not demonstrated in vitro for imazethapyr.