Differential Intraspecific Responses of Soybean Cultivars to Bentazon

Studies of the differential responses of soybean [ Glycine max (L.) Merr.] cultivars to bentazon [3-isopropyl-1 H -2,1,3,-benzothiadiazin-(4)3 H -one 2,2-dioxide] revealed slightly greater translocation and about a twofold greater foliar absorption in the sensitive cultivar ‘PI 229.342 (Nookishirohana),’ than in the tolerant cultivar, ‘Clark 63.’ Clark 63 metabolized bentazon faster than did PI 229.342. Bentazon was metabolized to unidentified polar metabolites I and II in Clark 63, and only to metabolite II in PI 229.342. The differential absorption and metabolism resulted in a tenfold greater concentration of bentazon in the treated leaflet of PI 229.342 than in that of Clark 63 at 8 days after foliar treatment. Leaflets contained similar amounts of 14 C in petiole uptake studies, but no bentazon was detected in Clark 63 leaflets 24 hr after pulse treatment, whereas about 20% of the methanol-extractable 14 C from PI 229.342 leaflets cochromatographed with bentazon. Metabolite I was labile to selected enzymes with β -glycosidic activity, whereas metabolite II was unaffected. Bentazon concentrations of 30 to 50 μM inhibited O 2 evolution about 50% in isolated chloroplasts from the two cultivars. Differential bentazon metabolism appears to be the main cause of the differential responses of these soybean cultivars to bentazon.