Acetolactate Synthase Herbicides Research Articles

Sulfonylurea Selectivity and Safener Activity in ′Landmark′ and ′Merit′ Sweet Corn

The sulfonylurea herbicides primisulfuron and nicosulfuron are phytotoxic to the sweet corn cultivar Merit, whereas most other cultivars, including ′Landmark,′ are relatively resistant. The mechanism of herbicide resistance between ′Merit′ and Landmark was studied. The sensitivity of acetolactate synthase (ALS) activity to primisulfuron and nicosulfuron in protein extracts from Merit and Landmark was compared as a possible mechanism of resistance. The rates of ALS activity from the two varieties were similar, at 0.58 and 0.57 μmol · (mg protein · hr)−1 for Merit and Landmark, respectively. Enzyme activity from both varieties was sensitive to herbicide inhibition. I50 values for primisulfuron were 16 and 12 nM for inhibition of ALS activity from Merit and Landmark, respectively. I50 values for nicosulfuron were 340 and 295 nM for inhibition of ALS activity from Merit and Landmark, respectively. The relative levels of primisulfuron and nicosulfuron metabolism in Merit and Landmark were also considered as a mechanism of resistance. Excised shoots from Landmark had a much greater capacity to metabolize primisulfuron and nicosulfuron than Merit. With Merit, after 8 hr, 8.3 and 13.3% of the extracted radioactivity was found as primisulfuron and nicosulfuron metabolites, respectively, whereas 60.5 and 63.5% of the extracted radioactivity in Landmark was found as primisulfuron and nicosulfuron metabolites, respectively. The effect of the safener BAS 145 138 on ALS activity and herbicide metabolism in both cultivars was also studied. BAS 145 138 did not affect ALS activity or herbicide sensitivity in either species. BAS 145 138 did not affect primisulfuron or nicosulfuron metabolism in Merit, whereas BAS 145 138 increased the metabolism of both herbicides in Landmark. Safener treatment increased the metabolism of primisulfuron from 33% in untreated to 49% in treated Landmark seedlings. The metabolism of nicosulfuron increased from 61% in the untreated Landmark to 84% in the treated seedlings. Because ALS activity from both Merit and Landmark was equally sensitive to both nicosulfuron and primisulfuron, differential sensitivity at the proposed biochemical site of action does not account for the observed selectivity at the whole plant level. The greater level of sulfonylurea metabolism in Landmark than Merit suggests that resistance to these herbicides is due to metabolic detoxication of the parent herbicide. BAS 145 138 enhanced herbicide metabolism in Landmark but not Merit.