The antagonistic and synergistic effects of naphthalic anhydride and 1-aminobenzotriazole to imazethapyr toxicity in maize

Ten-day-old maize seedlings, grown from grains dressed with naphthalic anhydride (NA, 0.4% w/w by seed weight), were treated with imazethapyr (Imz), 1-aminobenzotriazole (ABT) or their combination and samples were collected after 5 days. Imz provoked significant reduction in growth parameters concomitant with inhibition in acetohydroxyacid synthase (AHAS) activity, protein, branched-chain amino acids, glutathione (GSH) and ascorbate (AsA). However, lipid peroxides and H2O2 were elevated while the activities of nitrate reductase/nitrite reductase (NR/NiR), glutamine synthetase/glutamate synthase (GS/GOGAT) and catalase/ascorbate peroxidase (CAT/APX) were inhibited. The application of NA alleviated these impacts and declined Imz residues while ABT led to high levels; nonetheless, NA became no longer effective when ABT was present. These findings indicate that Imz toxicity in maize is antagonized by NA and synergized by ABT, such antagonism or synergism could be concluded to be attributed to enhanced detoxification or protraction of herbicide persistence, respectively. On the other hand, the changes in both AHAS activity and the branched-chain amino acids due to treatments were highly correlated. The slight changes in NR/NiR and the inhibition in GS/GOGAT reveal that protein drop is not due to ammonia demand but to decreased branched-chain amino acids. Moreover, Imz decreased Vmax of AHAS, NR, NiR, GS, GOGAT, CAT and APX but unchanged their Km except the increased Km of only AHAS indicating that the inhibition is mixed for AHAS and noncompetitive for the others.