Effect of metribuzin, butachlor and chlorimuron-ethyl on amino acid and protein formation in wheat and maize seedlings

Application of the recommended field dose of metribuzin, butachlor and chlorimuron-ethyl to 10-days-old wheat and maize seedlings differentially reduced shoot fresh and dry weights during the following 16 days. Metribuzin was the most reductive while butachlor was the least. The herbicides slightly affected the activities of nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.7.7.1) but greatly inhibited glutamine synthetase (GS, EC 6.3.1.2) and glutamate synthase (GOGAT, EC 1.4.7.1) activities. Meanwhile, there were significant accumulations of ammonia and soluble-N accompanied by diminutions in total-N and protein contents; metribuzin exerted the greatest changes. Additionally, aliphatic, aromatic and total amino acids in both species were mostly elevated by the three herbicides; however, valine, leucine and isoleucine were decreased by only chlorimuron-ethyl. These results could conclude that herbicides, particularly metribuzin, cause a shortage in ammonia assimilation and subsequently a decrease in protein formation. Moreover, the elevation of soluble-N and amino acids appeared to result from breakdown of the pre-existing protein, a state that seemed consistent in seedlings treated with metribuzin and, to some extent chlorimuron-ethyl but recovered in those treated with butachlor.