Effect of metribuzin, butachlor and chlorimuron-ethyl on amino acid and protein formation in wheat and maize seedlings M.M. Nemat Alla * , A.M. Badawi, N.M. Hassan, Z.M. El-Bastawisy, E.G. Badran Botany Department, Faculty of Science at Damietta, Mansoura University, Damietta, Egypt Received 21 January 2007; accepted 10 July 2007 Available online 31 July 2007 Abstract 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, partic- ularly 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. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Wheat; Maize; Herbicides; Nitrogen metabolism; Nitrogen-related enzymes 1. Introduction Herbicides drastically influence all aspects of primary and secondary metabolism in crops when given to control undesired weeds. Metribuzin [4-amino-6-tert-butyl-4,5- dihydro-3-methylthio-1,2,4-triazin-5-one], a triazinone her- bicide, inhibits photosynthesis [1]. It interferes with photo- synthetic electron transport between the primary and secondary acceptor of PSII [2]. Thus CO 2 assimilation is decreased leading to starvation and reactive oxygen species are formed causing oxidative stress [3,4]. Butachlor [N-but- oxymethyl-(2-chloro-2,6-diethylacetanilide], the chloroace- tanilide herbicide, affects seed germination, lipid metabolism, pigment and gibberelic acid synthesis, cell division, cell permeability, mineral uptake and disturb the absorption and incorporation of amino acids into protein [5,6]. The sufonylurea herbicide, chlorimuron-ethyl [Ethy- 2-(((((4-chloro-6-methoxypyrimidin-2-yl)amino)carbonyl) amino) sufonyl) benzoate], inhibits acetohydroxyacid syn- thase (AHAS), the key enzyme for biosynthesis of valine, leucine and isoleucine [7,8]. These herbicides have varied modes of action, however, they could interfere with nitro- gen cycle either directly or indirectly. Ammonia is regarded as very important for plant survival; however, its accumu- lation would be harmful. Among the major processes liber- ating ammonia is the reduction of nitrate and nitrite by NR and NiR, respectively [9]. Then ammonia is assimilated into an organic form by a two-step reaction. In the first step, glutamine is produced from the amination of L-gluta- mate by GS while in the second, two molecules of L-gluta- mate are produced from the reaction of glutamine and 2-oxoglutarate by GOGAT [10,11]. So, the changes of these activities might disturb the synthesis of amino acids and protein. Many plant species could tolerate such modi- fications. Therefore, the present work was aimed to ascer- tain the differential tolerance of wheat and maize to 0048-3575/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.pestbp.2007.07.003 * Corresponding author. Fax: +20 57 2403868. E-mail address: mamnematalla@mans.edu.eg (M.M. Nemat Alla). www.elsevier.com/locate/ypest Available online at www.sciencedirect.com Pesticide Biochemistry and Physiology 90 (2008) 8–18 PESTICIDE Biochemistry & Physiology