Ion chromatographic analysis of selected free amino acids and cations to investigate the change of nitrogen metabolism by herbicide stress in soybean (glycine max).

A simple and reliable method for the determination of NH 4 + , K + , Na + , aspartic acid, asparagine, glutamine, and alanine by ion chromatography has been developed. It is suitable for monitoring changes of nitrogen metabolism in soybean because it can accurately measure concentrations of asparagine and NH 4 + , two key substances for nitrogen storage and transport in this plant species. Analysis of asparagine distribution in soybean indicated that higher levels (up to 18.4 μmol g -1 of fresh mass) occur in stems and lower levels in roots (2.0 μmol g -1 of fresh mass) and leaves (1.6 μmol g -1 of fresh mass). When the herbicide metsulfuron-methyl (0.5, 5, and 50 ppb) was applied via the nutrient solution to the root system, asparagine concentrations increased 3-6 times in stems, roots, and leaves. Metsulfuron-methyl is known to impair the synthesis of branched amino acids and, in consequence, protein synthesis. Thus, nitrogen consumption was limited, leading to an accumulation of asparagine. The possible use of this physiological response in agricultural practice to identify herbicide stress in soybean and to detect low-level residues of sulfonylurea herbicides in the soil is discussed.