A Biological-chemical Test System to Investigate the Influence of Sulfonylurea Herbicides such as Metsulfuron-methyl on Nitrogen Metabolism in Soybean

Summary A simple and reliable method for the identification of residues of sulfonylurea herbicides in rotational farming systems has been developed using metsulfuron-methyl as a test substance. Metsulfuron-methyl such as other sulfonylurea herbicides impairs the synthesis of branched-chain amino acids and, in consequence, protein synthesis. Thus, nitrogen consumption is reduced, leading to an accumulation of amino acids and a reduction of nitrogen uptake. The biological-chemical test system is based on the determination of aspartic acid, asparagine, glutamine, and NH4 + by ion chromatography and is suitable for monitoring changes of nitrogen metabolism in soybean seedlings. Of the measured free amino acids, glutamine and aspartic acid represent key substances that reflect changes in nitrogen uptake, assimilation, transport, and storage. Their concentrations increased due to metsulfuron-methyl 2–15 times in roots, stems, and leaves. The asparagine content is a measure of the nitrogen status of soybean. However, the sensitivity of asparagine content to metsulfuron-methyl depended on the nutritious status of the plant. In case the asparagine level was already high, the herbicide did not induce a significant increase in asparagine. The NH4 + concentration may also add valuable information in case NH4 + is present in the root medium. With respect to the test system, the lower stem section of soybean seedlings represents the optimal plant section for free amino acid analyses, because samples can reproducibly be taken, the concentration of the cell sap is, compared to roots, less influenced by the water status, and this section is, compared to leaves, rather sensitive to sulfonylurea herbicides in the root medium. A reduction of leaf transpiration rate is a further effect of metsulfuron-methyl, so that its measurement may represent a valuable additional parameter to the described biological-chemical test system.