Effects of Glyphosate Application on Nitrogen Assimilation, Nitrogen Fixation, and Seed Composition in Glyphosate-Resistant Soybean.

The objective of this study was to investigate the effects of glyphosate application of 1.12  and 3.36 kg of ae ha^-1 on nitrogen assimilation, nitrogen fixation, and seed composition in glyphosate-resistant (GR) soybean. There was no effect of glyphosate application on nitrogen fixation as measured by acetylene reduction assay, yield, or seed nitrogen percentage. However, there were significant effects of glyphosate application on nitrogen assimilation, as measured by in vivo nitrate reductase activity (NRA) in leaves, roots, and nodules, especially at high rate. Transient lower leaf nitrogen or ¹⁵N natural abundance in high glyphosate application soybean supports the inhibition of NRA. With the higher glyphosate application level protein was significantly higher (10.3% increase) in treated soybean compared to untreated soybean. Inversely, total oil and linolenic acid were lowest at the high glyphosate application rate, but oleic acid was greatest (22% increase) in treated soybean. These results suggest that glyphosate application may alter nitrogen and carbon metabolism.