Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 2008
Publication Date: March 26, 2008
Citation: Bellaloui, N., Zablotowicz, R.M., Reddy, K.N., Abel, C.A. 2008. Nitrogen Metabolism and Seed Composition as Influenced by Glyphosate Application in Glyphosate-Resistant Soybean. Journal of Agriculture and Food Chemistry. 56: 2765-2772. Interpretive Summary: Most soybean cultivars now in U.S.A. are resistant to the herbicide or weed killer called glyphosate. Although the use of glyphosate resistant soybean cultivars is economical and important for weed management, visual injuries caused by herbicide has been of concern to growers and the scientific communities. Herbicides are known to influence nitrogen and carbon metabolism leading to a change in protein and oil in seed. The potential damage of glyphosate application on nitrogen assimilation (nitrogen derived from soil or fertilizer nitrogen) and nitrogen fixation (nitrogen derived from air) was investigated. There were no significant differences in yield or nitrogen fixation between glyphosate-treated and non-treated soybean. However, there was significant decrease in total nitrogen assimilation per plant. Higher glyphosate application resulted in higher protein and oleic acid percentages, but lower oil and linolenic percentages. The change in protein and oil appears to be a stress response to higher rates of glyphosate application. This research suggests that glyphosate application, especially at higher rates, can alter nitrogen and carbon metabolism, leading to protein and oil changes. This research provides researchers and growers with information on the potential changes in soybean seed from glyphosate application.
Technical Abstract: Our previous research demonstrated that glyphosate drift affected nitrogen fixation and nitrogen assimilation in glyphosate-sensitive soybean at early growth stage. The objective of the present study was to investigate the effects of glyphosate application (Gly) of 1.12 kg ae ha-1 and 3.36 kg ae ka ha-1 on nitrogen metabolism and seed composition in glyphosate-resistant (GR) soybean. Results from a 2-yr field trial, conducted in 2006 and 2007, showed that there was no effect of Gly on nitrogen fixation as measured by acetylene reduction assay and delta 15N natural abundance. However, there were significant effects of Gly on nitrogen assimilation, as measured by in vivo nitrate reductase (NR) assay, in leaves, roots, and nodules, especially at the higher rate (3.36 kg ae ha-1) at early growth stage. Protein percentage was significantly higher (10.3%), compared to untreated plants, in soybean treated with the higher Gly level . Inversely, percentage of total oil and linolenic acid was lower at the high Gly rate. Oleic acid percentage was greatest (22% increase) in treated soybean. Gly application had no significant effect on soybean yield or saturated fatty acids (palmitic and stearic) percentage of seed at both rates. These results suggest that nitrate assimilation in soybean was more affected than nitrogen fixation under higher rates of GLY. The inhibition of NRA may be due to the absence of substrate (nitrate), in leaves and roots or a temporary inhibition of de novo synthesis of the NR enzyme. Higher protein percentage and higher oleic acid and lower linolenic acids in treated plants compared to non-treated may be due to altered carbon and nitrogen metabolism from the higher glyphosate rate.