Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/8/2007
Publication Date: 4/7/2007
Citation: Nandula, V.K., Reddy, K.N., Rimando, A.M., Duke, S.O., Poston, D.H. 2007. Glyphosate resistant and susceptible soybean (Glycine max) and canola (Brassica napus) dose response and metabolism relationships with glyphosate. Journal of Agricultural and Food Chemistry 55:3540-3545. Interpretive Summary: Genetically modified crops resistant to the herbicide glyphosate (GR, Roundup Ready®) are widely used in North America. Glyphosate may occasionally damage GR soybeans due to accumulation of AMPA, a derivative of glyphosate formed by plants. Scientists from Delta Research and Extension Center, Mississippi State University, and USDA-ARS, Southern Weed Science Research Unit, Stoneville, MS; and USDA-ARS, Natural Products Utilization Research Unit, Oxford, MS have studied response and fate of glyphosate in GR and glyphosate-sensitive (non-GR) soybean and canola. The results show that GR soybean and canola were about 50-fold more tolerant to glyphosate than non-GR varieties. Glyphosate caused injury in GR soybean, but did not injure GR canola, despite the fact that canola is quite sensitive to AMPA. These results demonstrated that AMPA derived from glyphosate can behave different in GR soybean and GR canola.
Technical Abstract: Experiments were conducted to determine 1) dose response of glyphosate-resistant (GR) and –susceptible (non-GR) soybean [Glycine max (L.) Merr.] and canola (Brassica napus L.) to glyphosate, 2) if differential metabolism of glyphosate to aminomethylphosphonic acid (AMPA) is the underlying mechanism for differential resistance to glyphosate among GR soybean varieties, and 3) extent of metabolism of glyphosate to AMPA in GR canola and to correlate it to injury from AMPA. GR50 (glyphosate dose required to cause a 50% reduction in plant dry weight) values for GR (Asgrow 4603RR) and non-GR (HBKC 5025) soybean were 22.8 kg ae ha-1 and 0.47 kg ha-1, respectively, with GR soybean exhibiting a 49-fold level of resistance to glyphosate compared with non-GR soybean. Differential reduction in chlorophyll by glyphosate was observed between GR soybean varieties, but there were no differences in shoot fresh weight reduction. No significant differences were found between GR varieties in metabolism of glyphosate to AMPA, and in shikimate levels. These results indicate that GR soybean varieties were able to outgrow the initial injury from glyphosate, which has been shown previously to be caused at least in part by AMPA. GR50 values for GR (Hyola 514RR) and non-GR (Hyola 440) canola were 14.1 kg ha-1and 0.30 kg ha-1, respectively, with GR canola exhibiting a 47-fold level of resistance to glyphosate when compared with non-GR canola. Glyphosate did not cause reduction in chlorophyll content and shoot fresh weight in GR canola, unlike GR soybean. Less glyphosate (per unit leaf weight) was recovered in glyphosate-treated GR canola compared to glyphosate-treated GR soybean. External application of AMPA caused similar injury in both GR and non-GR canola. Presence of a bacterial glyphosate oxidoreductase gene in GR canola contributes to breakdown of glyphosate to AMPA. However the AMPA from glyphosate breakdown could have been metabolized to nonphytotoxic metabolites before causing injury to GR canola. Injury in GR and non-GR canola from exogenous application of AMPA was similar.