Effect Of Glyphosate On Symbiotic Nitrogen Fixation In Glyphosate Resistant Soybean.

Most of the soybean (Glycine max L. Merr.) grown in the U.S. is Roundup Ready, genetically modified to be resistant to glyphosate herbicide. There is concern that the application of glyphosate may negatively affect nitrogen fixation in glyphosate resistant (GR) soybean. Greenhouse experiments were conducted to examine the impact of glyphosate application on nitrogen fixing activity in GR soybean. Glyphosate treatments (1.68 kg a.e./ha) to GR soybean were: none, once, or twice during the study period. The isogenic conventional cultivar did not receive glyphosate and served as a control. Plants were harvested 2 days after each glyphosate application. In addition to soybean growth parameters, the nitrogenase activity of root nodules and nifH gene abundance in the rhizosphere soil were determined using the acetylene reduction assay and quantitative polymerase chain reactions, respectively. Glyphosate treated GR soybean had lower chlorophyll content, root mass, nodule mass, total plant nitrogen, and nitrogenase activity than the conventional cultivar, especially for the second harvest (V5 to V6 stage). Effects of glyphosate on shoot mass and nodule number did not show a consistent trend. The nifH gene abundance in the rhizosphere of GR soybean was not affected by glyphosate application. For most of the parameters measured in this study, no significant differences were observed between the conventional and non-treated resistant cultivars. Field experiments will be conducted to confirm the findings of greenhouse experiments.