Submitted to: Applied Soil Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/2/2009
Publication Date: 12/2/2009
Citation: Zobiole, L.H., Oliveira, R.S., Kremer, R.J., Constantin, J., Yamada, T., Castro, C., Oliveira, F.A., Oliveria, A. 2010. Effect of Glyphosate on Symbiotic N2 Fixation and Nickel Concentration in Glyphosate-Resistant Soybean. Applied Soil Ecology. 44:176-180. Interpretive Summary: The widespread use of the herbicide glyphosate and glyphosate-resistant (GR) crops represents a major advancement in effective weed management in conventional agricultural systems. One chemical property of the glyphosate molecule is the ability to tie up or immobilize certain nutrients essential for crop growth. An important micronutrient that might be affected by glyphosate is nickel (Ni), which is a critical component in the soybean-bacteria (rhizobia) interaction in root nodules that traps nitrogen from the atmosphere and makes it available to the soybean plant, a process known as symbiotic nitrogen fixation. Ni helps make the process energy efficient by enabling the rhizobia to recycle energy to drive nitrogen fixation that otherwise would be lost as gaseous hydrogen, a detrimental side effect of the process. Glyphosate applied to GR soybean can be released through roots and interact with soil nutrients. If Ni is immobilized by glyphosate, the efficiency of nitrogen fixation might be reduced and soybean may obtain less than adequate nitrogen, possibly reducing plant growth and seed yield. We investigated the effect of glyphosate on nodule formation and its interrelation with Ni in GR soybean plants growing in two different soils to determine if nitrogen fixation might be affected in cropping systems relying on glyphosate for weed management. We conducted greenhouse studies showing root nodulation, plant Ni concentration, and plant dry weights were consistently lower for GR soybean treated with glyphosate compared with GR soybean without glyphosate and the soybean parental lines that were not genetically modified to be GR. Photosynthesis was also reduced by glyphosate in all GR soybean varieties except the late-maturing variety, which is important because this process provides the food source for nitrogen fixation symbiosis. Although future field trials are required to determine if glyphosate affects Ni availability and the plant processes under variable environmental conditions, these preliminary results help partially explain some of the non-target effects of glyphosate that have been observed in many soybean production fields. The results are useful to other researchers, extension personnel, and farmers by providing an understanding of complex factors involved in the plant-soil-microbial relationships with which glyphosate, uniformly applied one or more times during the growing season, interacts. The knowledge derived from this work is essential for developing approaches to minimize or eliminate potential adverse effects of glyphosate in a GR soybean management system.
Technical Abstract: The impact of widespread cultivation of glyphosate-resistant (GR) soybean and the use of one herbicide class on biological processes has received considerable attention. Decreased biological nitrogen fixation in GR soybean has been attributed directly to toxicity of glyphosate or its metabolites to N2 -fixing bacteria. As a strong metal chelator, glyphosate may influence symbiotic N2 fixation by lowering the concentration of nickel (Ni) essential for the symbiotic bacteria. The objective of the present study was to investigate the effect of glyphosate on nodule formation and its relationship with Ni in GR soybean plants. Evaluation of different cultivars grown in different soils revealed significant reductions in photosynthetic parameters (chlorophyll, photosynthetic rate, transpiration and stomatal conductance) and Ni contents when glyphosate was used (single or sequential application). This work demonstrated that glyphosate might influence symbiotic N2 fixation by lowering Ni content available to symbiotic bacteria involved in the N2-fixation process. Interference with this process could lead to less than optimum soybean growth and seed yield.