Submitted to: Journal of Plant Nutrition and Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2011
Publication Date: 4/5/2012
Citation: Zobiole, L.H., Kremer, R.J., Oliveira, R.S., Constantin, J. 2012. Glyphosate effects on photosynthesis, nutrient accumulation, and nodulation in glyphosate-resistant soybean. Journal of Plant Nutrition and Soil Science. 175(2):319-330. Interpretive Summary: The widespread use of the herbicide glyphosate and glyphosate-resistant (GR; popularly known as ‘Roundup Ready’) crops represents a major advancement in effective weed management in conventional agricultural systems. Although glyphosate is applied to kill weeds infesting the crop, it also contacts and is absorbed into the soybean plant. The glyphosate molecule is also systemic, implying that it is moved throughout the living plant after absorption. Several reports suggest that as glyphosate is transported within the GR soybean plant, it may interact with a number of metabolic processes and reduce efficiencies of photosynthesis, accumulation of essential nutrients, and fixation of atmospheric nitrogen for protein production. Previous studies used the early or ‘first generation’ (GR1) soybean first released in 1997; ‘second generation’ (GR2) soybean varieties selected for higher grain yields than GR1 were released in 2008. We conducted a field study to investigate if effects of glyphosate on mineral nutrient content, photosynthetic activity, and nodulation (nitrogen fixing structures on roots) of a currently available GR2 soybean variety were similar to effects previously reported for GR1 varieties. The GR2 variety was planted during summer 2009 in central Missouri and received glyphosate at low, medium, and high rates representing the amount of herbicide that could be applied during the growing season depending on weed densities in the field. The glyphosate rates were applied at different growth stages of the soybean: early vegetative (two fully developed leaves); mid-vegetative (four leaves); and late vegetation (six leaves). We found that many essential nutrients were consistently lower for GR2 soybean treated with glyphosate compared with GR2 soybean without glyphosate. Photosynthesis and root nodulation were similarly reduced in plants receiving glyphosate. Decreases in all measurements were more pronounced with higher glyphosate rates applied at later stages of soybean growth (later in the growing season). Although future field trials are required to determine if glyphosate affects photosynthesis and plant nutrition under variable environmental conditions, these preliminary field results suggest that recently introduced GR2 varieties are affected by some of the non-target effects of glyphosate similarly observed in GR1 varieties. The results are useful to other researchers, extension personnel, nutritionists, and farmers by providing an understanding of potential non-target effects of glyphosate in the development of the whole soybean plant. Further studies are needed to understand the mechanisms of glyphosate effects on mineral nutrition, photosynthesis and nitrogen fixation (root nodulation). An immediate outcome of this research is the implication that the lowest rates of glyphosate providing effective weed control should be applied at early growth stages to avoid potential adverse effects of glyphosate on soybean growth and development.
Technical Abstract: Previous studies demonstrated that the photosynthesis of some cultivars of first (GR1) and second generation (GR2) glyphosate-resistant soybean was reduced by glyphosate. The reduction in photosynthesis caused by glyphosate might affect nutrient uptake and lead to lower plant biomass production and ultimately reduced grain yield. Therefore, a study was conducted to determine if glyphosate-induced damage to plants observed under greenhouse conditions in previous studies might occur under field conditions. The present study evaluated photosynthetic rate, nutrient accumulation, nodulation and biomass production of GR2 soybean receiving different rates of glyphosate (800, 1200, 2400 g a.e. ha-1) applied at V2, V4 and V6 growth stages. In general, plant damage observed in the field study was similar to that in previous greenhouse studies. Increasing glyphosate rates and applications at later growth stages decreased nutrient accumulation, nodulation, leaf area, and shoot biomass production. Thus, to reduce potential undesirable effects of glyphosate on plant growth, applications of the lowest glyphosate rate for weed control efficacy at early growth stages (V2 to V4) is suggested as an advantageous practice within current weed management in GR soybean for optimal crop productivity.