Submitted to: Acta Physiologiae Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2010
Publication Date: 2/17/2010
Citation: Zobiole, L.H., Bonini, E.A., Oliveira, R.S., Kremer, R.J., Ferrares-Fihlo, O. 2010. Glyphosate affects lignin content and amino acid composition in glyphosate-resistant soybean. Acta Physiologiae Plantarum. 32(5):831-837. Interpretive Summary: The use of the herbicide glyphosate and glyphosate-resistant (GR) crops for effective weed management is a widely accepted practice in current cropping systems. The primary aim of glyphosate application is to kill weeds infesting the crop, however, the herbicide also contacts and is absorbed by the GR plant. The glyphosate molecule is systemic, so it is transported throughout the living plant. Several reports suggests that as glyphosate is moved within GR soybean plants, it may interact with a number of metabolic processes that might lead to adverse effects on plant structural composition. Based on these previous studies, we investigated the effect of glyphosate on amino acid and lignin contents and general growth of GR soybean growing under greenhouse conditions. Various rates of glyphosate were applied to soybean at 24 days of growth; plants were sampled to assess effects of the herbicide at 45 days or at initiation of flowering. We found that lignin and total amino acid contents were consistently lower for GR soybean after treatment with glyphosate at all rates compared with GR soybean without glyphosate as well as with soybean parental lines that were not genetically modified to be GR. Lignin is important for plant stature (erect growth) and defense against pathogens; amino acids are essential components of proteins. Also, leaf chlorophyll contents were reduced thereby reducing photosynthesis, the central metabolic pathway for producing plant biomass. Although future field trials with additional varieties are required to determine if glyphosate affects lignin and amino acid contents in plants under variable environmental conditions, these preliminary results help partially explain some of the non-target effects of glyphosate that have been observed in current soybean production fields. 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 whole plant composition and metabolic pathways. Understanding the implications of glyphosate on plant development would help in establishing programs for improving plant biomass composition and photosynthetic efficiency. For example, an outcome of this research was that reduced glyphosate rates applied at two different times (sequential application) had less effect on the plant measurements than when the total rate was applied at one time, suggesting that modifying herbicide application practices may maintain plant quality.
Technical Abstract: Farmers report that some glyphosate-resistant soybean varieties are visually injured by glyphosate. Glyphosate is the main herbicide that directly affects the synthesis of secondary compounds. In this work, we evaluated the effect of increasing rates of glyphosate on lignin and amino acid content, photosynthetic parameters and dry biomass in early maturity group cultivar BRS 242 GR soybean. Plants were grown in half-strength complete nutrient solution and subjected to various rates of glyphosate either as a single or in sequential applications. All parameters evaluated were affected by increasing glyphosate rates. The effects were more pronounced as glyphosate rate increased, and were more intense with a single total application than sequential applications at lower rates.