Glyphosate and Aminomethylphosphonic Acid Content in Glyphosate-Resistant Soybean Leaves, Stems, and Roots and Associated Phytotoxicity Following a Single Glyphosate-Based Herbicide Application.

Glyphosate-based herbicide (GBH) applications were reported to induce physiological damages to glyphosate-resistant (GR) soybean, which were mainly attributed to aminomethylphosphonic acid (AMPA). In order to study glyphosate and AMPA dynamics in plants and associated phytotoxic effects, a greenhouse experiment was set where GR soybeans were exposed to GBH (0.7 to 4.5 kg glyphosate ha-1) and sampled over time (2, 7, 14, and 28 days after treatment (DAT)). Hydrogen peroxide content increased 2 DAT, while a decrease was observed for the effective quantum yield (2, 7, 14 DAT), stomatal conductance (2 DAT), and biomass (14 DAT). Glyphosate content was higher in leaves, followed by stems, and then roots. AMPA content tended to increase with time, especially in roots, and the amount of AMPA in roots was negatively correlated to mostly all phytotoxicity indicators. This finding is important since AMPA residues are measured in agricultural soils several months after GBH applications, which could impact productivity in GR crops.

Potential Efficiency of Grassy or Shrub Willow Buffer Strips against Nutrient Runoff from Soybean and Corn Fields in Southern Quebec, Canada.

Riparian buffer strips (RBS) are encouraged to control agricultural diffuse pollution. In Quebec Province, Canada, a policy promotes 3-m-wide RBS. Abiding farmers minimally maintain herbaceous vegetation, but nutrient retention efficiency could be improved with woody biomass. This work aimed to assess if fast-growing willows ( Seemen 'SX64') could reduce nutrient loads to a stream, in addition to yielding biomass. Triplicate treatments of two stem densities and a herbaceous control plot were monitored from 2011 to 2013 in a randomized block design on agricultural fields of the St. Lawrence Lowlands with sandy loam (Saint-Roch-de-l'Achigan [SR]) and organic-rich (Boisbriand [BB]) soils. Runoff, interstitial water, and water from the saturated zone were sampled 16 (SR) and 14 (BB) times to quantify nutrient buffering (NO, NH, P, and K). Sampling campaigns followed (i) snowmelt or ≥15-mm natural precipitation events after (ii) fertilization and (iii) glyphosate-based herbicide applications. Concentration reduction before and after the RBS was highest for nitrates (77-81% in runoff at BB, 92-98% at 35- to 70-cm depth at SR) just after fertilization, when edge-of-field concentrations peaked. Total P removal was observed in runoff after fertilization at SR, and K removal was punctually witnessed at BB. Riparian buffer strips were inefficient for NH and dissolved P removal, and RBS effluents exceeded aquatic life protection standards. plantations, irrespective of stem density, were not more efficient than herbaceous RBS. This shows that without fertilizer input reductions, narrow RBS are insufficient to protect streams from excess nutrients in corn ( L.) and soybean [ (L.) Merr.] crops.

Effects of low concentrations of glyphosate-based herbicide factor 540® on an agricultural stream freshwater phytoplankton community.

Residual glyphosate from glyphosate based herbicides (GBH) are ubiquitously detected in streams draining agricultural fields, and may affect phytoplankton communities present in these ecosystems. Here, the effects of the exposure (96 h) of a phytoplankton community collected in an agricultural stream to various glyphosate concentrations (1, 5, 10, 50, 100, 500 and 1000 µg l-1) of Factor 540® GBH were investigated. The lowest GBH concentration of 1 µg l-1 reduced chlorophyll a and carotenoid contents. Low glyphosate concentrations, such as 5 and 10 µg l-1, promoted changes in the community's structure and reduced the diversity of the main algal species. At glyphosate concentrations ranging from 50 to 1000 µg l-1, the phytoplankton community's composition was modified and new main species appeared. The highest glyphosate concentrations (500 and 1000 µg l-1) affected the shikimate content, the lipid peroxidation and the activity of antioxidant enzymes (superoxide dismutase, catalase and ascorbate peroxidase). These results indicate that GBH can modify structural and functional properties of freshwater phytoplankton communities living in streams located in agricultural areas at glyphosate concentrations much inferior to the 800 µg l-1 threshold set by the Canadian guidelines for the protection of aquatic life.

Phytoplankton growth and PSII efficiency sensitivity to a glyphosate-based herbicide (Factor 540®).

The use of glyphosate-based herbicides in agriculture has increased steadily since the mid 90's and there is now evidence of glyphosate leaching and contamination of aquatic ecosystems. The aim of this study was to evaluate the effects of a glyphosate-based herbicide (Factor 540®) on growth and photosynthetic capacity of algae and cyanobacteria. Six algal and three cyanobacterial species/strains, of three different taxonomic groups, were exposed to five glyphosate concentrations (10, 50, 100, 500 and 1000µgl-1) during 48h. All species have significant growth inhibition at concentrations varying between 50 and 500µgl-1. The photosynthetic response, after glyphosate exposure, varied among species, but a general pattern has emerged. There was an increase in the amount of photons absorbed (ABS/RC), in dissipated (DIO/RC) and trapped (TRO/RC) energy in the photosystem II reaction centers, along with a decreased of the maximum photosystem II quantum yield (FV/FM) and electron transport per reaction center (ETO/RC). The EC50 and LOEC values for growth and photosynthesis were calculated and established that growth was the most affected parameter by glyphosate-based herbicide, while parameter TRO/RC was the least affected. All species showed reduced growth at glyphosate concentrations lower than the Canadian standard for the protection of aquatic life, set at 800µgl-1 or the American aquatic life benchmark for acute toxicity in non vascular plants of 12 100µgl-1 questioning the validity of these thresholds in assessing the risks related to the presence of glyphosate and glyphosate-based herbicides in aquatic systems.

Herbaceous or Salix miyabeana 'SX64' narrow buffer strips as a means to minimize glyphosate and aminomethylphosphonic acid leaching from row crop fields.

Glyphosate is the most widely used herbicide worldwide and is now frequently detected in surface waters of agricultural regions, notably in Quebec (Canada). Numerous legislations require vegetated riparian buffer strips (RBS) along agricultural streams. Quebec policy requires 3-meter-wide RBS. The present study evaluates the efficiency of narrow herbaceous and low- or high-density (33,333 and 55,556stumps/ha) willow, Salix miyabeana 'SX64', RBS, to minimize leaching of glyphosate and its main degradation product (AMPA) from agricultural fields to streams. Our study compared triplicate treatments of herbaceous and willow-planted RBS located in an organic-rich soil at Boisbriand (BB) and in a mineral soil at Saint-Roch-de-l'Achigan (SR). Runoff water was sampled with surface collectors and interstitial water was collected with 35cm or 70cm tension lysimeters. Potential efficiency of the RBS is reported as the percent reduction between edge-of-field and edge-of-stream concentrations in runoff and interstitial waters. Neither glyphosate nor AMPA in runoff were significantly intercepted by the RBS. After field herbicide spraying, glyphosate measured in SR surface soils (0-20cm) was on average 210µg·kg-1 dw (range from undetected to ≤317µg·kg-1 dw). Glyphosate was found to be persistent and its infiltration towards groundwater may be enhanced by the RBS. Contrary to runoff trends, soil glyphosate was significantly less concentrated on the SR edge-of-stream compared to edge-of-field (27-54% potential efficiency). The potential efficiency of herbaceous, low- and high-density willow RBS were undifferentiated.

Glyphosate Can Decrease Germination of Glyphosate-Resistant Soybeans.

We investigated the effects of different concentrations of glyphosate acid and one of its formulations (Roundup) on seed germination of two glyphosate-resistant (GR) and one non-GR variety of soybean. As expected, the herbicide affected the shikimate pathway in non-GR seeds but not in GR seeds. We observed that glyphosate can disturb the mitochondrial electron transport chain, leading to H2O2 accumulation in soybean seeds, which was, in turn, related to lower seed germination. In addition, GR seeds showed increased activity of antioxidant systems when compared to non-GR seeds, making them less vulnerable to oxidative stress induced by glyphosate. The differences in the responses of GR varieties to glyphosate exposure corresponded to their differences in enzymatic activity related to H2O2 scavenging and mitochondrial complex III (the proposed site of ROS induction by glyphosate). Our results showed that glyphosate ought to be used carefully as a pre-emergence herbicide in soybean field crop systems because this practice may reduce seed germination.

Impact of phosphate on glyphosate uptake and toxicity in willow.

Phosphate (PO4(3-)) has been shown to increase glyphosate uptake by willow, a plant species known for its phytoremediation potential. However, it remains unclear if this stimulation of glyphosate uptake can result in an elevated glyphosate toxicity to plants (which could prevent the use of willows in glyphosate-remediation programs). Consequently, we studied the effects of PO4(3-) on glyphosate uptake and toxicity in a fast growing willow cultivar (Salix miyabeana SX64). Plants were grown in hydroponic solution with a combination of glyphosate (0, 0.001, 0.065 and 1 mg l(-1)) and PO4(3-) (0, 200 and 400 mg l(-1)). We demonstrated that PO4(3-) fertilization greatly increased glyphosate uptake by roots and its translocation to leaves, which resulted in increased shikimate concentration in leaves. In addition to its deleterious effects in photosynthesis, glyphosate induced oxidative stress through hydrogen peroxide accumulation. Although it has increased glyphosate accumulation, PO4(3-) fertilization attenuated the herbicide's deleterious effects by increasing the activity of antioxidant systems and alleviating glyphosate-induced oxidative stress. Our results indicate that in addition to the glyphosate uptake, PO4(3-) is involved in glyphosate toxicity in willow by preventing glyphosate induced oxidative stress.

Alteration of plant physiology by glyphosate and its by-product aminomethylphosphonic acid: an overview.

It is generally claimed that glyphosate kills undesired plants by affecting the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzyme, disturbing the shikimate pathway. However, the mechanisms leading to plant death may also be related to secondary or indirect effects of glyphosate on plant physiology. Moreover, some plants can metabolize glyphosate to aminomethylphosphonic acid (AMPA) or be exposed to AMPA from different environmental matrices. AMPA is a recognized phytotoxin, and its co-occurrence with glyphosate could modify the effects of glyphosate on plant physiology. The present review provides an overall picture of alterations of plant physiology caused by environmental exposure to glyphosate and its metabolite AMPA, and summarizes their effects on several physiological processes. It particularly focuses on photosynthesis, from photochemical events to C assimilation and translocation, as well as oxidative stress. The effects of glyphosate and AMPA on several plant physiological processes have been linked, with the aim of better understanding their phytotoxicity and glyphosate herbicidal effects.