Photolysis of tembotrione and its main by-products under extreme artificial conditions: comparison with another β-triketone herbicide

Degradation, adsorption, and bioaccumulation of novel triketone HPPD herbicide tembotrione

Tembotrione is a new triketone HPPD herbicide widely used in Europe, USA, and other areas. However, tembotrione is moderately to highly toxic to algae and daphnia in aquatic ecosystems. In this study, hydrolysis, photolysis, soil degradation, soil adsorption, and bioaccumulation of tembotrione were systematically studied. Hydrolysis experiment revealed that tembotrione was stable in acidic, neutral, and alkaline conditions with half-lives of 231-289 days. The photolysis half-lives of tembotrione were 112-158 days and 76-107 days in pH 4, 7, 9 buffer solutions and on three soils surface, respectively, which demonstrated that tembotrione could be persisted in soil and water. Meanwhile, tembotrione K_foc was 128-196 mL/g, indicating that tembotrione was not easily adsorbed to soil, and the adsorption capacity increased with the decrease in pH. The half-lives of tembotrione in the test soil were 32-48 days, and high organic matter soil is conducive to microbial activity and accelerates the degradation of tembotrione. Moreover, bioaccumulation experiment demonstrated that tembotrione with a BCF of 0.664 to 0.724 had a low risk of exposure to zebrafish. This study is very helpful for the evaluation environmental risk and safe use of tembotrione.

Water-Active Titanium/Molybdenum/Mixed-Oxides: Removal Efficiency of Organic Water Pollutants by Adsorption and Photocatalysis and Toxicity Assessment

A new titanium/molybdenum/mixed-oxides (TMO) contact-type heterojunction photocatalyst was prepared by a simple, low-cost, and environmentally-friendly mixing-calcination solid-state method. A microstructural investigation by scanning electron microscopy (SEM) showsirregularly shaped agglomerated morphology of TMO that consists of firmly connected globular TiO2 and rod-like MoO3 particles. The detailed structure and optical bandgap investigation by X-ray diffraction, Raman, and UV-Vis spectroscopy revealed the TMO’s composition of ~37 wt.% rutile TiO2, ~25 wt.% of anatase TiO2, and ~38 wt.% of molybdite MoO3 phase and an absorption threshold of around 380 nm, which implies more probability of desirable higher visible light absorption. The removal efficiency of pesticides quinmerac (QUI) and tembotrione (TEM), and pharmaceuticals metoprolol (MET), amitriptyline (AMI), ciprofloxacin (CIP),and ceftriaxone (CEF) from water in the presence of starting pure TiO2, MoO3, and prepared TMO were investigated under different pH values and UV irradiation/simulated sunlight (SS). Each starting metal-oxide precursors and prepared TMO showed a different affinity for adsorption of tested pesticides and pharmaceuticals, and, in general, better photocatalytic degradation efficiency under UV irradiation than under simulated sunlight. The highest photocatalytic degradation efficiency under UV irradiation was 81.6% for TEM using TMO; using TiO2 was 65.0% for AMI, and using MoO3 was 79.3% for CEF after 135 min. However, TMO showed a very high synergic adsorption/photocatalytic under-SS efficiency in the removal of CIP of almost 80% and under UV irradiation of 90% CIP removal after 75 min. The toxicity of catalysts, starting compounds, and their intermediates formed during the removal process was assessed using a rat hepatoma cell line (H-4-II-E). The highest hepatotoxic effects were obtained by using UV irradiated QUI and MET suspension with TMO for up to 60 min.

Dissipation, Fate, and Toxicity of Crop Protection Chemical Safeners in Aquatic Environments

Safeners are a group of chemicals applied with herbicides to protect crop plants from potential adverse effects of agricultural products used to kill weeds in monocotyledonous crops. Various routes of dissipation of safeners from their point of applications were evaluated. Despite the large numbers of safeners (over 18) commercially available and the relatively large quantities (~2 × 10⁶ kg/year) used, there is little information on their mobility and fate in the environment and occurrence in various environmental matrices. The only class of safeners for which a significant amount of information is available is dichloroacetamide safeners, which have been observed in some rivers in the USA at concentrations ranging from 42 to 190 ng/L. Given this gap in the literature, there is a clear need to determine the occurrence, fate, and bioavailability of other classes of safeners. Furthermore, since safeners are typically used in commercial formulations, it is useful to study them in relation to their corresponding herbicides. Common routes of dissipation for herbicides and applied safeners are surface run off (erosion), hydrolysis, photolysis, sorption, leaching, volatilization, and microbial degradation. Toxic potencies of safeners vary among organisms and safener compounds, ranging from as low as the LC₅₀ for fish (Oncorhynchus mykiss) for isoxadifen-ethyl, which was 0.34 mg/L, to as high as the LC₅₀ for Daphnia magna from dichlormid, which was 161 mg/L. Solubilities and octanol-water partition coefficients seem to be the principal driving force in understanding safener mobilities. This paper provides an up-to-date literature review regarding the occurrence, behaviour, and toxic potency of herbicide safeners and identifies important knowledge gaps in our understanding of these compounds and the potential risks posed to potentially impacted ecosystems.

Evaluation of oxidative stress responses and primary DNA damage in blood and brain of rats exposed to low levels of tembotrione

Tembotrione is a rather novel pesticide, usually used for post-emergence weed control. Even though its use is rapidly growing, it is not followed by an adequate flow of scientific evidence regarding its toxicity towards non-target organisms. We evaluated the potential of low doses of tembotrione to induce oxidative stress and cytogenetic damage in blood and brain cells of adult male Wistar rats. Parameters of lipid peroxidation, glutathione levels, activities of antioxidant enzymes and primary DNA damage were assessed following 28-day repeated oral exposure to doses comparable with the currently proposed health-based reference values. The results of the alkaline comet assay showed that such low doses of tembotrione have the potency to inflict primary DNA damage in both peripheral blood leukocytes and brain of treated rats, even with only slight changes in the oxidative biomarker levels. The DNA damage in blood and brain cells of Wistar rats significantly increased at all applied doses, suggesting that tembotrione genotoxicity is mainly a result of direct interaction with DNA while the induction of oxidative stress responses contributes to DNA instability in a lesser extent. The findings of the present study call for further research using other sensitive biomarkers of effect and different exposure scenarios.

Tembotrione detoxification in 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor-resistant Palmer amaranth (Amaranthus palmeri S. Wats.)

BACKGROUND

Resistance to the 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide tembotrione in an Amaranthus palmeri population from Nebraska (NER) has previously been confirmed to be attributable to enhanced metabolism. The objective of this study was to identify and quantify the metabolites formed in Nebraska susceptible (NES) and resistant (NER) biotypes.

RESULTS

NER and NES formed the same metabolites. Tembotrione metabolism in NER differed from that in NES in that resistant plants showed faster 4-hydroxylation followed by glycosylation. The T₅₀ value (time for 50% production of the maximum 4-hydroxylation product) was 4.9 and 11.9 h for NER and NES, respectively. This process is typically catalyzed by cytochrome P450 enzymes. Metabolism differences between NER and NES were most prominent under 28 °C conditions and herbicide application at the four-leaf stage.

CONCLUSION

Further research with the aim of identifying the gene or genes responsible for conferring metabolic resistance to HPPD inhibitors should focus on cytochrome P450s. Such research is important because non-target-site-based resistance (NTSR) poses the threat of cross resistance to other chemical classes of HPPD inhibitors, other herbicide modes of action, or even unknown herbicides. © 2017 Society of Chemical Industry.

Evidence for photolytic and microbial degradation processes in the dissipation of leptospermone, a natural β-triketone herbicide

Bioherbicides appear as an ecofriendly alternative to synthetic herbicides, generally used for weed management, because they are supposed to have low side on human health and ecosystems. In this context, our work aims to study abiotic (i.e., photolysis) and biotic (i.e,. biodegradation) processes involved in the fate of leptospermone, a natural β-triketone herbicide, by combining chemical and microbiological approaches. Under controlled conditions, the photolysis of leptospermone was sensitive to pH. Leptospermone has a half-life of 72 h under simulated solar light irradiations. Several transformation products, including hydroxy-leptospermone, were identified. For the first time, a bacterial strain able to degrade leptospermone was isolated from an arable soil. Based on its 16S ribosomal RNA (rRNA) gene sequence, it was affiliated to the Methylophilus group and was accordingly named as Methylophilus sp. LS1. Interestingly, we report that the abundance of OTUs, similar to the 16S rRNA gene sequence of Methylophilus sp. LS1, was strongly increased in soil treated with leptospermone. The leptospermone was completely dissipated by this bacteria, with a half-life time of 6 days, allowing concomitantly its growth. Hydroxy-leptospermone was identified in the bacterial culture as a major transformation product, allowing us to propose a pathway of transformation of leptospermone including both abiotic and biotic processes.

Direct photolysis mechanism of pesticides in water

Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.

Fate and ecotoxicological impact of new generation herbicides from the triketone family: An overview to assess the environmental risks

Triketones, derived chemically from a natural phytotoxin (leptospermone), are a good example of allelochemicals as lead molecules for the development of new herbicides. Targeting a new and key enzyme involved in carotenoid biosynthesis, these latest-generation herbicides (sulcotrione, mesotrione and tembotrione) were designed to be eco-friendly and commercialized fifteen-twenty years ago. The mechanisms controlling their fate in different ecological niches as well as their toxicity and impact on different organisms or ecosystems are still under investigation. This review combines an overview of the results published in the literature on β-triketones and more specifically, on the commercially-available herbicides and includes new results obtained in our interdisciplinary study aiming to understand all the processes involved (i) in their transfer from the soil to the connected aquatic compartments, (ii) in their transformation by photochemical and biological mechanisms but also to evaluate (iii) the impacts of the parent molecules and their transformation products on various target and non-target organisms (aquatic microorganisms, plants, soil microbial communities). Analysis of all the data on the fate and impact of these molecules, used pure, as formulation or in cocktails, give an overall guide for the assessment of their environmental risks.

Monopersulfate photocatalysis under 365 nm radiation. Direct oxidation and monopersulfate promoted photocatalysis of the herbicide tembotrione

Oxone(®) (potassium monopersulfate, MPS) has been used to oxidize the herbicide tembotrione in aqueous solution. Tembotrione elimination kinetics by MPS direct oxidation has been studied. The influence of the main operating variables affecting the process (MPS concentration, temperature and pH) has been evaluated. The process follows 2/3 and first orders in MPS and tembotrione concentrations, respectively. Optimal pH is located around circumneutral conditions. MPS decomposition in the presence of 365 nm UVA radiation and titanium dioxide has also been studied. A kinetic mechanism that simulates MPS decomposition has been proposed, showing the positive effect of titania load and MPS concentration. The system MPS/UVA/TiO2 significantly improves tembotrione and mineralization rate abatement if compared to runs conducted in the absence of MPS. Tembotrione total abatement was achieved in 20 min when 0.05 g L(-1) of titania and 10(-4) M of Oxone(®) were used. TOC conversion was roughly 70% in 90 min under similar operating conditions. An experimental design (Plackett-Burman) has been considered to study the influence of the main variables affecting tembotrione photocatalytic oxidation promoted by MPS.

Transformation of the Herbicide Sulcotrione into a Root Growth Enhancer Compound by Sequential Photolysis and Hydrolysis

Xanthene-1,9-dione-3,4-dihydro-6-methylsulfonyl (1), the main product of sulcotrione phototransformation on plant leaves, was slowly hydrolyzed into 2-hydroxy-4-methylsulfonylbenzoic acid (2) and 1,3-cyclohexanedione (3) in aqueous solution. Interestingly, the rate of hydrolysis was significantly enhanced in the presence of roots of monocotyledonous plants, while the same treatment showed adverse effects on broadleaf weeds. Root growth enhancement varied according to the plant species and concentrations of compound 2, as shown with Zea mays roots. Compound 2 is a derivative of salicylic acid that is known to be a plant signaling messenger. Compound 2 was, therefore, able to mimic some known effects of this phytohormone. This work showed that a pesticide like sulcotrione was transformed into a compound exhibiting a positive impact on plant growth. This study exemplified a rarely reported situation where chemical and biological chain reactions transformed a xenobiotic into a compound exhibiting potential beneficial effects.

Cytotoxicity on Allium cepa of the two main sulcotrione photoproducts, xanthene-1,9-dione-3,4-dihydro-6-methylsulphonyl and 2-chloro-4-mesylbenzoic acid

The cytotoxic effects of 2-chloro-4-mesylbenzoic acid (CMBA) and xanthene-1,9-dione-3,4-dihydro-6-methylsulphonyl (XDD), the two main photoproducts of sulcotrione, were investigated on Allium root meristematic cells at different concentrations. Degradation of sulcotrione was correlated to mitotic index decrease, together with increasing anomaly and c-mitosis frequencies. Mitotic index significantly decreased with increasing XDD and CMBA concentrations. Cell frequency with abnormal chromosomes increased with CMBA or XDD application rates. In contrast, CMBA induced a low micronucleus rate even for high concentrations while XDD increased the micronucleus ratio. C-mitoses, chromosomal aberrations due to an inactivation of the spindle, were enhanced by CMBA treatments but not by XDD. The photochemical degradation process of the pesticide can change the risk for the environment.

Photolysis of natural β-triketonic herbicides in water

The fate of four natural β-triketones (leptospermone, isoleptospermone, grandiflorone and flavesone, pKa = 4.0-4.5) in aqueous solution, in the dark and upon simulated solar light irradiation was investigated. In anionic form, β-triketones undergo slow dark oxidation and photolysis with polychromatic quantum yields varying from 1.2 × 10(-4) to 3.7 × 10(-4). Leptospermone and grandiflorone are the most photolabile compounds. In molecular form, β-triketones are rather volatile. Polychromatic quantum yields between 1.2 × 10(-3) and 1.8 × 10(-3) could be measured for leptospermone and grandiflorone. They are 3-5 times higher than for the anionic forms. Photooxidation on the carbon atom bearing the acidic hydrogen atom is the main oxidation reaction, common to all the β-triketones whatever their ionization state. However, leptospermone shows a special photoreactivity. In molecular form, it mainly undergoes photoisomerization. Based on this work, the half-lives of β-triketones in surface waters should be comprised between 7 and 23 days.

Genotoxicity of sulcotrione pesticide and photoproducts on Allium cepa root meristem

Contamination by toxic agents in the environment has become matters of concern to agricultural countries. Sulcotrione, a triketone herbicide used to control dicotyledonous weeds in maize culture is rapidly photolyzed on plant foliage and generate two main photoproducts the xanthene-1,9-dione-3,4-dihydro-6-methylsulfonyl and 2-chloro-4-mesylbenzoic acid (CMBA). The aim of this study was to analyze the potential toxicity of the herbicide and the irradiated herbicide cocktail. Cytotoxicity and genotoxicity of non irradiated and irradiated sulcotrione were investigated in Allium cepa test. The sulcotrione irradiation was monitored under sunlight simulated conditions to reach 50% of phototransformation. Concentrations of sulcotrione in the range 5 × 10(-)(9)-5 × 10(-)(5)M were tested. Cytological analysis of root tips cells showed that both non irradiated and irradiated sulcotrione caused a dose-dependent decrease of mitotic index with higher cytotoxicity for the irradiated herbicide which can lead to 24.2% reduction of mitotic index compared to water control. Concomitantly, chromosomal aberrations were observed in A.cepa root meristems. Both non irradiated sulcotrione and irradiated sulcotrione induced a dose-dependent increase of chromosomal abnormalities frequencies to a maximal value of 33.7%. A saturating effect in anomaly frequencies was observed in meristems treated with high concentrations of non irradiated sulcotrione only. These data suggest that photolyzed sulcotrione cocktail have a greater cytotoxicity and genotoxicity than parent molecule and question about the impact of photochemical process on environment.