Integrating transcriptome and physiological analyses to elucidate the molecular responses of sorghum to fluxofenim and metolachlor herbicide

The extensive use of herbicides has raised concerns about crop damage, necessitating the development of effective herbicide safeners. Fluxofenim has emerged as a promising herbicide safener; however, it's underlying mechanism remains unclear. Here, we screened two inbred lines 407B and HYZ to investigate the detoxication of fluxofenim in mitigating metolachlor damage in sorghum. Metolachlor inhibited seedling growth in both 407B and HYZ, while, fluxofenim could significantly restore the growth of 407B, but not effectively complement the growth of HYZ. Fluxofenim significantly increased the activities of glutathione-S-transferase (GST) to decrease metolachlor residue in 407B, but not in HYZ. This implys that fluxofenim may reduce metolachlor toxicity by regulating its metabolism. Furthermore, metolachlor suppressed AUX-related and JA-related genes expression, while up-regulated the expression of SA-related genes. Fluxofenim also restored the expression of AUX-related and JA-related genes inhibited by metolachlor and further increased expression of SA-related genes. Moreover, we noted a significant increase in the content of trans-zeatin O-glucoside (tZOG) and Gibberellin1 (GA1) after the fluxofenim treatment. In conclusion, fluxofenim may reduce the injury of herbicide by affecting herbicide metabolism and regulating hormone signaling pathway.

Adaptation of the metolachlor-degrading fungus Trichoderma harzianum to the simultaneous presence of low-density polyethylene (LDPE) microplastics

Although it is known that microplastics (MPs) in soils cause a threat to this complex environment, the actual effects of MPs on soil microorganisms and their catabolic activities, particularly with the biodegradation of herbicides, remain unclear. Hence, the objective of this study was to investigate the effects of a simultaneous presence of metolachlor and low-density polyethylene (LDPE) microplastics on growth inhibition and adaptive responses of Trichoderma harzianum in soil microcosms. Using ergosterol content as an indicator of fungal biomass, it was observed that MPs alone had a marginal inhibitory effect on the growth of the fungus, whereas MET exhibited a dose-dependent inhibitory effect on T. harzianum. However, the presence of MPs did not influence the fungal transforming activity toward the herbicide. Conversely, analysis of lipid profiles in the presence of MPs and herbicides revealed a reduction in the overall fluidity of phospholipid fatty acids, primarily attributed to an increase in lysophospholipids. The activities of six extracellular enzymes in the soil, measured using methylumbelliferone-linked substrates, were significantly enhanced in the presence of MET. These findings contribute to a broader understanding of the alterations in fungal activity in soil resulting from the influence of MPs and MET.

Palmelloid-like phenotype in the alga Raphidocelis subcapitata exposed to pollutants: A generalized adaptive strategy to stress or a specific cellular response?

This work focuses on the formation of palmelloid-like phenotype in the freshwater alga Raphidocelis subcapitata (formerly known as Pseudokirchneriella subcapitata and Selenastrum capricornutum), when exposed to adverse conditions generated by the presence of organic [the antibiotic erythromycin (ERY) and the herbicide metolachlor (MET)] or inorganic [the heavy metals, cadmium (Cd) and zinc (Zn)] pollutants, at environmentally relevant concentrations. This alga in absence of stress or when exposed to ERY or Zn, up to 200 µg/L, essentially showed a single-nucleus state, although algal growth was reduced or stopped. R. subcapitata "switched" to a multinucleated state (palmelloid-like morphology) and accumulated energy-reserve compounds (neutral lipids) when stressed by 100-200 µg/L MET or 200 µg/L Cd; at these concentrations of pollutants, growth was arrested, however, the majority of the algal population (≥83 %) was alive. The formation of palmelloid-like phenotype, at sub-lethal concentrations of pollutants, was dependent on the pollutant, its concentration and exposure time. The multinucleated structure is a transitory phenotype since R. subcapitata population was able to revert to a single-nucleus state, with normal cell size, within 24-96 h (depending on the impact of the toxic in the alga), after being transferred to fresh OECD medium, without pollutants. The obtained results indicate that the formation of a palmelloid-like phenotype in R. subcapitata is dependent on the mode of action of toxics and their concentration, not constituting a generalized defense mechanism against stress. The observations here shown contribute to understanding the different strategies used by the unicellular alga R. subcapitata to cope with severe stress imposed by organic and inorganic pollutants.

Effects of the herbicide metolachlor and pond drying on growth and development of wood frog tadpoles (Lithobates sylvaticus)

Pesticides and climate change are both thought to contribute to the global amphibian decline, yet their combined effects are still poorly understood. Metolachlor is a widespread herbicide applied across North America, but little is known about its effects on amphibians. Here, we used a replicated mesocosm experimental design with different levels of drying (i.e., no drying, medium and rapid drying) and metolachlor concentrations (0, 0.8, 8 and 80 µg/L) to assess their respective and combined effects on wood frog (Lithobates sylvaticus) larvae throughout metamorphosis. Metolachlor had no significant effect on survival and development of tadpoles. However, metolachlor significantly interacted with drying levels to reduce growth of tadpoles, which was mainly due to a difference detected among metolachlor concentrations under the rapid drying treatment. Drying also directly reduced growth and body mass at metamorphosis. Our results suggest that environmental stressors, such as drying, should be considered in toxicological experiments to provide relevant exposure conditions to pesticides for ephemeral pond species in the context of global climate change.

Effects of three surfactants on the degradation and environmental risk of metolachlor in aquatic environment

Surfactants and pesticides can be simultaneously detected in the environment by the reason of their widespread use and large amounts of emissions. Due to the special amphipathicity of surfactants, it may have special effects on the environmental behaviors and toxic effects of other substances in the environment. There are few relevant studies at present. In this study, the effects of three surfactants on the degradation of the amide pesticide metolachlor in water-sediment system were investigated. The study found that the three surfactants had no significant effect on the degradation of metolachlor in the system at environmental concentrations. However, at critical micelle concentration, cationic surfactant octadecyl trimethyl ammonium bromide and nonionic surfactant nonylphenol polyoxyethylene ether promoted the degradation of metolachlor in water-sediment system. Anionic surfactant odium dodecylbenzene sulfonate (SDBS) prolonged the degradation half-life of metolachlor. The presence of surfactants not only affected the environmental behavior of pesticides. When they coexisted with pesticides, the joint toxicity to aquatic organisms cannot be ignored. This study found that the combined effects of three surfactants and metolachlor on the acute developmental toxicity of zebrafish embryos were all synergistic effects. The combined effects of two ionic surfactants and metolachlor on the acute toxicity of adult zebrafish were synergistic effects. Further study showed that co-exposure of SDBS and metolachlor increased the absorption of metolachlor by zebrafish. Combined exposure of SDBS and metolachlor caused oxidative stress in brain, gill and liver of zebrafish. The results showed that the simultaneous presence of anionic surfactants and pesticides in the environment may increase the environmental risk of pesticides.

The statistical power to detect regional temporal trends in riverine contaminants in the Chesapeake Bay Watershed, USA

Chemical contamination of riverine ecosystems is largely a result of urbanization, industrialization, and agricultural activities occurring on adjacent terrestrial landscapes. Land management activities (e.g., Best Management Practices) are an important tool used to reduce point and non-point sources of pollution. However, the ability to confidently make inferences about the efficacy of land management activities on reducing in-stream chemical concentrations is poorly understood. We estimated regional temporal trends and components of variation for commonly used herbicides (atrazine and metolachlor), total estrogenicity, and riverine sediment concentrations of total PCBs for rivers in the Chesapeake Bay Watershed, USA. We then used the estimated variance components to perform a power analysis and evaluated the statistical power to detect regional temporal trends under different monitoring scenarios. Scenarios included varying the magnitude of the annual contaminant decline, the number of sites sampled each year, the number of years sampled, and sampling frequency. Monitoring for short time periods (e.g., 5 years) was inadequate for detecting regional temporal trends, regardless of the number of sites sampled or the magnitude of the annual declines. Even when monitoring over a 20-year period, sampling a relatively large number of sites each year was required (e.g., >50 sites) to achieve adequate statistical power for smaller trend magnitudes (declines of 5-7%/year). Annual sampling frequency had little impact on power for any monitoring scenario. All sampling scenarios were underpowered for sediment total PCBs. Power was greatest for total estrogenicity, suggesting that this aggregate measure of estrogenic activity may be a useful indicator. This study provides information that can be used to help (1) guide the development of monitoring programs aimed at detecting regional declines in riverine chemical contaminant concentrations in response to land management actions, and (2) set expectations for the ability to detect changes over time.

Adsorption of metolachlor by a novel magnetic illite-biochar and recovery from soil

In this study, we investigated a highly efficient adsorbent that can be recycled from the soil. Walnut shells were used as raw materials to prepare original ecological biochar (OBC), illite modified biochar (IBC), FeCl₃ modified biochar (magnetic biochar; MBC), and illite and FeCl₃ modified biochar (IMBC), which were tested as low-cost adsorbents. The agents were used to remove metolachlor (MET) from soil. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, magnetic sensitivity curve analysis, and a series of adsorption experiments were conducted to study the interaction between illite and MBC, and the effect on MET adsorption. Compared with OBC, IMBC had more adsorption sites on the surface. IMBC improved the hole filling effect during the adsorption process. IMBC had more oxygen-containing functional groups and it performed better at removing organic matter through π-π interactions. According to the Langmuir model, the Q₀ values for IBC, MBC, and IMBC were 91.74 mg g^-1, 107.53 mg g^-1, and 129.87 mg g^-1, respectively, which were significantly higher than that for OBC (72.99 mg g^-1). The response surface model was used to explore the optimal adsorption conditions for IMBC. After three regeneration cycles, the MET adsorption rate with IMBC was still 81.38% and the MET recovery rate was 98.12%. Therefore, IMBC was characterized as an adsorbent with high efficiency, low cost, and good recyclability. In addition, we propose a suitable agricultural system for recovering MBC on site in the field.

Bioaccumulation, metabolism and endocrine-reproductive effects of metolachlor and its S-enantiomer in adult zebrafish (Danio rerio)

The aim of the present study was to evaluate the enantioselective bioaccumulation, metabolism, and toxic effects of metolachlor and S-metolachlor in zebrafish. Five-month-old zebrafish were exposed to metolachlor and S-metolachlor for 28 days, then transferred to clean water and purified for 7 days. In the uptake phase, S-metolachlor was preferentially accumulated at low concentrations, while metolachlor was preferentially accumulated at high concentrations. The two chemicals were metabolized by >70% in zebrafish on the first day and showed same metabolic process. At the accumulation endpoint, S-metolachlor had no significant inhibitory effect on the enzymes activities of superoxide dismutase (SOD), catalase (CAT) and glutathione S-transferase (GST) and developmental indicators of zebrafish. However, 300 μg/L metolachlor significantly inhibited the enzymes activities of SOD, CAT and GST and affected the liver development. The preferential enrichment of metolachlor at the high concentration may be the reason for its higher toxicity to zebrafish. Further research demonstrated that metolachlor significantly altered the expression of hypothalamic-pituitary-gonadal (HPG) axis-related genes, including gnrh2, gnrh3, lhβ, 17βhsd and cyp19a, thereby reducing the levels of testosterone (T) in females and sex hormones (estradiol and testosterone) in males. S-metolachlor increased the levels of estradiol (E2) in females by altering the expression of HPG axis-related genes such as fshβ, cyp17, 17βhsd and cyp19a. The mechanism of metolachlor and S-metolachlor on the endocrine disrupting effects of zebrafish is different, which may be sex-specific. 7 days after transferring the exposed zebrafish to clean water, most of the enzymes activities, sex hormone levels and related gene expression levels returned to normal, which may be related to the rapid metabolism of the two chemicals.

UV and temperature effects on chloroacetanilide and triazine herbicides degradation and cytotoxicity

The purpose of this study was to explore the stability and toxicity of the herbicides and their degradation byproduct after exposure to different environmental factors. Triazines (atrazine, propazine, simazine) and chloroacetanilides (acetochlor, alachlor, metolachlor) which are commonly used herbicides were evaluated for cytotoxicity in different UV (254 nm and 365 nm) and temperature (4 °C, 23 °C, and 40 °C) conditions as well as degradation rates. Atrazine with the highest LD₅₀ (4.23 μg mL^-1) was less toxic than the other tested triazine herbicides Chloroacetanilides tested were more toxic than tested triazines, with LD₅₀ 0.08-1.42 μg mL^-1 vs 1.44-4.23 μg mL^-1, respectively. Alachlor with LD₅₀ 0.08 μg mL^-1 showed the strongest toxic response as compared with other tested herbicides. Temperatures only did not alter cytotoxicity of the tested herbicides, except for acetochlor and alachlor showing about 45 % more cell death after exposure to 40 °C for 2 h. At all 3 tested temperatures, 2 h of UV treatments did not affect cytotoxic effects of the tested herbicides, except for acetochlor and alachlor. At 4 °C, acetochlor toxicity was attenuated about 63 % after UV 365 nm exposure; but alachlor toxicity was enhanced after either UV 254 or 365 nm exposure for about 40 % and 24 %, respectively. At 23 °C, acetochlor toxicity was enhanced about 35 % after UV 254 nm exposure, but attenuated about 48 % after UV 365 nm exposure. Alachlor toxicity was enhanced about 34 % after UV 254 nm and 23 °C exposure. In combination of UV 254 nm and 40 °C, acetochlor toxicity was lowered by 63 % and alachlor toxicity was no change as compared with 4 °C, no UV group. After co-treatment with UV 365 nm and 40 °C both acetochlor and alachlor toxicity was enhanced 55 % and 80 %, respectively. Through degradation analysis by LC-MS/MS, alachlor showed the most dramatic degradation (only 0.58 %-10.58 % remaining) after heat and UV treatments.

Pulsed exposure of the macrophyte Lemna minor to herbicides and the mayfly Neocloeon triangulifer to diamide insecticides

Pesticides applied to agricultural land can enter aquatic ecosystems through runoff or leaching during precipitation events. In a lotic system, these events result in a pulse of exposure to biota living in these systems. The concentration of pesticide increases, peaks, and then gradually declines, and this pulsed exposure may occur multiple times over the course of a growing season. The dynamic nature of exposure to pesticides in the environment is not often mimicked in the laboratory testing of the toxicity of pesticides. The present study investigated the potential latent effects of a 24-h pulsed exposure of metolachlor, metribuzin, MCPA (2-methyl-4-chlorophenoxyacetic acid), MCPP (methylchlorophenoxypropionic acid or mecoprop), dicamba, and 2,4-D to the aquatic macrophyte Lemna minor followed by a 5-day recovery period. The relative sensitivity of L. minor to the herbicides were, in this decreasing order: metolachlor > metribuzin >2,4-D > MCPA > MCPP > dicamba. This study also investigated the effects of short-term exposures of the diamide insecticides cyantraniliprole and chlorantraniliprole on the survival of the larvae of the parthenogenetic mayfly Neocloeon triangulifer. The median lethal concentrations (96-h LC50s) for cyantraniliprole and chlorantraniliprole were 8.60 and 2.92 μg/L, respectively.

Magnitude of Diffusion- and Transverse Dispersion-Induced Isotope Fractionation of Organic Compounds in Aqueous Systems

Determining whether aqueous diffusion and dispersion lead to significant isotope fractionation is important for interpreting the isotope ratios of organic contaminants in groundwater. We performed diffusion experiments with modified Stokes diaphragm cells and transverse-dispersion experiments in quasi-two-dimensional flow-through sediment tank systems to explore isotope fractionation for benzene, toluene, ethylbenzene, 2,6-dichlorobenzamide, and metolachlor at natural isotopic abundance. We observed very small to negligible diffusion- and transverse-dispersion-induced isotope enrichment factors (ε < -0.4 ‰), with changes in carbon and nitrogen isotope values within ±0.5‰ and ±1‰, respectively. Isotope effects of diffusion did not show a clear correlation with isotopologue mass with calculated power-law exponents β close to zero (0.007 < β < 0.1). In comparison to ions, noble gases, and labeled compounds, three aspects stand out. (i) If a mass dependence is derived from collision theory, then isotopologue masses of polyatomic molecules would be affected by isotopes of multiple elements resulting in very small expected effects. (ii) However, collisions do not necessarily lead to translational movement but can excite molecular vibrations or rotations minimizing the mass dependence. (iii) Solute-solvent interactions like H-bonds can further minimize the effect of collisions. Modeling scenarios showed that an inadequate model choice, or erroneous choice of β, can greatly overestimate the isotope fractionation by diffusion and, consequently, transverse dispersion. In contrast, available data for chlorinated solvent and gasoline contaminants at natural isotopic abundance suggest that in field scenarios, a potential additional uncertainty from aqueous diffusion or dispersion would add to current instrumental uncertainties on carbon or nitrogen isotope values (±1‰) with an additional ±1‰ at most.

Novel phenylpyrimidine derivatives containing a hydrazone moiety protect rice seedlings from injury by metolachlor

One strategy for solving the phytotoxicity of herbicides is to apply herbicide safeners that can efficiently alleviate the injuries of agricultural crops caused by herbicides. When metolachlor, a chloroacetamide herbicide, is applied with paddy rice, for example, the mechanisms associated with metolachlor and its residue negatively impact on the growth and yields of rice. To identify novel high-activity herbicide safener candidates for metolachlor, a series of (E)-4-(2-substituted hydrazinyl)-6-chloro-2-phenyl pyrimidines were synthesized and their structures were confirmed using IR (infrared radiation), ¹H NMR, ¹³C NMR, and HRMS (high resolution mass spectrometry). The herbicide safener activities were then evaluated via primary tests. Compounds 3i and 3t were found to have the best herbicide activity on plant height. These compounds were then further screened for their activities at lower concentrations and showed better or similar activities compared to the positive control fenclorim, a commercial herbicide safener. The compounds 3i and 3t significantly enhanced glutathione S-transferase (GST) activity related with the herbicide safener activity in both shoots and roots tissues. Moreover, a qPCR (Real-time quantitative polymerase chain reaction) analysis found that the 3i and 3t treatments enhanced the expressions of OsGSTU3, OsGsTU39, and OsGSTF5. Finally, the results of an acute toxicity assessment with zebrafish (Danio rerio) embryos using treatments 3i and 3t indicated they are relatively safe to aquatic organisms.

Response of soil bacterial and fungal community structure succession to earthworm addition for bioremediation of metolachlor

Synergistic biodegradation of earthworms and soil microorganisms plays a key role in the removal of organic pollutants in soil, yet microbially mediated processes remain unclear, especially regarding the succession of soil microbial interactions. Herein, soil biochemical evaluation, microbial community characterization, and interaction network construction were combined to understand the mechanisms dominating microbial community succession during synergistic bioremediation of metolachlor-polluted soils. The results of the network analysis indicated that metolachlor could render more complex relations but weaker connection strength among soil microorganisms. The addition of earthworms significantly alleviated the stress of metolachlor on soil microbial interactions and resulted in the restoration of interactions to a great extent. Additionally, the soil physicochemical properties, enzyme activities, and microbial community changed greatly with the addition of metolachlor and earthworms. Some soil microorganisms became significantly correlated with soil properties, metolachlor concentrations, and enzyme activities. These results, dominated by the succession of soil microbial communities, provide a new perspective for assessing the remediation effect of contaminated soil by organic pollutants.

Biotransformation and detoxification of chloroacetanilide herbicides by Trichoderma spp. with plant growth-promoting activities

Due to the increasing use of chlorinated organic compounds, environmental pollution is a key issue in agricultural and industrial areas. In this study, biodegradation of chloroacetanilide herbicides, such as alachlor and metolachlor, by eight fungal strains of Trichoderma spp. originating from different microorganism collections was investigated. The tested fungi converted 80-99% of alachlor and 40-79% of metolachlor after 7 days of incubation. Biotransformation of herbicides was performed mainly by dechlorination and hydroxylation reactions. Eight alachlor metabolites and four byproducts of metolachlor conversion were detected in Trichoderma cultures, including two metolachlor intermediates for the first time identified in fungi. Moreover, in the cultures of six Trichoderma strains supplemented with chloroacetanilides, a decrease in toxicity was observed toward tested Artemia franciscana crustaceans. Simultaneously, 7 days after the application of the spores of T. koningii IM 0956, T. citrinoviride IM 6325, T. harzianum KKP 534, T. viride KKP 792 and T. virens DSM 1963 the length of roots and shoots of rapeseed seedlings treated with alachlor or metolachlor significantly increased. All tested strains exhibited plant growth-promoting traits, such as siderophore production, 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity, and phosphate solubilization, even in the presence of chloroacetanilide herbicides.

A strategy to reduce the dose of multichiral agricultural chemicals: The herbicidal activity of metolachlor against Echinochloa crusgalli

Agricultural chemicals are normally used as mixtures of several isomers, e.g., enantiomers. In theory, in order to minimize the pesticides dose, it is desirable to use the most target-active isomer. Metolachlor is a typical multichiral herbicide belonging to amide herbicides. An asymmetric carbon atom and a chiral axis yield four stereoisomers. In this study, a novel laboratory method was developed to prepare the S-metolachlor and the four stereoisomers using high performance liquid chromatography. The separated isomers had a purity of >99%, with their absolute configurations assigned by electronic circular dichroism. The enantioseparation by ultra performance convergence chromatography tandem mass spectrometry was also performed for the rapid and sensitive detection of metolachlor stereoisomers. The enantioselective herbicidal activity toward the target weed (Echinochloa crusgalli) was systematically assessed for the first time by measuring the morphology of the weed after treatment with rac-, S-metolachlor and the four stereoisomers, respectively. Among the commercial pesticides, S-metolachlor was more effective in weed inhibition than rac-metolachlor, and to the four stereoisomers, the herbicidal activities were ranked as: SS > SR ≫ RS > RR, and the RR-isomer even had some stimulative effect to the weed growth at lower concentration (1 ppm). Thus, we concluded that in these cases, the chiral carbon feature played a major role in herbicidal activity rather than the chiral axis feature, and the higher bioactivity of the S-isomers was confirmed by more effective uptake and stronger interaction with target enzymes that were involved in the gibberellic acid biosynthesis. Although the SS-isomer shows the highest herbicidal activity, controlling the major chiral feature is still much easier and more economical than controlling two chiral features.

Stimulation of earthworms (Eisenia fetida) on soil microbial communities to promote metolachlor degradation

Degradation of metolachlor in surface soil is extremely important to its potential mobility and overall persistence. In this study, the effects of earthworms (Eisenia fetida) on the degradation of metolachlor at two concentration levels (5 and 20 mg kg^-1) in soil were investigated via the column experiment. The degradation kinetics of metolachlor indicate that addition of earthworms enhances metolachlor degradation significantly (P < 0.05), with the enhanced degradation rate of 30% and 63% in the low and high concentration treatments at the 15th day, respectively. Fungi rather than bacteria are primarily responsible for metolachlor degradation in soil, and earthworms stimulate metolachlor degradation mainly by stimulating the metolachlor-degrading functional microorganisms and improving fungal community structure. Earthworms prefer to promote the possible fungal degraders like order Sordariales, Microascales, Hypocreales and Mortierellales and the possible bacteria genus Rubritalea and strengthen the relationships between these primary fungi. Two metabolites metolachlor oxanilic (MOXA) and moetolachlor ethanesulfonic acid (MESA) are detected in soil and earthworms in the high concentration treatments. Earthworms stimulate the formation of MOXA and yet inhibit the formation of MESA in soil. Another metabolite metolachlor-2-hydroxy (M2H) is also detected in earthworms, which is reported firstly. The study provides an important information for the remediation of metolachlor-polluted soil.

Enantioselective Oxidative Stress Induced by S- and Rac-metolachlor in Wheat (Triticum aestivum L.) Seedlings

The unfounded use of chiral pesticides has caused widespread concern. In this study, the enantioselective effects of S- and racemic (Rac)-metolachlor on the oxidative stress of wheat seedlings was determined based on physiological and gene transcription differences. Growth inhibition increased with increasing concentrations of tested metolachlor, and S-metolachlor had a stronger inhibitory effect than did Rac-metolachlor. Root growth was also significantly inhibited, but no enantioselective effects from the tested concentrations of the metolachlor enantiomers were observed. At a concentration of 5 mg L^-1, the maximal fresh weight inhibition reached 63.7% and 53.8% for S-metolachlor and Rac-metolachlor, respectively. In response to the S-metolachlor treatment, the maximum level of superoxide anions and malondialdehyde (MDA) increased to 1.73 and 2.55 times that in response to the control treatment, both of which were greater than those in response to the Rac-metolachlor treatment. The activity of superoxide dismutase (SOD) also increased in response to the S-metolachlor treatment, but the activity of peroxidase (POD) decreased. Real-time polymerase chain reaction (PCR) revealed that, compared with the Rac-metolachlor treatment, the S-metolachlor treatment attenuated the expression of several antioxidant genes. Together, these results demonstrate that S-metolachlor has a greater effect than does Rac-metolachlor on wheat seedlings.

Fenton-based electrochemical degradation of metolachlor in aqueous solution by means of BDD and Pt electrodes: influencing factors and reaction pathways

This work explores the role of electrode material and the oxidation ability of electrochemical advanced oxidation processes (EAOPs), such as electro-oxidation (EO) with or without H₂O₂ production, electro-Fenton (EF), and UVA photoelectron-Fenton (PEF), in the degradation of metolachlor. The performance of the EAOPs using Boron-doped diamond (BDD) or Pt as anode has been compared from the analysis of decay kinetics, mineralization profile, and energy consumption using small undivided batch cell. Metolachlor concentration always decays following a pseudo-first-order kinetics. Using the Pt anode, none of the processes reaches 30% mineralization, including PEF. In contrast, the BDD anode showed a higher mineralization rate allowing almost total mineralization in PEF due to the synergetic action of UVA light and oxidant hydroxyl radicals formed in the bulk from Fenton's reaction, as well as in the BDD, which has large reactivity to oxidize the pollutants. The increase in current density and decrease in metolachlor concentration accelerated the mineralization in PEF, although lower current efficiency and higher energy consumption was obtained. The GC-MS and HPLC analysis allowed the identification of up to 17 aromatics intermediates and 7 short-chain carboxylic acids. Finally, a reaction pathway for metolachlor mineralization by EAOPs is proposed. PEF with BDD allowed total removal of the herbicide in real water matrix and a high mineralization (83.82%).

Holistic assessment of occurrence and fate of metolachlor within environmental compartments of agricultural watersheds

BACKGROUND

Metolachlor [(RS)-2-Chloro-N-(2-ethyl-6-methyl-phenyl)-N-(1-methoxypropan-2-yl)acetamide] and two degradates (metolachlor ethane-sulfonic acid and metolachlor oxanilic acid) are commonly observed in surface and groundwater. The behavior and fate of these compounds were examined over a 12-year period in seven agricultural watersheds in the United States. They were quantified in air, rain, streams, overland flow, groundwater, soil water, subsurface drain water, and water at the stream/groundwater interface. The compounds were frequently detected in surface and groundwater associated with agricultural areas. A mass budget approach, based on all available data from the study and literature, was used to determine a percentage-wise generalized distribution and fate of applied parent metolachlor in typical agricultural environments.

RESULTS

In these watersheds, about 90% of applied metolachlor was taken up by plants or degraded, 10% volatilized, and 0.3% returned as rainfall. One percent was transported to surface water, while an equal amount infiltrated into the unsaturated zone soil water. <0.02% reached the groundwater. Subsurface flow paths resulted in greater degradation of metolachlor because degradation reactions had more time to proceed.

CONCLUSIONS

An understanding of the residence times of water in the different environmental compartments, and the important processes affecting metolachlor as it is transported along flowpaths among the environmental compartments allows for a degree of predictability of metolachlor's fate. Degradates with long half-lives can be used (in a limited capacity) as tracers of metolachlor, because of their persistence and widespread occurrence in the environment.

Characterization of glutathione S-transferases in the detoxification of metolachlor in two maize cultivars of differing herbicide tolerance

Glutathione S-transferases (GSTs) have been widely studied in relation to their role in herbicide tolerance and detoxification. However, a detailed characterization of GSTs from herbicide tolerant and sensitive maize cultivars is still lacking. In this study, we determined the mechanism of differential tolerance between two maize cultivars which had 4-fold difference tolerance to metolachlor. The metabolism rate of metolachlor was more rapid in the tolerant cultivar (Zea mays L. cv Nongda86) than the susceptible one (Zea mays L. cv Zhengda958). Addition of the GST inhibitor ethacrynic acid reduced the metabolism of metolachlor indicating the involvement of GSTs in the differential detoxification of metolachlor. The expression profiles of 32 GST isozymes were measured using quantitative RT-PCR. The results showed the expression of GST genes were slightly up-regulated in Nongda86, but severely inhibited in Zhengdan958 24h after metolachlor treatment. The genes GSTI, GSTIII, GSTIV, GST5, GST6 and GST7, which can detoxify chloroacetanilide herbicides, were all expressed higher in Nongda86 compared to Zhendgan958. The result of GST activity was consistent with the gene expression profiles. Collectively, higher-level expression of GST genes, leading to higher GST activity and faster herbicide detoxification, appears to be responsible for the difference in tolerance to metolachlor in two maize cultivars.

Pesticide and nitrate transport in an agriculturally influenced stream in Indiana

Agrochemicals can be transported from agricultural fields into streams where they might have adverse effects on water quality and ecosystems. Three enrichment experiments were conducted in a central Indiana stream to quantify pesticide and nitrogen transport dynamics. In an enrichment experiment, a compound solution is added at a constant rate into a stream to increase compound background concentration. A conservative tracer (e.g., bromide) is added to determine discharge. Water and sediment samples are taken at several locations downstream to measure uptake metrics. We assessed transport of nitrate, atrazine, metolachlor, and carbaryl through direct measurement of uptake length (S _w ), uptake velocity (V _f ), and areal uptake (U). S _w measures the distance traveled by a nutrient along the stream reach. V _f measures the velocity a nutrient moves from the water column to immobilization sites. U represents the amount of nutrient immobilized in an area of streambed per unit of time. S _w varied less than one order of magnitude across pesticides. The highest S _w for atrazine suggests greater transport to downstream ecosystems. Across compounds, pesticide S _w was longest in August relative to October and July. V _f varied less than one order of magnitude across pesticides with the highest V _f for metolachlor. U varied three orders of magnitude across pesticides with the highest U associate with sediment-bound carbaryl. Increasing nitrate S _w suggests a lower nitrate demand of biota in this stream. Overall, pesticide transport was best predicted by compound solubility which can complement and improve models of pesticide abundance used by water quality programs and risk assessments.

Assessing pesticide wet deposition risk within a small agricultural watershed in the Southeastern Coastal Plain (USA)

Pesticide volatilization and deposition with precipitation is widely documented and has been connected to adverse ecological impact. Here we describe a 3-yr study of current use and legacy pesticides in event-based rain samples within a 123-ha agricultural watershed. Crops in farm fields were documented quarterly with data used to estimate target compound use. The median number of pesticide detections in samples was 6. The fungicide, chlorothalonil which was used most intensively was detected in nearly all samples. It had the highest mean and peak concentrations with total deposition ≈0.1% of the estimated amount applied. The insecticide endosulfan also had relatively high use with behavior mirroring chlorothalonil. There was strong seasonal variation in concentration and depositional dynamics with the highest values measured during growing seasons. Similar behavior was observed with other compounds detected in rain samples with a general decrease in deposition and mean concentrations as use decreased. Comparison of measured concentrations to values associated with toxic impact on aquatic organisms indicated that chlorothalonil, endosulfan, chlorpyrifos, malathion and atrazine may contribute to adverse impact. The number of samples exceeding risk endpoints ranged from 1 to 77%. The highest value was for endosulfan; however its on-going phase-out is expected to reduce risks. Another finding was that the wet deposition of the herbicide, metolachlor exceeded measured runoff rates in the watershed by 5-fold. The study has demonstrated that localized pesticide wet deposition may present ecological risks and that volatilization and wet deposition is an important pesticide transport pathway at the local scale. Findings point to the need to include wet deposition in assessments of pesticide ecological risk and environmental fate.

Biochar characteristics produced from rice husks and their sorption properties for the acetanilide herbicide metolachlor

Rice husk biochar (RHBC) was prepared for use as adsorbents for the herbicide metolachlor. The characteristics and sorption properties of metolachlor adsorbed by the RHBC prepared at different pyrolysis temperatures were determined by analysis of physico-chemical characteristics, Fourier transform infrared spectroscopy (FTIR), Boehm titration, scanning electron microscopy (SEM), and thermodynamics and kinetics adsorption. With increasing pyrolysis temperature, the RHBC surface area greatly increased (from 2.57 to 53.08 m² g^-1). RHBC produced at the highest temperature (750 °C) had the greatest surface area; SEM also showed the formation of a porous surface on RH-750 biochar. The sorption capacity of RHBC also increased significantly with increasing pyrolysis temperature and was characterized by the Freundlich constant K _f for the adsorption capacity increasing from 125.17-269.46 (pyrolysis at 300 °C) to 339.94-765.24 (pyrolysis at 750 °C). The results indicated that the surface area and pore diameter of RHBC produced with high pyrolysis temperature (i.e., 750 °C) had the greatest impact on the adsorption of metolachlor. The FTIR, Boehm titration, and SEM analysis showed that the greatest number of surface groups were on RHBC produced at the lowest temperature (300 °C). The biochars produced at different pyrolysis temperatures had different mechanisms of adsorbing metolachlor, which exhibited a transition from hydrogen bonds dominant at low pyrolytic temperature to pore-filling dominant at higher pyrolytic temperature.

Effect of fly ash amendment on metolachlor and atrazine degradation and microbial activity in two soils

The study reports the effect of Inderprastha (IP) and Badarpur (BP) fly ashes on degradation of metolachlor and atrazine in Inceptisol and Alfisol soils. Metolachlor dissipated at faster rate in Alfisol (t1/2 8.2-8.6 days) than in Inceptisol (t1/2 13.2-14.3 days). The fly ashes enhanced the persistence of metolachlor in both the soils; however, the extent of effect was more in Inceptisol (t1/2 16.6-33.8 days) than Alfisol (t1/2 8.4-12 days) and effect increased with fly ash dose. 2-Ethyl-6-methylacetanilide was detected as the only metabolite of metolachlor. Atrazine was more persistent in flooded soils (t1/2 10.8-20.3 days) than nonflooded soils (t1/2 3.7-12.6 days) and fly ash increased its persistence, but effect was more pronounced in the flooded Inceptisol (t1/2 23.7-31 days) and nonflooded Alfisol (t1/2 6.3-10.1 days). Increased herbicide sorption in the fly ash-amended soils might have contributed to the increased pesticide persistence. The IP fly ash inhibited microbial biomass carbon at 5 % amendment levels in both the soils, while BP fly ash slightly increased microbial biomass carbon (MBC) content. Dehydrogenase activity was inhibited by both fly ashes in both the soils with maximum inhibition observed in the IP fly ash-amended Alfisol. No significant effect of fly ash amendment was observed on the fluorescein diacetate activity.

Effect of sugarcane cropping systems on herbicide losses in surface runoff

Herbicide runoff from cropping fields has been identified as a threat to the Great Barrier Reef ecosystem. A field investigation was carried out to monitor the changes in runoff water quality resulting from four different sugarcane cropping systems that included different herbicides and contrasting tillage and trash management practices. These include (i) Conventional - Tillage (beds and inter-rows) with residual herbicides used; (ii) Improved - only the beds were tilled (zonal) with reduced residual herbicides used; (iii) Aspirational - minimum tillage (one pass of a single tine ripper before planting) with trash mulch, no residual herbicides and a legume intercrop after cane establishment; and (iv) New Farming System (NFS) - minimum tillage as in Aspirational practice with a grain legume rotation and a combination of residual and knockdown herbicides. Results suggest soil and trash management had a larger effect on the herbicide losses in runoff than the physico-chemical properties of herbicides. Improved practices with 30% lower atrazine application rates than used in conventional systems produced reduced runoff volumes by 40% and atrazine loss by 62%. There were a 2-fold variation in atrazine and >10-fold variation in metribuzin loads in runoff water between reduced tillage systems differing in soil disturbance and surface residue cover from the previous rotation crops, despite the same herbicide application rates. The elevated risk of offsite losses from herbicides was illustrated by the high concentrations of diuron (14μgL(-1)) recorded in runoff that occurred >2.5months after herbicide application in a 1(st) ratoon crop. A cropping system employing less persistent non-selective herbicides and an inter-row soybean mulch resulted in no residual herbicide contamination in runoff water, but recorded 12.3% lower yield compared to Conventional practice. These findings reveal a trade-off between achieving good water quality with minimal herbicide contamination and maintaining farm profitability with good weed control.

Combined effects of pollutants and salinity on embryo-larval development of the Pacific oyster, Crassostrea gigas

For several years, low larval recruitment has been observed in Arcachon Bay, in southwest France. Exposure to pollutants could partly account for the reduction of early life stages of the Pacific oyster. This study evaluated the effects of copper and S-metolachlor in combination with salinity on the early life stages of Crassostrea gigas. Embryos were exposed to concentrations of copper (1, 10 and 50 μg L(-1)) or S-metolachlor (10, 100 and 1000 ng L(-1)) and six salinities (18, 21, 24, 27, 30 and 33 u.s.i). Embryotoxicity was measured by considering both the percentage of abnormalities and arrested development in D-shaped larvae. Embryo-larval development was only affected at salinities ≤24 u.s.i, which have never been observed during C. gigas reproduction period in Arcachon Bay. Both contaminants had an effect at environmental concentrations. Our results suggest that copper and metolachlor toxicity was enhanced with decreasing salinity.

Assessment of exposure to pesticides during mixing/loading and spraying of tomatoes in the open field

Some evidence of exposure-response of metolachlor and pendimethalin for lung cancer and an association of metribuzin with risk of glioma have been reported. The primary objectives in this study were to evaluate exposure and occupational risk during mixing/loading of pesticides and during their application to tomatoes cultivated in open fields. Sixteen farmers were sampled. Respiratory exposure was estimated by personal air sampling using fiberglass filters in a IOM device. Dermal exposure was assessed using skin pads and hand washing. Absorbed doses were estimated assuming 100% lung retention, and 50% or 10% skin absorption for metribuzin, and pendimethalin and metolachlor, respectively. The three pesticides were quantified by gas chromatography tandem mass spectrometry in all matrices. Metolachlor was used as a tracer of contamination of clothes and tractors unrelated to the exposure monitored. Respiratory exposure to metribuzin, used in granular form, was on average more than one order of magnitude higher than exposure to pendimethalin, used in the form of microencapsulated liquid. The actual doses were 0.067-8.08 µg/kg bw, 0.420-12.6 µg/kg bw, and 0.003-0.877 µg/kg bw for pendimethalin, metribuzin, and metolachlor, respectively. Dermal exposure was about 88% of the actual dose for metribuzin and more than 95%, for pendimethalin and metolachlor. For risk assessment, the total absorbed doses (sum of respiratory and skin absorbed doses) were compared with the AOEL for each compound. The actual and absorbed doses of the three pesticides were always lower than the acceptable operator exposure level (AOEL), which are reported to be 234 µg/kg bw, 20 µg/kg bw, and 150 µg/kg bw for pendimethalin, metribuzin, and metolachlor, respectively. In any case, personal protective equipment and spraying devices should be chosen with care to minimize exposure.

Comparative assessment of herbicide and fungicide runoff risk: a case study for peanut production in the Southern Atlantic Coastal Plain (USA)

Peanut (Arachis hypogaea) is produced intensively in the southern Atlantic Coastal Plain of the eastern USA. To effectively protect the region's water quality data are needed which quantify runoff of pesticides used to protect these crops. Fungicides are used intensively yet there is little published data which describe their potential for loss in surface runoff. This study compared runoff of a fungicide, tebuconazole (α-[2-(4-chlorophenyl)ethyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol), and an herbicide, metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide) from 0.2 ha fields in strip (ST), a commonly used conservation-tillage practice, and conventional tillage (CT) near Tifton, GA (USA). Following their first application, metolachlor and tebuconazole were detected at high frequency in runoff. Concentrations and their annual losses increased with application frequency and runoff event timing and frequency with respect to applications, and when fields were positioned at the top of the slope and CT was practiced. Runoff one day after treatment (DAT) contributed to high tebuconazole runoff loss, up to 9.8% of the amount applied on an annual basis. In all cases, metolachlor loss was more than 10 times less even though total application was 45% higher. This was linked to the fact that the one metolachlor application to each crop was in May, one of the region's driest months. In sum, studies showed that fungicide runoff rates may be relatively high and emphasize the need to focus on these products in future studies on peanut and other crops. The study also showed that peanut farmers should be encouraged to use conservation tillage practices like ST which can substantially reduce pesticide runoff.

A novel function of sanshools: the alleviation of injury from metolachlor in rice seedlings

Szechuan peppers are extensively used as a spice and in traditional medicine in Asia, primarily because of its active compounds, sanshools (S). However, there is only limited mention in agriculture, and there are no papers reporting its use as an herbicide safener. In this study, we provide the first evidence that S can effectively alleviate rice-seedling injury from metolachlor (M). We observed that the M-treated (0.25 μM) rice seedlings, which were 56.0%, 66.0%, and 57.0% of the non-treated control in shoot height, root length, and fresh biomass, respectively, were recovered by S to 93.1%, 97.6%, and 94.8%, respectively. The emergence rate was enhanced to over 80% in the M+S treatment, whereas it was below 60% in the M treatment. This M+S mixture elevated the rice-seedling root activity to higher than 87.0% of the value for the non-treated control. The activity of glutathione transferases in the combined treatments approximately doubles that of the M treatment and quadruples that of the non-treated controls. This effect was positively correlated with the induced expression of OsGSTU3. Our results suggest that S may represent a new group of safeners and enable the possibility of using these compounds for improving plant production or protecting rice from the phytotoxicity of metolachlor.

Comparative responses of sperm cells and embryos of Pacific oyster (Crassostrea gigas) to exposure to metolachlor and its degradation products

Metolachlor is one of the most intensively used chloroacetanilide herbicides in agriculture. Consequently, it has been frequently detected in coastal waters as well as its major degradation products, metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOA) which are encountered at higher concentrations than metolachlor. Although a few studies of metolachlor toxicity have been conducted on marine organisms, little is known about the environmental toxicity of metolachlor degradation products. In this study, the deleterious effects of metolachlor and its degradation products on spermatozoa and embryos of Crassostrea gigas have been compared using biomarkers of developmental defects, DNA damage and gene transcription levels. After 24h exposure, significant increases in the percentage of abnormal D-larvae and DNA damage were observed from 0.01 μg L(-1) for S-metolachlor and 0.1 μg L(-1) for MESA and MOA. Results showed that S-metolachlor was more embryotoxic and genotoxic than its degradation products. Oyster sperm was also very sensitive to metolachlor exposure and followed the pattern: metolachlor (0.01 μg L(-1))>MOA (0.1 μg L(-1))>MESA (1 μg L(-1)). Metolachlor and MESA mainly triggered variations in the transcription level of genes encoding proteins involved in oxidative stress responses (mitochondrial superoxide dismutase and catalase). Overall, no significant variation in transcription levels could be detected in C. gigas embryos exposed to MOA. This study demonstrates that metolachlor and its main degradation products have the potential to impact several steps of oyster development and therefore recruitment in coastal areas exposed to chronic inputs of pesticides.