Joint action of sulfonylurea herbicides and MCPA

The joint action of mixtures of the sulfonylurea herbicides chlorsulfuron, metsulfuron and tribenuron and MCPA as a dimethylamine salt was assessed on Stellaria media L. and Lamium purpureum L. Two statistical analyses, and appropriate experimental designs, were used—the additive dose model (ADM) and the principle of parallel dose-response curves. The ADM analyses revealed that mixtures of the sulfony-lureas and MCPA were less potent than expected on Stellaria while the parallel-line assay analyses showed that activity of the sulfonylurea herbicides on Stellaria and Lamium was reduced in mixtures with MCPA. The antagonism was found to be dependent on the ratio of the two herbicides. Activity decreased with increasing MCPA ratios in the mixtures. Sequential spraying and selective placement of droplets revealed that MCPA only antagonized the sulfonylureas when the herbicides were in physical contact with each other. The benefits of applying both statistical methods when studying the joint action of two herbicides both possessing activity on the test plants are discussed. Action de melanges d'herbicides sulfonylurées et de MCPA L'action de melanges des herbicides sulfonylurées chlorsulfuron, metsulfuron et tribénuron avec le MCPA formulé en sel de diméthylamine, a été etudiée sur Stellaria media L. et Lamium purpureum L. Deux analyses statistiques asso-ciées a deux systèmes expérimentaux appropriés ont été utilisés: le modèle à doses additives (ADM) et le principe du parallélisme des cour-bes dose-effet. Les analyses ADM ont montré que les melanges de sulfonylurées et de MCPA ont été rnoins efficaces qu'attendu sur Stellaria. Les analyses en courbes parallèles ont montré que 1'activité des herbicides sulfonylurées sur Stellaria et Lamium était réduite dans des mélanges avec le MCPA.L'antagonisme dépendait du rapport entre les doses des deux herbicides, 1'activité décroissant avec 1'augmentation du taux de MCPA. Des traitements séquentiels et des dépôts localisés de gouttelettes ont montré que le MCPA n'était an-tagoniste des sulfonylurées que lorsque les herbicides étaient physiquement en contact. L'intérêt est discuté, d'utiliser les deux méthodes statistiques de concert pour étudier les interactions entre deux herbicides actifs sur les plantes test. Gemeinsame Wirkung von Sulfonylharnstoff-Herbiziden und MCPA Die Wirksamkeit von Mischungen der Sulfonyl-hamstoff-Herbizide Chlorsulfuron, Metsulfuron und Tribenuron mil MCPA als Dimethylamin-salz wurde an Stellaria media L. und Lamium purpureum L. bestimmt. Die Versuche wurden so angelegt, um 2 statistische Analysen anzuwen-den: das Additiv-Dosis-Modell (ADM) und das Prinzip paralleler Dosis-Wirkungs-Kurven. Die ADM-Analyse ergab, daß diese Mischungen gegenüber Stellaria weniger wirksam als erwartet waren, während die Parallel-Analyse zeigte, daß die Wirksamkeit der Sulfonylharnstoff-Herbiz-ide in Mischungen mil MCPA gegeniiber den beiden Arten herabgesetzt war. Es ergab sich, daß der Antagonismus vom Verhältnis der Mischungspartner abhängt. Die Wirkung nahm mit steigenden MCPA-Anteilen in den Mischungen ab. In Spritzfolgen und bei separater Applikation zeigte sich, daß MCPA nur dann gegenüber Sulfonylharnstoffe antagonistisch wirkte, wenn sich die Herbizide vermischten. Die Vorzüge der Anwendung der beiden statistischen Methoden zur Untersuchung des gemeinsamen Effektes von 2 Herbiziden ähnlicher Wirkung werden diskutiert.     

Joint action of amino acid biosynthesis-inhibiting herbicides

The joint action of binary mixtures of the amino acid biosynthesis‐inhibiting herbicides glyphosate, glufosinate‐ammonium, metsulfuron‐methyl and imazapyr was assessed in pot experiments applying the Additive Dose Model (ADM). Plants of Sinapis arvensis or S. alba were sprayed with seven doses of the herbicides alone and binary fixed‐ratio mixtures of the four herbicides. In total, 73 binary mixtures were studied in six separate experiments. Mixtures of glyphosate and glufosinate‐ammonium were less phytotoxic than predicted by ADM whether commercial formulations or technical grade products were applied. In contrast, mixtures of glyphosate and metsulfuron‐methyl, glyphosate and imazapyr, glufosinate‐ammonium and metsulfuron‐methyl, glufosinate‐ammonium and imazapyr, and metsulfuron‐methyl and imazapyr either followed ADM or were synergistic. Synergism was observed most frequently for mixtures of glyphosate or glufosinate‐ammonium with metsulfuron‐methyl. Synergism was also more pronounced for commercial formulations of glyphosate and glufosinate‐ammonium than for the corresponding technical grade formulations, implying that synergism was caused by the presence of the formulation constituents of the commercial glyphosate and glufosinate‐ammonium formulations in the spray solution.     

Sulfur assimilation in plants and weed control: Potential targets for novel herbicides and action sites of certain safeners

Sulfur is an indispensable element for plants. It is found in sulfur-containing amino acids, cysteine and methionine, and in various other important biochemical components and processes. Inhibitors of sulfur assimilation, or cysteine and methionine synthesis, could be potential herbicides. In the present paper, the sulfur assimilation pathway in plants is described, followed by the introduction of several compounds (inhibitors and safeners) acting on this pathway. Uptake of inorganic sulfate through the roots is the first step of sulfur assimilation in plants. Sulfate is reduced mainly in chloroplasts to sulfide by a multistep process, and sulfide is then incorporated into cysteine. Cysteine is converted to cystathionine, homocysteine and methionine. Cysteine is incorporated into glutathione (GSH) by γ-glutamylcysteine synthetase and GSH synthetase. Three enzymes involved in cysteine and methionine biosynthesis, cysteine synthase, cystathionine γ-synthase and cystathionine β-lyase, have been investigated as target sites for herbicides. Several inhibitors of these enzymes (e.g. rhizobitoxine and propargylglycine) were also phytotoxic, suggesting that the synthetic pathway of sulfur-containing amino acids could be a new target site for herbicides. Some safeners for herbicides were found to act on the sulfur assimilation pathway and on GSH synthesis to increase GSH, which can be involved in herbicide metabolism and detoxification. Several safeners elevate GSH levels by increasing the activities of enzymes involved in sulfur assimilation and GSH synthesis. Further studies on plant sulfur metabolism may lead to the discovery of new herbicides and to the comprehensive understanding of the mode of action of safeners.     

Phytotoxic sites of action for molecular design of modern herbicides (Part 1): The photosynthetic electron transport system

A bird's eye review was tried to select the bio‐rational targets from known and novel plant‐specific ones for the molecular design of modern herbicides, which exhibit efficient phytotoxicity at a low‐use rate and preserve a good environment in the 21st century. In phytotoxic sites in the photosynthetic electron transport (PET) system discussed in the present article (Part 1), the generally called bleaching herbicides interfering with the biosynthesis of photosynthetic pigments, chlorophylls and carotenoids, and the biosynthesis of plastoquinone, were considered to be good models for the molecular design of modern herbicides. The PET itself was still considered as an interesting target site for new herbicides, although they need to exert their action in all green leaves of weeds to achieve herbicidal efficacy. Because these herbicides never form a tight binding with D1‐protein, their use‐rate cannot be expected to be as low as the herbicides inhibiting chlorophyll or branched amino‐acid biosynthesis. Other herbicidal targets found in chloroplasts, namely ATP and NADPH formations, have already been omitted from the worldwide biorational molecular design program of herbicides targeting the PET system.     

Phytotoxic sites of action for molecular design of modern herbicides (Part 2): Amino acid, lipid and cell wall biosynthesis, and other targets for future herbicides

A bird's eye review was tried in Part 2 of this series, 'Phytotoxic sites of action for molecular design of modern herbicides', in order to select the best selection of known and some novel plant-specific targets for molecular design of modern herbicides, which affect amino acid, lipid and cell wall biosynthesis. Although amino acid biosynthesis pathways, particularly those for aromatic amino acids, ammonia assimilation and branched amino acids, have been confirmed as reasonable herbicidal target domains, the other targets affecting plant growth more markedly than inhibition of 5-enolpyruvylshikimate-3-phosphate synthase, glutamine synthetase and acetolactate synthase are discussed. In three essential enzymes involved in fatty acid biosynthesis in or in the vicinity of chloroplasts, acetyl-CoA carboxylase (ACCase), elongase(s) for very long chain fatty acids (VLCFA) and linolate monogalactosyldiacylglycerol desaturase, ACCase and elongase are more important targets for new herbicides. Although the effect of cellulose biosynthesis inhibitors is restricted to cell wall formation in growing plant cells only, there is a good chance to design the low-use rate herbicides also in this class of inhibitors. Other possible targets for new herbicides are also discussed.     

Target sites for herbicides: entering the 21st century

At present the use-rate of modern herbicides is in the range of 100-300 g AI ha-1, with a tendency to decline. The low use-rate (ca 10 g AI ha-1) of the original sulfonylurea and cyclic imide herbicides prompted agrochemical scientists to look for even more active compounds which led to the successive discoveries of many new herbicidal acetolactate synthase inhibitors and no less than 18 cyclic imides in the class of protoporphyrinogen-IX oxidase inhibitors in the 1990s. In this paper, mechanisms of action related to function and biosynthesis of chlorophylls, carotenoids, plastoquinone, amino acids, fatty acids and photosynthetic electron transport and other metabolic processes are discussed as plant-specific herbicidal target domains.     

Auxin herbicides: current status of mechanism and mode of action

Synthetic compounds that act like phytohormonal ‘superauxins’ have been among the most successful herbicides used in agriculture for more than 60 years. These so‐called auxin herbicides are more stable in planta than the main natural auxin, indole‐3‐acetic acid (IAA), and show systemic mobility and selective action, preferentially against dicot weeds in cereal crops. They belong to different chemical classes, which include phenoxycarboxylic acids, benzoic acids, pyridinecarboxylic acids, aromatic carboxymethyl derivatives and quinolinecarboxylic acids. The recent identification of receptors for auxin perception and the discovery of a new hormone interaction in signalling between auxin, ethylene and the upregulation of abscisic acid biosynthesis account for a large part of the repertoire of auxin‐herbicide‐mediated responses, which include growth inhibition, senescence and tissue decay in sensitive dicots. An additional phenomenon is caused by the quinolinecarboxylic acid quinclorac, which also controls grass weeds. Here, the accumulation of phytotoxic levels of tissue cyanide, derived ultimately from quinclorac‐stimulated ethylene biosynthesis, plays a key role in eliciting the herbicidal symptoms in sensitive grasses. Copyright © 2009 Society of Chemical Industry     

The relationship of metabolism studies to the modes of action of herbicides

Metabolism studies are necessary in the understanding of the mode of action of herbicides, their loss of activity and their selectivity. Molecular change of a herbicide may involve photochemical degradation, and non-enzymic or enzymic reactions that normally inactivate, but in some cases activate, a compound. Molecular change resulting in activation is discussed with regard to the (2,4-dichlorophenoxy)-alkanoic acids, the pyridines paraquat and diquat, the anilides benzoylprop-ethyl and flamprop-methyl, and the thiocarbamates. Inactivation due to metabolism is reviewed with regard to herbicides applied to soil and foliage. Of the soil-applied herbicides, the 1,3,5-triazines and the nitrile dichlobenil are exemplified as cases in which selectivity may be due largely to interspecific differences in metabolism. Of the foliage-applied translocated compounds, the metabolism of the phenoxyalkanoic acid herbicides has been extensively investigated and may involve conjugation, side-chain degradation or extension, ring hydroxylation and protein binding. The mode of action of glyphosate, asulam, dalapon and aminotriazole in perennial weeds is discussed with particular reference to their metabolism.     

Mechanisms of action and structure activity relations of herbicides that inhibit photosynthesis

A survey and chemical classification are given for the most important herbicidal inhibitors of photosynthesis. The photochemical reactions involved in photosynthesis are reviewed. The herbicides discussed here all interfere with photosynthetic electron transport in the light reactions. Comments are made on the redox catalysts and the electron transport chain. The mode of action of the herbicides due to inhibition of the light reactions I and II or of photophosphorylation are described. Structure activity correlations according to the regression analysis (Hansch-approach) are discussed with examples in the classes of acetanilides, benzimidazoles and triazinones. The correlation studies in the series of the Hill-reaction inhibitors have led to a model for the essential structural elements. Some work on the problem of selectivity and its importance is reviewed.     

Sites of action of photosynthetic inhibitor herbicides; Experiments with trypsinated chloroplasts

The effect of four photosynthetic inhibitor herbicides, bromacil, ioxynil, metribuzin and monuron, on chloroplast electron transport was investigated. All four compounds completely inhibited electron flow with tripotassium hexacyanoferrate as oxidant, but the inhibition caused by bromacil, metribuzin and monuron was almost totally reversed by trypsin treatment. With ioxynil, only a partial degree of reversability was shown. With a molybdosilicate as oxidant, electron transport was not completely inhibited by any of the herbicides. Whereas the partial inhibition was reversed by tryptic digestion in the presence of bromacil, metribuzin and monuron, there was virtually no reversal in the presence of ioxynil. The results suggest a common site of action for all four herbicides and an additional site for ioxynil nearer to photosystem II.     

Joint action of some root-absorbed herbicides in oats (Avena sativa L.)

In growth chambers the phytotoxicity of binary mixtures of four herbicides was compared using an Additive Dose Model as reference. Of the four herbicides used, lenacil was the most potent whereas ethofumesate was the least potent herbicide; chloridazon and metamitron were equally potent. The Additive Dose Model implicitly presupposes that at any one response level the herbicides of a mixture can replace each other in proportion to their relative potency when applied separately. Metamitron and chloridazon mixtures appeared to follow the reference model. The efficacy of mixtures of lenacil and chloridazon or lenacil and metamitron, however, were increased in comparison with the herbicides applied separately. On the other hand, lenacil and ethofumesate or metamitron and ethofumesate were less phytotoxic than expected from the Additive Dose Model. The results are discussed in relation to the mode of action of the compounds. Action combinée de certains herbicides absorbés par les racines en culture d'avoine Avena sativa L. La phytotoxicité de mélanges binaires de quatre herbicides a fait l'objet d'une comparaison en phytotron, utilisant comme témoin un Modèle Dose Additive. Parmi les quatre herbicides utilisés, le lénacile s'est révélé le plus et l'éthofumésate le moins actif; le chloridazone et le métamitrone ont fait preuve d'une activitéégale. Le Modèle Dose Additive présuppose qu'à tout niveau d'activité chaque herbicide faisant partie d'un mélange puisse se substituer à l'autre en fonction de son efficacité relative en traitement simple. Les mélanges métamitrone–chloridazone semblent s'accorder au modèle de référence. Cependant, l'efficacité des mélanges chloridazone–lénacile et lénacile–métamitrone a augmenté par rapport à l'application de chaque élément séparément. D'autre part, les mélanges lénacile–éthofumesate et métamitrone–éthofumesate ont fait preuve d'une phytotoxicité moindre que celle qu'annonçait le modèle. Ces résultats sont discutés par rapport au mode d'action des composés. Kombinierte Wirkung verschiedener wurzelwirksamer Herbizide auf Avena sativa L. In Klimakammern wurde die Phytotoxizität von Zweier-Mischungen von vier Herbiziden untersucht und mit einem Modell additiver Dosierungen verglichen. Von den vier untersuchten Herbiziden war Lenacil das wirksamste und Ethofumesat das schwächste; Chloridazon und Metamitron wirkten gleich stark. Das Modell additiver Dosierungen setzt stillschweigend voraus, dass auf jedem Wirkungsniveau die Herbizide einer Mischung einander im Verhältnis ihrer relativen Wirksamkeiten, wenn allein appliziert, ersetzen können. Metamitron + Chloridazon Mischungen scheinen dem Vergleichsmodell zu folgen. Die Wirkungen der Mischungen von Lenacil + Chloridazon oder Lenacil + Metamitron, hingegen, waren im Vergleich zu den Effekten der einzeln applizierten Komponenten gesteigert. Andrerseits waren Lenacil + Ethofumesat oder Metamitron + Ethofumesat weniger phytotoxisch, als das Modell hätte erwarten lassen. Die Resultate werden mit Bezug auf die Wirkungsmechanismen der Komponenten diskutiert.     

A review of the activity,fate and mode of action of sulfonylurea herbicides

The sulfonylurea herbicides are a relatively new group of compounds which control broad-leaved weeds and some grasses in cereal crops. This literature review emphasises work reported on chlorsulfuron and met-sulfuron-methyl. The activity of the herbicides, their fate in soil and in plants, and their mode of action are discussed. In addition some of the methods of assaying these compounds are described.     

Mode of action,crop selectivity,and soil relations of the sulfonylurea herbicides

The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2–75 g ha^?1), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results from rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range from 1–5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives > 20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative O-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combination of bridge hydrolysis and microbial degradation. Hydrolysis is significantly faster under acidic (pH 5) than alkaline (pH 8) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of 1–6 weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensulfuron-methyl) have been developed.     

Changes in free amino acid pools can predict the mode of action of herbicides

Studies were conducted to determine the short-term changes in free amino acid levels in the meristematic zone of maize after treatment with various herbicides with different modes of action. These herbicides included inhibitors of various amino acid biosynthetic pathways, photosynthesis, and fatty acids biosynthesis. Inhibitors of various amino acid biosynthetic pathways caused specific reduction in the pools of amino acids being produced by the particular pathway. Inhibitors of other metabolic pathways also caused significant changes in pools of various amino acids. Very similar changes in the pools of amino acids were seen in plants treated with different chemical classes of inhibitors affecting a certain metabolic pathway. However, the changes were quite different between inhibitors of different metabolic processes. The data generated from these studies could be used as a diagnostic tool to determine the mode of action of novel herbicides.     

The action of herbicides on fatty acid biosynthesis and elongation in barley and cucumber

BACKGROUND: Herbicides that affect lipid metabolism have been used commercially for many years. Here, napropamide, diphenamid, dimethachlor and cafenstrole are compared; these have all been classified by the Herbicide Resistance Action Committee (HRAC) as K₃ herbicides and inhibitors of cell division and/or synthesis of very‐long‐chain fatty acids (VLCFAs). In addition, spiro‐decanedione A and pinoxaden dione are compared as inhibitors of lipid synthesis through inhibition of acetyl‐CoA carboxylase (ACCase). RESULTS: Whereas the chloracetamide dimethachlor and the carboxyamide cafenstrole potently inhibited VLCFA synthesis in both barley and cucumber, the acetamides napropamide and diphenamid which are also classified as K₃ herbicides and likewise the unclassified herbicide cinmethylin did not. The graminicide pinoxaden dione inhibited de novo fatty acid synthesis in barley, but not in cucumber, and correspondingly inhibited the plastid form of maize ACCase much more than the cytosolic form (IC₅₀ values of 0.1 and 17 µM ). By contrast, spiro‐decanedione A exhibited herbicidal effects not only on grasses but also on broad leaves, strongly inhibited maize cytosolic ACCase and inhibited synthesis of VLCFAs in cucumber. CONCLUSIONS: The acetamides napropamide and diphenamid, which do not inhibit VLCFA synthesis, should be classified separately from K₃ herbicides that do. Pinoxaden dione and spiro‐decanedione A represent new classes of chemicals acting on plant lipid synthesis. Copyright © 2010 Society of Chemical Industry     

Deltamethrin: A neurophysiological study of the sites of action

Recent experiments on the mode of action of pyrethroids have indicated that those pyrethroids containing an α-cyano phenoxybenzyl group may act on GABA-mediated chloride channels. The crayfish stretch receptor neuron provides a useful preparation for examining the effects of pyrethroids on these channels and on sodium channels. The lowest concentration of deltamethrin to have an effect on sodium channels was 10⁻¹² M, but the response of the preparation to GABA appeared to be unaffected by concentrations of deltamethrin up to 10⁻⁷ M. Although 10⁻⁶ M deltamethrin had a slight effect on the GABA response of the dactyl abductor muscle, it appears that the majority of the effects of cyano pyrethroids in invertebrates could be accounted for solely by their action on sodium channels.     

Nitro free radical formation of diphenyl ether herbicides is not necessary for their toxic action

The diphenyl ether herbicides MC 15608 {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-chloromethylbenzoate} and MC 10878 {5-[2-chloro-4-(trifluoromethyl)phenoxy]methyl benzoate} are structurally similar to acifluorfen-methyl (methyl ester of 5-[2-chloro-4-(trifluoromethyl)phenoxy]-nitrobenzoic acid), except that the NO₂ is replaced by a Cl and H, respectively. These diphenyl ether herbicides required light for herbicide toxicity to the green unicellular alga Chlamydomonas eugametos and three major weeds (Xanthium pennsylvanicum, Abutilon theophrasti, and Ipomoea sp.). Acifluorfen-methyl and MC 15608 toxicity in Chlamydomonas decreased in an atmosphere of nitrogen, and in the presence of the free radical scavengers α-tocopherol and ethanol. Therefore, the mechanism of toxic action of these three different diphenyl ether herbicides is similar and appears to involve some type of free radical reaction. As confirmed by cyclic voltammetry studies, MC 15608 and MC 10878, unlike AFM, cannot readily accept electrons to become free radicals. Therefore, initiation of free radical reactions in polyunsaturated fatty acids of membranes does not necessarily involve direct reduction and reoxidation of the diphenyl ether molecule.     

Diaryloxyacetate herbicides

The discovery and investigation of a novel family of herbicides containing a diaryl acetal are described. The stability of the acetal limited herbicidal efficacy and recognizing that fact led to the design of analogs with commercial levels of post-emergence activity on broadleaf weeds. These compounds inhibited acetolactate synthase and in vitro activity data were used to guide target design. However, no members of this family provided a commercially valuable combination of herbicidal efficacy and crop selectivity.