Herbicide Modes Of Action Research Articles

Weed Dynamics and Management Strategies for Cropping Systems in the Northern Great Plains

Cropping systems in the northern Great Plains (NGP) have evolved from wheat (Triticum aestivum L.)–fallow rotations to diversified cropping sequences. Diversification and continuous cropping have largely been a consequence of soil moisture saved through the adoption of conservation tillage. Consequently, weed communities have changed and, in some cases, become resistant to commonly used herbicides, thus increasing the complexity of managing weeds. The sustainability of diverse reduced tillage systems in the NGP depends on the development of economical and effective weed management systems. Utilizing the principle of varying selection pressure to keep weed communities off balance has reduced weed densities, minimized crop yield losses, and inhibited adverse community changes toward difficult‐to‐control species. Varied selection pressure was best achieved with a diverse cropping system where crop seeding date, perennation, and species and herbicide mode of action and use pattern were inherently varied. Novel approaches to cropping systems, including balancing rotations between cereal and broadleaf crops, reducing herbicide inputs, organic production, fall‐seeded dormant canola (Brassica napus and B. rapa), and the use of cover crops and perennial forages, are discussed in light of potential systems‐level benefits for weed management.

Weed Dynamics and Management Strategies for Cropping Systems in the Northern Great Plains

Cropping systems in the northern Great Plains (NGP) have evolved from wheat (Triticum aestivum L.)–fallow rotations to diversified cropping sequences. Diversification and continuous cropping have largely been a consequence of soil moisture saved through the adoption of conservation tillage. Consequently, weed communities have changed and, in some cases, become resistant to commonly used herbicides, thus increasing the complexity of managing weeds. The sustainability of diverse reduced tillage systems in the NGP depends on the development of economical and effective weed management systems. Utilizing the principle of varying selection pressure to keep weed communities off balance has reduced weed densities, minimized crop yield losses, and inhibited adverse community changes toward difficult-to-control species. Varied selection pressure was best achieved with a diverse cropping system where crop seeding date, perennation, and species and herbicide mode of action and use pattern were inherently varied. Novel approaches to cropping systems, including balancing rotations between cereal and broadleaf crops, reducing herbicide inputs, organic production, fall-seeded dormant canola (Brassica napus and B. rapa), and the use of cover crops and perennial forages, are discussed in light of potential systems-level benefits for weed management.

Triaziflam and Diaminotriazine derivatives affect enantioselectively multiple herbicide target sites.

Enantiomers of triaziflam and structurally related diaminotriazines were synthesized and their herbicidal mode of action was investigated. The compounds caused light and dark-dependent effects in multiple test systems including heterotrophic cleaver and photoautotrophic algal cell suspensions, the Hill reaction of isolated thylakoids and germinating cress seeds. Dose-response experiments revealed that the (S)-enantiomers of the compounds preferentially inhibited photosystem II electron transport (PET) and algae growth with efficacies similar to that of the herbicide atrazine. In contrast, the (R)-enantiomers of the diaminotriazines were up to 100 times more potent inhibitors of growth in cleaver cell suspensions and cress seedlings in the dark than the (S)-enantiomers. The most active compound, the (R)-enantiomer of triaziflam, inhibited shoot and root elongation of cress and maize seedlings at concentrations below 1 microM. The meristematic root tips swelled into a club shape which is typical for the action of mitotic disrupter herbicides and cellulose biosynthesis inhibitors. Microscopic examination using histochemical techniques revealed that triaziflam (R)-enantiomer blocks cell division in maize root tips 4 h after treatment. The chromosomes proceeded to a condensed state of prometaphase but were unable to progress further in the mitotic cycle. Disruption of mitosis was accompanied by a loss of spindle and phragmoplast micotubule arrays. Concomitantly, cortical microtubules decreased which could lead to isodiametric cell growth and consequently to root swelling. In addition, a decline in cellulose deposition in cell walls was found 24 h after treatment. Compared to the (R)-form, triaziflam (S)-enantiomer was clearly less active. The results suggest that triaziflam and related diaminotriazines affect enantioselectively multiple sites of action which include PET inhibitory activity, mitotic disruption by inhibiting microtubule formation and inhibition of cellulose synthesis.

Rational herbicide design by inhibition of tryptophan biosynthesis

Compounds designed to mimic the tryptophan synthase α subunit reactive intermediate were found to be potent inhibitors of the enzyme. These compounds are herbicidal and the herbicidal mode of action was demonstrated to be due to disruption of trytophan biosynthesis.

Herbicide Metabolism and Cross-Tolerance in Transgenic Potato Plants Expressing Human CYP1A1

Transgenic potato plants expressing human CYP1A1 and human CYP1A1/yeast NADPH–cytochrome P450 reductase (YR) fused enzyme were generated from microtubers by the use of an Agrobacterium transformation system. The transgenic plants S1384 expressing human CYP1A1 and both F1386 and F1515 expressing the fused enzyme were selected by kanamycin resistance, PCR analysis, chlortoluron (CT) resistance, and Western blot analysis. The integration and transcription of the corresponding CYP1A1 genes were confirmed in these selected transgenic plants by Southern and Northern blot analyses. CYP1A1 and its fused proteins were found to be produced in the transgenic plants S1384 and F1515, respectively. The P450-dependent monooxygenase activity of the transgenic plants S1384, S1386, and F1515 was 3.5, 4.2, and 3.8 times higher in 7-ethoxycoumarin 0-deethylation in vitro and 6.4, 5.8, and 5.3 times higher in [14C]CT metabolism in vivo than those of the control plants, respectively. In the metabolism of [14C]atrazine (AT), four metabolites were found in both control and transgenic plants. The deisopropylated deethylated metabolite DIDE, which is nonphytotoxic, was produced to a higher extent in S1384 and F1515 compared with the control. With herbicide tolerance tests, S1384 showed tolerance toward both AT and pyriminobac-methyl (PM), and F1386 and F1515 were tolerant toward PM, while the control died by treatment with both herbicides. Thus, it was found that the transgenic potato plants expressing human CYP1A1 metabolized the herbicides CT and AT with different structures and herbicide modes of action and resulted in cross-tolerance to both herbicides as well as PM.

Photobleaching Activity of 2-(Phenylamino)methylidenecyclohexane-1,3-diones in Tobacco (Nicotiana tabacum) Cultured Cells

A series of phenylalkylidenecyclohexane-1,3-dione derivatives were designed and synthesized as vinylogous analogs of phthalimides, which are known photobleaching herbicides. The bleaching activity of synthesized compounds was assayed with photomixotrophic tobacco cultured cells under either light or dark conditions. Both the chlorophyll and the carotenoid contents of the cells treated with the cyclic diones decreased to almost zero within 24 h under the light condition but not under the dark condition. This rapid emergence of bleaching activity with light is one of the most distinctive actions of Protox-inhibiting herbicides; however, the cyclic dione did not inhibit protoporphyrinogen oxidase in vitro. Thus, we concluded that the cyclic diones possessed a different herbicidal mode of action from Protox-inhibiting herbicide.

Comparative Sensitivity of Selenastrum capricornutum and Lemna minor to Sixteen Herbicides

Aquatic plant toxicity tests are frequently conducted in environmental risk assessments to determine the potential impacts of contaminants on primary producers. An examination of published plant toxicity data demonstrates that wide differences in sensitivity can occur across phylogenetic groups of plants. Yet relatively few studies have been conducted with the specific intent to compare the relative sensitivity of various aquatic plant species to contaminants. We compared the relative sensitivity of the algae Selenastrum capricornutum and the floating vascular plant Lemna minor to 16 herbicides (atrazine, metribuzin, simazine, cyanazine, alachlor, metolachlor, chlorsulfuron, metsulfuron, triallate, EPTC, trifluralin, diquat, paraquat, dicamba, bromoxynil, and 2,4-D). The herbicides studied represented nine chemical classes and several modes of action and were chosen to represent major current uses in the United States. Both plant species were generally sensitive to the triazines (atrazine, metribuzin, simazine, and cyanazine), sulfonureas (metsulfuron and chlorsulfuron), pyridines (diquat and paraquat), dinitroaniline (trifluralin), and acetanilide (alachlor and metolachlor) herbicides. Neither plant species was uniformly more sensitive than the other across the broad range of herbicides tested. Lemna was more sensitive to the sulfonureas (metsulfuron and chlorsulfuron) and the pyridines (diquat and paraquat) than Selenastrum. However Selenastrum was more sensitive than Lemna to one of two thiocarbamates (triallate) and one of the triazines (cyanazine). Neither species was sensitive to selective broadleaf herbicides including bromoxynil, EPTC, dicamba, or 2,4-D. Results were not always predictable in spite of obvious differences in herbicide modes of action and plant phylogeny. Major departures in sensitivity ofSelenastrum occurred between chemicals within individual classes of the triazine, acetanilide, and thiocarbamate herbicides. Results indicate that neither species is predictively most sensitive, and that a number of species including a dicot species such as Myriophyllum are needed to perform accurate risk assessments of herbicides.

1‐Aminobenzotriazole increases glutathione‐s‐transferase activity of maize

Abstract Glutathione‐S‐transferase (GST) activity of maize (Zea mays L.) seedlings treated with 1‐aminobenzotriazole (ABT) derivatives and/or EPTC were measured using EPTC‐sulfoxide as substrate. Both safeners and ABT derivatives significantly elevated the GST activity in the concentrations needed for effective safening action. ABT is considered as an inhibitor of plant cytochrome P‐450 monooxygenases (P‐450) and, because of this, used to study herbicide mode of action. Our data indicate that ABT has multiple effects on plants influencing not only P‐450 but GST as well. Thus the role of ABT in herbicide metabolism needs reconsideration.

Isomerization and Peroxidizing Phytotoxicity of Thiadiazolidine-thione Compounds

Abstract Eight 5-arylimino-3,4-tetramethylene-1,3,4-thiadiazolidine-2-thiones and eight 4-aryl-1,2- tetramethylene-1,2,4-triazolidine-3,5-dithiones were synthesized and their phytotoxic activities were investigated using sawa millet (Echinochloa utilis), green microalgae (Scenedesmus acutus) and protoporphyrinogen-IX oxidase isolated from etiolated corn (Zea mays) seedlings. 5-Arylimino-3,4-tetramethylene-1,3,4-thiadiazolidine-2-thiones showed strong phytotoxic activities and the same herbicidal mode of action as known for peroxidizing herbicides. 5-Arylimino-3,4-tetramethylene-1,3,4-thiadiazolidine-2-thiones were not or very little converted into 4-aryl-1,2-tetram ethylene-1,2,4-triazolidine-3,5-dithiones either with E. utilis seedlings present for 7 days, with S. acutus cells, or using glutathione 5-transferase (GST) and glutathione (GSH). The phytotoxic activities of 4-aryl-1,2-tetram ethylene-1,2,4-triazolidine- 3,5-dithiones were stronger than those of 5-arylimino-3,4-tetram ethylene-1,3,4-thiadiazolidine- 2-thiones [cf. Sato, Y., et al., Z. Naturforsch. 49c, 49-56 (1994)].

A Novel Class of Herbicides (Specific Inhibitors of Imidazoleglycerol Phosphate Dehydratase).

A new mode of herbicidal action was established by finding specific inhibitors of imidazoleglycerol phosphate dehydratase, an enzyme of histidine (His) biosynthesis. Three triazole phosphonates inhibited the reaction of the enzyme with Ki values of 40 [plus or minus] 6.5, 10 [plus or minus] 1.6, and 8.5 [plus or minus] 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. This effect was completely reversed by the addition of His, proving that the cytotoxicity was primarily caused by the inhibition of His biosynthesis. These inhibitors showed wide-spectrum, postemergent herbicidal activity at application rates ranging from 0.05 to 2 kg/ha.

SC-0051, a 2-benzoyl-cyclohexane-1,3-dione bleaching herbicide, is a potent inhibitor of the enzyme p-hydroxyphenylpyruvate dioxygenase

Growth inhibition of Lemna gibba plantlets by the bleaching herbicide, SC-0051 (2-(2-chloro-4-methanesulfonylbenzoyl)-1,3-cyclohexanedione)) was alleviated by the addition of homogentisic acid to the growth medium. Homogentisic acid is a key intermediate in the biosynthesis of tyrosine-derived plant quinones as well as in tyrosine metabolism. The herbicide prevented the incorporation of radioactivity from [ 14C]tyrosine into lipophilic plant metabolites and, in rat liver extracts, the herbicide inhibited the conversion of tyrosine to homogentisic acid. The enzyme p-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) from both Zea mays seedlings and liver tissues, was found to be subject to strong inhibition by SC-0051. Inhibition of plant quinone biosynthesis is a new mode of herbicidal action. One of the consequences of quinone depletion in plants in vivo is apparently an indirect inhibition of phytoene desaturation. The enzyme phytoene desaturase itself, however, is not afflicted by the herbicide.

Inhibition of δ-Aminolevulinic Acid Synthesis by Glyphosate

The effect of glyphosate [N-(phosphonomethyl) glycine] on barley (Hordeum vulgareL.) and corn (Zea maysL.) shoot δ-aminolevulinic acid (ALA) production was examined by monitoring ALA content in the tissue and measuring incorporation of14C precursors into ALA and chlorophylla. Barley shoot ALA content was significantly decreased by 1 mM glyphosate after 9, 11, and 15 h of illumination. ALA production by treated barley shoots was 30 nmoles•g fresh weight-1•h-1at each interval tested, compared with 75 to 120 nmoles•g fresh weight-1•h-1for the control. In corn shoots, ALA content was reduced 32, 45, and 58% by 0.1, 1.0, and 10.0 mM glyphosate, respectively, after 12 h illumination. Incorporation studies with14C-glutamate,14C-α-ketoglutarate, and14C-glycine into ALA showed a 77, 92, and 91% inhibition, respectively, in barley shoots treated with 1 mM glyphosate. Incorporation of14C-ALA into chlorophyllawas not affected by 1 mM glyphosate. Thus, the site of action of glyphosate may involve two enzyme pathways:one controlling the conversion of α-ketoglutarate to ALA, and the other controlling the condensation of glycine with succinyl CoA to form ALA and carbon dioxide. Inhibition of ALA synthesis blocks synthesis of chlorophyll, as well as all other porphyrin ring compounds found in higher plants. Thus, inhibition of ALA synthesis may be an integral component of the herbicidal mode of action of glyphosate.

Multifunctional Mode of Action of Substituted Nitrodiphenylethers in Scenedesmus Cells

Abstract Nitrodiphenylethers substituted with Cl, CF3, and other comparatively simple substituents are compared with respect to their primary effects on the photosynthetic apparatus. These are direct electron-transport inhibition, energy-transfer inhibition (of ATP synthesis), and pigment bleach­ ing, the latter accompanied by decrease of electron transport activity. Certain substituents and their positions at the diphenylether skeleton can be correlated with a particular influence on these phytotoxic effects. The unicellular alga, Scenedesmus, is an appropriate species to fumisch quantitative data on these primary herbicidal modes of action within a 24-h assay period.

Sites of Herbicidal Action on Photosynthesis: A Fluorescence Assay Study

The effect of the bleaching and non-bleaching phenylpyridazinones, norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] and BAS 29095 [4,5-dimethoxy-2-phenyl-3(2H)-pyridazinone], on chlorophyll fluorescence of the green alga,Scenedesmus acutus, was investigated and compared to other bleaching herbicides, difunon3[EMD-IT 5914, 5(dimethylamino-methylene)-2-oxo-4-phenyl-2,5-dihydrofuranecarbonitrile-(3)] and the diphenyl ether, oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene]. Following chlorophyll fluorescence during short-term cultivation in the presence of herbicides and comparing it to physiological parameters such as chlorophyll content and packed cell volume allowed for rapid screening to detect different primary herbicidal modes of action. Two primary bleaching effects caused by either inhibition of carotene biosynthesis (certain substituted pyridazinones and difunon) or peroxidative degradation of membrane lipids (certain diphenyl ethers) led to completely different fluorescence signals. Growth of algae in the presence of a carotene biosynthesis inhibitor resulted in a rapid rise to maximum fluorescence, followed by a single decay phase, whereas bleaching diphenyl ethers led to a rapid loss of total fluorescence. Non-bleaching phenylpyridazinones, which act as weak electron transport inhibitors, inhibited variable fluorescence. Detoxication during algal growth became evident by recovery of the fluorescence induction.