Herbicidal action

Bleaching Herbicides. Membrane-based modes of herbicidal action relevant to photosynthesis (37) include those of inhibitors of carotenoid biosynthesis, eg, norflura2on, diftmon, y -phenoxyben2amines inhibitors of chlorophyll biosynthesis, eg, oxadia2on, DTP or... 

C. R dSR, Biochemistry and Physiology of Herbicide Action, Spriuger-Vedag, New York, 1982. 

Agrochemical Products. Hydrazones of vanillin have been shown to have a herbicidal action similar to that of 2,4-D, and the zinc salts of dithiovanillic acid. Made by the reaction of vanillin and ammonium polysulfide in alcohoHc hydrochloric acid, dithiovanillic acid is a vulcanization inhibitor. 5-Hydroxymerciirivanillin, 5- a cetoxym erci iri va n ill in, and 5-ch1oromercurivani11in have been prepared and found to have disinfectant properties. 

So far as the author is aware, the first statement of the herbicidal action of 2,4-dichlorophenoxyacetic acid, made in the United States, was in a publication by Hamner and Tukey (14) in 1944. They sprayed 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid on bindweed and obtained a complete kill down to the root tips. At about the same time Hamner and Tukey (13) and Marth and Mitchell (24) demonstrated the effect of 2,4-dichlorophenoxyacetic acid as a differential herbicide on lawns. All plants growing on the lawns (except the grasses) were destroyed with apparently no noticeable ill effect on the grasses. 

Herbicides, 11 868 13 281-345 13 524. See also Environmental fate of herbicides Growth retardants Herbicide action modes Herbicide groups... 

Most of today s practices are based on empirical data. Further investigation into the mechanism of herbicidal action is of fundamental importance in establishing the use of herbicides on a sound basis. More study into the synergistic effects of combining various chemicals is needed, as well as studies on methods of formulation, optimum quantities, times of application, and the influence of environmental conditions on the herbicidal action. Secondary effects, such as the reported tainting of coffee from 2,4-D deserve further inquiry. 

Proherbicides. Thio- and dithiocarbamates probably require metabolic activation prior to exerting their herbicidal effects. Sulfoxide metabolites of the -alkyl thiocarbamates are generally more potent herbicides than the parent compounds ( -5). The herbicidal action of these sulfoxides probably results from their carbamylating action for thiols, although the specific target site or receptor is not defined (23, 24). It is conceivable that the -chloroallyl thiocarbamate herbicides may act in the same way, since their sulfoxides are also potent carbamylating agents... 

Herbicidal activity generally consists of interference with plant-specific biochemical reactions. Thus, mammalian toxicity is generally low and not predictable from the mechanism of herbicidal action. In contrast, rodenticide target selectivity is not based on differences in biochemistry between humans and rodents but rather on differences in physiology or behavior, especially feeding behavior. For example, an emetic may be included in a rodenticide formulation to promote vomiting in humans who accidentally consume the product rodents do not have a vomit reflex. 

Imidazolium salts 25 react with chlorine to yield chloro-2//-imidazoles 26 and 27 (Scheme 9). - When R = alkyl, R = polychloroalkyl in 27. The action of phosphorus oxychloride on 2-ethylimidazole gives 27 (R = CCl2Me). Some of the products have herbicidal action. ... 

Quality control standards have been developed for the new petroleum weed killers and laboratory techniques have been worked out tor testing and establishing the suitability of a given petroleum fraction for a specific herbicidal use. Although attempts to correlate herbicidal action with physical and chemical tests have not been entirely successful, the use of additional specific t ts to determine the chemical composition of the fraction has served as an indicator of toxicity in the specific composition under study. 

Devine, M.D., Duke, S.O. and Fedtke, C. (1993) Physiology of Herbicide Action, PTR Prentice Hall, Englewood Cliffs, NJ. 

Gardner, G. (1981). Azidoatrazine Photoaffinity label for the site of triazine herbicide action in chloroplasts. Science, 211 937-940. Gingrich, J.C., J.S. Buzby, V.L. Stirewalt, and D.A. Bryant (1988). Genetic analysis of two new mutations resulting in herbicide resistance in the cyanobacterium Synechococcus-sp pcc 7002. Photosyn. Res., 16 83-100. 

Duke, S.O., Romagni, J.G., Dayan, F.E. Natural products as sources for new mechanisms of herbicidal action. Crop Prot 2000 19 583-589. 

Mitsutake, K.-I., Iwasmura, H., Shimizu, R., Fujita, T. (1986) Quantitative structure-activity relationships of photosystem II inhibitors in chloroplasts and its link to herbicidal action. J. Agric. Food Chem. 34, 725-732. 

C. L. Hamner and H.B. Tukey, Selective herbicidal action of midsummer and fall applications of 2,4 - Dichlorophe-noxyacetic acid, Botanical Gazette 106 (1944) 232-245. 

G.Sandman, P.Boger (Eds.), Target Sites of Herbicide Action, CRC Press, Boca Raton, FL, 1989. 

Despite the chemical diversity of the several hundred structures representing herbicidal activity, most reactions of herbicides fall within only a limited number of mechanistic types oxidation, reduction, nucleophilic displacements (such as hydrolysis), eliminations, and additions. "Herbicides", after all, are more-or-less ordinary chemicals, and their principal transformations in the environment are fundamentally no different from those in laboratory glassware. Figure 2 illustrates three typical examples which have received their share of classical laboratory study—the alkaline hydrolysis of a carboxylic ester (in this case, an ester of 2,4-dichlorophenoxyacetic acid, IX), the cycloaddition of an alcohol to an olefin (as in the acetylene, VI), and the 3-elimination of a dithiocarbamate which provides the usual synthetic route to an isothiocyanate (conversion of an N.N-dimethylcarbamic acid salt, XI, to methyl isothiocyanate). Allow the starting materials herbicidal action (which they have), give them names such as "2,4-D ester" or "pronamide" or "Vapam", and let soil form the walls of an outdoor reaction kettle the reactions and products remain the same. 

However, controlled or specific environmental degradation sometimes is necessary for herbicidal action. For example, the phenoxy herbicide sesone (sodium 2,4-dichlorophenoxyethyl sulfate) has no effect on plants until it can be oxidized to 2,4-D by a specific soil microorganism, Bacillus cereus (38). The growth regulator ethophon (Ethrel) relies upon slow environmental conversion into ethylene for its activity (39). And metham (Vapam) depends upon hydrolysis in soil to release toxic methyl isothiocyanate (40). 

Phosphinotricylalanylalanine 105 (bialaphos), isolated by Zahner et al. 2l5-232> from the culture filtrates from Streptomyces species, has a strong herbicidal action. 

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