Mechanism of herbicidal action

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. 

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. 

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. 

The ability of voltammetric responses to predict the herbicidal activity can be explained by the mechanism of herbicidal action for the nitrodiphenylethers. It is thought that these compounds are involved in the initiation of destructive free radical reactions with the phospholipid molecules which make up cellular membranes... 

Mechanisms of herbicide action and resistance have been well studied and they have often been dependent on a single gene. 

The biochemical mechanism of herbicidal action of the alkylphosphonates was explored on the basis of biochemical experiments. Representative herbicidal active alkylphosphonates were selected to examine their inhibition against plant PDHc through enzyme activity assays, kinetic experiment, and enzyme-selective inhibition experiment. Based on the above study we can give the following conclusions ... 

S. Murata, A. Yuda, A. Nakano, Y. Kimura, K. Motoba, T. Mabuchi, Y. Miura, H. Nishizawa, and S. Funayama, Mechanisms of selective action of the peroxidizing herbicide ET-751 on wheat and Galium aparine, in Proceedings of the 1995 Brighton Crop Protection Conference-Weeds, vol. 1, pp. 243-248 (1995). 

Before Radosevich and De Villiers found in 1975 that isolated chloroplasts of resistant common groundsel were insensitive to atrazine and simazine (2), it had been erroneously assumed that all living plants would die if the herbicides could reach their target site intact. We now know that mechanisms of selectivity in crops can be due to differences in metabolism rates, uptake, translocation, site of action or avoidance mechanisms. However, the mechanisms of herbicide resistance that have evolved in weeds are usually different from the mechanisms of herbicide selectivity in most crops. This is certainly true with the most prevalent and thoroughly studied cases of herbicide resistance, including the triazines, dinitroanilines, and AHAS inhibitors. 

Presented experimental results suggest that application of herbicide-binding protein in sensor technology has a high potential. Several detection systems were tested in combination with D1 protein electrochemical (amperometry and cyclic voltammetry), optical (surface plasmon resonance and ellipsometty) and assays (ELISA and D1 protein- containing liposomes and DELFIA fluori-metric assay). The main mechanisms of D1 action are either on the ability of herbicides to replace the plastoquinone molecule in D1 protein and in this way change the electrochemical and optical... 

Mechanism. Degradation of a-linolenic acid (a-lin) as proposed by (29,50,21) is demonstrated in Figure 6. The initial step is a hydrogen abstraction from an a-linolenic molecule by a radical species that was formed as a result of herbicidal action. In the following radical-chain reaction the w-3 alkyl peroxide is formed via the peroxy radical. Subsequently, this peroxide is decomposed in a Fenton-type reaction to the oj-3 alkoxy radical in the presence of transition metals that can undergo one-electron transfer reaction, e.g., Cu(I/II), Fe(Il/III), Ti(IIl/IV), or Ce(III/IV). The w-3 alkoxy radical can split by 8-sclssion to an unsaturated aldehyde and the ethyl radical. The latter is either oxidized to ethylene or reduced to ethane. 

In summary, it can be said that from present experience only in a few cases was herbicide uptake by the crop reduced in combination with a safener, and even then doubts remained as to whether the reduction of herbicide uptake was the mechanism of safener action. It is, therefore, concluded that interference with herbicide uptake by the crop has no importance as a mechanism of safener action, though it cannot be excluded that there may be cases where it plays an auxiliary role. 

The mechanism of the action of glyphosate is thought to be associated with the metabolism of aromatic amino acids. iV-(phosphonomethyl) glycine is a highly effective herbicide because of its potent and specific inhibition of 5-enolpyruvoylshikimate 3-phosphate synthase (EPSPS) [28]. This enzyme, which is found in plants and microorganisms but not in other forms of life, catalyzes the penultimate step of the shikimic acid pathway [29], that is, the interaction of shikimate-3-phosphate 8 with phosphenolpyruvate 9 to give 5-enolpyruvoylshikimate 3-phosphate 11 and inorganic phosphate (Pi)[30,31-33]. 

Mechanism of toxicolo action. The dipyridyl herbicides are reduced by reduced... 

The sites and mechanisms of surfactant action have been considered by Holloway and Stock. Surfactants may act at four possible sites in treated leaves, namely, on the surface, cuticle, and epidermis or within the internal tissues. As mentioned previously, the addition of surfactants to formulations of agrochemicals invariably improves their wetting and spreading characteristics as measured by contact angle, deposit area, spread, and surface tension. Generally, however, the effect of surfactants on coverage can seldom be related to herbicide uptake or activity. 

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