Mode of 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... 

From Table XI, note that the tt optimum for both 3- and 4-substituted TFMS series members acting on Foxtail grass in the presence of surfactant is 7t0 = —0.77. Examination of Figures 2c and 2d indicates, however, that gross differences in the herbicidal activity surfaces for the meta- and para-TFMS series exist. On this basis, it would appear that coincidence of tt0 values for 3- and 4-substituted series is not a sufficient condition for assuming that modes of herbicidal action for the two series are also identical. 

Modes of Herbicide Action as Determined with Chlamydomonas reinhardii and Coulter Counting... 

Synchronously grown Chlamydomonas reinhardii cells were used for the differentiation between different physiological modes of herbicide action. The cells were counted with a Coulter Counter and the volume spectrum was obtained by an adapted Channelyzer,... 

If great persistence is required in a soil-acting herbicide, it should have, in addition to stability, a low water solubility and a low vapour pressure. The mode of herbicide action is in many cases unknown, but it is likely to involve the inhibition of metabolic processes such as photosynthesis, respiration, DNA or lipid synthesis. 

Keywords Baseline toxicity Ecotoxicology Environmental transformation products Metabolites Mode of toxic action Pharmaceuticals Pesticides Herbicides QSAR... 

Studies on the mode of safener action in wheat indicated that the safener enhanced the metabolic degradation of both herbicides in the crop species, while it did not significantly alter their rate of degradation in the target weed species wild oats and blackgrass Alopecurus myosuroides) [14, 21, 49]. 

The differences in sensitivity of organisms and test systems, although a possible complication for interspecies extrapolations, do not affect the reliability of these correlations. Much more critical are the actual limitations of their validity stemming from the different modes of toxic action of environmental contaminants in various species. Chemicals (e.g. herbicides and AChE inhibitors) that have different toxicity mechanisms in fish and algae, respectively (Figure 8.4), reveal distinct QSARs the interspecies correlations thus break down (e.g. Gehring and Rao, 1977 Smissaert and Jansen, 1984 Wallace and Niemi, 1988 Nendza and Wenzel, 1993). 

Transgenic soybean plants expressing the CTP-CP4 EPSPS display commercial levels of Roundup tolerance. These results vaUdate the importance of substrate kinetics of EPSPS in order to maintain adequate rates of aromatic biosynthesis. Furthermore, the fact that glyphosate tolerance can be obtained by expression of a glyphosate-tolerant EPSPS illustrates that the herbicidal mode of action of glyphosate is related solely to inhibition of the EPSPS reaction. 

The mode of action has not yet been elucidated but the manufacturer states that it probably behaves like the herbicide triflurolin and its congeners. These materials inhibit cell division by binding to tubuHn thereby internipting micro-tubule development. This, in turn, stops spindle fiber formation essential to mitosis and cell division. Experiments with C-labeled Prime+ show that it is acutely toxic to fish with estimated LC q (96 h) of less than 100 ppb for rainbow trout and bluegiU. sunfish. However, channel catfish did not exhibit any toxic response at the maximum attainable water concentration (10). 

Herbicides are also sometimes classified according to mode of action, selectivity, registered uses, and toxicity. The ever-increasing importance of herbicides and other pesticides and agrochemicals to a wide range of users, regulators, and researchers has led to the development of multiple and extensive computer databases. The primary database resources contain collected information relevant to herbicides, and numerous resource pubHcations are available to those needing information on the various aspects of herbicides (2). 

Biorational approaches have proven useful in the development of classes of herbicides which inhibit essential metaboHc pathways common to all plants and thus are specific to plants and have low toxicity to mammalian species. Biorational herbicide development remains a high risk endeavor since promising high activities observed in the laboratory may be nullified by factors such as limitations in plant uptake and translocation, and the instabiHty or inactivity of biochemical en2yme inhibitors under the harsher environmental conditions in the field. Despite these recogni2ed drawbacks, biorational design of herbicides has shown sufficient potential to make the study of herbicide modes of action an important and growing research area. 

A second class of herbicides primarily affects ( -carotene desaturase. These herbicides are apparent feedback inhibitors of PD as well. This class of compounds includes dihydropyrones like LS 80707 [90936-96-2] (56) and 6-methylpyridines (57,58). The third class consists of the ben2oylcyclohexane-diones, eg, 2-(4-chloro-2-nitroben2oyl)-5,5-dimethyl-cyclohexane-I,3-dione. This class of atypical bleaching herbicides induces phytoene accumulation when appHed either pre- or post-emergence. However, it does not inhibit phytoene desaturase activity in vitro (59). Amitrole also has been considered a bleaching herbicide, though its main mode of action is inhibition of amino acid synthesis. 

Cell Division Inhibitors. The most common mode of action of soil-appHed herbicides is growth inhibition, primarily through dkect or indkect interference with cell division (163). Such growth inhibitory activity is the basis for most pre- or post-emergent herbicides intended to control germinating weed seeds. In germinating seeds, cell division occurs in the meristems of the root and the shoot. Meristematic cells go through a cycle... 

The inhibitors of amino acid synthesis, sulfonylureas, imidazolinones, and glyphosate, were first recognized as general growth inhibitors that prevent mitotic entry (188,189). Whatever the mode of action, herbicides that inhibit amino acid synthesis also cause a rapid inhibition of cell growth, usually through inhibition of mitotic entry. 

Hassall, K.A. (1990). A readable student text, particularly useful for the metabolism and mode of action of herbicides and fungicides. 

The following brief account identifies only major groups of herbicides not mentioned elsewhere in the text, and is far from comprehensive. Their mode of action is only dealt with in a superficial way. From an ecotoxicological point of view, there has not been as much concern about their sublethal effects upon plants as there has been in the case of mammals, and there has not been a strong interest in the development of biomarker assays to establish their effects. The major concern has been whether weeds, or nontarget plants, have been removed following herbicide application—a rather easy matter to establish as plants are fairly sedentary. For a more detailed account of herbicide chemistry and biochemistry, see Hassall (1990). 

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