Simazine metabolism

Beynon et al. (1972a) compared the breakdown of cyanazine, atrazine, and simazine in soils and com. Residues of parent compound in leaf and stem tissue of corn plants 70 days after application at a rate of 1.5 kg a.i./ha to a medium loam soil were barely detectable. Cyanazine metabolized to chlorotriazines and hydroxytriazines, including their dealkylated derivatives. Atrazine and simazine metabolized to hydroxytriazines and unidentified polar components. 

Plant. Simazine is metabolized by plants to the herbicidally inactive 6-hydroxysimazine which is further degraded via dealkylation of the side chains and hydrolysis of the amino group releasing carbon dioxide (Castelfranco et al., 1961 Humburg et al, 1989). 

Simazine and atrazine have little effect on com, sorghum, sugarcane, and some other grasses, apparently because of the ability of the plant to metabolize the parent molecule. Com and wheat gave a cyclic hydrox-amate, namely, the 2-D-glucoside of 2,4-dihydroxy-7-methoxy-l, 4-benzox-azin-3-one (11), m.p. 168-70°, aglycon m.p. 156-7 (dec.), believed to... 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

Exer, B. (1958). Plant growth regulators The effect of simazine on metabolism. Experientia, 14 136-137. 

With the approval of simazine in 1957 by the USFDA, USDA, and USEPA, the basis and procedures for successful introductions of other chlorotriazines were established. Although additional development work was necessary for approval and registration of the subsequent chlorotriazines, the procedures to optimize the production, formulation, and directions for use and the protocols to analyze and understand metabolism and toxicology remained similar. Approval for the first commercial uses of simazine and atrazine in various countries are given in Tables 3.3 and 3.4, respectively. 

The triazine herbicides can be divided into four different structural classes chlorotriazines, methylthiotriazines, methoxytriazines, and atypical or asymmetrical triazines. The chlorotriazine group includes atrazine, simazine, pro-pazine, terbuthylazine, and cyanazine. The methylthiotriazine group includes ametryn, prometryn, and terbutryn. The methoxytriazine group will include prometon and secbumeton. Hexazinone and metribuzin were chosen to represent the atypical triazine group. The plant metabolism of the most researched member of each triazine group will be discussed in detail to cover all major biological and chemical transformations reported in the literature. 

The metabolism of s-triazines has been the subject of extensive research since the 1950s to the present time. Much of this research has been the subject of review articles published over the years since s-triazines were introduced. The metabolism of -triazine herbicides in animals and plants and their degradation in soil were the subject of a review by Knuesli et al. (1969), later updated and revised by Esser et al. (1975) as a second edition. The metabolism of. v-triazines in plants was also reviewed by Shimabukuro et al. (1971a). Naylor (1976) published a review of herbicide metabolism in plants that included the. v-triazines. Lamoureux et al. (1998) reported on the identification of several plant metabolites of atrazine and simazine. 

Thompson Jr. L. (1972b). Metabolism of simazine and atrazine by wild cane. Weed Sci., 20 153-155. 

Soon after the discovery of triazine-resistant common groundsel, another equally important discovery was made. Radosevich and DeVilliers (1976) found that the mechanism of resistance in this weed was due to insensitive chloro-plasts that were capable of photosynthesis, even in the presence of simazine or atrazine. This was surprising because earlier research had confirmed that there were no differences in plant selectivity or susceptibility due to the origin of chloroplasts. Moreland (1969) had reported that isolated chloroplasts were equally inhibited to simazine whether they came from tolerant com or susceptible spinach. Radosevich and Appleby (1973) had confirmed there were no differences between the susceptible and resistant biotypes of common groundsel due to herbicide uptake, distribution, or metabolism, whereas it is known that com metabolizes triazine herbicides (Shimabukuro, 1985). 

Lund-Hoie, K. (1969a). Uptake, translocation and metabolism of simazine in Norway spruce (Picea abies). Weed Res., 9(2) 142-147. 

Couch, R.W., J.V. Gramlich, D.E. Davis, and H.H. Funderburk (1965). The metabolism of atrazine and simazine by soil fungi. Proc. Southern Weed Control Conf, 18 623. 

Eno, C.F. (1962). The effect of simazine and atrazine on certain of the soil microflora and their metabolic processes. Soil Sci. Soc. Florida Proc., 22 49. 

It is useful at this point to review briefly the current status of plant metabolism of simazine [2-chloro-4,6-bis (ethylamino) -s-triazine] and then consider this compound in soils. The conversion of simazine to hydroxysimazine [2-hydroxy-4,6-bis (ethylamino) -s-triazine] in com apparently is catalyzed by a nonenzymatic reaction (4,15,37) (Figure 6). 

Figure 7. Proposed metabolic pathway for simazine decomposition in soils from Kearney et al. (26)]...

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. 

Remediation of simazine and 4-nonylphenol with transgenic plants carrying drug-metabolizing cytochrome P450 genes (H. Inui, Kobe Univ., Japan)... 

Two other Lolium ri dum biotypes from Australia (WLR2 and VLR69) developed metabolism-based resistance to PSII inhibitors. WLR2 came from a field with selection pressure by atrazine and amitrole, but never by phenylureas, and VLR69 from a field with selection pressure by diuron and atrazine. Both biotypes were resistant to triazines, and, despite the field selection by atrazine, resistance was more pronounced to the structurally related simazine. Furthermore, both biotypes were resistant to chlorotoluron, though only VLR69 was previously exposed to phenylureas. Analytical work revealed that in both resistant biotypes... 

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