In studies of herbicide metabolism

The mixed-function oxidase inhibitors aminobenzotriazole and piperonyl butoxide can synergize herbicide activity in resistant Lolium growing in a hydroponic system. This indicates that at least one aspect of cross-resistance in Lolium rigidum may be related to enhanced metabolic activity of mixed-function oxidazes acting to detoxify herbicides. We are now concentrating on direct studies of herbicide metabolism in resistant biotypes. 

In the study of herbicide metabolism, it has been observed that the parent compound and its metabolites can be incorporated into lignin, cellulose, and some proteins. These products are insoluble, resistant to hydrolytic enzymes, immobile, and consequently nontoxic. Rigorous procedures may be needed to release the metabolite for analysis. ... 

The study of xenobiotic metabolism has developed rapidly during the past few decades. These studies have been fundamental in the assessment of drug efficacy, safety, and design of dosage regimens in the development of food additives and the assessment of potential hazards of contaminants in the evaluation of toxic chemicals and in the development of pesticides and herbicides and their metabolic fate in insects, other animals, and plants. The metabolism of many xenobiotics is fundamental to many toxic processes such as carcinogenesis, teratogenesis, and tissue necrosis. Often the same enzymes involved in... 

N-Dealkylation also plays a role in the metabolism and selectivity of 5-triazine herbicides such as atrazine," simazine, simetryn, and ter-butryn. In species in which this pathway was prominent, the major products identified were monodealkylated triazines, which retained phytotoxicity. However, in a study of atrazine metabolism in C. album, C. strictum, and Amaranthus ponelli, Khan et al. identified the bound residue from resistant biotypes as N-desethylatrazine and suggested that sequestration of ph3 otoxic metabolites contributed to tolerance to the herbicide. " ... 

Cl4-DBpD) may occur in trace amounts in the herbicide, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (I, 2). Radiolabeled preparations of this dioxin are needed to facilitate studies of its degradation chemistry, metabolism, and mode of action. 

The complexity of the metabolism of alachlor, acetochlor, butachlor, and propachlor has led to the development of degradation methods capable of hydrolyzing the crop and animal product residues to readily quantitated degradation products. Alachlor and acetochlor metabolites can be hydrolyzed to two major classes of hydrolysis products, one which contains aniline with unsubstituted alkyl groups at the 2- and 6-positions, and the other which contains aniline with hydroxylation in the ring-attached ethyl group. For alachlor and acetochlor, the nonhydroxylated metabolites are hydrolyzed in base to 2,6-diethylaniline (DBA) and 2-ethyl-6-methylaniline (EMA), respectively, and hy-droxylated metabolites are hydrolyzed in base to 2-ethyl-6-(l-hydroxyethyl)aniline (HEEA) and 2-(l-hydroxyethyl)-6-methylaniline (HEMA), respectively. Butachlor is metabolized primarily to nonhydroxylated metabolites, which are hydrolyzed to DEA. Propachlor metabolites are hydrolyzed mainly to A-isopropylaniline (NIPA). The base hydrolysis products for each parent herbicide are shown in Eigure 1. Limited interference studies have been conducted with other herbicides such as metolachlor to confirm that its residues are not hydrolyzed to the EMA under the conditions used to determine acetochlor residues. Nonhydroxylated metabolites of alachlor and butachlor are both hydrolyzed to the same aniline, DEA, but these herbicides are not used on the same crops. 

Chemical manipulation of phenolic allelochemical production in plants has two potential values 1) for study of the role of phenolic allelochemicals in plant interactions with other organisms and 2) to alter such interactions for agricultural purposes. The first of these uses has already been accomplished on a limited scale (21, 22, 50, 51, 84, 86), however, there is no published evidence of the latter. This does not mean that herbicide and growth regulator-influences on plant secondary metabolism do not affect agricultural ecosystems by changing allelochemic compositions of plants. It is likely that this is the case, but it... 

Kearney et al. [254] in a study of persistence, binding and metabolism of six dinitroaniline herbicides (Trifluralin, Profluralin, Dinitramine, Butralin, Fluchloralin and Chlornidine) showed that the parent herbicide was the major compound extracted from herbicide-treated soil after three, five and seven months. Trifluralin, Fluchloralin and Profluralin were the most persistent of the six pesticides. 

Alkylthiotriazines. In our laboratory we have studied the metabolic fate of 2-(4-ethylamino-6-methylthio- -triazin-2-ylamino)-2-methylpropionitrile (cyanatryn, 1, Fig. 1). This compound is a member of a class of herbicidal -triazines which also includes ametryne, prometryne and terbutryne. We were interested to note ( ) that two of the major metabolites of cyanatryn were the mercapturic acids 2-[A-ethylamino-6-(N-acetylcysteinyl)- -triazin-2-ylamino]-2-methylpropionitrTle (2.1) and its N-de-ethyl derivative (2.2) (Fig. 2). This pathway had not hitherto been reported for this class of compound. 

Dichlorophenpxy) acetic acid-14C is metabolized by plant tissue to inactive, water-soluble substances, some of which regenerate the parent acid on hydrolysis. The rate of metabolism differs with different species, but is usually low in comparison with the indole or naphthalene auxins. One of the earlier studies described a single major metabolite in corn, wheat, peas, and tomatoes,121 the identity of which is still uncertain. In another study of differences between species which were susceptible and those which were resistant to the herbicide, the resistant red currant metabolized 50% of the carboxyl-labeled and 20% of the methylene-labeled (2,4 dichlorophenoxy)acetic acid to 14COi in seven days, whereas the susceptible black currant altered122 only 2%. Similar effects were found with apple varieties. Strawberry and lilac decarboxylated the herbicide readily however, 16 other species, of various susceptibilities to herbicide, were unable to decarboxylate the acid to any significant extent. In both varieties of currant, 5 to 10% of the herbicide was converted into water-soluble derivatives of the parent acid, and 10 to 30% was bound in the leaf tissue in an unextractable form. 

Numerous studies on the metabolism of 2,1t-dichlorophenoxy-acetic acid (2,1+-D) and related herbicides in animals have shown that these chemicals are absorbed and distributed rapidly in the body, and are excreted, undegraded, relatively quantitatively in the urine within a week after administration (M Pharmacokinetic studies with 2,1+,5-T in rats and dogs (5.) and in humans (6J supported these findings, and demonstrated that rates of clearance from plasma and elimination in urine depend on dosage level, animal species, and chemical structure of the phenoxy acid being studied ( + ). Corresponding chlorinated phenol metabolites were detected only in ruminants (M or in trace amounts in urine of rats fed very high doses of phenoxy herbicides (7.) ... 

A-Dealkylation Montgomery et al. (1969) studied the further metabolism of hydroxysimazine in plants. They demonstrated that the primary metabolite was the result of dealkylation to produce 2-amino-4-ethylamino-6-hydroxy-.y-triazine (GS-17792). There was also chromatographic evidence for a second dealkylation step that possibly produced ammeline, 2,4-diamino-6-hydroxy-.v-triazine (GS-17791) andammelide, 2-armno-4,6-dihydroxy-y-triazine (G-35713). The authors concluded that the dealkylation of these herbicides appears to be an important pathway of detoxification. 

Metribuzin is a member of the substituted as-triazinone group of herbicides. Activity is due to interference with photosystem II electron transport in plant chloroplasts (Dodge, 1983). The metabolism of metribuzin in plants has been the subject of many short-term and long-term studies dating back to the early 1970s. 

The metabolism of triazine herbicides in plants has been diligently studied since their introduction. Much of what is currently known about the metabolic pathways was obtained only after newer and more advanced methods of chromatography and spectral analysis were discovered. The study of triazine herbicides has resulted in improved methodologies and understanding of plant proteins, biochemistry, and metabolic pathways. These pathways will serve as a reference point for future researchers in their quest for a complete understanding of plant metabolic chemistry. 

This method can be used to compensate for inhibition of a biochemical pathway which results in a deficiency of an essential metabolic product. Detailed variations of the method are provided by Dayan et al.7 and Amagasa et al.1 The inhibitor concentration should be no higher than that required for strong herbicidal effect. Metabolite concentrations should be below that which is phytotoxic. For example, certain amino acids such at methionine, are growth inhibitors at relatively low concentrations. So, in preliminary work, dose-response studies should be done with amino acids to find the maximum concentrations that do not inhibit growth. Then, seeds of test plants should be imbibed in solutions of the phytotoxin with and without metabolite solutions. Amino acids, tricarboxylic acid cycle intermediates, vitamins, nucleotides, and reducing agents have all been used in complementation studies to elucidate modes of action of a variety of phytotoxins. Examples of each of these is provided by Dayan et al.7... 

Through the further combined use of radiolabeled herbicides and additives in tracer and metabolic studies, it may be possible to develop new concepts regarding formulation of herbicides for differential absorption, translocation, and selective action, while at the same time minimizing chemical residues. 

The structure/activity studies seemed to support the idea that the antidote R-25788 (or its analogues) may act as a competitive inhibitor at site(s) of EPTC action in corn. These latter two theories of "antidote enhanced herbicide metabolism" versus "competitive inhibition" were hard to reconcile with each other. 

At this point, evidence that similar molecules acted as effective antidotes by inducing needed metabolic pathways for herbicide detoxication was at most very speculative. However, another hypothesis emerged. Could early herbicide pretreatments increase crop tolerance to these herbicides by elevating the substrates and enzymes needed for detoxication While not a new concept in animal systems, such an idea has received little attention in plant systems and it certainly has not been exploited in any practical way. The whole idea has seemed much more credible with the study by Jacetta and Radosevich (19) of photosynthetic recovery in corn after treatment with atrazine. More specifically, they showed that inhibition of photosynthesis was reduced and the rate of recovery enhanced in corn plants treated for the second or third time with atrazine compared to "first exposed" plants (Figure 2). Furthermore, the faster recovery was related to enhanced rates of atrazine metabolism in the previously treated plants (Table III). 

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