Herbicides Interference

Herbicides are designed to kill plants, not animals, and in general have lower mammalian toxicity than insecticides. Most herbicides interfere with plant hormones or enzymes that do not have any direct counterpart in animals. The most serious human health concerns have been related to contaminants of the primary chemical herbicide. There is an enormous amount of animal and some human toxicity data on 2,4-D and 2,4,5-T, but it now appears that much of this toxicity is caused by the contaminant TCDD. Military personnel exposed to Agent Orange, often contaminated with TCDD, reported birth defects, cancers, liver disease, and other illness. These concerns led to improvement in the manufacturing process to reduce TCDD contamination and ultimately to a reduction in use of 2,4-D herbicides. There is also concern that some herbicides may affect wildlife. For example, atrazine, a persistent herbicide, may adversely affect frogs. Persistence of herbicides may also... 

A number of herbicides interfere with the flow of electrons from singlet chlorophyll through the electron transport chain. The main herbicide groups and examples from these groups are shown in Table 2.1. 

Application of this procedure improved LOD down to 5 nM atrazine [114], There are several other examples of conductometric determination of atrazine based on MIPs [114, 165, 176]. These determinations discriminated atrazine from analogous herbicide interferants, such as trazine, simazine and prometryn [114],... 

JVeed killer 1. Dichlorophenyldimethylurea (DCMU), a herbicide, interferes with photophosphorylation and O2 evolution. However, it does not block O2 evolution in the presence of an artificial electron acceptor such as ferricyanide. Propose a site for the inhibitory action of DCMU. 

It has been demonstrated that peroxidizing herbicides interfere with the chlorophyll biosynthetic pathway (22) by accumulation of tetrapyrroles (17,18,23,24). It has also been shown that soluble plastidic cytochrome decreases when oxadiazon is present in a concentration when no peroxidation is yet apparent (25). For the identification of the tetrapyrrole the tolerant Bumilleriopsis was helpful. This species excretes large amounts of a tetrapyrrole into the liquid culture medium when treated with peroxidizing herbicides like oxadiazon, chlorophthalim, oxyfluorfen, or LS 82-556. The product found in the medium could be identified by mass and NMR spectrometry as protoporphyrin IX (17). 

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

Another process of Importance to plant science Is amino acid biosynthesis. Plants and most microbes share the capacity to synthesize the twenty common amino acids from central, key metabolites (see Figure 1). In contrast animals must Ingest ten amino acids "essential to their diet they are unable to produce leucine, valine, Isoleuclne, threonine, methionine, lysine, histidine, tryptophan, tyrosine and phenylalanine. A sufficiently specific chemical Inhibiting the biosynthesis of an essential amino acid thus might control weed growth and display little toxicity towards mammals. Indeed a number of herbicides interfere with the biosynthesis of the essential amino acids (, see Table I). 

A large number of commercial herbicides interfere with electron transport and ATP production in isolated chloroplasts and mitochondria (1.). These herbicides can be divided into two groups electron transport inhibitors and inhibitory uncouplers ( 1, 2, The dimethylphenylureas, substituted uracils, s-triazines, and pyridazinones have been classified as electron... 

Whereas inhibition of chloroplast electron transport has been correlated with binding to a protein(s), the sites and mechanisms through which herbicides interfere with mitochondrial and chloroplast mediated phosphorylations remain to be identified. When lipophilic herbicides partition into the lipid phases of membranes, they could perturb lipid-lipid, lipid-protein, and protein-protein interactions that are required for membrane functions such as electron transport, ATP formation, and active transport. Evidence for general membrane perturbations caused by chlorpropham, 2,6-dinitroanilines, perfluidone, and certain phenylureas have been reported previously (8-11). 

The degree of resistance was the same in intact cells as in the enzyme assay. This indicates that the mutants contain a phytoene-desaturase enzyme which is modified in a way that herbicide interference is less effective. Further studies have excluded that the resistance in both mutants is caused by overexpression of the phytoene desaturase gene leading to an overproduction of phytoene desaturase. vVe observed cross resistance with some other hercicides interfering with phytoene desaturase like fluorochloridone. In contrast, our mutants were not resistant against fluridone. 

Herbicide interference with respiration has been extensively reviewed. " However, several recent studies have been published which will form the major contribution to this section. 

Herbicide Interference with Microtubule Structure and Function... 

Inhibition of protein, RNA, and DNA synthesis by herbicides has been the subject of numerous reports and reviews (see, e.g.. Refs. 3, 4, and 137-139). Inhibition of the biosynthesis of any of these macromolecules would lead to an effective and abrupt cessation of plant growth. However, with very few exceptions, the reports on herbicidal interference with protein and nucleic acid synthesis reflect secondary effects resulting from an effect at a target site elsewhere in the cell. For example, any interruption of the production of metabolic energy, as a result of respiratory uncoupling and/or inhibition, would lead to a rapid cessation of macromolecular synthesis. In several surveys of herbicide effects on C-labeled precursor incorporation into protein and RNA in excised plant tissues and isolated mesophyll cells, dinitrophenols and hydroxybenzonitriles were the most potent inhibitors (Section 5.1 see, e.g.. Refs. 138 and 140). 

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 influences of herbicides on cell division fall into two classes, ie, dismption of the mitotic sequence and inhibition of mitotic entry from interphase (G, S, G2). If ceU-cycle analyses indicate increases in abnormal mitotic figures, combined with decreases in one or more of the normal mitotic stages, the effect is upon mitosis. Mitotic effects usually involve the microtubules of the spindle apparatus in the form of spindle depolymerization, blocked tubulin synthesis, or inhibited microtubule polymerization (163). Alkaloids such as colchicine [64-86-8J,viahla.stiae [865-21-4] and vincristine [57-22-7] dismpt microtubule function (164). Colchicine prevents microtubule formation and promotes disassembly of those already present. Vinblastine and vincristine also bind to free tubulin molecules, precipitating crystalline tubulin in the cytoplasm. The capacities of these dmgs to interfere with mitotic spindles, blocking cell division, makes them useful in cancer treatment. 

An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

E. A. Hogendoorn, E. Dijkman, B. Baumann, C. Hidalgo, J. V. Sancho and E. Hernandez, Strategies in using analytical restricted access media columns for the removal of humic acid interferences in the trace analysis of acidic herbicides in water... 

Emulsions are frequently used in the formulation of herbicides. Such emulsions have to form stable concentrates. They will be diluted with water before applying on plants. In this form too the surfactants used have to support the formation of fine droplets at the nozzles and provide a sufficient wetting of leaves. Surfactants have to be used which will not interfere with water hardness. Therefore phosphate esters and phosphonic esters are often applied. 

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. 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

GC/MS. GC/MS is used for separation and quantification of the herbicides. Data acquisition is effected with a data system that provides complete instrument control of the mass spectrometer. The instrument is tuned and mass calibrated in the El mode. Typically, four ions are monitored for each analyte (two ions for each herbicide and two ions for the deuterated analog). If there are interferences with the quantification ion, the confirmation ion may be used for quantification purposes. The typical quantification and confirmation ions for the analytes are shown in Table 4. Alternative ions may be used if they provide better data. 

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