MODE OF ACTION The control of pests by pesticides depends on several factors like the mode of action

1.5 MODE OF ACTION The control of pests by pesticides depends on several factors like the mode of action

of these compounds, the crop stage and the environmental conditions, moisture, soil type and temperature, among others, and numerous works have been published on these subjects [10 –13]. The main modes of action of pesticides are summarized later.

1.5.1 H ERBICIDES

1.5.1.1 Amino Acid Synthesis Inhibitors Amino acid synthesis inhibitors act on a specific enzyme to prevent the production of

certain amino acids, which are the key building blocks for normal plant growth and development.

One type of herbicide causes the inhibition of acetolactate synthase (ALS), the first common enzyme in the branched-chain amino acid biosynthetic pathway. ALS

Pesticides: TABLE 1.22 Chemical Names and Properties of Miscellaneous Fungicides

Water Solubility Half-Life in Classi Common Name

Vapor Pressure

K ow

IUPAC Name

mPa (258C)

log P

mg=L (258C) Soil (Days)

Azoxystrobin C 22 H 17 N 3 O 5 (E)-2-{2-[6-(2-Cyanophenoxy) pyrimidin-4-yloxy]phenyl}-

1.1 3 10 7a 2.5 6 a 7 –56 fi cation

3-methoxyacrylate

Captan C 9 H 8 Cl 3 NO 2 S

N -(Trichloromethylthio) cyclohex-4-ene-1,2-dicarboximide

Chlorothalonil C 8 Cl 4 N 2 Tetrachloroisophthalonitrile

2.92 0.81 5 –36 and

Cyprodinil C 14 H 15 N 3 4-Cyclopropyl-6-methyl-N-phenylpyrimidin-2-amine

0.51 4 13 20 –60 Properties

Fenhexamid C 14 H 17 Cl 2 NO 2 N -(2,3-Dichloro-4-hydroxy phenyl)-

4.0 3 10 4a 3.51 20 a

1-methylcyclohexanecarboxamide

Folpet C 9 H 4 Cl 3 NO 2 S

N -(Trichloromethylthio) phthalimide

Iprodione C 13 H 13 Cl 2 N 3 O 3 3-(3,5-Dichlorophenyl)-N-isopropyl-2,4-dioxoimidazo

5 3 10 4 3 13 a 20 –160

lidine-1-carboxamide

3.3 1.71 26,000 5 –30 Ofurace C 14 H 16 ClNO 3

Metalaxyl-M C 15 H 21 NO 4 Methyl-N-(methoxy acetyl)-N-(2,6-xylyl)- D -alaninate

2 3 10 2a 1.39 146 a 26 Orthophenylphenol C 12 H 10 O

Biphenyl-2-ol

Procymidone C 13 H 11 Cl 2 NO 2 N -(3,5-Dichlorophenyl)-1,2-dimethylcyclopropane-

1,2-dicarboximide Pyrimethanil C 12 H 13 N 3 N -(4,6-Dimethylpyrimidin-2-yl)aniline

2.2 2.84 121 7 a a –54

Tolylfluanid C 10 H 13 Cl 2 FN 2 O 2 S 2 N -Dichlorofluoromethylthio-N 0 ,N 0 -dimethyl-N-p-tolylsulfamide

Triforine C 10 H 14 Cl 6 N 4 O 2 N-N 0 -[Piperazine-1,4-diylbis [(trichloromethyl)methylene] ]

80 2.2 9 a 21

diformamide

Sources: Data from Tomlin, C. (Ed.) in The Pesticide Manual, British Crop Protection Council, 2000; http:==ec.europa.eu=food=plant=protection=evaluation=exist _subs_rep_en.htm; http:==www.epa.gov=opprd001=factsheets=; Hornsby, A.G., Wauchope, R.D., and Herner, A.E. in Pesticide Properties in the Environment, Springer-Verlag, New York, 1996; De Liñan, C. in Farmacología Vegetal, Ediciones Agrotecnicas S.L., 1997.

a 208C. 29

30 Analysis of Pesticides in Food and Environmental Samples inhibitors include, among others, herbicides of the sulfonylurea family. These

compounds vary greatly in selectivity; some of them remain extremely active. The aromatic ring amino acids, tryptophan, phenylalanine, and tyrosine, are synthesized by plants through the shikimic acid pathway. Only one herbicide, glyphosate, inhibiting that pathway has been commercialized. The mode of action of glyphosate is the inhibition of the enzyme, 5-enolpyruvoyl-shikimate-3-phosphate synthase (EPSPS). This enzyme is present in plants, fungi, and bacteria, but absent in animals, which need to ingest those amino acids in the diet because they are not produced by them.

Another enzyme involved in amino acid synthesis used as a target for herbicides is the glutamine synthase (GS), which makes glutamine from glutamate and ammonia. This enzyme is present in plants, where it plays an important role in nitrogen assimilation, as well as in animals, as glutamate is a neurotransmitter that can be inactivated by GS. The mode of action of the herbicide glufosinate is the inhibition of the enzyme glutamine synthase.

1.5.1.2 Cell Division Inhibitors This type of herbicide reacts with tubulin, a protein essential for building the

intracellular skeleton in eukaryotic cells forming the wall of microtubules. These compounds disturb normal cell division by binding with tubulin.

Inhibitors of cell division are herbicides belonging to various chemical classes, such as dinitroanilines, benzoic acids, and pyridines.

1.5.1.3 Photosynthesis Inhibitors Photosynthesis is a key process for plants and consequently is a main target for many

herbicides. There are different mechanisms involved in the inhibition of photosyn- thesis, such as free radical generators, blockage of the electron transport system, and inhibition –destruction of protective pigments, but, in general, most herbicides inter- fere with the transfer of electrons to the plastoquinone pool by binding to a specific protein that regulates electron transfer.

The herbicides acting as photosynthesis inhibitors are all nitrogen-containing compounds with a diversity of chemical composition. These compounds, including phenyl ureas, triazines, pyridazines, phenyl carbamates, nitriles, and amides, are represented by various herbicide families, although some of these chemical classes also have specific herbicides that do not act as photosynthesis inhibitors.

1.5.2 I NSECTICIDES

1.5.2.1 Signal Interference in the Nervous System Chemicals that disturb signal systems are frequently potent poisons. Pyrethroids and

organochlorines are the most important insecticides in this category. Their mode of action is to inhibit the proper closing of the channels by acting at the voltage-gated sodium channels. Pyrethroids modify axonal conduction within the central nervous system of insects by altering the permeability of the nerve membrane to sodium and

Pesticides: Classification and Properties 31 potassium ions. Organochlorines may interact with the pores of the lipoprotein

structure of the insect nerve causing distortion and consequent excitation of nerve impulse transmission. The toxic properties of chlorinated cyclodiene insecticides, as lindane, reside in the blockade of the g-aminobutyric acid-gated chlorine channels, inducing convulsions in insects.

1.5.2.2 Inhibitors of Cholinesterase The target for many insecticides is an enzyme called acetylcholinesterase (AChE).

This enzyme is an essential constituent of the nervous system and plays an important role in animals, but not in plants as they lack nervous system. AChE hydrolyzes acetylcholine, an ester released when nerve impulses are transmitted. Synapses, myoneural functions, and ganglia of the nervous system transmit neural impulses by the mediation of acetylcholine.

The organophosphorus insecticides have the capacity to phosphorylate the esteratic active site of the AChE. The phosphorylated enzyme is irreversibly inhib- ited and is not able to carry out its normal function of the rapid removal of acetylcholine (ACh). As a result, ACh accumulates and disrupts the normal function- ing of the nervous system. Carbamates are also strong inhibitors of AChE and may also have a direct effect on acetylcholine receptors.

1.5.2.3 Inhibitors of Chitin Synthesis Chitin is a very abundant polysaccharide in nature, although it is present in arthro-

pods and fungi, but absent in plants and mammals. Benzoylureas affect chitin synthesis in the insect cuticle by disrupting the process of connecting the N-acetylglucosamine units to the chitin chain, preventing in this way the normal moulting process of insects.

1.5.3 F UNGICIDES

1.5.3.1 Sulfhydryl Reagents Sulfhydryl (SH) groups are important reactive groups often found in the active sites

of many enzymes. Dithiocarbamate fungicides react with the SH-containing enzymes and coenzymes of fungal cells. Enzyme inhibition may also occur by complex formation of the active substance with the metal atoms of metal-containing enzymes.

The perhalogen mercaptans, captan and folpet, are good examples of pesticides that react with sulfhydryl groups in many enzymes. These fungicides affect the structure and functions of the cell membranes and inhibit the enzyme system causing tumors in the mitochondria.

1.5.3.2 Cell Division Inhibitors Benzimidazole fungicides react with tubulin, a protein that is the building block of

the intracellular skeleton in cells. The impairment of cell division is produced in most cases by inhibiting the formation of the microtubules. Benzimidazoles, such as

32 Analysis of Pesticides in Food and Environmental Samples benomyl, carbendazim, and thiabendazole, as well as other fungicide groups like

carbamates, have this mode of action.

1.5.3.3 Inhibitors of Ergosterol Synthesis The ergosterol inhibitor fungicides are active against many different fungi.

Although they disturb sterol synthesis in higher plants, as well as the synthesis of gibberellins, their phytotoxicity is low. The synthesis of sterols is very complex and various groups of fungicides act on different targets of that synthesis. One large group of fungicides, called demethylase inhibitors, includes various compounds having a heterocyclic N-containing ring, such as azoles, morpholines, pyridines, and piperazines.