All biological organisms have defense mechanisms to protect them from the negative effects of small quantities of foreign compounds. These foreign compounds in plants include pesticides. Herbicides are a type of pesticide toxic to plants specifically, because they inhibit metabolic pathways unique to plants (e.g. photosynthesis). Herbicides are used successfully in weed management systems because they selectively harm sensitive weeds while leaving crops undamaged. Many factors contribute to successful herbicide action; however, the major reason herbicides are selective against weeds in crops is because crop plants are able to metabolize the herbicide to a non- toxic form. The basis for herbicide selectivity relies on enzymatic systems used in the plant’s normal metabolic processes.

The relative rates of herbicide absorption, translocation, and metabolism usually determine whether or not a herbicide will elicit a phytotoxic response. To better understand how these processes may each influence herbicidal action, they should be considered separately. Absorption has primary control over translocation, metabolism and phytotoxic action because the total amount of herbicide available for these processes is determined by the amount of herbicide absorbed by the plant. Metabolism influences both herbicide absorption and phytotoxic action. Metabolism generally converts the herbicide to a form with reduced phytotoxicity.

Herbicide selectivity between a weed and crop can be related to differences in: absorption, translocation within the plant, and metabolism In the majority of cases selectivity is due to differences in metabolism between the crop and weed. The crop unlike the weed is able to metabolize the herbicide to an inactive form.

1. metabolism, molecular modification metabolism and molecular modification, both of them referring to a change in the chemical structure of the herbicide molecule. Any minor change in the herbicide molecule can drastically change the herbicide activity. 2. Metabolism of a herbicide may result in: inactivation of the herbicide or deactivation; this is the most common result of herbicide metabolism. since plants vary in their ability to modify chemical structures, this determines tolerance. atrazine is rapidly degraded by corn but not by weeds. Hence the corn is tolerant and the weeds are susceptible. This makes atrazine a selective herbicide.

Specific example of herbicide inactivation of Simazine The simazine molecule is dechlorinated and the chlorine is replaced with a hydroxyl group. The end product of the inactivation reaction is hydroxysimazine, which is 1000 times less toxic than simazine.

In most cases, crops are able to metabolize the herbicide, but weeds are not. activation of the herbicide - in some cases a herbicide is metabolized to a more active form. This is less common than inactivation. Specific example of herbicide activation through conversion of 2,4-DB to 2,4-D

2,4-DB (2,4-dichlorophenoxy butyric acid) is a phenoxy herbicide used in soybeans and alfalfa, which are both legume crops. β-oxidation is a pathway in plants whereby long chain carbon segments are degraded by removing 2 carbons for each cycle of the pathway. By removing 2 carbons from 2,4-DB we have the more active form, 2,4-D. The 2,4-D is what kills the weed. Why are the soybeans and alfalfa not injured? β-oxidation occurs in all plants, but in legume crops the process occurs at a much slower pace. Hence, the susceptible species convert nontoxic 2,4-DB to toxic 2,4-D through β- oxidation, whereas soybeans and alfalfa do not and are therefore not affected.

3. Inactivation of herbicides - herbicide is made less toxic by altering the molecular configuration. biochemical reactions - primarily enzymatic reactions that occur within a living plant. chemical reactions - breakdown herbicide, but do not necessarily require enzymes. conjugation - the herbicide is bound to another compound. adsorption - herbicide weakly bound to cell wall constituents. 4. Specific biochemical or chemical reactions decarboxylation - removal of -COOH group from the herbicide; part of the degradation sequence for many phenoxy, benzoic acid, and substituted urea herbicides hydroxylation - the addition of a -OH group to the molecule; often accompanied by removal or shifting of another atom such as chlorine. hydrolysis - splitting of a molecule through the addition of water.

dealkylation - removal of alkyl side chains from the herbicide; examples include the triazine, carbamate, thiocarbamate, and dinitroaniline herbicides. deamination - removal of an amine group (NH2)from the herbicide. ring hydroxylation - the addition of a -OH group to the ring structure of the molecule. dealkyloxylation - removal of an alkyl group with an attached “O” beta-oxidation - pathway in plants whereby long chain carbon segments are degraded by removing 2 carbons for each cycle of the pathway see Figure 4-18 (Some common reactions involved in herbicide degradation.)

5. The metabolism or molecular fate of herbicides was described using a 3 phase process. A Phase I reaction is the first step needed to make a herbicide less toxic; this reaction modifies the molecule so it is more water soluble and ready for Phase II and Phase III reactions which further detoxify the chemical. Phase I reactions - dechlorination, hydroxylation, decarboxylation, dealkylation, oxidation or reduction, and hydrolysis are the initial chemical reactions that occur. These reactions detoxify the herbicide and predispose the resultant metabolites to conjugation. Phase II reactions - conjugation with sugars, amino acids or natural plant constituents. In general, plants "immobilize" these metabolites whereas animals excrete them. In animals, toxins are degraded by the liver and excreted through the kidneys. see Figure 6.14 (Examples of oxidation and conjugate transformations)

Herbicide metabolism is the primary mechanism of selectivity where the desirable plant is not injured because it has rendered the herbicide less toxic and the undesirable weed dies because the herbicide stays in its active form. Herbicide metabolism proceeds in plants through a three phase process. The first phase introduces reactive groups which then can be acted on by Phase II reactions. Phase II reactions usually increase the size and polarity of the molecule. Phase III reactions which are unique to plants because they cannot excrete metabolites. Metabolism-based selectivity is also important in herbicide resistance and can be altered through the addition of synergists or antagonists.

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