**4. Herbicides**

Herbicides are products intended for the control of weeds of a certain crop that are not desired due to their negative impact on production and yields. These are within the group of phytosanitary products, which of the World Health Organization (WHO) are all those substances or mixture of substances, designed to prevent the action, control, or directly destroy weeds, insects, fungi, mites, molluscs, bacteria, rodents, and other forms of animal or vegetable life that may be harmful to both public health and agriculture.

The growth and development of the food industry have had an effect on the current daily diet, diversifying the foods available in the diet. This progressive increase in production has been accompanied by surveillance and food laws in the countries, regulating and unifying processes and products [18].

In general, all the herbicides belonging to the same generic group act in the same way, for example: once the effect of atrazine on a weed is known, it is also known how simazine, ametryn, and prometryn act, since all these products belong to the same family of triazines [19].

#### **4.1. Mechanisms of action of herbicides**

A common feature that may have the herbicides is that they act on physiological processes of plants, being its toxicity very low on other species.

The most useful way of classifying herbicides is according to their mode of action [20]. The mode of action is the sequence of events that occur from the absorption of the herbicide to the death of the plant.

Herbicides with the same mode of action have the same absorption and transport behavior and produce similar symptoms in the treated plants [21].

In addition, the classification of herbicides according to their mode of action allows us to predict, in general terms, their weed control spectrum, application season, crop selectivity, and persistence in the soil [22].

#### *4.1.1. Inhibitors of photosynthesis*

on the type of soil or water content and approaches a cylinder 10 cm high with a diameter of

All capacitance sensors installed in floors, even with similar characteristics, must be calibrated with the aim of improving their accuracy given the influence on the measurement of other factors independent of the moisture content such as pH variability or electrical conductivity

Capacitance sensors are the most economical and easy to install. In addition, it allows a continuous recording of the moisture values in the soil, enabling direct information and in real time. They are very useful for the planning of alert monitoring systems. They can be used as

Herbicides are products intended for the control of weeds of a certain crop that are not desired due to their negative impact on production and yields. These are within the group of phytosanitary products, which of the World Health Organization (WHO) are all those substances or mixture of substances, designed to prevent the action, control, or directly destroy weeds, insects, fungi, mites, molluscs, bacteria, rodents, and other forms of animal or vegetable life

The growth and development of the food industry have had an effect on the current daily diet, diversifying the foods available in the diet. This progressive increase in production has been accompanied by surveillance and food laws in the countries, regulating and unifying

In general, all the herbicides belonging to the same generic group act in the same way, for example: once the effect of atrazine on a weed is known, it is also known how simazine, ametryn, and prometryn act, since all these products belong to the same family of triazines [19].

A common feature that may have the herbicides is that they act on physiological processes of

The most useful way of classifying herbicides is according to their mode of action [20]. The mode of action is the sequence of events that occur from the absorption of the herbicide to the

Herbicides with the same mode of action have the same absorption and transport behavior

In addition, the classification of herbicides according to their mode of action allows us to predict, in general terms, their weed control spectrum, application season, crop selectivity,

about 25 cm, assuming there are no spaces with air [17].

64 Soil Contamination and Alternatives for Sustainable Development

that may be harmful to both public health and agriculture.

inside of the porous matrix.

substitutes for neutron probes.

processes and products [18].

death of the plant.

and persistence in the soil [22].

**4.1. Mechanisms of action of herbicides**

plants, being its toxicity very low on other species.

and produce similar symptoms in the treated plants [21].

**4. Herbicides**

The inhibitors of photosynthesis can be classified into mobile or systemic herbicides and nonmobile or contact herbicides. The inhibitors of photosynthesis include the chemical families of the triazines, triazinones, triazolinones, phenylureas, and uracils and the contact ones to the nitriles, benzothiadiazoles, and amides [23].

It is subdivided into four groups, the first three being those that act on the luminous phase of photosynthesis. Besides, they are not only used in agriculture but also to clean land, railroad tracks, industrial zones, and warehouses [24].

(a) Herbicides that inhibit the transfer of electrons by inhibiting photosynthesis.

Ureas, uracils, and triazines correspond to this group of herbicides. This type of herbicides are applied to the soil and absorbed by the roots, transported via xylem to the aerial part, reach the chloroplasts of the leaves, and there inhibit the light phase. Any plant can be affected by this type of herbicide.

(b) Herbicides that uncouple the electron transport chain.

They have the ability to capture the electrons preventing oxidation and forming free radicals "superoxides." Superoxides are very powerful oxidants, which oxidize the unsaturated lipids of the chloroplast membranes, losing structure, and the chloroplast stops working. These molecules are formulated as bromides and chlorides; they are very soluble and very easily absorbed by the roots.

(c) Herbicides that prevent the formation of adenosine triphosphate (ATP).

Acylanilides, hydroxybenzonitriles, dinitrophenols, pyridazines, N-phenylcarbamates belong to this group. In photosynthesis, ATP is synthesized from the thylakoid membrane (sites of the photochemical reactions of photosynthesis) of chloroplast cells of plants. The photon particles from the sunlight excite the chloroplast thylakoid membrane, which in turn converts this excitation into ATP's chemical energy. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer. It is used by the plant for metabolic processes. They are applied differently. They can present an important toxicity for animals. Hence, some can be used as herbicides and fungicides.

(d) Herbicides that alter the biosynthesis of carotenoids.

They act at some point in the synthesis of lycopene. The most important is the amino triazole.

The most used inhibitors of photosynthesis are type (a).

#### *4.1.2. Those that alter the biosynthesis of metabolites other than carbohydrates*

They are subdivided into three groups:

**a.** Herbicides that alter the biosynthesis of aromatic amino acids: These amino acids are then part of proteins. The herbicide capable of inhibiting the synthesis of aromatic amino acids is glyphosate. Glyphosate in animals can be degraded; on the ground it can be inactivated, so that environmentally it has very good behavior.

in some crops when the crops resist these oils and only the weeds will be eliminated. They

Flow in Unsaturated Soils and Transport of Herbicides in Agricultural Areas

http://dx.doi.org/10.5772/intechopen.82329

67

These are the inhibitors of carotenoid biosynthesis (PDS). They interfere in the formation of chlorophyll, either by inhibiting the synthesis of typhoid, of carotenoids, or of the amino acid

They are applicable to soil or foliage. The characteristic symptom is foliar albinism after application. They are important herbicides when plants are exposed to full sun and depend on the light intensity; its effect begins to be close to 2 hours when the amount of carotenoids has

These are the disruptors of cell growth. Synthetic Auxins belong to this class with probable action towards indoleacetic acid. These herbicides interfere in the synthesis of nucleic acids, controlling the protein synthesis in different stages; affecting the regulation of DNA during the formation of RNA, an effect that can be achieved by the depression of a gene or activation of RNA polymerase; or simply affecting the RNA message to proteins. They are characterized by having a greater phytotoxicity toward dicotyledons and Cyperaceae than toward grasses; they act as growth regulators; transport occurs via symplast with the assimilated from the source of production to the organs in consumption or storage. They generally exhibit a short residual effect. The grasses are tolerant because they do not have cambium, besides the knots

Pesticides, fertilizers, herbicides, and animal waste are sources of contamination of groundwater of agricultural origin. These sources of agricultural pollution are very varied and numerous: spillage of fertilizers and pesticides during handling, runoff by loading and washing spray pesticides or other application equipment, and the use of chemicals up the slope and a few 100 meters of wells or well-feeding waters. Contamination can also occur when chemicals are stored in uncovered areas, which are not protected from wind and rain, or stored in places

The movement of water in the soil is the main mechanism for the transfer of pollutants to surface and groundwater [25]. The physics of water in the soil and the movement of solutes

The movement of solutes through the unsaturated, vadose zone is particularly important in

The limited availability of data obtained from field studies represents the main limitation to assess the degree of contamination of groundwater against the effects of herbicide applica-

and internodes hinder the arrival of the herbicide to the site of action [24].

where groundwater flows from chemical storage to wells.

can be used to determine the behavior of these materials.

tions, especially in agricultural areas.

relation to environmental and agronomic contamination [26].

were the first substances used as herbicides.

(b) Herbicides that act on pigments

(c) Herbicides with hormonal activity

histidine.

decreased enough [24].

**5. Conclusions**


#### *4.1.3. Plant growth regulators*

They alter the elongation and cell division. When they are incorporated into a plant, they give rise to an abnormal growth of the plant, and as a consequence it causes deformations, lack of functionality, and the death of the plant.

(a) Herbicides that alter cell elongation

Cell elongation occurs by the action of the auxins in the meristematic cells. The main effect of the auxins is the elongation of the cells, mainly due to the fact that the cell wall becomes more plastic. These at high concentration have herbicidal effects, cause excessive cell elongation with malformations in the apices, and the death of the vegetable. They are not used too much since in their synthesis dioxins are released. They are contact foliar herbicides that act at the point where they fall, they do not translocate. This allows many dicotyledonous weeds to be controlled.

(b) Herbicides that inhibit the synthesis of gibberellins

Gibberellins are phytohormones responsible for the growth of the plant, since they give rise to the internodes having a certain length. If the synthesis of gibberellins is inhibited, the internode distance is shortened, giving rise to the stunting of the plant and loss of functionality, in addition the petioles shorten, and the root system increases. All this causes the plant to lose functionality and die. Chlormequat (is a quaternary ammonium salt) inhibits the synthesis of gibberellins.

(c) Herbicides capable of inhibiting cell division

There are many types, but the most important ones are the N-phenyl carbamates and maleica hydrazida.

They are used in the soil and have little mobility and alter cell division as they prevent the correct organization of proteins that are part of the microtubules of achromatic use; this causes cells with giant nuclei without functionality. These herbicides affect meristematic cells, preventing them from thickening, there is no cell differentiation, and the plant stops growing and dies.

#### *4.1.4. Herbicides with other mechanisms of action*

#### (a) Herbicides that cause disruption of the cell membrane

These are the so-called herbicidal mineral oils. They are complex mixtures of long-chain CH that come from the fractional distillation of oil + dry fraction of coal. They are substances with very soluble lipids. They are used as total herbicides; they are also used as selective herbicides in some crops when the crops resist these oils and only the weeds will be eliminated. They were the first substances used as herbicides.

(b) Herbicides that act on pigments

is glyphosate. Glyphosate in animals can be degraded; on the ground it can be inactivated,

**c.** Herbicides that inhibit lipid synthesis: Thiocarbamates that inhibit the conversion of shortchain fatty acids into long-chain AG belong to this group. As a result, they slow down the growth of the vegetable. These molecules can be used in soil treatments for seeds that are germinating; thiocarbamates are easily degraded by enzymes and are poorly absorbed in the soil.

They alter the elongation and cell division. When they are incorporated into a plant, they give rise to an abnormal growth of the plant, and as a consequence it causes deformations, lack of

Cell elongation occurs by the action of the auxins in the meristematic cells. The main effect of the auxins is the elongation of the cells, mainly due to the fact that the cell wall becomes more plastic. These at high concentration have herbicidal effects, cause excessive cell elongation with malformations in the apices, and the death of the vegetable. They are not used too much since in their synthesis dioxins are released. They are contact foliar herbicides that act at the point where they fall, they do not translocate. This allows many dicotyledonous weeds to be controlled.

Gibberellins are phytohormones responsible for the growth of the plant, since they give rise to the internodes having a certain length. If the synthesis of gibberellins is inhibited, the internode distance is shortened, giving rise to the stunting of the plant and loss of functionality, in addition the petioles shorten, and the root system increases. All this causes the plant to lose functionality and die. Chlormequat (is a quaternary ammonium salt) inhibits the synthesis of gibberellins.

There are many types, but the most important ones are the N-phenyl carbamates and maleica

They are used in the soil and have little mobility and alter cell division as they prevent the correct organization of proteins that are part of the microtubules of achromatic use; this causes cells with giant nuclei without functionality. These herbicides affect meristematic cells, preventing them from thickening, there is no cell differentiation, and the plant stops growing and dies.

These are the so-called herbicidal mineral oils. They are complex mixtures of long-chain CH that come from the fractional distillation of oil + dry fraction of coal. They are substances with very soluble lipids. They are used as total herbicides; they are also used as selective herbicides

so that environmentally it has very good behavior.

**b.** Herbicides that alter the biosynthesis of glutamine.

66 Soil Contamination and Alternatives for Sustainable Development

(b) Herbicides that inhibit the synthesis of gibberellins

(c) Herbicides capable of inhibiting cell division

*4.1.4. Herbicides with other mechanisms of action*

(a) Herbicides that cause disruption of the cell membrane

hydrazida.

*4.1.3. Plant growth regulators*

functionality, and the death of the plant. (a) Herbicides that alter cell elongation

These are the inhibitors of carotenoid biosynthesis (PDS). They interfere in the formation of chlorophyll, either by inhibiting the synthesis of typhoid, of carotenoids, or of the amino acid histidine.

They are applicable to soil or foliage. The characteristic symptom is foliar albinism after application. They are important herbicides when plants are exposed to full sun and depend on the light intensity; its effect begins to be close to 2 hours when the amount of carotenoids has decreased enough [24].

(c) Herbicides with hormonal activity

These are the disruptors of cell growth. Synthetic Auxins belong to this class with probable action towards indoleacetic acid. These herbicides interfere in the synthesis of nucleic acids, controlling the protein synthesis in different stages; affecting the regulation of DNA during the formation of RNA, an effect that can be achieved by the depression of a gene or activation of RNA polymerase; or simply affecting the RNA message to proteins. They are characterized by having a greater phytotoxicity toward dicotyledons and Cyperaceae than toward grasses; they act as growth regulators; transport occurs via symplast with the assimilated from the source of production to the organs in consumption or storage. They generally exhibit a short residual effect. The grasses are tolerant because they do not have cambium, besides the knots and internodes hinder the arrival of the herbicide to the site of action [24].
