**4. Allelopathic compounds**

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

their modes of action [18].

different forms. One of the mechanisms is through the production of a new generation of weeds of different morphological and physiological characteristics with shorter life cycles that can survive from the application of the systemic herbicides. Thus, the number of these weeds surviving from the continuous application of herbicides which will gradually increase. While the other weed species in the same area will be controlled by applying herbicides [16]. Another way in the development of weed resistance comes from the application of contact herbicides, which are responsible for inhibiting the process of photosynthesis, such as attrition. The application of such herbicides, especially during unfavorable weather conditions can contribute to the absorption of a small amount of the active ingredient but ineffective to the targeted weeds. Over time, the continuous accumulation of contact herbicides developed a new generation of weed that is immune to the synthetic herbicides [17]. In September 2010, Dow AgroSciences' scientists stated that weeds which have become resistant to the glyphosate herbicide and the ensuing production of genetically modified plants are one of the solutions to resolve issues pertaining to weed competition [7]. Nowadays, there are more than 604 species of weed plants considered as resistance to synthetic herbicides, most of these weed species are resistant to herbicides which are responsible for inhibiting acetolactate synthase (ALS enzyme), photosystem II and acetyl-CoA carboxylase [18, 19]. **Figure 1**, showed the number of resistant plant species for several herbicides according to

In this regard, Sekutowski [20] and Weber and Golebiowska [21] reported that there are 10 species of weeds that pose the biggest threat to crops by causing yield losses. The weeds include the most important herbicide-resistant species which are characterized by multiple resistances such as rigid rye-grass (*Lolium rigidum* Gaud.), wild oat (*Avena fatua* L.) and redroot pigweed (*Amaranthus retroflexus*). As a result of weed resistance, many types of synthetic herbicide are re-evaluated as useless. Hence, a lot of conventional herbicides have been identified as resistance and ineffective against common weeds. For example, there are 116 useless conventional herbicides identified in Europe, where the highest number of herbicides is found in France (30 types) followed by Spain (26 types), United Kingdom (24

types), Belgium (18 types) and Germany (18 types) [22].

*Number of resistant plant species for several herbicides according to their modes of action [18].*

**192**

**Figure 1.**

The word allelopathy comes from the Greek words "Allelon" meaning "each other" and "Pathos" refers "to suffering". This allelopathic phenomenon whereby a plant response in defense of the neighboring plants, insects, microorganism and animals results in the production of natural chemicals called allelochemicals or phytochemicals [25, 26]. Allelopathy phenomenon is defined by Kato-Noguchi [27] as an important defense mechanism of the plant which results in the manufacturing of secondary metabolites. This term is also defined by the International Allelopathy Society in 1996th as a science that study any process involving secondary metabolites produced by plants, micro-organisms, viruses, and fungi that influence growth and development of agricultural and biological systems, excluding animals [28]. Usually, the allelochemical compounds released from donor plants caused negative effects on organisms found within the surrounding environment. However, some of the recent researchers reported the effects of the allelochemical produced by plants belonging to the plant family (Fabaceae) can be positive to the surrounding environment. As for an instance, the allelochemicals residues produced by these plant types such as legumes help to fix nitrogen from the air thus enriching the soil [29]. Most of the plants including weeds release allelochemical compounds as defense mechanisms. These compounds can be released from donor plant parts into the environment by leaching, volatilization, exudate from living plant tissue or by the decomposition of plant residues as shown in **Figure 2**. Hence, it was responsible for inhibiting the germination and growth of neighboring organisms [30]. One of the major advantages of allelopathy involves the release of plant biochemical compounds into the environment that inhibits germination or suppresses the growth of the surrounding vegetation. Another form of allelopathic potential can be tapped from trees that produce biochemical compounds for its survival and hence its reproduction. An example of such plants is eucalyptus; these types of plants can be used in agricultural production as cover crops to control weeds [31]. Lately, the allelopathic phenomenon has gained prominent attention and has been successfully employed in field crop production toward the improvement of crop productivity and for the protection of the environment through eco-friendly control of weeds, pests and crop diseases [32, 33].

The motivation for the use of the allelochemical compound in weed control is attributed to its phytotoxic effects similar to the phytotoxic effects of the synthetic herbicides in inhibiting seed germination and reducing seedling growth. The

#### **Figure 2.**

*Methods of allelochemical compounds released from the donor plant into the environment.*

#### **Figure 3.**

inhibition process includes several action sites such as cell division, nutrient uptake, photosynthesis and specific enzymatic functions [34]. The benefit of using allelopathy will not be just from being an attractive alternative to conventional herbicides, but also in the possibility of applying at places where the use of synthetic herbicides is illegal such as in organic farming. Thus, the use of allelochemical compounds could be adopted to reduce damage resulting from weed competition of the areas where the use of synthetic herbicides is not allowed [35]. Plants produce secondary metabolites exhibited a few ecological advantages such as pollinator attractants, determinants of vegetation patterning, provide protection against predators and other enemies and more importantly in mediating plant-plant interactions known as allelopathy [36]. The responsible chemical compounds for demonstrating allelopathic influences are called allelochemicals or biochemicals compounds produced as offshoots from the primary metabolic pathways of plants [37].

Allelochemicals have been defined as compounds derived as metabolic byproducts of that certain plant which, when introduced into the environment can cause growth inhibition as a result of different malfunctions inside targeted plants such as respiration, cell division, water and nutrient uptake. The symptoms of the "allelopathic effects" include leaf wilting and yellowing, or death of part or all of a

**195**

wood [43].

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent…*

that have been identified by Li et al. [40] can be summarized as below:

3.Effects on plant photosynthesis and respiration.

6.Effects on protein synthesis.

production [41].

**5. Essential oils**

tion using organic solvents [42].

4.Effects on various enzymatic functions and activities.

5.Effects on the synthesis of plant endogenous hormones.

1.Changes in membrane permeability and inhibition of plant nutrient uptake.

Allelochemical compounds have been classified into 10 categories depending on the structures and properties of these compounds according to [22, 40] namely the flavonoids, terpenoids, alkaloids, phenolics, tannins, coumarins, cinnamic acid and its derivatives, simple lactones, water-soluble organic acids and long-chain fatty acids. A wide range of these biochemicals are synthesized through the shikimate pathway or the isoprenoid pathway which are responsible for the essential oil

Essential oil is a concentrated volatile liquids consisting of different types of secondary plant metabolites but mainly composed of terpenoids and phenolics. Technically, essential oils are defined as odiferous bodies by oily nature obtained from plants by different ways, such as cold and hot pressing, distillation and extrac-

Essential oils produced from specific types of plants can be used for different purposes. Most of the essential oil usage is influenced by donor or producer plants and their surroundings such as scent to attract certain animals and insects, aiding in pollination, protection or as repellent agents, energy reserve, wound healing and prevent water evaporation. Essential oils can be obtained from different parts of plants such as the leaves, flowers, fruit, seeds, roots, rhizomes, bark and

2.Inhibition of cell division, elongation, and submicroscopic structure.

plant [3]. Allelochemical compounds, therefore, can act directly and indirectly when releasing from donor to the receiver plant as described in **Figure 3** which showed the direct and indirect allelopathic mechanisms of donor plant to the targeted plants [19]. The direct action of allelochemicals similar to the action of conventional herbicides hence may lead to being a new approach in herbicides technique to discover and select the most effective allelopathic plants which are the commonly used as natural herbicides [24]. The allelopathic potential on targeted or receiver plants are shown in different ways, such as the reduction in both the length and mass of radicle and roots, extension shoot and coleoptile, swelling or necrosis of root tips, destruction of cell wall, curling of the root axis, lack of root hairs, decrease in the number of seminal roots, reduced in plant dry weight accumulation, leaf discoloration and lower in reproductive capacity [25, 33, 38]. Allelopathic inhibition is complex and can involve the interaction of different classes of allelochemicals such as phenolics, flavonoids, terpenoids, alkaloids, steroids, carbohydrates and amino acids [37, 39]. The allelopathic mechanisms of allelochemical compounds on targeted plants

*DOI: http://dx.doi.org/10.5772/intechopen.91322*

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent… DOI: http://dx.doi.org/10.5772/intechopen.91322*

plant [3]. Allelochemical compounds, therefore, can act directly and indirectly when releasing from donor to the receiver plant as described in **Figure 3** which showed the direct and indirect allelopathic mechanisms of donor plant to the targeted plants [19]. The direct action of allelochemicals similar to the action of conventional herbicides hence may lead to being a new approach in herbicides technique to discover and select the most effective allelopathic plants which are the commonly used as natural herbicides [24]. The allelopathic potential on targeted or receiver plants are shown in different ways, such as the reduction in both the length and mass of radicle and roots, extension shoot and coleoptile, swelling or necrosis of root tips, destruction of cell wall, curling of the root axis, lack of root hairs, decrease in the number of seminal roots, reduced in plant dry weight accumulation, leaf discoloration and lower in reproductive capacity [25, 33, 38]. Allelopathic inhibition is complex and can involve the interaction of different classes of allelochemicals such as phenolics, flavonoids, terpenoids, alkaloids, steroids, carbohydrates and amino acids [37, 39].

The allelopathic mechanisms of allelochemical compounds on targeted plants that have been identified by Li et al. [40] can be summarized as below:


Allelochemical compounds have been classified into 10 categories depending on the structures and properties of these compounds according to [22, 40] namely the flavonoids, terpenoids, alkaloids, phenolics, tannins, coumarins, cinnamic acid and its derivatives, simple lactones, water-soluble organic acids and long-chain fatty acids. A wide range of these biochemicals are synthesized through the shikimate pathway or the isoprenoid pathway which are responsible for the essential oil production [41].
