**5. Essential oils**

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

*Direct and indirect allelopathic mechanisms of donor plant to the targeted plants [19].*

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

offshoots from the primary metabolic pathways of plants [37].

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

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

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**Figure 3.**

**Figure 2.**

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 extraction using organic solvents [42].

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 wood [43].

Biosynthetically, essential oil components composed of two groups. The first group is the terpenoids, which is considered the main group; mostly, of the monoterpenoids, sesquiterpenoids. The second group is non-terpenoids, which may contain aromatic compounds such as phenylpropanoids, short-chain aliphatic structures, nitrogenated and sulfuric substances [42]. Essential oils can be isolated from plants by several processes such as expressed oils, steam distillation, solvent extraction, fractional distillation and percolation and carbon dioxide extraction. The process of steam distillation is the most widely accepted method for the extracting of essential oils on a large scale.

The steam distillation process considered lower risk as compared with another process due to absence chemical compounds such as solvents and the thermal degradation as temperature generally not above 100°C [42]. Considering the multiple properties demonstrated with essential oils, such as pharmaceutical applications, antioxidant, food and cosmetic uses [44, 45]. Nowadays, essential oils are becoming increasingly important as a safer alternative to synthetic chemical products [46]. Essential oils also showed a broad spectrum of advantages against the pest, plant pathogenic and fungi ranging from bactericidal, fungicidal, insecticidal, antifeedant or repellent, oviposition deterrent, and growth regulatory and antivector activities [47, 48]. The application of essential oils and their constituents mainly; terpenoids for weed and pest management is currently being explored and is viewed as an important source of lead molecules in agriculture [49, 50].

Recently, the effectiveness of essential oils has been investigated on some weed species, demonstrating the ability to inhibit germination and the development of seedlings. The reasons that encouraged the use essential oils as alternative compounds to conventional herbicides are due to a less harmful effect on the environment and almost as effective as the synthetic herbicides. Furthermore, there are no contradictions and obstacles to be used as bioherbicides in all aspects of agriculture, specifically in organic farming as compared to the use of synthetic pesticides, which has attracted a lot of interest in the safety and health of the consumers [35].
