**6. Terpenoids**

Terpenoids or terpenes are the most structurally varied classes and they are the largest family of compounds of plant products. Previous studies estimated the exiting of more than 23,000 of known terpenoid compounds, including carotenoids, tocopherol, phytol, steroids and hormones [51]. Terpenoids play an important and essential role in a broad range of biological functions like respiration, chain electron transport, cell wall and membrane biosynthesis. Also, terpenoids are involved extensively in the fields of pharmaceuticals, cosmetics, colorants, disinfectants, scents, flavorings and agrichemicals [52]. Biosynthetically, terpene compounds are classified as unsaturated hydrocarbons and basically synthesized from the isoprene unit which has the molecular formula (CH2〓C(CH3)▬CH〓CH2). The molecular formulae of terpenoids are multiples of that, (C5H8) and where (n) is the number of linked isoprene units called the isoprene rule or the C5 rule; **Figure 4** shows the chemical structure of the isoprene unit.

The isoprene units may be joined together head-to-tail to form linear chains or they may be arranged to form rings. Chains of isoprene units are linked to building the terpene structure that synthesized sequentially to form hemiterpenes, monoterpenoids, sesquiterpenoids, diterpenes, sesterterpenes, triterpenes, and tetraterpenes depending on the number of isoprene units [53].

**197**

**Table 1.**

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

Terpenoids play a defensive role in trees. They are the major component of essential oils in many trees which are responsible for the aroma of the trees. They are released into the air by vaporization or leached in small amounts by water. These compounds caused poor growth of neighboring plants in addition to aggravating other problems site when released [54]. Among the terpenoids compounds, Monoterpenoids and sesquiterpenoids are the most available compounds in the secondary plant metabolites and widely used as antimicrobials and cosmetics. Monoterpenoids and sesquiterpenoids produce in plants as defensive chemicals

Terpenoids can be classified according to the number of isoprene units in the molecule. A prefix in the name indicates the number of terpene units needed to assemble the molecule; **Table 1** lists of various classifications of isoprene units as well as the examples of the compounds associated with the classification. All the above different secondary plant metabolites are produced through the metabolism of primary metabolites depending on the pathway and the type of primary

> **Number of isoprene units**

5 Sesterterpenoids Five C25H40 Geranylfarnesol

7 Sesquarterpenoids Seven C35H56 Ferrugicadiol and

8 Tetraterpenoids Eight C40H64 Cyclic lycopene, carotenoids

6 Triterpenoids Six C30H48 Sterols

eight

9 Polyterpenes More than

*Classification of terpenoids based on the number of isoprene units.*

1 Hemiterpenes Single C5H8 Prenol and isovaleric acid

2 Monoterpenoids Two C10H16 Geraniol, limonene, terpineol and

3 Sesquiterpenoids Three C15H24 Humulene, caryophyllene, and

4 Diterpenoids Four C20H32 Cafestol, kahweol, cembrene and

**Structure formula**

**Example**

myrcene

farnesol

— Natural rubber

taxadiene and phytol

tetraprenylcurcumene

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

against insects, fungi and surrounding plants [51].

metabolites.

**Figure 4.**

*Structure of one isoprene unit.*

**No. Type of** 

**classification**

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

Terpenoids play a defensive role in trees. They are the major component of essential oils in many trees which are responsible for the aroma of the trees. They are released into the air by vaporization or leached in small amounts by water. These compounds caused poor growth of neighboring plants in addition to aggravating other problems site when released [54]. Among the terpenoids compounds, Monoterpenoids and sesquiterpenoids are the most available compounds in the secondary plant metabolites and widely used as antimicrobials and cosmetics. Monoterpenoids and sesquiterpenoids produce in plants as defensive chemicals against insects, fungi and surrounding plants [51].

Terpenoids can be classified according to the number of isoprene units in the molecule. A prefix in the name indicates the number of terpene units needed to assemble the molecule; **Table 1** lists of various classifications of isoprene units as well as the examples of the compounds associated with the classification. All the above different secondary plant metabolites are produced through the metabolism of primary metabolites depending on the pathway and the type of primary metabolites.

**Figure 4.** *Structure of one isoprene unit.*

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

extracting of essential oils on a large scale.

in agriculture [49, 50].

**6. Terpenoids**

chemical structure of the isoprene unit.

penes depending on the number of isoprene units [53].

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

The steam distillation process considered lower risk as compared with another

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].

Terpenoids or terpenes are the most structurally varied classes and they are the largest family of compounds of plant products. Previous studies estimated the exiting of more than 23,000 of known terpenoid compounds, including carotenoids, tocopherol, phytol, steroids and hormones [51]. Terpenoids play an important and essential role in a broad range of biological functions like respiration, chain electron transport, cell wall and membrane biosynthesis. Also, terpenoids are involved extensively in the fields of pharmaceuticals, cosmetics, colorants, disinfectants, scents, flavorings and agrichemicals [52]. Biosynthetically, terpene compounds are classified as unsaturated hydrocarbons and basically synthesized from the isoprene unit which has the molecular formula (CH2〓C(CH3)▬CH〓CH2). The molecular formulae of terpenoids are multiples of that, (C5H8) and where (n) is the number of linked isoprene units called the isoprene rule or the C5 rule; **Figure 4** shows the

The isoprene units may be joined together head-to-tail to form linear chains or they may be arranged to form rings. Chains of isoprene units are linked to building the terpene structure that synthesized sequentially to form hemiterpenes, monoterpenoids, sesquiterpenoids, diterpenes, sesterterpenes, triterpenes, and tetrater-

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

**196**


#### **Table 1.**

*Classification of terpenoids based on the number of isoprene units.*

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

**Figure 5.**

*General schematic biosynthetic pathways to produce major secondary metabolites including terpenoids [60].*

#### **6.1 Biosynthesis of terpenoids**

Secondary metabolites including terpenoids are derived essentially from the modification of primary metabolites by different main pathways; the pathways responsible for synthesizing the primary metabolites. The secondary metabolite biosynthetic pathways are too numerous and cannot be easily determined. Nevertheless, there are a few typical pathways involved in the biosynthesis of major classes of these compounds. The shikimate pathway is considered the major pathway used by plants as well as different organisms like bacteria and fungi to synthesize primary metabolites which in turn form the basic building block for a wide range of phenolic and flavonoid compounds [55, 56]. The shikimate pathway involving multiple isoprene units (C5H8) linked together in a head-to-tail pattern can synthesize to terpenoids according to the number of isoprene units incorporated in the molecular skeleton [57]. Terpenoids can also be synthesized through an isopentenyl diphosphate pathway, which arose from the intermediate substrate, particularly, mevalonic acid (MVA) via the mevalonate pathway and a methylerythritol phosphate/deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP) pathway. **Figure 5** showed the biosynthesis of terpenoids [58, 59].

## **7. Terpenoids as natural weed killers; mode of action**

The term "mode of action" refers to the sequence of herbicides action beginning from absorption by plant tissues until death. Understanding herbicide mode of action is helpful in knowing what groups of weed killers. Generally, herbicides classified depending on their mode of action and the toxicity into two groups; contact and systemic herbicides [56]. Contact herbicides only kill the plant tissue which comes into contact with the spray solution. While systemic herbicides need to translocated in plant tissues until reaching the active site for causing the injury. Contact herbicides quite often the fastest acting by causing acute toxicity but the whole plant must be sprayed to be effective [60]. Herbicides also can further

**199**

**Figure 6.**

*Natural weed killers; mode of action [23].*

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

classify according to their selectivity to selective or non-selective herbicides. Nonselective herbicides can kill most plants while selective herbicides designed to kill specific types of plants depends on the morphological and physiological differences between the two major plant groups, grassy or broadleaf. Moreover, herbicides can be divided into two groups as a result of its application timing; pre and postemergence. Preemergence herbicides normally targeted seeds by preventing the germination and suppress seedling development. While postemergence herbicides target weed biomass by reducing or inhibiting the biological processes in plant tissues. Study of the injury symptoms of the targeted plant tissues resulting from the application of herbicides helps to determine how herbicides interact with the biological and physical systems of the targeted plant. Injury symptoms in targeted weed species depend on the type of herbicide, the rate of application, stage of growth, type of exposure, and the plant species receptor involved. All herbicides work by disrupting one or more than one of the natural mechanisms of the targeted plant tissues such as the stomatal system through the influence of the guard cells, photosynthesis by the distraction of chlorophyll pigment and targeting cell mem-

Herbicidal mechanisms of the secondary plant metabolites as post-contact formulations weed killers are strictly fast-acting. They generally disrupt the cuticular layer of the foliage which resulting in the rapid desiccations or burndown of young tissues [35]. Membrane disruption can be considered as one of the underlying mechanisms of plants' phytotoxic effects, which result in cell death and growth inhibition. Secondary metabolites such as terpenoids are less specific and attack a multitude of proteins by building hydrogen, hydrophobic and ionic bonds and as a result of this, modulating their 3D structures and in consequence

Monoterpenes are considered lipophilic compounds; hence, there is, therefore,

the possibility of plant cell membrane expansion as a result of accumulation monoterpenes, thereby destroying the membrane structure [62, 63]. **Figure 7** showed the interaction of terpenoids with the plant cell membrane [64]. Moreover, the monoterpenes compounds in essential oils uncoupled the oxidative photophosphorylation (transform ADP to form ATP using the energy of sunlight). As a result, monoterpenes cause a reduction in cellular respiration leading to a perturbation in the ATP production. Thus, disorders in physiological processes in plants are induced

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

brane and other cellular systems.

their bioactivities [61] (**Figure 6**).

[33, 65] (**Table 2**).

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

classify according to their selectivity to selective or non-selective herbicides. Nonselective herbicides can kill most plants while selective herbicides designed to kill specific types of plants depends on the morphological and physiological differences between the two major plant groups, grassy or broadleaf. Moreover, herbicides can be divided into two groups as a result of its application timing; pre and postemergence. Preemergence herbicides normally targeted seeds by preventing the germination and suppress seedling development. While postemergence herbicides target weed biomass by reducing or inhibiting the biological processes in plant tissues.

Study of the injury symptoms of the targeted plant tissues resulting from the application of herbicides helps to determine how herbicides interact with the biological and physical systems of the targeted plant. Injury symptoms in targeted weed species depend on the type of herbicide, the rate of application, stage of growth, type of exposure, and the plant species receptor involved. All herbicides work by disrupting one or more than one of the natural mechanisms of the targeted plant tissues such as the stomatal system through the influence of the guard cells, photosynthesis by the distraction of chlorophyll pigment and targeting cell membrane and other cellular systems.

Herbicidal mechanisms of the secondary plant metabolites as post-contact formulations weed killers are strictly fast-acting. They generally disrupt the cuticular layer of the foliage which resulting in the rapid desiccations or burndown of young tissues [35]. Membrane disruption can be considered as one of the underlying mechanisms of plants' phytotoxic effects, which result in cell death and growth inhibition. Secondary metabolites such as terpenoids are less specific and attack a multitude of proteins by building hydrogen, hydrophobic and ionic bonds and as a result of this, modulating their 3D structures and in consequence their bioactivities [61] (**Figure 6**).

Monoterpenes are considered lipophilic compounds; hence, there is, therefore, the possibility of plant cell membrane expansion as a result of accumulation monoterpenes, thereby destroying the membrane structure [62, 63]. **Figure 7** showed the interaction of terpenoids with the plant cell membrane [64]. Moreover, the monoterpenes compounds in essential oils uncoupled the oxidative photophosphorylation (transform ADP to form ATP using the energy of sunlight). As a result, monoterpenes cause a reduction in cellular respiration leading to a perturbation in the ATP production. Thus, disorders in physiological processes in plants are induced [33, 65] (**Table 2**).

**Figure 6.** *Natural weed killers; mode of action [23].*

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

Secondary metabolites including terpenoids are derived essentially from the modification of primary metabolites by different main pathways; the pathways responsible for synthesizing the primary metabolites. The secondary metabolite biosynthetic pathways are too numerous and cannot be easily determined. Nevertheless, there are a few typical pathways involved in the biosynthesis of major classes of these compounds. The shikimate pathway is considered the major pathway used by plants as well as different organisms like bacteria and fungi to synthesize primary metabolites which in turn form the basic building block for a wide range of phenolic and flavonoid compounds [55, 56]. The shikimate pathway involving multiple isoprene units (C5H8) linked together in a head-to-tail pattern can synthesize to terpenoids according to the number of isoprene units incorporated in the molecular skeleton [57]. Terpenoids can also be synthesized through an isopentenyl diphosphate pathway, which arose from the intermediate substrate, particularly, mevalonic acid (MVA) via the mevalonate pathway and a methylerythritol phosphate/deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP) pathway.

*General schematic biosynthetic pathways to produce major secondary metabolites including terpenoids [60].*

The term "mode of action" refers to the sequence of herbicides action beginning from absorption by plant tissues until death. Understanding herbicide mode of action is helpful in knowing what groups of weed killers. Generally, herbicides classified depending on their mode of action and the toxicity into two groups; contact and systemic herbicides [56]. Contact herbicides only kill the plant tissue which comes into contact with the spray solution. While systemic herbicides need to translocated in plant tissues until reaching the active site for causing the injury. Contact herbicides quite often the fastest acting by causing acute toxicity but the whole plant must be sprayed to be effective [60]. Herbicides also can further

**6.1 Biosynthesis of terpenoids**

**Figure 5.**

**Figure 5** showed the biosynthesis of terpenoids [58, 59].

**7. Terpenoids as natural weed killers; mode of action**

**198**

#### **Figure 7.**

*Interaction of terpenoids with plant cell membrane [60].*


**201**

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

**Affected weed species Ref.**

*Secale cereale* [75]

*Phalaris minor* [77]

*Silybum marianum* [78]

*Phalaris canariensis* [79]

[72]

[73]

[74]

[76]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

*S. arvensis Trifolium campestre L. rigidum P. canariensis*

*Lactuca serriola*

*C. dactylon*

*L. sativa Lepidium sativum P. oleracea*

*S. marianum*

*A. retroflexus Avena fatua Datura stramonium Lepidium sativum*

*L. sativa Sorghum bicolor*

*Ph. minor A. viridis*

*Chenopodium murale*

*Digitaria australe A. hybridus*

*Digitaria australe A. hybridus*

*D. longiflora Stachytarpheta indica Aster subulatus*

*P. virgatum Chloris barbata, Euphorbia hirta Stachytarpheta indica*

*Sinapis arvensis Diplotaxis harra Trifolium campestre Desmazeria rigida Phalaris canariensis*

**compounds**

*N. meyeri* Nepetalactone *Bromus danthoniae*

*Eucalyptus globulus* 1,8-Cineole *Amaranthus blitoide*

*Cymbopogon citratus* Citral *E. crus-galli* [64]

*Eucalyptus citriodora* Citronellol *A. viridis* [67]

β-Pinene

Carvacrol Thymol

α-Pinene β-Pinene

γ-Muurolene

Chrysanthenone

Geraniol

Limonene

Carvacrol

Thymol

Thujene Thymol

Eudesmol

Geranial

α-Thujene α-Pinene

*Melaleuca bracteata* Methyl eugenol *Panicum virgatum*

α-terpinolene Piperitone (E)-Tagetone (Z)-Ocimenone

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

**Donor plant Involved terpenoid** 

*Pinus pinea* Limonene α-pinene

*Eupatorium adenophorum* γ-Cadinene

*Pelargonium graveolens* Citronellol

*Artemisia judaica* Thujone

*Carum carvi* Carvone

*Thymus daenensis* Thymol

*Plectranthus amboinicus* Carvacrol

*Cupressus macrocarpa* Citronellal

*Pelargonium radula* Cis-Geraniol

*C. citratus* Neral

*Eucalyptus lehmannii* 1,8-Cineole

*Tagetes minuta* Limonene piperitenone

*Satureja khuzestanica Satureja rechingeri*

*Pinus brutia Pinus pinea*

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


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

**Donor plant Involved terpenoid** 

*Interaction of terpenoids with plant cell membrane [60].*

*Artemisia scoparia* p-Cymene

**Figure 7.**

*Peumus boldus* Ascaridole

*Cistus ladanifer* Trans-pinocarveol

*Eucalyptus salubris* 1,8-Cineole

*Cupressus sempervirens* α-Pinene

**compounds**

Caryophyllene Germacrene D Limonene α-Pinene

p-Cymene 1,8-Cineole

*Anisomeles indica* α-Bisabolol oxide *Bidens pilosa*

Viridiflorol Bornyl acetate Ledol

α-Pinene ρ-Cymene Predominant

α-Cedrol δ-3-Carene Germacrene D

*Nepeta meyeri* Nepetalactone *Amaranthus retroflexus* 

*Leptospermum scoparium Digitaria sanguinalis* [35] — Limonene *Amaranthus viridis* L [67]

**Affected weed species Ref.**

[26]

[66]

[68]

[69]

[50]

[71]

*Achyranthes aspera, Cassia occidentalis Parthenium hysterophorus Echinochloa crus-galli, Ageratum conyzoides*

*Portulaca oleracea Bromus danthoniae, Agropyron cristatum Lactuca serriola Bromus tectorum Bromus intermedius Chenopodium album Cynodon dactylon Convolvulus arvensis*

*Amaranths hybrids P. oleracea*

*C. occidentalis, A. viridis E. crus-galli*

*A. hybridus Conyza canadensis Parietaria judaica*

*L. rigidum Phalaris canariensis Trifolium campestre Sinapis arvensis*

*Solanum elaeagnifolium* [70]

**200**


#### **Table 2.**

*Allelopathic effects of terpenoids compounds in essential oils on seed germination and seedling development on different weed species 2010–current.*
