**2. Biosynthesis and classifications of plant terpenes**

The name "terpene" is derived from the old French word "turpentine" and means "resin" [5]. Plants use two pathways to produce terpenes: the plastidial

### *Influence of Salinity on* In Vitro *Production of Terpene: A Review DOI: http://dx.doi.org/10.5772/intechopen.111813*

2-C-methylD-erythritol-4-phosphate (MEP) pathway and the acetyl-CoA-linked cytosolic mevalonate (MVA) pathway. One of the basic terpene biosynthesis building blocks (C5 isoprene unit) is isopentyl pyrophosphate (IPP), and the other is allylic isomer dimethylallyl pyrophosphate (DMAPP). DMAPP and IPP building blocks combine to form mono-, di-, tri-, tetra-, and polyterpenes with higher molecular weights [21]. Terpenes are volatile unsaturated hydrocarbons with various structural properties [22]. They are commonly found in the leaves, flowers, stems and roots of higher plants, aromatic medicinal plants, citrus, conifers, and eucalyptus species.

The development of chromatographic and spectroscopic methodologies has accelerated the discovery of terpenes and terpenoids [18]. Terpenes and terpenoids are terms that are often used interchangeably. But terpenes undergo oxygenation, hydrogenation, or dehydrogenation to form terpenoids. Terpenes, such as pinene, myrcene, limonene, terpinene, and p-cymene, are compounds with simple hydrocarbon structures. However, terpenoids are a class of modified terpenes with different functional groups and oxidized methyl groups that have been moved or removed [23].

Terpene compounds are classified according to the number of isoprene units they contain. The chemical formula of terpenes is (C5)n. The n, in this formula, indicates the number of isoprenes present in the compound. The number of isoprene units forms the groups that provide the structural diversity of terpenes [14]. Accordingly, terpenes, as shown in **Figure 1**, are hemiterpenes (C5 isoprene), monoterpenes (C10 limonene, menthol, etc.), sesquiterpenes (C15 atractylone, caryophyllene, etc.),

**Figure 1.** *Classification of terpenes.*

diterpenes (C20 taxol, etc.), sesterpenes (C25 disidiolide), triterpenes (C30 masticadienolic acid, ursolic acid, etc.), tetraterpene (C40, zeaxanthin, carotene, etc.) and polyterpenes (C˃40 resin, etc.)
