*2.1. Terpenoids*

Terpenoids, or terpenes, represent the largest group of plant floral volatiles [27]. They play key roles in attracting pollinators for successful reproduction [32,33] and in defense against pathogens and florivores [34,35]. Moreover, from their natural roles, terpenoids are widely used in the cosmetic and perfume industries and as food additives because of their distinctive aromas and flavors [36,37]. Studies on floral scents have mainly focused on isolation and characterization of *terpene synthase* (*TPS*) genes encoding the key enzymes responsible for the synthesis of terpenes. All terpenoids are produced from isopentenyl diphosphate (IPP) and dimethyl allyl diphosphate (DMAPP), which are five-carbon (C5) precursors [38]. IPP and DMAPP are derived from two alternative biosynthetic pathways localized

in different cellular compartments. The classical mevalonic-acid (MVA) pathway, which is localized in the cytosol, gives rise to IPP from three molecules of acetyl-CoA. In contrast, the methylerythritol phosphate (MEP) pathway takes place in plastids and produces IPP from pyruvate and glyceraldehyde 3-phosphate. In plants, monoterpenes, diterpenes, carotenoids, ubiquinones, and phytols are produced in the plastid via the MEP pathway, while all other plant terpenoids (sesquiterpenes, triterpenes, and polyterpenes) are produced using the MVA pathway. Floral volatiles with TPSs have been identified in such orchids as *P. bellina* [24] and *C. goeringii* [28].

Terpenoids are dominant in floral volatiles including those emitted by orchids. Geraniol, linalool, and their derivatives are major compounds of scented *P. bellina* flowers. Monoterpenes (Table 1) play a key role in the volatile profile [23,24]; in *C. goeringii*, floral volatile organic compounds include farnesol, methyl epi-jasmonate, (*E*)-β-farnesene, and nerolidol. Sesquiterpenes play a key role in the scent profile [28]. In the *Cymbidium* hybrid "Sunny Bell," linalool is the major compound found in the petal [39]. The volatile floral scents inside species and cultivars of *Cymbidium* have been reported [28,39]. Among the volatiles α-pinene, eucalyptol, trans-β-ocimene, α-copaene, and β-caryophyllene terpenoid were leading components in the volatile mixture [40,41]. The Vanda Mimi Palmer flower mainly contains cimene, linalool oxide, and linalool, which are classified as monoterpenes [42]. In addition, nerolidol is a sesquiterpene [40,41]. Compared with model plants, there are few reports on floral scent terpenoids in orchids.


**Table 1.** Major volatile organic components in orchid flowers.


**Table 1.** *Cont.*
