# GC-MS Analysis of the Volatile Constituents in the Leaves of 14 Composite Plants
## 3 Discussion  
The chemical constituents, mainly consisting of volatile compounds, in the leaves of 14 Compositae plants with unique scents were investigated qualitatively and quantitatively. Most of the sampled plants contained abundant terpenoids, primarily monoterpenoids, sesquiterpenoids, and derivatives (Figure 6). The terpenoid contents were highest in the leaves of _A. yunnanensis_, _A. abrotanum_, _A. argyi_, _C. indicium_ var. _aromaticum_, and _O. tailangensis_. Since monoterpenoids and sesquiterpenoids  
Figure 5: Dendrogram of the hierarchical cluster analysis of 14 samples based on the relative content of terpenoid compounds. The abscissa denotes the Euclidean distances and the ordinate denotes the sample numbers, which are the same as in Table 2.  
with lower molecular weight volatilize easily and have specific odors [8; 31], these samples have greater potential to be used for extraction of perfume oils. Considering those characteristic constituents (Figure 6C), the tested samples could be defined by their distinct scent via sensory evaluation in a later study, just as _Chrysanthemum_ essential oils were assessed based on six sensory attributes including floral, woody, grassy, fruity, sour, and minty [31]. On the other hand, because terpen play important roles in protecting plants against pests, these compounds can be divided into different types based on their functions, such as in indirect defense or direct repulsion of pests [32]. For example, eucalypt and camphor have activity as insect repellents or pesticid agents [33; 34], whereas (_E_)-\(\beta\)-caryophyllene and (_E_)-\(\beta\)-farnesene can additionally attract the natural enemies of herbivores [35; 36]. Therefore, the above results could serve as a reference for the utilization of the volatile compounds from these Compositae plants to exploit the specific activities of certain constituents. In plants, the metabolism of terpenoids use five-carbon isopentenyl diphosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) as precursors, which originate from the mevalonic acid (MVA) and methylerythitol phosphate (MEP) pathways (Figure 6B). One, two, or three units of IPP combine with one unit of DMAPP to generate GPP (or NPP), FPP, and GGPP, respectively, which are subsequently converted to monoterenes, sesquiterpenes, and diterpenes by the activity of the terpene synthases (TPSs) [37]. Generally, FPP (the precursor of sesquiterpenes) is generated via the MVA pathway in the cytoplasm, whereas GPP and GGPP (the precursor of monoterenes and diterpenes, respectively) are synthesized from the MEP pathway in plastids [38]. After synthetic processing by TPS enzymes, these terpen can be further modified by oxidation, hydroxylation, methylation, acylation, or cleavage to form various terpene derivatives [37; 38]. For instance, more oxygenated terpenids than terpenes were detected in many species in our research (Figure 3). Therefore, the diversity of terpenoids in different plant species is due to the action of TPSs and multiple modification of terpenes. The family size and functions of TPSs, which include seven subfamilies are related to species phylogeny [37], suggesting that a diversity of TPSs may exist in different genera of Composite plants. According to the present research, based on the terpenoid composition in leaves, the genera  
Figure 6: Local diagram of the terpenoid biosynthetic pathway and the chemical structures of the main compounds found in 14 Compositae plants. Graph (**A**) displays the representative samples used in this research; Graph (**B**) was modified according to the literature [37]; and (**C**) the sample numbers which are the same as in Table 2, denote the samples with the highest concentrations of constituents (above 100 ng/g FW). Chrysanthemum_ and _Opisthopapppus_ have greater chemical similarity, whereas the _Artemisia_ plants, _C. chinense_ and _T. vulgare_ were classified together. The phylogeny based on the sequence variation in both the nuclear ribosomal (ITS) and chloroplast (trnL-F ICS) DNA constructed by [PERSON] et al.