## 4 Discussions
### Association between Differential Metabolites and Differential Expressed Genes  
_GPPS_ was generally considered as a structural gene in terpenoid backbone biosynthesis; it was responsible for the head-to-tail condensation of one dimethylallyl diphosphate (DMAPP) with one isopentenyl diphosphate (IPP) molecule to form geranyl diphosphate (GPP), which is the precursor of monoterpenes [10]. In the TH, the expression trend of _GPPS_ was consistent with the abundance of monoterpenoids in floral scent. Nevertheless, the integrated analysis indicated that the downstream pathway of _GPPS_, monoterpenoid biosynthesis (ko00902), showed no strong correlation to DMs except for geraniol. Further analysis found that many screened monoterpenoids DMs from two varieties of aromatic \(D\). _loddigesii_ could not be mapped into the KEGG reference pathway, because their related biosynthesis and metabolism pathways have not been studied clearly, including 3-carene, \(p\)-menthene, and o-cymene. In addition, the types of terpenoids in \(D\). _loddigesii_ were very different to those common components seen in other orchids, such as limonene, myrcene, linalool, and ocimene [26, 27].  
LOX is a key branchpoint enzyme leading to the biosynthesis of short-chain aldehydes and alcohols in alpha-linolenic acid metabolism (ko00592). In the pathway, linoleic acid could be formed to 9-hydroperoxy and 13-hydroperoxy intermediates under the oxidation of LOX, and these intermediates were further converted to c6/c9 aldehydes and alcohols such as (Z)-3-hexenal with a 'fresh green' smell; this main branch pathway is called the 'LOX pathway' [28]. Meanwhile, 13-hydroperoxy intermediates could be catalyzed by a series of enzymes such as AOC, allene oxide synthase (AOS), and 12-oxophyllenoate reductase (OPR) to generate jasmonate and its derivatives; this branch pathway was called\(\beta\)-oxidation [29]. Combining the results of metabolomic and transcriptomic analysis, the relationship between (Z)-3-hexenal and _LOX_ could be established. The expression of _LOX_ and _AOC_ genes was all increased, and the downstream metabolites of _LOX_, (Z)-3-hexenal, and (E,E)-2,4-nonadienal were accumulated more in TL. Moreover, (Z)-3-hexenal has not been reported in the study of volatile components in _Dendrobium_ species before. It indicated that the alpha-linolenic acid metabolism pathway and its related genes played a significant role in the formation of volatile components in light-scented _D. loddigesii_.  
Phenylpropanol compounds were the second-largest class of plant VOCs; the first committed step in the biosynthesis of the majority of phenylpropanoids is catalyzed by-PAL, which deaminates phenylalanine (Phe) to trans-cinnamic acid (CA) [10]. Then, CA is catalyzed by a series of methyltransferases and acyltransferases to form the volatile phenylpropenes, such as eugenol and isoeigenenol. _PAL_ and its downstream _C4H_, _4 CL_ genes constituted the common phenylpropanoid pathway, which played a significant role in the synthesis of phenylpropanoids [30]. Nevertheless, the expression change trends of _PAL_, _C4H_, and _4 CL_ were opposite to that of the relative content of eugenol in TH. To further explain this inconsistency, the expression level of _PAL_ and _C4H_ in the three flowering stages of the TH sample were analyzed using a real-time PCR method. The results showed that the expression levels of _PAL_ and _C4H_ in the flowering stage were much lower than those of the bud stage and half-open stage. It implied that the biosynthesis and release of phenylpropanoid volatiles might not be synchronized in TH.