# Trepenes and Trepenoids in Plants: Interactions with Environment and Insects
## 1 Plant Trepenes in Abiotic Stress
### Effects of Plant Terpenes on Abiotic Stress  
VTs mitigate the effects of oxidative stress by modulating the oxidative status of plants, and the volatile emission profile could be one of the signature responses of a plant under stress conditions. Considering that there is a convergence of various stress pathways at the level of oxidative signaling [35], it has been proposed [29] that protection against abiotic stress is mediated through direct reactions of terpenoids with oxidants either intracellularly or at the leaf-atmosphere interface/boundary layer, membrane stabilization, and indirect alteration of ROS signaling [35; 36]. For instance, thermal stress induces membrane destabilization of the thylakoid membrane, resulting in fragmentation of the membrane and disintegration of protein complexes (e.g., PSII). It is hypothesized that, due to the amphipathic nature of isoprene, they could alleviate stress by transiently inserting in the membrane, which enhances hydrophobic interactions of large protein complexes between themselves or with membrane lipids [37]. On the other hand, membrane-bound antioxidants such astocopherol and carotenoids (zeaxanthin, neoxanthin, and lutein) may directly scavenge ROS in response to photoinhibition [38; 39; 40; 41; 42]. The photosynthetic output is compromised under certain abiotic stresses. Plants have adapted a mechanism to re-focus resources to more important pathways, and it is hypothesized that VTs together with phytohormones are key players in this process called premature or stress-induced senescence [43; 44; 45]. This is a coping mechanism whereby plants economize available resources. A study [43] has proposed that during abiotic stress, VT production is upregulated by re-routing resources to the MEP pathway and, together with phytohormones, may promote plant senescence and organ abscission. Production of VTs is considered a substantial and irretrievable investment for plants; however, even if photosynthesis is inhibited, VT production and emission are sustained, suggesting they are beneficial to plants under stress conditions [29]. Under one or more abiotic stresses, plants may eventually undergo stress-induced senescence. An article [43] has also proposed that abiotic stress induces VT production and emission, which initiate signaling pathways leading to senescence, apoptosis, and abscission in plants. Senescence in _Arabidopsis_ is induced by citral, peppermit, \(\beta\)-pinene, \(\alpha\)-pinene, and camphene [43; 46; 47]. Plants gamble to ensure survival by diverting resources from older organs to synthesize stress-combating secondary metabolites. A correlation has been reported in _Salvia mirzayanni_ where increased production of monoterpenoids (1,8-cineole, \(\alpha\)-terpinyl acetate, and linalyl acetate) resulted in a subsequent decrease of photosynthetic productivity (rate, stomatal conductance, internal CO\({}_{2}\) partial pressure, and transpiration) [48]. This observation suggests a resource partitioning that diverts more carbon to secondary metabolite synthesis. Furthermore, an interesting observation during plant senescence of expended leaves is that, while they become more prone to aphid infestation, re-allocation of resources could oppose the ability of aphids to otherwise redirect the flow of photosynthates to them [44]. It could be that re-routing of nutrients and secondary metabolites makes the leaves less deterring to herbivores because of infestation by making it a bountiful source of nutrient leftovers. While this is a compromise that plants have to make, most plants can reverse the process once the environmental conditions become favorable [49]. Abiotic stress-associated terpen and terpenoids currently known are listed in Table 1.