## 3 Factors Influencing PSMs Production in Plants
### Effects of Heat Stress on PSMs  
Warming causes the accumulation of terpenoids, which usually have protective functions in mitigating environment-induced oxidative stress in plants [87; 190]. For instance, tomato (_S. Lycopersicum_) grown under heat stress (at 46 \({}^{\circ}\)C) emits higher levels of monoterpenes such as \(\alpha\)-thujene, \(\alpha\)-pinene, camphene, 2-carene, \(\alpha\)-phellandrene, \(\delta\)-3-carene (car-3-ene), \(\alpha\)-terpinene, limonene, \(\beta\)-phellandrene, (_E_)-\(\beta\)-ocimene, and terpionlene; and also sesquiterpenenes such as \(\delta\)-elemene, \(\beta\)-elemene, \(\alpha\)-humulene, and \(\beta\)-caryophyllene (Table 1) compared to controls [97]. In contrast, [PERSON] et al. [191] observed decreased emission of terpenes in _Citrus nonspeliensis_ grown under laboratory conditions at 35 \({}^{\circ}\)C; instead, increased assimilation of water-soluble antioxidant ascorbate indicates a shift from terpenenedicated to ascorbate-mediated ROS scavenging mechanism. Moreover, when _C. nonspeliensis_ was grown in the field, total terpene emission was higher during winter than in summer [191]. These contrasting findings suggest that terpene emissions under heat conditions could be species-specific and vary seasonally. Additionally, free fatty acids released by membrane phospholipase in response to heat (and cold) form lipoxygenase (LOX) products via lipoxygenase pathway, out of which C6 compounds (_Z_)-3-hexenal and (_E_)-2-hexenal are most common [97]. Wounded plants also release these two compounds within a few minutes [192; 193]. Notably, (_E_)-2-hexenal acts as a chemical signal inducing the expression of stress-related transcription factors such as HSFA2 (heat stress transcription factor A-2) and MBF1c (multiprotein-bridging factor 1c) [194]. Heat stress may cause the melting of cuticular lipids, thus increasing cuticular permeability [195], and extreme temperatures may rupture terpene-containing-glandular trichomes, releasing the contents into the air [97]. After exposure to cold and heat stresses, favorable pH conditions inside plastids favor increased terpene synthesis [97] (Figure 5). Under simulated environmental conditions, heat stress damages membranes (e.g., thylakoid membrane) and disintegrates membrane protein complexes (e.g., photosystem II) [196], consequently decreasing the rate of photosynthesis. Plants counteract such damage through sustained synthesis and emission of terpen [87; 197]. [PERSON] et al. [197] proposed that plants produce more terpenes under stressful conditions by diverting carbon to a non-mevalonate pathway, which otherwise could have been used in photosynthesis. Monoterpenes such as 1,8-cineole, \(\alpha\)-terpinyl acetate, linalyl acetate, limonene, sabinene, myrcene, \(\alpha\)-terpinen, \(\beta\)-ocimene, \(\alpha\)-terpionlene, and \(\gamma\)-terpinene are most produced following decreased photosynthesis in plants [191; 198]. Non-targeted PSMs profiling in tomato revealed higher concentrations of \(\alpha\)-tocopherol and plastoquinone under 38 \({}^{\circ}\)C compared to lower temperatures (20 and 10 \({}^{\circ}\)C) [199]. Taken together with other studies [200; 201], this suggests that these compounds function as electron carriers and facilitate photosynthesis in addition to their anti-oxidative functions. The photosynthetic rate also decreases under the increasing temperature as in _Pueraria lobata_ [Willd.] Ohwi., and _Quercus_ spp. when isoprene synthesis (non-mevalonate pathway) was inhibited with fosmidomycin [202]. They suggest that isoprene improves thermotolerance in plants and helps photosynthetic apparatus recover after experiencing heat shock (i.e., temperature \(>\) 40 \({}^{\circ}\)C). Studies [203; 204] suggest that plants tolerant to sunlight-induced heat flecks, O\({}_{3}\), and ROS produce more isoprene than non-tolerant species. However, not all plants seem to produce isoprenoid compounds, but it varies among different plant species.