## 3 Factors Influencing PSMs Production in Plants
### Effects of Ozone on PSMs  
Ozone (O\({}_{3}\)) in the lower atmosphere (troposphere) acts as a greenhouse gas and is phototoxic to plants [233]. It is usually produced by reactions between primary pollutants (such as carbon oxides, sulphur oxides, nitric oxides, and hydrocarbons) catalyzed by sunlight. Although O\({}_{3}\) is neither a free radical nor a ROS, its strong oxidizing properties enable it to react with biomacromolecules, including lipids, proteins, nucleic acids, and carbohydrates [234]. Generally, O\({}_{3}\) enters through stomata and damages leaf tissues, mainly in the upper (adaxial) layers resulting in chlorosis and lesions. Physiologically, exposure to O\({}_{3}\) impairs stomatal function (dysfunction of transpiration and water use efficiency) and reproductive development, CO\({}_{2}\) assimilation, and subsequently photosynthetic activity. In snap bean (_Phaseolus vulgaris_), exposure to an ambient concentration of O\({}_{3}\) (\(\leq\)150 ppb, 1 h) [along with water stress (\(\leq\)15%)] induces sluggishness in stomatal closure, subsequently causing more significant loss of leaf surface water [235]. In addition to changes in plant physiological functions, O\({}_{3}\) triggers pathways responsible for producing defensive molecules, such as flavonoids. When [PERSON] et al. exposed soybean leaves to elevated O\({}_{3}\) (110 \(\pm\) 10 nmol mol\({}^{-1}\) for 8 h daily, for 54 days), the concentrations of rutin, quercetin, and total flavonoids increased significantly [236]. Ozone also enhances the activity of enzymes involved in flavonoid biosynthesis. Plants fumigated with O\({}_{3}\) show increased activities of phenylalanine-ammonium lyase (PAL), and chalcone synthase (CHS) enzymes involved in phenylpropanol and flavonoid biosynthesis pathways [237] and subsequently produce protective compounds that can scavenge ROS [56]. The general phenylpropanol pathway and flavonoid biosynthesis pathways are outlined in Figure 6 below. These pathways, in turn, contribute significantly towards plant defense response by producing protective phenolic compounds such as condensed tannins and flavonoids that can scavenge ROS [57]. For instance, when Arabidopsis thaliana is exposed to O\({}_{3}\) (300 ppb daily for 6 h), PAL mRNA levels increase 3-fold compared to their control plants [238]. Similarly, O\({}_{3}\) treatment (200 nL/L for 10 h) increases both PAL and CHS activities resulting in a 2-fold increase of total leaf funcoucumarins and flavone glycosides in parsley (_Petroselinum crispum_) [239]. Lignin deposition in O\({}_{3}\) exposed leaves is also linked to increased PAL activity [240], whereas in sage (_Salvia officinalis_), both PAL and PPO (phenol oxidase) activities were suppressed after 24 h exposure to O\({}_{3}\)[241]. However, rosmarinic acid synthase (RAS) activity is accompanied by the increased transcription level of genes (e.g., RAS) encoding biosynthesis enzymes, suggesting that the sage plant mediates oxidative damage through synthesizing phenolic compounds. Studies have shown that plant chemical responses to O\({}_{3}\) exposure variably depend on the O\({}_{3}\) concentration [242]. Ozone alone enhances the production of phenolic compounds more significantly than in response to the increased CO\({}_{2}\) concentration, while the combination of these two factors resulted in higher diterpenes, but not mono- and sesquiterpene, synthesis in plants [243]. However, some experiments showed contrasting results from O\({}_{3}\) fumigation. Leaves of _Ginkgo biloba_, upon fumigation with an elevated level of O\({}_{3}\), increased the concentrations of terpen (Table 1), but phenolics decreased [152]. Ozone also enhances the accumulation of salicylic acid (SA) in plant tissues; for instance, in the tobacco plant (Nicotiana tabacum), emission of SA-derived methyl salicate increases upon exposure to O\({}_{3}\)[244; 245]. In _Arabidopsis_, SA accumulation is necessary for forming O3-induced mRNAs, such as PAL and pathogenesis-related protein 1 (PAR1) transcripts [245].