## 2 Plant Secondary Metabolites and Their Biological Roles
### Phenolic Compounds  
More than 8000 phenolic compounds are reported from plants, of which half of them are flavonoids (approximately 4000-4500 compounds) such as aglycone, glycosides, and methylated derivatives [6; 49]. Phenolics exhibit diverse structures from single aromatic rings (e.g., in phloroglucinol, gentisic acid, ferulic acid, caffeic acid, and vanillin) to complex polymeric structures such as in lignins (e.g., coniferyl alcohol), coumarins (e.g., scopoletin), phenolic quinones (e.g., juglone), tannins (e.g., ellagic acid), and flavonoids [50; 51]. Among phenolic group compounds, flavonoids are the most abundant, and stilbenes and lignans are less common.  
Flavonoids are a diverse secondary metabolite group with a wide array of functions, including protection against stress. Flavononoids comprise seven sub-groups (Figure 2) (flavones, flavonols, flavanones, isoflavonoids, flavan-3-ols or catechins, and anthocyanins) [52; 53] based on the C-ring carbon to which B-ring is attached, and also based on the degree of oxidation and unsaturation of their C-ring [53]. Flavones contain a double bond between positions 2 and 3 and a ketone functional group in position 4 of the C-ring. In comparison, flavonols have a hydroxy group at position 3 of the C-ring and are sometimes glycosylated. Unlike flavones, flavonones are saturated with a double bond between positions 2 and 3 of the C-ring. Flavan-3-ols have a hydroxyl group in position 3 of the C-ring, but there is no double bond between positions 2 and 3 [53]. Although the metabolic role of phenolics is not well-defined, their protective functions in plants are attributed to their ability to scavenge free radicals and filter harmful UV radiations [54; 55]. Ferulic acid, caffeic acid, and \(p\)-coumaric acid (hydroxycinnamic acid derivatives) are some of the best-known UV-B attenuators in plants [56]. Flavonoids help plants adjust to extreme heat and cold [57] through increasing accumulation. When [PERSON] et al. (2016) [58] analyzed the expression of flavonoids in 20 mutants of two different _Arabidopsis thaliana accessions_ (Col-0 and Ler) in response to freezing and cold acclimation (14 days at 4 \({}^{\circ}\)C), 19 mutants, which are gene-knock outs, did not exhibit flavonoid biosynthesis, with an exception to pap1-D mutant. A similar observation of increasing concentrations in flavonoids (anthocyanins and flavonoids) was also reported by [PERSON] et al. (2017) [59] in grapevine berries, but tannins did not show any changes. The role of flavonoids in UV protection is also supported by [PERSON] and [PERSON] work [60], in which they have isolated an _Arabidopsis_ mutant tolerant to high levels of UV-B radiations. Such protective flavonoids are reported more in plants thriving in colder climates at higher elevations and semi-arid environments [61]. Flavonoid-based plant pigments, such as anthocyanins synthesized in the last step of the flavonoid biosynthesis pathway under UV stress upon acylation, can absorb UV radiation and scavenge ROS [62; 63]. If not kept under control, ROS can cause direct damage to plants through the oxidation of essential biomolecules, leading to the accumulation of more ROS and ultimately programmed cell death [64].  
Flavonoids such as quercetin can chelate transition metals (for example, Fe), consequently inhibiting Fenton reaction (conversion of H\({}_{2}\)O\({}_{2}\) to toxic OH\({}^{\bullet}\) radical), thereby creating a robust antioxidative environment in the plants [65]. Phenolics are also known to play a strategic role in reproduction as frugivore attractants that promote seed dispersal (e.g., anthocyanins and anthocyanins such as cyanidin-3-glucoside) [66; 67].