**1. Introduction**

Phenolics are a type of secondary metabolite that can be found almost all over in plants. They are an aromatic molecule with a benzene ring (C6) and one or more hydroxyl groups that belong to a broad and diversified group. In general, phenolics are classified according to the number of carbon atoms in the molecule. Three different biosynthetic pathways produce phenolics: (i) the shikimate/chorizmate or succinylbenzoate pathway, which produces phenylpropanoid derivatives (C6–C3); (ii) the acetate/ malonate or polyketide pathway, which produces side-chain-elongated phenylpropanoids, including the large group of flavonoids (C6–C3–C6) and some quinones; and (iii) the acetate/mevalonate pathway, which produces the aromatic terpenoids, mostly monoterpenes, by dehydrogenation reactions [1].

The content of a certain phenolic in plant tissue varies depending on the season and stage of growth and development. Trauma, wounding and pathogen infection are only a few of the internal and external variables that alter phenolic production and accumulation. Furthermore, light increases the production of phenolics in chloroplasts and their accumulation in vacuoles. In some plant species, photoinhibition, as well as nutrient stressors such as nitrogen, phosphate, potassium, sulphur, magnesium, boron, and iron deficiency, cause the synthesis of phenylpropanoid chemicals [1].

The distribution of phenolics in plants is not consistent at the tissue, cellular, and subcellular levels. Plant cell walls contain insoluble phenolics, while plant cell vacuoles contain soluble phenolics. Certain polyphenols, such as quercetin, can be found in all plant products, including vegetables, fruit, cereals, tea, wine, fruit juices, infusions, and so on, whereas isoflavones and flavanones are found only in specific foods. Polyphenols are found in most foods in complex combinations. Higher levels of phenolics compounds found in outer layers of plants than inner layers. Plant polyphenol content is influenced by a variety of factors, including ripeness at harvest, environmental factors, processing, and storage. Environmental and edaphic factors, such as soil type, sun exposure, and rainfall, have a significant impact on the polyphenolic content of foods. The quantities and amounts of different polyphenols are greatly influenced by the degree of ripeness. In general, phenolic acid content declines as ripening progresses, although anthocyanin concentrations increase. Many polyphenols, particularly phenolic acids, are directly engaged in plants' responses to many types of stress: they aid in the healing of damaged areas by lignification, have antimicrobial capabilities, and their concentrations may rise following infection. Storage is another element that has a direct impact on the polyphenol content in foods. Polyphenolic content in foods changes during storage, according to studies, due to the simple oxidation of these polyphenols. Oxidation reactions result in the creation of more or less polymerised compounds, which affect food quality, especially colour and organoleptic qualities. Such alterations can be useful, as with black tea, or damaging, as with fruit browning. When wheat flour is stored, it loses a significant amount of phenolic acids. In terms of quality, flour after six months of storage had the same phenolic acids, although their concentrations were 70% lower than when it was fresh. Cold storage, on the other hand, has only a minor impact on the polyphenol content of apples, onions or pears. Cooking has a significant impact on polyphenol concentrations. After boiling for 15 minutes, onions and tomatoes lose between 75 and 80 percent of their initial quercetin content, 65 percent after cooking in a microwave oven, and 30 percent after frying [2].

## **2. Phenolics in plant defence**

Phenolics perform a dual function in the plant's environment, repelling and attracting various organisms. They act as inhibitors, natural animal toxicants, and pesticides against invading organisms, such as herbivores, phytophagous insects, nematodes, fungal and bacterial pathogens. On the plant surface, simple phenolic acids, complex tannins, and phenolic resins deter birds by interfering with the gut microflora and impairing their digestive ability. Low-molecular-weight phenylpropanol derivatives attract symbiotic microbes, pollinators, and animals that disperse fruit [3].

Phenolics have long been recognised in animals as phytoestrogens and as allelochemicals for competitive weeds and plants. Allelochemicals that are widely effective include volatile terpenoids, toxic water-soluble hydroquinones, hydroxybenzoates, hydroxycinnamates, and 5-hydroxynapthoquinones. Numerous simple and complex phenolic compounds accumulate in plant tissues and function as phytoalexins, phytoanticipins, and nematicides against soil-borne pathogens and phytophagous insects. Thus, phenolic compounds have been proposed as useful alternatives to chemical control of agricultural crop pathogens for some time. The majority of polyphenols have been shown to have a negative effect on microbes. Plants accumulate phytoalexins

*Medicinal Plants and Phenolic Compounds DOI: http://dx.doi.org/10.5772/intechopen.99799*

in response to pathogen attacks, such as hydroxycoumarins and hydroxycinnamate conjugates. The synthesis, release, and accumulation of phenolic compounds—in particular, salicylic acid are critical for a variety of plant defence strategies against microbial invaders. Phenolics are synthesised when plant pattern recognition receptors recognise potential pathogens via conserved pathogen-associated molecular patterns (PAMPs), resulting in PAMP-triggered immunity. As a result, the pathogen's progress is slowed significantly before it takes complete control of the plant [1].
