**2. Types of stresses**

The environment around the plant can influence the physiological condition of the plant and any disturbance in the external environment including physical factors and biological factors can influence the metabolic pathways in the cell. Accordingly, the types of stress conditions are defined as biotic and abiotic stresses, such as pathogen infection, water deprivation, salinization, high/low temperature stress, heavy metal toxicity, nutrient deficiency, atmospheric pollution and UV irradiation [10]. In addition to this endogenously generated stress also can influence the production of PSM which are consequential effect of external factors or molecules generated due to various physiological activities within the plant like reactive oxygen and nitrogen species (RONS) burden [10].

### **3. Phenolic compounds as secondary metabolites**

The adoption of plants from aquatic habitat to terrestrial occurred at about 480–300 million years ago and to cope up with this change they adopted protective UV screens called phenolic compounds [11]. They constitute an important set of secondary metabolites which are ubiquitously spread in plant kingdom and the type of phenolic compounds differs in different genre of plant kingdom. Polyphenols are one of the important classes of specialized metabolites that play crucial physiological roles throughout the plant life cycle including responses to stress [12]. Therefore, as an adaptive response to adverse environmental conditions, phenolics are accumulated in various plant tissues which confers evolutionary fitness to the plant. Plants are continuously exposed to various biotic and abiotic stresses like intense light, low temperature nutrient deficiency, microbial infections with

*Effect of Biotic and Abiotic Stresses on Plant Metabolic Pathways DOI: http://dx.doi.org/10.5772/intechopen.99796*

increased free radical and other oxidative species and plant phenolics are compounds which play a defense role by scavenging these high reactive species [13]. Plants adapt themselves their phenolic patterns to a changing environment through the emergence of new genes brought about by gene duplication and mutation and subsequent recruitment for adaptation to specific functions [14]. Many of the genes related to secondary metabolism are duplicated in plant genome and many of these secondary metabolites production demands change in the amounts of precursors supplied by primary metabolism to balance the perturbations in chemical ecology [15, 16]. The induction of secondary metabolism gene expression by biotic and abiotic stress is often mediated by integrating signaling molecules such as salicylic acid, jasmonic acid, and their derivatives [17]. Flavonoids, stilbenes and proanthocyanidins are collectively grouped in polyphenols, the name indicating both the compounds with a second aromatic ring and those arising from the polymerization of flavonoidic/catechin units. The main structure in flavonoids is the flavan nucleus consisting of 15 carbon atoms which are arranged into three rings with two benzene rings (A & B) connected by an oxygen containing pyran ring (C). Various flavonoids differ in the level of oxidation and saturation of the C ring and accordingly are classified into flavanones, flavones, flavanals, flavanols and anthocyanidins. The individual compounds in a particular class of flavonoids differ in the sunstitution pattern of the A and B rings [18]. Due to the heterogeneous structures of these phenolic acids which range from low molecular weight single aromatic ring structure to high molecular weight polymeric compounds, they can be broadly classified into simple and complex phenolics (**Figure 1**).
