**5. Role of flavonoids in plants**

Plants are the key source of natural products and plants had already yielded a vast number of phytochemicals and still continue to a major source of biologically active molecules. Numerous plants have already established their potentiality as a source of naturally occurring insecticides, pesticides, fungicides and agro-chemicals as an alternative to toxic and hazardous synthetic chemicals. Owing to ever-increasing awareness to the hazardous side effects of synthetic chemicals, more and more emphasis is being given on the use of products obtained from natural sources so that ecological balance is well-maintained. The WHO has already called for an immediate ban on the use of many synthetic chemicals viz. endosulfan is a dangerous synthetic pesticide that causes severe damage to the eyes, kidneys, and liver. The Government of India had already banned the use of 12 highly toxic and hazardous pesticides and imposed restriction on the use of many others to prevent environmental pollution. To minimize the hazardous effects and to control environmental pollution, attempts are now being made to develop naturally occurring plant-based pesticides. Many phytochemicals such as phytoecdysones, and azadirachtin (from Indian neem) have been reported to possess pesticidal and insecticidal properties and are being widely used in protecting the loss of crop from the attack of insects and parasites in place of their synthetic analogues because of their non-toxic, non-pollutant, readily bio-degradable character and harmless nature of their residues.

As a result of changes in plant growth, conditions, and maturity, there are more than 10,000 types of flavonoid compounds found in vascular plants, which vary in type and quantity according to a variety of factors. There is still a lack of systematic analysis of the flavonoid content of many plant species, which makes it difficult to identify and quantify all the flavonoids humans consume [57]. In order to defend themselves against herbivores, pathogens, oxidative cell damage, and fungal parasites, plants have evolved to synthesize flavonoids [58]. On the other hand, flavonoids act as a stimulant that aids in pollination and guides insects on their way to food sources. For example, flavonoid compounds anthocyanins are responsible for the pink, blue red, light purple and violet colors in flowers, fruits, and vegetables [56, 59].

#### **5.1 Combating oxidative stress of flavonoids**

It has long been reported that flavonoids have a variety of functions in plants [60]. Both abiotic and biotic factors contribute to oxidative stress in plants as a result of ROS being generated in plants. A high level of oxidative stress commonly enhances the synthesis of flavonoids in plants. The pigments are capable of absorbing the most energetic rays of the sun (i.e., UV-B and UV-A), inhibiting ROS production, and quenching ROS once they have been generated [61]. When plants moved from water to soil, flavonoids were primarily responsible for screening UV-B. Different flavonoids have different antioxidant capacities and UV-wavelength-absorbing capabilities based on their substitutions. There is an increase in antioxidant capacity in flavonoids with dihydroxy B rings substituted for these rings, whereas flavonoids with monohydroxy B rings have a greater ability to absorb UV wave lengths. Glycosylation is generally the hallmark of the most reactive hydroxyl groups of flavonoids (7-OH in flavones or 3-OH in flavanols). Flavonoids can be transported from the endoplasmic reticulum to various cellular compartments and secreted from their plasma membrane and cell wall through glycosylation, thereby increasing their solubility in the aqueous cellular environment, protecting the reactive hydroxyl groups from auto-oxidation [62]. Studies have shown that antioxidant flavonoids are found in cells of the mesophyll and in chloroplasts, which generate ROS. Using this method, they are able to easily quench H2O2, hydroxyl radicals, and singlet oxygen [61, 63]. Conditions that restrict CO2 diffusion to carboxylation sites and carboxylation efficiency may exacerbate oxidative stress caused by an excessive amount of excitation energy in chloroplasts [61, 64]. A number of environmental factors can restrict CO2 assimilation, including drought/ salinity, temperature fluctuations, and nutrient scarcity. This can decrease the activity of ROS detoxifying enzymes in the chloroplast [65], which increases the production of antioxidant flavonoids. Flavonoids are highly important for plants under severe stress conditions because of their reducing properties. In addition to their functional roles, dihydroxy B ring substitutes are also highly concentrated [66]. It has been suggested that flavonoids represent a secondary antioxidant defense system in plants under stress [61]. In response to oxidative stress, lipid peroxidation occurs, causing cell membrane degradation. It has been suggested that quercetin-3-*O*-rutinoside (Rutin) interacts with phospholipids polar head at the water lipid interface, increasing membrane inflexibility and thereby protecting membranes from oxidative damage [67].

#### **5.2 Role of flavonoids as growth regulator**

Role of Flavonoids as Growth Regulator: in plant-environment interactions, flavonoids play an essential role. There is evidence that flavonoids control auxin catabolism
