**1.2 Structure and types of flavonoids**

The name 'Flavonoids' refers to a group of plant pigments generated mostly from benzo-γ-pyrone (**Figure 1**). Advanced techniques such as [1H-1H]-correlated spectroscopy, [1H]- and [13C]-NMR spectrometry, X-ray diffraction, mass spectrometry, circular dichroism, and optical rotatory dispersion help us to analyze and elucidate the flavonoid structures and configurations [4]. The class of flavonoids primarily comprises of anthocyanidins, proanthocyanins, flavonols, iso-flavonoids, chromones, flavones, iso-flavones, flavanes, flavanones, flavanols, catechins, aurones, benzofurones, and coumarins.

The variability observed in the flavonoids mainly occurs due to differences in the following features:


Flavonoids exhibit a range of beneficial effects in human health like


**Figure 1.** *Basic structure of flavonoid.*

*Flavonoids: Recent Advances and Applications in Crop Breeding DOI: http://dx.doi.org/10.5772/intechopen.107565*


#### **1.3 Role of flavonoids in plants**

Flavonoids are responsible for distinct flavor and color, which draw pollinators, as well as characteristic color and fragrance of flowers. Additionally, they facilitate the germination of seeds and spores, as well as the growth and development of seedlings, by fruit dispersal. Plants can be protected from biotic and abiotic challenges by flavonoids, which also operate as UV filters, signal molecules, phytoalexins, detoxifying agents, and antimicrobial defense components. They are recognized for their ability to frequently play a useful role in the capacity of plants to adapt to heat, to cold, to frost, to drought, and to both. Early advances in floral genetics have mostly been made by mutation techniques that impact flower colors that are produced from flavonoids, and it has been proven that plants that are involved in flavonoid production are capable of functional gene silencing [1].

#### *1.3.1 Role in pigmentation*

Flavonoids play a prominent role in floral coloration, as well as pollinator attractiveness and UV protection [9, 10]. A study on Papaver flowers has revealed that the spatial occurrence of flavonoids is responsible for its wide-range variation in flower color [11]. Approximately 8000 flavonoids contribute to the vibrant colors seen in fruits, herbs, vegetables, and medicinal plants.

#### *1.3.2 Role as a growth regulator*

Flavonoids have the ability to regulate auxin movement and catabolism. Recent research has revealed that flavonoids are capable of modulating protein activity during cell growth [12].

#### *1.3.3 Role in nitrogen metabolism*

Flavonoids, through inducing root nodulation, play a significant role in nitrogen metabolism in nitrogen-fixing plants. Dinitrogen-fixing bacteria, such as the *Rhizobium* strain, exist in symbiosis with leguminous plants and are found in these nodules. The major action of flavonoids is likely to be the stimulation of genes that express proteins necessary by nodule cells, but because they are antioxidative, they are also well adapted to participate in dioxygen removal [4].

#### *1.3.4 Role in combating oxidative stress*

As a result of a variety of biotic and abiotic stimuli causing oxidative stress, reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced in plants. Nearly all of the increase in flavonoid synthesis in plants comes from oxidative stress. Flavonoids like mono-apigenin and mono-kaempferol and dihydroxy B-ringsubstituted (Luteolin and Quercetin) flavonoid glucosides reduce ROS generation,

quench ROS once they have formed, and absorb the UV-B and UV-A wavelengths. When early plants went from the water to the land, flavonoids performed key UV-B screening roles. The type of substitution on distinct rings of flavonoids determines the extent of antioxidant capability and ability to absorb UV wavelengths [12].

### *1.3.5 Role in defense against pathogens and insects*

Flavonoids in plants are also helpful in protecting plants from harmful bacteria and fungus. Catechins and other flavanols potentially act as a plant's defensive mechanism against insects that are damaging to it [13–15].
