**1. Introduction**

Nature is an extremely rich source of highly diverse and innovative chemical structures with a variety of structural arrangement and interesting biological activities, which have played a significant role in the process of drug discovery and design. Chemistry of Natural Products has lately undergone explosive growth; natural products are of much interest and of promise in the present day research directed particularly toward drug-design and drug-discovery. Much research works were already carried out and also intensive works are now going on world-wide in the perspective of academic as well as pharmacological/therapeutic scenario. Statistically, only less than ~10–15% of the plants have been investigated so far; a major portion of them is still being left. To gather more knowledge on the natural availability of chemical compounds, their structural variety, properties, and the isolated compounds for detailed studies in regards to biological and pharmacological potentials, more and more research is demanded for the exploration of chemical nature of plants, particularly those ones which have been used as traditional medicines all over the world. Hence, the present investigator has been motivated to undertake this work on some plants traditionally used as medicine in India.

Research into secondary metabolism has long centered on flavonoids. Scientists from a wide variety of fields are interested and intrigued by flavonoids, which are

widely found throughout the plant kingdom. Over the past few years, it has been revealed that plant flavonoids play a vital role in our lives and in the health of our plants. As a result of ongoing research on chemistry, occurrence, natural distribution and biological function of flavonoids, a number of reviews have already been published time to time [1–10]. A PubMed search incorporating the term "flavonoid" returns more than 58,180 articles from last 5 years. The role of flavonoids in plants has received considerable attention in recent decades [11].

Human culture is facing the greatest threat because of global climate change. Increasing global food prices and global warming put the future of humanity at risk. Scientists from NASA's Goddard Institute for Space Studies (GISS) estimate that global temperatures have risen by around 1°C since 1880 [12]. Every 2°C increase in global temperature could annihilate up to a hundred million people and wipe out up to a million species [13]. In addition to using fossil fuels to generate energy, agricultural activities are among the biggest contributors to climate change through the emission of greenhouse gases [14]. In spite of the convenience, ease of use, and rapid soil nutrient recharge, commercial fertilizers have become viewed as a source of toxic and residual soil issues. Using less mineral fertilizer may lower GHG emissions by 20% [15]. Global warming has made it necessary to rethink outdated and ineffective policies. Ecofriendly farming practices and a more sustainable agricultural system are urgently needed. Bio-based products, for example, might usher in organic farming, bio-fertilizers, and bio-control, all of which would be significant steps toward assuring global food security in the long run. Flavonoids are one type of biostimulant discussed in this chapter, and their role in sustainable agriculture. The flavonoids are an important class of polyphenolic secondary metabolites involved in plant physiological function, and show protection against biotic and abiotic stresses, including ultraviolet radiation, salt stress, and drought [16–18], at least in part by detoxifying the reactive oxygen species (ROS) produced when plants are under stress conditions [19]. Flavonoids may help protect Mediterranean endemic species from UV radiation and drought stress, as evidenced by recent studies that show polyphenol concentrations fluctuate monthly with the maximum values occurring at midday during the summer when drought, temperature, and UV radiation are high [20, 21]. The flavonoids in some plants play a critical role in plant defense and growth. There are several flavonoids that comprise plant pigments, including anthocyanins (red, orange, blue, and purple pigments); chalcones and aurones (yellow pigments); and flavonols and flavones (white and pale yellow pigments), which contribute to a diverse range of plant colors [22]. The flavonoids are also crucial in symbiotic associations between plants and microbes, such as rhizobial and arbuscular mycorrhizal symbioses [23]. As a signaling compound, certain flavonoids trigger the induction of nodule induction in rhizobia, which is the first step in legume-rhizobia symbiotic relationships [24]. In addition to preventing pests and pathogens, some flavonoids have antimicrobial properties [25]. The color pigments contained in some classes of flavonoids make leaf and flower petals distinctive, aiding plants in attracting pollinators [26]. Further, flavonoids have indirect effects on nutrient availability and supply since they enhance mycorrhizal symbiosis and enhance rhizosphere colonization by beneficial microbes [27].
