**3. Ecology chemicals**

#### **3.1. Pigments and pollination**

Plant compounds that are perceived by humans to have color are generally referred to as "pigments." The three main classes of pigments for coloration in plants are: betalains, carotenoids and flavonoids (anthocyanins) [43, 44]. All three classes of pigments act as visible signals to attract insects, birds and animals for pollination and seed dispersal [11, 43].

The pigments that color most flowers, fruits and seeds are flavonoids, which have the widest color range, from pale-yellow to blue [12]. Anthocyanins occur in all plant tissues and provide a wide range of colors ranging from orange/red to violet/blue [44]. They are formed by glycosides that may have several sugars in position 3; when there are no glycosides, the pigments are called anthocyanidins [12]. In addition to various modifications to their structures, their specific color also depends on pH, copigments and metal ions [11, 45].

The basic chromophore of anthocyanins is the flavylium ion [45, 46]. In acidic medium (pH below 2.5), anthocyanins show intensely reddish coloration or orange due the presence of flavylium cation form. When the pH increases from 2.5 to 4–6, the violet anhydrobase is formed first, but it decolorizes rapidly due the predominance of pseudobase carbinol formed by hydration (**Figure 5**) [11, 39, 47].

Since the flower cell sap is usually weakly acidic, in this pH region, most of the anthocyanins are in colorless form Ref. [47]. Hydration of the flavylium cation, which causes decoloration, may be prevented by formation of a complex between this ion and other substances. This phenomenon is called copigmenting [11, 47]. Such complexes are formed by intermolecular, intramolecular rearrangements and self-association, with organic molecules such as flavonoids, tannins, aromatic acids or metal for chelation [46, 47]. The copigmentation has a stabilizing effect as well as a bathochromic effect on anthocyanins [47]. Various flavonols and flavones act as copigments, with anthocyanins leading to an intensification of flower color [32, 45, 46]. So far, the main pigments targeted for flower and fruit color modifications are anthocyanins that contribute to a variety of colors such as red, pink and blue [44].

Humans recognize the color of a compound by perceiving reflected or transmitted light of wavelengths between 380 and 730 nm, while insects recognize light of shorter wavelengths [43]. Anthocyanins contribute to the UV patterns that are visible to insects and serve to signal flowers and fruits that are attractive to pollinators [45]. The light absorption of anthocyanins extends over most of the spectrum. Particularly, anthocyanins have an intense absorption in the 450–560 nm region (visible region), attributed to the hydroxyl cinnamoyl system of the B ring, while the absorption in the 240–280 nm region (UV region), characteristic of all flavonoids, corresponds to the A ring [32, 39].

Flavonoids: Classification, Biosynthesis and Chemical Ecology http://dx.doi.org/10.5772/67861 9

**Figure 5.** A change in the structure of an anthocyanin in aqueous solution as a function of pH [47].

lipid oxidation products [40]. The radical scavenging properties associated with the structure of flavonoids defend against oxidative stress and in doing so reduce heart disease, prevent cancer and slow down the aging processes in cells responsible for degenerative diseases [40, 42].

Plant compounds that are perceived by humans to have color are generally referred to as "pigments." The three main classes of pigments for coloration in plants are: betalains, carotenoids and flavonoids (anthocyanins) [43, 44]. All three classes of pigments act as visible sig-

The pigments that color most flowers, fruits and seeds are flavonoids, which have the widest color range, from pale-yellow to blue [12]. Anthocyanins occur in all plant tissues and provide a wide range of colors ranging from orange/red to violet/blue [44]. They are formed by glycosides that may have several sugars in position 3; when there are no glycosides, the pigments are called anthocyanidins [12]. In addition to various modifications to their structures, their

The basic chromophore of anthocyanins is the flavylium ion [45, 46]. In acidic medium (pH below 2.5), anthocyanins show intensely reddish coloration or orange due the presence of flavylium cation form. When the pH increases from 2.5 to 4–6, the violet anhydrobase is formed first, but it decolorizes rapidly due the predominance of pseudobase carbinol formed

Since the flower cell sap is usually weakly acidic, in this pH region, most of the anthocyanins are in colorless form Ref. [47]. Hydration of the flavylium cation, which causes decoloration, may be prevented by formation of a complex between this ion and other substances. This phenomenon is called copigmenting [11, 47]. Such complexes are formed by intermolecular, intramolecular rearrangements and self-association, with organic molecules such as flavonoids, tannins, aromatic acids or metal for chelation [46, 47]. The copigmentation has a stabilizing effect as well as a bathochromic effect on anthocyanins [47]. Various flavonols and flavones act as copigments, with anthocyanins leading to an intensification of flower color [32, 45, 46]. So far, the main pigments targeted for flower and fruit color modifications are

Humans recognize the color of a compound by perceiving reflected or transmitted light of wavelengths between 380 and 730 nm, while insects recognize light of shorter wavelengths [43]. Anthocyanins contribute to the UV patterns that are visible to insects and serve to signal flowers and fruits that are attractive to pollinators [45]. The light absorption of anthocyanins extends over most of the spectrum. Particularly, anthocyanins have an intense absorption in the 450–560 nm region (visible region), attributed to the hydroxyl cinnamoyl system of the B ring, while the absorption in the 240–280 nm region (UV region), characteristic of all flavo-

anthocyanins that contribute to a variety of colors such as red, pink and blue [44].

nals to attract insects, birds and animals for pollination and seed dispersal [11, 43].

specific color also depends on pH, copigments and metal ions [11, 45].

**3. Ecology chemicals**

**3.1. Pigments and pollination**

8 Flavonoids - From Biosynthesis to Human Health

by hydration (**Figure 5**) [11, 39, 47].

noids, corresponds to the A ring [32, 39].

The different colors produced by pigments are visible only to animals with the right photoreceptors, and many insects have limited color vision at the red end of the spectrum [40]. Due to the structural diversity of anthocyanins, the presence of one determined anthocyanin in the flower might affect the type of pollinators visiting the plant. The color preferences are different for different pollinators, and blue anthocyanins, for example, appear to attract bees more than red ones. Some butterfly and birds species visit red flowers, suggesting that both groups of animals are attracted to red anthocyanins [12, 40, 48].

A study of anthocyanins in two species of the genus *Schizanthus* Ruiz & Pav (Solanaceae) showed that the hummingbird-pollinated red flowers of *S. grahamii* contained a higher proportion of delphinidin 3-O-rutinoside (anthocyanin), whereas the bee-pollinated bluish-pink flowers of *S. hookeri* contained a higher proportion of petanin-derivatives (anthocyanin) [48].

Flavones and flavonols also contribute to flower color hue. Both groups of compounds comprise unpigmented or pale yellow flavonoids and are mostly invisible to the human eye [43, 44]. Studies on flavonoids by spectroscopy have revealed that most flavones and flavonols exhibit two major absorption bands: band I (320–385 nm) represents the B ring absorption, while band II (250–285 nm) corresponds to the A ring absorption [39]. As they absorb UV, which insects recognize, they give color and patterns to flowers to attract insects [43].

Chalcones and aurones, which provide yellow pigmentation in the flowers of several ornamental species, are relatively rare types of flavonoids [49]. The UV spectra of both compounds are characterized by an intense band I and diminished band II absorption [50]. The major absorption band in chalcones (band I) usually occurs in the range 340–390 nm. Band II is usually a minor peak in the 220–270 nm region, while the long wavelength absorption band in aurones is usually found in the 370–430 nm region. They produce the strongest yellow colors owing to their absorbance at longer wavelengths compared to the other types [50, 51]. Chalcones, flavonols, flavones or anthocyanins usually accumulate in sex organs of flowers, including the pollen. In contrast to man, some insects, especially bees, can perceive in the near ultraviolet (340–380 nm) as well as in the visible region. However, insects are possibly attracted to pollen whose color contrasts against petals due to UV reflective or absorptive flavonoids [46].

Pollination is an essential step in the reproductive process of the world's nearly 300,000 species of flowering plants because it is usually required for the production of seeds. Pollination can result from the action of abiotic forces such as wind and water, but 80% of the Angiosperms rely on animals, including bats, flies, butterflies, beetles and other insects [52]. Such diversity is acquired through evolutionary processes to ensure successful reproduction [44].

#### **3.2. Allelopathy**

The interactions between organisms are fundamental for the determination of plant abundance and distribution pattern in the community, of the productivity of several cultivated species and of the degree of interference on weeds [53]. Weeds are one of the most important factors that impose limitations on the development of agricultural activity in the world and are difficult to eradicate. The success of weeds in different cropping systems is associated, in part, with their ability to produce, store and release to the environment chemicals with allelopathic properties [54].

Allelopathy can be defined as a process by which compounds from the metabolism of a plant are released, preventing the germination, growth and development of other neighboring plants [55]. These compounds are involved in plant-plant interactions or allelopathy [56] and may influence, for example, in the vegetation of a local, in the succession of plants, in the germination of seeds and in the cultures productivity, among others [57]. Among the main groups of compounds with allelopathic potential are highlighted the benzoquinones, coumarins, flavonoids, terpenoids, glycosides, phenolic acids, alkaloids, rotenoids, catechins and tannins [58, 59].

Although flavonoids have many roles in plants, in relation to their role in allelopathy and the inhibition of seedling root growth [56], the activity of flavonoids in plant-plant interactions can be positive or negative [60]. The negative relations are mainly based on inhibiting germination and growth of other plants seedlings [56], as depicted in **Table 1**.

Some flavonoids present a level of phytotoxicity, indicating that allelopathy could be a beneficial function of the flavonoids to the producing plant [65]. Although the relative role of flavonoids in allelopathic interference has been less well-characterized than of some secondary metabolites, some examples of their involvement in autotoxicity and allelopathy are reported


**Table 1.** Flavonoids of different classes with allelopathic potential.

[56]. In a previous study, see [65], presented flavonoids as are at least partly responsible for the strong phytotoxic effects of *Stellera chamaejasme* L. The potential allelopathic behavior may facilitate this weed to become a good competitor against other plant species in the environment.

Allied to the need for understanding the mechanism action of flavonoids, the importance of the study of allelopathy gains more and more attention in agriculture because these interactions could be employed for reducing weed growth.

Biopesticides based on flavonoids displaying allelopathic properties against weeds can potentially be an efficient natural defense against them [62]. In the study [63], the inhibiting activity against weeds of the species *Echinochloa crus-galli, Cyperus difformis* and *Cyperus iria* using the 5,7,4′-trihydroxy-3′,5′-dimethoxyflavone is shown.
