**4. Fabaceae**

#### **4.1 Taxonomy**

The pea family Fabaceae, also known as Leguminosae, is the third largest family of flowering plants after the Orchidaceae (orchid family) and the Asteraceae (aster family), representing about 7% of the global number of flowering plant species [69]. The Fabaceae consists of more than 700 genera and about 20,000 species of annual, biennial, or perennial trees, shrubs, herbaceous plants, vines, and lianas, which are encountered in all ecosystems throughout the world except Antarctica and the high Arctic [69, 70]. Most woody trees are found in tropical regions, while the herbaceous plants and shrubs are predominant outside the tropics [70]. Fabaceae members are readily recognizable by their fruits known as legumes or pods, which split open as they dry, releasing the seeds, and by their compound, stipulated leaves [71].

The leaves of many species (such as those of the common vetch *Vicia sativa* L.) have glands that produce nectar (extrafloral nectaries) through which they attract ants, which protect them from attacks by herbivores [72]. In other species (such as some in the genus *Acacia*), the stipules (outgrowths on both sides of the base of the

#### *Phenolic Compounds and Antioxidant Activities of Eight Species of Fabaceae… DOI: http://dx.doi.org/10.5772/intechopen.106076*

leafstalk) are modified to tiny chambers called domatia, which accommodate "body guard" ants [73]. Many Fabaceae (such as species of groundnut in the genus *Apios*) also host symbiotic bacteria in their roots—called rhizobia—which convert atmospheric nitrogen into a form that they can use for their metabolism (such as nitrate or ammonia) in a process referred to as nitrogen fixation [74]. The flowers of most species are conspicuous and colorful to attract pollinator insects [71]. The ovary itself matures into a legume or pod that encloses the seeds [71].

The Fabaceae includes six subfamilies [75], namely the Faboideae (or Papilionoideae), Caesalpinioideae, Detarioideae, Cercidoideae, Dialioideae, and Duparquetioideae [75]. The largest subfamily is that of the cosmopolitan Faboideae that harbors 503 genera and about 14,000 species including species of milkvetch in the genus *Astragalus*, species of lupin in the genus *Lupinus*, and species of pea in the genus *Pisum* [75]. The pantropical subfamily Caesalpinioideae includes about 4400 species in 148 genera such as the peacock flower *Caesalpinia pulcherrima* (L.) Sw., the candle bush *Senna alata* (L.) Roxb., the shy plant *Mimosa pudica* L., and the soap pod *Senegalia tenuifolia* (L.) Britton & Rose [75].

The subfamilies Detarioideae and Cercidoideae are mainly tropical and include 84 genera and about 760 species, and 12 genera and about 335 species, respectively [75]. Well-known examples in the Detarioideae are the ornamental pride of Burma *Amherstia nobilis* Wall and the tamarind *Tamarindus indica* L. that bears edible fruit. Renowned species in the Cercidoideae are well-appreciated ornamentals such as the pom pom orchid tree *Bauhinia divaricata* L. and the Judas tree *Cercis siliquastrum* L. [75]. The 85 species in 17 genera of the subfamily Dialioideae are widespread throughout the tropics [75]. A well-known example is the western African velvet tamarind *Dialium cochinchinense* Pierre, the velvety black pods of which contain a vitamin-rich acidic pulp that is chewed to relieve thirst or macerated in water to produce a beverage [75]. The subfamily Duparquetioideae is the smallest, consisting of one genus and one species, the liana *Duparquetia orchidacea* Baill that is native to western and central Africa [75].

The five largest genera of the Fabaceae family are *Astragalus* (milkvetches, subfamily Faboideae; over 3000 species), *Acacia* (acacias, subfamily Caesalpinioideae; over 1000 species), *Indigofera* (true indigos, subfamily Faboideae; around 700 species), *Crotalaria* (rattlepods, subfamily Faboideae; around 700 species), and *Mimosa* (sensitive plants or touch-me-nots, subfamily Caesalpinioideae; around 400 species), which constitute about a quarter of all legume species [75].

#### **4.2 Economic value**

Together with cereals, some vegetables and fruits, roots and tubers, oil-bearing crops, and sugar crops, various Fabaceae have been a staple food for humans for millennia, and their use and subsequent domestication and cultivation have been critical to the development of human civilization settlements [76, 77]. There are records dating the use of several species and varieties of beans in Asia, the Americas, and Europe to about 6000 BC, when they were becoming an essential staple as a source of protein (see, for instance, references [78, 79]). Contributing to the importance of the Fabaceae to human civilization were their extraordinary diversity and abundance and the broad variety of other uses they can be put to, ranging from ornamentals to medicines [69, 70]. In fact, species of Fabaceae are still among the economically and culturally most important plants in the world, providing foods, natural fertilizers, and forage; medicines; ornamentals; as well as materials for the pharmaceutical, cosmetic, and textile industries (see, for instance, reference [80]).

Examples of Fabaceae that are food crops of global importance are *G. max* (soybean), *Phaseolus* (beans), *Pisum sativum* (pea), *Cicer arietinum* (chickpeas), *Medicago sativa* (alfalfa), *A. hypogaea* (peanut), *Ceratonia siliqua* (carob), and *Glycyrrhiza glabra* (liquorice) [81]. Notably, the Fabaceae plant family is the second most important economic producer of crop plants after the Poaceae, the rice family [80]. Furthermore, the ability of Fabaceae to fix atmospheric nitrogen makes them very suitable as natural fertilizers to replenish soil that has been depleted of nitrogen [82]. A few species used for this purpose are leadtrees (*Leucaena* spp.) and riverhemps (*Sesbania* spp.) [82]. The additional nitrogen they receive increases their protein content, making some of them (such as the alfalfa *M. sativa* L. as well as clovers (*Trifolium* spp.), vetches (*Vicia* spp.), and peanut-like species (*Arachis* spp.) suitable as fodder for livestock [83].

Examples of Fabaceae with therapeutic properties are gum Arabic from the gum acacia *Senegalia senegal* (L.) Britton that has antitussive and anti-inflammatory properties [84] and tragacanth from *Astragalus gummife*r Labill that can be used as a demulcent in burn wounds [85]. Other species of Fabaceae are used for the production of vegetable oils for cooking. Well-known examples are the oils extracted from the soya bean *G. max* [86] and the peanut *A. hypogaea* [87].

Still other Fabaceae members are industrially farmed to produce dyes. Examples are the logwood *Haematoxylon campechianum* L., the heartwood of which produces red and purple dyes such as the histological stain hematoxylin [88], and the true indigo *Indigofera tinctoria* L., the leaves of which give the blue dye indigotin [89]. Furthermore, the roots of species in the genus *Derris* such as *D. elliptica* found in Southeast Asia and the southwest Pacific islands are a source of the strong insecticide rotenone [90]. And well-known ornamentals are the cockspur coral tree *Erythrina crista-galli* L., the national tree of Argentina, and the national flower of Argentina and Uruguay [91], and the Chinese wisteria *Wisteria sinensis* (Sims) DC that is a much appreciated ornamental vine [92].

#### **4.3 Phenolic compounds and antioxidant activity**

In addition to the applications mentioned above, many species of Fabaceae are traditionally used for medicinal or invigorating purposes (as mentioned in Section 6 of this chapter). The pharmacological activities have been associated with the abundant presence in the plants of certain bioactive ingredients—particularly phenolic compounds such as phenolic acids, (iso)flavonoids, and anthocyanins—with relatively high antioxidant activity [93–96]. In fact, the phenolic compounds in many species of Fabaceae—mostly isoflavones such as genistein and daidzein—are involved in a variety of physiological and metabolic processes that are relevant to human health [97]. The seeds are often not only highly nutritious, but also contain the majority of the phenolics [97–102]. Several of the phenolics elicited high antioxidant potential, displaying the ability to scavenge free radicals, and the ability to interact with proteins [97, 98], as well as a diversity of pharmacological activities including, among others, antiinflammatory, vasodilatory, analgesic, antimicrobial, anti-allergenic, cardioprotective, anti-atherogenic, anticarcinogenic, and immunomodulating activities [97, 98].

However, as mentioned before, the Fabaceae comprise almost 20,000 species, and much of the information on the antioxidant activity and phenolic content extrapolated to the entire plant family is based on investigations with a relative handful of species (see, for instance, references [22, 23, 103–106]). Nevertheless, despite variable antioxidant activities among species, these studies have suggested a good correlation between total phenolic content and antioxidant activity [22, 23, 103–106].

*Phenolic Compounds and Antioxidant Activities of Eight Species of Fabaceae… DOI: http://dx.doi.org/10.5772/intechopen.106076*

The Fabaceae also constitute the greatest number of selenium-hyperaccumulating species, *that is*, plants that accumulate selenium in their cells at concentrations in excess of 1000 mg per kg dry weight [107]. Many selenium hyperaccumulators in this family belong to the genus *Astragalus* (milkvetches, subfamily Faboideae), the largest Fabaceae genus with over 3000 species of herbs and small shrubs [108]. The members of the much smaller genus *Neptunia* (subfamily Caesalpinioideae) also hyperaccumulate selenium [109].
