Douke (Leguminosae sp.), commonly known as the legume, pea, or bean family in English, is a large and economically important flowering plants family. It includes trees, shrubs, perennial or annual herbaceous plants, and can be easily identified by their fruits (Legumes), compounds, and stipules. Many legumes have unique flowers and fruits.

Some legumes, together with grains, certain fruits, and roots of certain tropical plants, have been the main food for humans for thousands of years, and their use is closely related to human evolution (Burkart, 1987). The legume family includes many important agricultural and edible plants, including soybean (Glycine max), kidney bean (Phaseolus), pea (Pisum sativum), chickpea (Cicer arietinum), alfalfa (Medicago sativa), peanut (Arachis hypogaea), carob tree (Ceratonia siliqua), and licorice (Glycyhiza glabra).

Legumes not only have extraordinary species diversity and richness, but also are an important source of starch, protein, oil, and other nutrients in human diets, playing an irreplaceable role in agriculture, horticulture, industry, and other fields (Allen and Allen, 1982; Graham and Vance, 2003). Planted legumes can be classified into many types, including feed, grain, horticulture, pharmaceutical/industrial, leisure/green manure, and timber, and most commercially grown legumes play two or more roles simultaneously.

1 Grain and Feed

The history of leguminous plants is closely related to human civilization. As early as 6,000 BC, leguminous crops have already appeared in Asia, America, and Europe, where legumes became the staple food and an important source of protein for human. Leguminous crops currently grown in agriculture include soybeans, peanuts, broad beans, peas, adzuki beans, mung beans, cowpeas, green beans, and lentils, among others. They usually have the ability to fix nitrogen in the atmosphere, thereby reducing the labor and fertilizer costs of growing leguminous crops, and also supplementing soil fertility, they are commonly used as natural fertilizer. The protein content of their seeds and leaves is relatively higher than that of non-leguminous crops due to their absorption of additional nitrogen during the nitrogen fixation process. Therefore, leguminous crops are ideal for intercropping (Sprent, 2009).

There are two main categories of leguminous plants used as forage. One category, such as purple clover, clover, peas, and peanuts, is sown in pastures for grazing. The other category, such as Leucaena or Albizia, is woody shrubs or trees used directly as animal feed or periodically cut for fodder (livestock feed).

2 Industry

Natural gum is a plant secretion secreted by plants due to damage caused by insects, natural or artificial cuts. These secretions are viscoelastic colloidal solutions containing heterogeneous polysaccharides formed by different sugars, usually containing uronic acid as well. Although there are many species that produce tree gum, the most important of these species is the leguminous plant. Leguminous gum is widely used in the pharmaceutical, cosmetics, food, and textile industries. For example, Arabic gum has cough suppression and anti-inflammatory effects. The most famous gums are tragacanth (Astragalus gummifer), gum arabic (Acacia senegal), and guar gum (Cyamopsis tetragonoloba) (Kuklinski, 2000).

3 Dyes

Species used for dye production include: Logwood (Haematoxylon campechianum), a large thorny tree that can grow up to 15 meters tall. Its bark is thin and soft, the wood is hard, and the heartwood is used to produce red and purple dyes, and the dye known as hematoxylin is derived from this plant. The Brazilwood tree (Caesalpinia echinata) is similar to the former tree, but smaller, with red or purple flowers, and the wood is also used to produce red or purple dyes. Madras thorn (Pithecellobium dulce) is another thorny tree native to Latin America which can grow up to 4 meters tal and has yellow or green small flowers. Its fruit is reddish and used to produce yellow dyes (Márquez, 1999). Indigo dye is extracted from Indigofera tinctoria, native to Asia, while in Central and South America, the dye comes from two species related to this plant, namely Indigofera suffruticosa and Indigofera arrecta. The yellow dye extracted from Butea monosperma is commonly referred to as “forest flame”.

4 Ornamental Plants

For centuries, leguminous plants have been used as ornamental plants around the world due to their height, shape, leaves, and colorful flowers. In Caesalpinioideae, there are Bauhinia forficata, Caesalpinia gilliesii, Caesalpinia spinosa, Ceratonia siliqua, Cercis siliquastrum, and others; in Mimosoideae, there are Acacia caven, Acacia cultriformis, Acacia dealbata, Acacia karroo, Acacia longifolia, Acacia melanoxylon, and others; in Faboideae, there are Clianthus puniceus, Citysus scoparius, Erythrina crista-galli, Erythrina falcata, Laburnum anagyroides, and others (Macaya, 1999). Some leguminous species have special symbolic meanings. Cockspur Coral Tree (Erythrina crista-galli) is the national flower of Argentina and Uruguay, Elephant ear tree (Enterolobium cyclocarpum) is the national tree of Costa Rica, Brazilwood tree (Caesalpinia echinata) has been the national tree of Brazil since 1978, Golden wattle (Acacia pycnantha) is the national flower of Australia, and the Hong Kong Orchid tree (Bauhinia blakeana) is the city flower of Hong Kong.

5 Others

In addition to the aforementioned uses, leguminous plants also have many other important uses that have not been mentioned or discovered. For example, Taiwan Acacia (Acacia confusa Merr.), Siamese Cassia (Cassia siamea Lam.), Royal Poinciana (Delonix regia), and Black Locust (Robinia pseudoacacia L.) are important timber tree species. The seed gum of Sesbania cannabina (Retz.) Poir. and Cyamopsis tetragonoloba can be formulated into water-based fracturing fluids to increase the permeability of oil-bearing formations, thereby increasing oil production in oil wells (Chinese Academy of Sciences, Flora of China Editorial Committee, 1988, Flora of China. Vol. 39, Science Press). Medicinal plants include Catechu (Acacia catechu (L.f.) Willd.), Licorice (Glycyrrhiza uralensis Fisch.), Kudzu (Pueraria lobata (Willd.) Ohwi), and Kadsurae Caulis. In addition, many leguminous plants are also good sources of honey, such as alfalfa, white clover, sweet clover, and various Prosopis species. Many species in Fabaceae are important pollen sources for the large earth bumblebee (Bombus hortorum), which is particularly fond of Trifolium pretense (also known as Red clover) (Goulson et al., 2011).

6 Typical Morphological Characteristics of Fabaceae

Fabaceae have a variety of growth forms, including trees, shrubs, herbs, and even vines. Herbaceous plants can be annuals, biennials, or perennials, with no basal or terminal leaf rosette, and most Fabaceae species are perennial herbs. Fabaceae have an indeterminate inflorescence, which sometimes reduces to a single flower. The flower has a short calyx and a short pistil with a gynoecium, which develops into the fruit of Fabaceae after fertilization. Many Fabaceae plants have tendrils, which are found in erect, epiphytic, or climbing plants, the latter supporting themselves by twining around support structures or using tendrils on the stem or leaves.

6.1 Leaf

The leaves of Fabaceae can be evergreen or deciduous and are usually alternate, rarely opposite. In most cases, the leaves of Fabaceae are odd or even pinnately compound (Caragana and Robinia), with a few having trifoliate (Trifolium and Medicago) or palmate (Lupinus) leaves. The leaves are usually accompanied by stipules, which can be leaf-like (Pisum), spiny (Robinia), or inconspicuous; the entire leaf margin or part has serrations; both leaves and leaflets usually have wrinkled pulvinus to allow for tropic movement. In some species, the leaflets have evolved into tendrils (Vicia) (Dimitri, 1987; Judd et al., 2010).

The leaves of many species have structures that attract ants, which protect the plants from herbivorous insects (a form of mutualism). Extrafloral nectaries are common in Mimosoideae and Caesalpinioideae, and are also found in Faboideae (Vicia sativa). In some Acacia species, modified hollow stipules serve as ant habitats, known as domatia.

6.2 Flower

The flowers of Fabaceae are usually bisexual, with a few being unisexual, radially or bilaterally symmetrical, usually arranged in racemes, umbels, spikes, heads, or panicles. The perianth consists of two whorls; the calyx has (3-)5(6) sepals, which may be separate or fused into a tube, sometimes two-lipped, and rarely reduced or absent; the corolla has (0-)5(6) petals, usually the same number as the sepals, with fewer or none being rare, and they may be separate or fused into a tubular corolla with unequal sizes or forming a butterfly-shaped corolla. The androecium typically has 10 stamens, sometimes 5 or numerous (in Mimosoideae), which may be separate or fused into a tube, with monadelphous or diadelphous stamens, anthers with two locules that open longitudinally or sometimes by pores, and pollen grains that are single or commonly joined into compound pollen. The gynoecium typically consists of a single carpel, with a superior ovary that is 1-loculed and often with a stalk or without, and ovules hanging or ascending in two rows along the ventral suture with lateral placental tufts, with 2 to several ovules per carpel, and the ovules may be horizontal, inverted, or curved. The style and stigma are single, terminal, and apical. Fabaceae are typical insect-pollinated plants, with their flowers often brightly colored to attract pollinators.

In Faboideae, the flowers are bilaterally symmetrical with a butterfly-shaped corolla and a unique structure that is arranged in a descending imbricate manner. The one petal on the adaxial axis is called the flag petal, the two petals on the lateral axis are called the wing petal, and the two abaxil petals are often united, forming a boat shaped structure called the keel petal, which covers the stamens and pistils. There are typically 10 stamens, and their filaments can fuse in different forms, usually with a set of nine stamens plus a single stamen. Various genes in the CYCLOIDEA (CYC)/DICHOTOMA (DICH) family express in the upper (also called dorsal or adaxial) petal. In some species, such as Cadia, these genes express throughout the flower, resulting in radially symmetrical flowers (Citerne et al., 2006).

In Caesalpinioideae, the flowers are usually bilaterally symmetrical, such as Cercis and Bauhinia. The flowers have a false butterfly-shaped corolla, arranged in an ascending imbricate manner. Unlike Faboideae, the upper petal is the innermost petal. However, some species, such as some Senna species, are not symmetrical, with one of the lower petals being larger than its opposite petal, and the style bending to one side.

In Mimosoideae, the flowers are radially symmetrical and arranged in capitulum, in which all the flowers bloom simultaneously. The petals are very small, with over 10 stamens and long colored filaments, which are the most magnificent part of the flower.

6.3 Fruit

The development of the ovary in leguminous plants usually forms pods, which crack or remain unbroken along the suture after maturity, or break into pods containing single seeds. The seeds usually have a leathery or membranous seed coat, which is born on the funicles of different lengths. Sometimes, the funiculus also form a fleshy aril. The embryo is large, and the endosperm is absent or very thin. There are a few species whose fruits have evolved into samaras, loments, follicles, indehiscent legumes, achenes, drupes, or berries.

6.4 Root

Many plants in the Fabaceae family have root nodules. The nodules are the result of a mutualistic symbiosis between bacteria and the plant root system. These bacteria are called rhizobia and have the ability to fix atmospheric nitrogen (N₂) and convert it into a form of nitrogen (NO₃⁻ or NH₃) that can be used by the host plant. This process is called nitrogen fixation.

Authors’ Contribution

LJH was responsible for the literature collection and writing of the paper; ZJ was the project leader and was responsible for revising and finalizing the paper; both authors read and approved the final manuscript.

Acknowledgements

This study was supported by Funding for Cuixi Innovation Research & Development Project of Cuixi Academy of Biotechnology, Zhuji.

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