Ecological and economic importance

The unique ecological role of Fabaceae is in nitrogen fixation. Nitrogen is an element of all proteins and is an essential component in both plant and animal metabolism. Although elemental nitrogen makes up about 80 percent of the atmosphere, it is not directly available to living organisms; nitrogen that can be metabolized by living organisms must be in the form of nitrates or ammonia compounds. Through a mutual benefit arrangement (symbiosis) between legumes and Rhizobium bacteria, nitrogen gas (N₂) is fixed into a compound and then becomes available to the biotic world. The legume plant furnishes a home and subsistence for the bacteria in root nodules. In a complex biosynthetic interaction between the host plant and the bacterium, nitrogen compounds are formed that are used by the host plant. These compounds are also available to other plants after decayed roots (and other plant parts) of the host plant have allowed these nitrogen products to be released into the soil. Animals obtain compound nitrogen by eating plants or other animals.

Consequently, the vegetation of the forests, prairies, and deserts of most of the world is primarily dependent on the legume component of their vegetation and could not exist without it. Only in a few ecosystems—those that include few legume species—have alternative biological nitrogen-fixing arrangements evolved. These include symbiotic relationships between miscellaneous woody species other than legumes, and certain actinomycetes or bacteria and are limited mostly to boreal evergreen forests, certain coastal areas, and acid bogs. Nitrogen fixation by free-living cyanobacteria seems to be important in aquatic ecosystems. On a worldwide scale, however, these alternate arrangements of nitrogen fixation are relatively minor compared with those supported by legumes.

Legume nitrogen fixation is of prime importance in agriculture. Before the use of synthetic fertilizers in the industrial countries, the cultivation of crop plants, with the exception of rice, was dependent on legumes and plant and animal wastes (as manure) for nitrogen fertilization. A common procedure was the use of crop rotation, usually the alternation of a cash grain crop such as corn (maize) with a legume, often alfalfa (Medicago sativa), in the temperate world. Apart from the nitrogen contribution, the legume in this case furnishes animal forage (hay or silage). Pastures or other grazing areas must have legume components, such as a clover (Trifolium), as well as a grass component.

The 20th-century substitution of petroleum-derived synthetic nitrogen fertilizers is partly a consequence of economics in that a cash grain, such as corn or wheat, planted every year provides a higher fiscal return than alternating it with a legume crop. In addition, legume-rhizobium nitrogen fixation is inhibited when the level of nitrogen in the soil is high and is not sufficient for maximum yields of a grass crop. Therefore, in developed countries chemical fertilizers have largely replaced biological fixation in row-crop culture. On a worldwide basis, however, dependence on legumes is still preeminent. Even in the United States, when rangeland and pasture agriculture are included, it has been estimated that nitrogen production by biological fertilizers still exceeds chemical application.

Other benefits accrue from the use of legumes to maintain soil nitrogen. Weed control is facilitated by a crop sequence that alternately changes the growing environment. Such legumes as alfalfa may be harvested for forage (hay or silage) or grazed by livestock. As cover crops, legumes prevent or reduce soil erosion and may be plowed under as “green manure.” Even though starch-producing grasses such as corn are more efficient under favourable conditions in producing energy foods, grain legumes are commonly grown in the tropics because they are more successful in depleted, nitrogen-deficient soils.

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Legume seeds constitute a part of the diet of nearly all humans. Their most vital role is that of supplying most of the protein in regions of high population density and in balancing the deficiencies of cereal protein (Poaceae). Except for the soybean and peanut, the order is not noted for the oil content of the seeds since most seeds have only about 10 percent oil content by weight. The legume seeds generally are highest in carbohydrate compounds, followed by protein and fat. Legumes are thus considered to be energy foods. Most legumes that are used for foods are multipurpose plants, serving for animal forage and soil improvement as well. Some, notably the soybean, are also important industrial crops. Fabaceae contains the more important crop plants, such as soybeans, beans, cowpeas (Vigna), pigeon peas (Cajanus cajan), chick-peas (Cicer arietinum), lentils (Lens culinaris), peas (Pisum sativum), and peanuts.

Forage legumes (which concentrate their vitamins and proteins in their young growing parts) also are grown as animal feed. Their role as such is especially common in countries that can afford the luxury of meat (luxury because livestock typically yield fewer calories than the plants they are fed). Some major forage legumes of the temperate world include clovers, alfalfa, bird’s-foot trefoil (Lotus corniculatus), and vetches. In the tropics or arid regions, some of the important elements of the habitat are species of Glycine (soybean), Stylosanthes, and Desmodium (tick trefoil).

Apart from the legume plants of worldwide importance, the following are examples of locally significant legume species that are cultivated or gathered from the wild. Some would plainly have substantial potential were they subject to genetic evaluation and development through modern breeding techniques. They are still in the same stage as teosinte (the ancestor of corn) or einkorn and emmer (the ancestors of the modern varieties of cultivated wheats) in yield and utilization potential.

Notable among the locally useful plants of the legume family is Vigna subterranea (Bambara groundnut), a leguminous plant that develops underground fruits in the arid lands of Africa. Important too are the seeds of Bauhinia esculenta; they are gathered for the high-protein tubers and seeds. Vigna aconitifolia (moth bean) and V. umbellata (rice bean) are much used in the tropics for forage and soil improvement, and their seeds are palatable and rich in protein. Psophocarpus tetragonolobus (winged bean) is collected in Southeast Asia for the edible fruits and protein-rich tubers. Pachyrhizus (yam bean) is a high-yield root crop of Central America.

Various forms of leucaena (such as Leucaena leucocephala) have been developed for animal forage, firewood, and construction, as well as for the high production of nitrogen that enriches impoverished soils, especially in the Asiatic tropics. Other important plants are acacia, used for animal food (both pods and leaf forage), for soil improvement and revegetation, and as a source of tannin and pulpwood; Cordeauxia edulis (yeheb), an uncultivated desert shrub of North Africa that has been so extensively exploited for food (seeds) that it is in danger of extinction; Ceratonia siliqua (carob), a Mediterranean plant whose fruits are used as animal and human food and in the manufacture of industrial gums; and Tamarindus indica (tamarind) of Africa, now primarily grown in India, which has food and medicinal uses and is also used as an industrial gum.

The soybean is a bushy annual whose seeds are an important source of oil and protein. An edible oil pressed from the seeds is used to make margarine and as a stabilizing agent in the processing of food and the manufacture of cosmetics and pharmaceuticals. The oil is employed in such industrial products as paint, varnish, printing ink, soaps, insecticides, and disinfectants. Oil cakes pressed from the seeds are used as protein concentrate in the mixed-feeds industry. The soybean is a good source of vitamin B and is dried to produce soy milk, which is used in infant formulas. Fermented pods are used in making soy sauce, a flavouring common in Asian cooking.

The peanut, a native of South America, is high in vitamin B complex, proteins, and minerals. The peanut is eaten raw or roasted or is processed into peanut butter. An edible oil is pressed from the seed and is used as a cooking oil and in processing margarine, soap, and lubricants. The oil also is employed by the pharmaceutical industry in making medications. Pressed oil cake is fed to livestock. Peanuts are commercially grown in the United States, Asia, Africa, and Central and South America.

Legumes in general are used to revitalize nutrient-depleted soils, especially abandoned or abused agricultural and grazing lands. A more stringent revegetational challenge is that following strip-mining. Generally speaking, native legumes are common in these habitats because they are better able to thrive in nitrogen-poor soils than other plants.

As mentioned above, the legumes produce secondary compounds of an irritating or poisonous nature that provide protection against predators. Some of these secondary compounds are being studied for their pharmacological potential. They are found in the leaves and fruiting parts and include flavonoids, alkaloids, terpenoids, nonprotein amino acids, and others. Some of these—for example, the amino acid canavanine—may comprise up to 5 percent of the dry weight of seeds. The chemical compound rotenone, which is toxic to a number of organisms, is sufficiently abundant in the roots and stems of certain species belonging to the Papilionoideae that primitive peoples often used these plants to poison fish. More recently it has been shown that serious bone and neural diseases afflicting humans (e.g., lathyrism) and livestock may be caused by the ingestion of unusually large amounts of certain free amino acids. In sheep, ingestion of large quantities of the amino acid mimosine, found in Leucaena glauca and some other species of the Mimosoideae, apparently halts the growth of hair or wool, and in certain cases the fleece itself has been observed to shed. A wide variety of alkaloids are found in the order, most of them restricted to Fabaceae, however. Some alkaloids occur in sufficient concentration in range plants to be poisonous to livestock, especially in species belonging to the large genus Astragalus. Species of Astragalus are commonly referred to as locoweed in North America because, following excessive consumption of these plants, cattle seem to become unmanageable and “go crazy” or “loco.” Astragalus is poisonous in any of three ways: by promoting selenium accumulation, through locoine, and through several nitrogen-containing toxins. In the early 20th century, several African species of Crotalaria were brought to the United States for use as soil-improvement plants. Their poisonous qualities were discovered in connection with animal stock loss, and development was then halted, but several persist as common noxious weeds.

An interesting biochemical component of the legume seed is phytohemagglutinin, a large protein molecule that is specific in its capacity to agglutinate certain human blood types. Approximately 60 percent of the several thousand seeds belonging to this order tested to date contain the compound. Phytohemagglutinin is particularly abundant in the common bean and has been extracted in a relatively pure state on a commercial scale from species belonging to this genus. In addition to its agglutination properties, the compound has been of interest because of its other biological effects. It is toxic to rats, inactivates some human tumor cells, and has beneficial effects in the treatment of aplastic anemia, the shortage of blood cells in humans due to the destruction of blood-forming tissues.

The subfamilies Caesalpinioideae and Mimosoideae do not contain many food crops and are perhaps best known for their shade and ornamental species, such as Cercis siliquastrum (the Judas tree, or redbud), Bauhinia bartlettii (orchid tree), and Acacia farnesiana (sweet acacia), although some of the more rapid-growing weedy species—for example, Leucaena leucocephala (white popinac) and Albizia species—are widely employed as green manure and fodder crops. Acacia species are used extensively in the production of gum exudates and wood, especially in South Africa and Australia, where the species are known as wattle trees.