Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consumption, but also as livestock forage and silage, and as soil-enhancing green manure. Legumes produce a botanically unique type of fruit — a simple dry fruit that develops from a simple carpel and usually dehisces (opens along a seam) on two sides.
Most legumes have symbiotic nitrogen-fixing bacteria, Rhizobia, in structures called root nodules. Some of the fixed nitrogen becomes available to later crops, so legumes play a key role in crop rotation.
Terminology
The term pulse, as used by the United Nations' Food and Agriculture Organization (FAO), is reserved for legume crops harvested solely for the dry seed.[1] This excludes green beans and green peas, which are considered vegetable crops. Also excluded are seeds that are mainly grown for oil extraction (oilseeds like soybeans and peanuts),[citation needed] and seeds which are used exclusively for sowing forage (clovers, alfalfa).[citation needed] However, in common usage, these distinctions are not always clearly made, and many of the varieties used for dried pulses are also used for green vegetables, with their beans in pods while young.[2]
The Food and Agriculture Organization (FAO) recognizes 11 primary pulses, excluding green vegetable legumes (such as green peas) and legumes used mainly for oil extraction (for example, soybeans and groundnuts) or used only as seed (such as clover and alfalfa).[3]
- Dry beans[a]
- Kidney bean, navy bean, pinto bean, black turtle bean, haricot bean (Phaseolus vulgaris)
- Lima bean, butter bean (Phaseolus lunatus)
- Adzuki bean, azuki bean (Vigna angularis)
- Mung bean, golden gram, green gram (Vigna radiata)
- Black gram, urad (Vigna mungo)
- Scarlet runner bean (Phaseolus coccineus)
- Ricebean (Vigna umbellata)
- Moth bean (Vigna aconitifolia)
- Tepary bean (Phaseolus acutifolius)
- Dry broad beans[b]
- Horse bean (Vicia faba equina)
- Broad bean (Vicia faba)
- Field bean (Vicia faba)
- Dry peas[c]
- Garden pea (Pisum sativum var. sativum)
- Protein pea (Pisum sativum var. arvense)
- Chickpea (also known as garbanzo and Bengal gram)[d]
- Dry cowpea, black-eyed pea, blackeye bean[e]
- Pigeon pea (aka Arhar/Toor, cajan pea, Congo bean, gandules)[f]
- Lentil[g]
- Bambara groundnut (aka earth pea)[h]
- Vetch, common vetch[i]
- Lupins[j]
- Pulses NES[k] including:
- Lablab, hyacinth bean (Lablab purpureus)
- Jack bean (Canavalia ensiformis), sword bean (Canavalia gladiata)
- Winged bean (Psophocarpus tetragonolobus)
- Velvet bean, cowitch (Mucuna pruriens var. utilis)
- Yam bean (Pachyrhizus erosus)
Distribution
Legumes are widely distributed as the third-largest land plant family in terms of number of species, behind only the Orchidaceae and Asteraceae, with about 751 genera and some 19,000 known species,[4][5] constituting about seven percent of flowering plant species.[6][7]
Ecology
Nitrogen fixation
Many legumes contain symbiotic bacteria called Rhizobia within root nodules of their root systems (plants belonging to the genus Styphnolobium are one exception to this rule). These bacteria have the special ability of fixing nitrogen from atmospheric, molecular nitrogen (N2) into ammonia (NH3).[8] The chemical reaction is: