Legume

Key takeaways

Legume belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Legume to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Legume from memory before moving on to harder problems.

Reference excerpt

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. 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), and seeds which are used exclusively for sowing forage (clovers, alfalfa). 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. 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).

Dry beans 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 Horse bean (Vicia faba equina) Broad bean (Vicia faba) Field bean (Vicia faba) Dry peas Garden pea (Pisum sativum var. sativum) Protein pea (Pisum sativum var. arvense) Chickpea (also known as garbanzo and Bengal gram) Dry cowpea, black-eyed pea, blackeye bean Pigeon pea (aka Arhar/Toor, cajan pea, Congo bean, gandules) Lentil Bambara groundnut (aka earth pea) Vetch, common vetch Lupins Pulses NES 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, constituting about seven percent of flowering plant species.

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). The chemical reaction is:

N2 + 8 H+ + 8 e− → 2 NH3 + H2 Ammonia is converted to another form, ammonium (NH+4), usable by (some) plants, by the following reaction:

NH3 + H+ → NH+4 This arrangement means that the root nodules are sources of nitrogen for legumes, making them relatively rich in nitrogenous amino acids and protein. Nitrogen is therefore a necessary ingredient in the production of proteins. When a legume plant dies in the field, for example following the harvest, all of its remaining nitrogen, incorporated into amino acids inside the remaining plant parts, is released back into the soil. In the soil, the amino acids are converted to nitrate (NO−3), making the nitrogen available to other plants, thereby serving as fertilizer for future crops.

In many traditional and organic farming practices, crop rotation or polyculture involving legumes is common. By alternating between legumes and non-legumes, or by growing both together for part of the growing season, the field can receive a sufficient amount of nitrogenous compounds to produce a good result without adding nitrogenous fertilizer. Legumes are often used as green manure. Sri Lanka developed the polyculture practice known as coconut-soybean intercropping. Grain legumes are grown in coconut (Cocos nuficera) groves in two ways: intercropping or as a cash crop. These are grown mainly for their protein, vegetable oil and ability to uphold soil fertility. However, continuous cropping after 3–4 years decrease grain yields significantly.

Pests and diseases A common pest of grain legumes that is noticed in the tropical and subtropical Asia, Africa, Australia and Oceania are minuscule and highly destructive flies that belong to the family Agromyzidae, dubbed "bean flies". The host range of these flies is very wide amongst cultivated legumes. Infestation of plants starts from germination through to harvest, and they can destroy an entire crop in early stage. Black bean aphids are a serious pest to broad beans and other beans. Common hosts for this pest are fathen, thistle and dock. Pea weevils and bean weevils damage leaf margins, leaving characteristic semi-circular notches. Stem nematodes are very widespread but will be found more frequently in areas where host plants are grown. Common legume diseases include anthracnose, caused by Colletotrichum trifolii; common leaf spot caused by Pseudomonas syringae pv. syringae; crown wart caused by Physoderma alfalfae; downy mildew caused by Peronospora trifoliorum; root rot caused by Fusarium spp.; rust caused by Uromyces striatus; crown and stem rot caused by Sclerotinia trifoliorum; Southern blight caused by Sclerotium rolfsii; pythium (browning) root rot caused by Pythium spp.; fusarium wilt caused by Fusarium oxysporum; root knot caused by Meloidogyne hapla. These are all classified as biotic problems. Abiotic problems include nutrient deficiencies, (nitrogen, phosphorus, potassium, copper, magnesium, manganese, boron, zinc), pollutants (air, water, soil, pesticide injury, fertilizer burn), toxic concentration of minerals, and unfavorable growth conditions.

… excerpt ends here. Continue reading the full article.

Illustrations

Legume: Legumes play a key role in the nitrogen cycle, making nitrates available to other plants in the soil. — Legumes play a key role in the nitrogen cycle, making nitrates available to other plants in the soil.

Worked examples

Example 1 — a first encounter with Legume

Start with the simplest possible case. Write down what Legume claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Legume before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Legume ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Legume

In research: Legume appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Legume in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Legume is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fruit morphology, Legumes, Staple foods, so understanding it makes those chapters shorter.
In everyday life: Look for Legume outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Legume in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Legume means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Legume out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Legume in simple terms?

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.

Why does Legume matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Legume?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Legume.