The lentil (Lens culinaris or Lens esculenta) is an edible legume. It is a bushy annual plant known for its lens-shaped seeds. It is about 40 cm (16 in) tall, and the seeds grow in pods, usually with two seeds in each. As a food crop, the majority of world production comes from Canada, India, and Turkey.
The cultivated lentil Lens culinaris was derived from its wild subspecies L. culinaris subsp. orientalis, although other species may also have contributed some genes, according to Jonathan Sauer (Historical Geography of Crop Plants, 2017.) Unlike their wild ancestors, domesticated lentil crops have indehiscent pods and nondormant seeds.
Lentils are the oldest pulse crop known, and among the earliest crops domesticated in the Old World, having been found as carbonized remains alongside human habitations dating to 11,000 BC in Greece. The lentil is indigenous to Western and Central Asia.
Many different names in different parts of the world are used for the crop lentil; lentil (English), adas (Arabic), mercimek (Turkish), messer (Amharic), masser or massur (Hindi) and hiramame (Japanese) are the most common names.
The first use of the word lens to designate a specific genus was in the 16th century by the botanist Tournefort. However, the word "lens" for the lentil is of classical Roman/Latin origin: McGee points out that a prominent Roman family took the name "Lentulus", just as the family name "Cicero" was derived from the chickpea, Cicer arietinum. 
Lens is a small genus which consists of the cultivated L. culinaris and six related wild taxa. Among the different taxa of wild lentils, L. orientalis is considered to be the progenitor of the cultivated lentil and is now generally classified as L. culinaris subsp. orientalis. Therefore, the genus Lens comprises seven taxa in six species:
- Lens culinaris subsp. culinaris (and L. culinaris subsp. orientalis)
- Lens odemensis
- Lens ervoides
- Lens nigricans
- Lens lamottei
- Lens tomentosus
The plant is a diploid, annual, bushy herb of erect, semierect, or spreading and compact growth and normally varies from 30 to 50 cm (10 to 20 in) in height. It has many hairy branches and its stem is slender and angular. The rachis bears 10 to 15 leaflets in five to eight pairs. The leaves are alternate, of oblong-linear and obtuse shape and from yellowish green to dark bluish green in colour. In general, the upper leaves are converted into tendrils, whereas the lower leaves are mucronate. If stipules are present, they are small. The flowers, one to four in number, are small, white, pink, purple, pale purple, or pale blue in colour. They arise from the axils of the leaves, on a slender footstalk almost as long as the leaves. The pods are oblong, slightly inflated, and about 1.5 cm long. Normally, each of them contains two seeds, about 0.5 cm in diameter, in the characteristic lens shape. The seeds can also be mottled and speckled. The several cultivated varieties of lentil differ in size, hairiness, and colour of the leaves, flowers, and seeds.
Lentils are self-pollinating. The flowering begins from the lowermost buds and gradually moves upward, so-called acropetal flowering. About two weeks are needed for all the flowers to open on the single branch. At the end of the second day and on the third day after the opening of the flowers, they close completely and the colour begins to fade. After three to four days, the setting of the pods takes place.
Types can be classified according to their size, whether they are split or whole, or shelled or unshelled. Seed coats can range from light green to deep purple, as well as being tan, grey, brown, black or mottled. Shelled lentils show the colour of the cotyledon which can be yellow, orange, red, or green.
- Nipper (Australia)
- Northfield (Australia)
- Cobber (Australia)
- Digger (Australia)
- Nugget (Australia)
- Aldinga (Australia)
- Masoor (unshelled lentils with a brown seed coat and an orange-red cotyledon)
- Petite crimson(shelled Masoor lentils)
- Red Chief (light tan seed coat and red cotyledon)
Small green/brown-seed coat types:
- Eston Green
- Pardina (Spain)
- Verdina (Spain)
Medium green/brown-seed coat types
- Avondale (USA)
- Matilda (Australia)
Large green/brown-seed coat types:
- Boomer (Australia)
- Brewer's: a large brown lentil which is often considered the "regular" lentil in the United States
- Castellana (Spanish)
- Laird: the commercial standard for large green lentils in western Canada 
- Mosa (Spain)
- Naslada (Bulgaria)
- Pennell (USA)
- Riveland (USA)
- Beluga: black, bead-like, lens-shaped, almost spherical, named for resemblance to beluga caviar. Called Indianhead in Canada.
- Macachiados: big yellow Mexican lentils
- Puy lentils: Lens esculenta puyensis, (small dark speckled blue-green), Protected Designation of Origin name
Saskatchewan is the most productive growing region in Canada (95% of Canadian lentils grown). For 2016, Statistics Canada reported a national production yield of 3.2 million tonnes from 5,700,000 acres (2,300,000 ha) harvested.
The Palouse region of eastern Washington and the Idaho panhandle, with its commercial center at Pullman, Washington, constitutes the most important lentil-producing region in the United States. Montana and North Dakota are also significant lentil growers.
Lentils can grow on various soil types, from sand to clay loam, growing best in deep sandy loam soils with moderate fertility. A soil pH around 7 would be the best. Lentils do not tolerate flooding or water-logged conditions.
Lentils improve the physical properties of soils and increase the yield of succeeding cereal crops. Biological nitrogen fixation or other rotational effects could be the reason for higher yields after lentils.
The conditions under which lentils are grown differ across different growing regions. In the temperate climates lentils are planted in the winter and spring under low temperatures and vegetative growth occurs in later spring and the summer. Rainfall during this time is not limited. In the subtropics, lentils are planted under relatively high temperatures at the end of the rainy season, and vegetative growth occurs on the residual soil moisture in the summer season. Rainfall during this time is limited. In West Asia and North Africa, some lentils are planted as a winter crop before snowfall. Plant growth occurs during the time of snow melting. Under such cultivation, seed yields are often much higher.
Seedbed requirements and sowing
The lentil requires a firm, smooth seedbed with most of the previous crop residues incorporated. For the seed placement and for later harvesting it is important that the surface is not uneven with large clods, stones, or protruding crop residue. It is also important that the soil is made friable and weed-free so that seeding could be done at a uniform depth.
The plant densities for lentils vary between genotypes, seed size, planting time and growing conditions and also from region to region. In South Asia a seed rate of 30–40 kg/ha is recommended. In West Asia countries a higher seed rate is recommended and also leads to a higher yield. The seeds should be sown 3–4 cm deep. In agriculturally mechanized countries, lentils are planted using grain drills, but many other areas still hand broadcast.
Cultivation management, fertilization
In intercropping systems – a practice commonly used in lentil cultivation – herbicides may be needed to assure crop health. Similar to many legume crops, lentils can fix atmospheric nitrogen in the soil with specific rhizobia. Lentils grow well under low fertilizer input conditions, although phosphorus, nitrogen, potassium, and sulfur may be used for nutrient-poor soils.
Below is a list of the most common lentil diseases.
|Aphanomyces root rot|
|Black root rot|
|Black streak root rot|
|Botrytis gray mold|
|Cercospora leaf spot|
|Cylindrosporium leaf spot and stem canker||
|Dry root rot|
|Helminthosporium leaf spot||
|Phoma leaf spot|
|Pythium root and seedling rot|
|Sclerotinia stem rot|
|Wet root rot|
|Cyst nematode||Heterodera ciceri|
|Reniform nematode||Rotylenchulus reniformis|
|Root knot nematode|
|Root lesion nematode||Pratylenchus spp.|
|Stem nematode||Ditylenchus dipsaci|
|Bean (pea) leaf roll virus||Beet western yellows virus|
|Bean yellow mosaic||Bean yellow mosaic virus|
|Broad bean mottle|
|Broad bean stain||Broad bean stain virus|
|Cucumber mosaic||Cucumber mosaic virus|
|Pea seedborne mosaic||Pea seed-borne mosaic virus|
A combination of gravity, screens and air flow is used to clean and sort lentils based on shape and density. After destoning, they may be sorted by a color sorter and then packaged.
A major part of the world’s red lentil production undergoes a secondary processing step. These lentils are dehulled, split and polished. In the Indian subcontinent, this process is called dhal milling. The moisture content of the lentils prior dehulling is crucial to guarantee a good dehulling efficiency. The hull of lentils usually accounts for 6 to 7% of the total seed weight, which is lower than most legumes. Lentil flour can be produced by milling the seeds, like cereals.
Lentils are consumed in many ways. They can be eaten soaked, germinated, boiled, fried and baked. The most common preparation method is boiling. The seeds require a cooking time of 10 to 40 minutes, depending on the variety; shorter for small varieties with the husk removed, such as the common red lentil. Most varieties have a distinctive, earthy flavor. Lentils with husks remain whole with moderate cooking; lentils without husks tend to disintegrate into a thick purée, which leads to quite different dishes. The composition of lentils leads to a high emulsifying capacity which can be even increased by dough fermentation in bread making.
In the Indian subcontinent, lentil curry is part of the everyday diet, eaten with both rice and roti. Boiled lentils and lentil stock are used to thicken most vegetarian curries. They are also used as stuffing in dal parathas and puri for breakfast or snacks. Lentils are also used in many regional varieties of sweets. Lentil flour is used to prepare several different bread varieties, such as papadum.
They are frequently combined with rice, which has a similar cooking time. A lentil and rice dish is referred to in Levantine countries as mujaddara or mejadra. In Iran, rice and lentil is served with fried raisin; this dish is called adas polo. Rice and lentils are also cooked together in khichdi, a popular dish in the Indian subcontinent (India and Pakistan); a similar dish, kushari, made in Egypt, is considered one of two national dishes.
Lentils are used to prepare an inexpensive and nutritious soup all over Europe and North and South America, sometimes combined with some form of chicken or pork. In western countries, cooked lentils are often used in salads.
Lentils are commonly eaten in Ethiopia in a stew-like dish called kik, or kik wot, one of the dishes people eat with Ethiopia's national food, injera flatbread. Yellow lentils are used to make a non-spicy stew, which is one of the first solid foods Ethiopian women feed their babies.
Lentils were a chief part of the diet of ancient Iranians, who consumed lentils daily in the form of a stew poured over rice.
According to the USDA National Nutrient Database, 100 g of raw lentils (variety unspecified) provide 353 calories; the same weight of cooked lentils provides 116 calories. Raw lentils are 8% water, 63% carbohydrates including 11% dietary fiber, 25% protein, and 1% fat (table). Lentils are a rich source (20% or more of the Daily Value, DV) of numerous essential nutrients, including folate (120% DV), thiamin (76% DV), pantothenic acid (43% DV), vitamin B6 (42% DV), phosphorus (40% DV), iron (50% DV), and zinc (35%), among others (table). When lentils are cooked by boiling, protein content declines to 9% of total composition, and B vitamins and minerals decrease due to the overall water content increasing (protein itself is not lost). Lentils have the second-highest ratio of protein per calorie of any legume, after soybeans. Lentils contain the carotenoids, lutein and zeaxanthin, and polyunsaturated fatty acids.
The low levels of readily digestible starch (5%) and high levels of slowly digested starch make lentils of potential value to people with diabetes. The remaining 65% of the starch is a resistant starch classified as RS1. A minimum of 10% in starch from lentils escapes digestion and absorption in the small intestine (therefore called "resistant starch"). Additional resistant starch is synthesized from gelatinized starch, during cooling, after the lentils were cooked.
Lentils also have antinutrient factors, such as trypsin inhibitors and a relatively high phytate content. Trypsin is an enzyme involved in digestion, and phytates reduce the bioavailability of dietary minerals. The phytates can be reduced by prolonged soaking and fermentation or sprouting.
Although lentils have been an important crop for centuries, lentil breeding and genetic research has a relatively short history compared to many other crops. Since the inception of The International Center for Agriculture Research in the Dry Areas (ICARDA) breeding programme in 1977 significant gains have been made. It supplies landraces and breeding lines for countries around the world, supplemented by other programmes in both developing (e.g. India) and developed (e.g. Australia and Canada) countries. In recent years, such collaborations among breeders and agronomists are becoming increasingly important.
The focus lies on high yielding and stable cultivars for diverse environments to match the demand of a growing population. In particular, progress in quantity and quality as well as in the resistance to disease and abiotic stresses are the major breeding aims. Several varieties have been developed applying conventional breeding methodologies. Serious genetic improvement for yield has been made, however, the full potential of production and productivity could not yet be tapped due to several biotic and abiotic stresses.
Wild Lens species are a significant source of genetic variation for improving the relatively narrow genetic base of this crop. The wild species possess many diverse traits including disease resistances and abiotic stress tolerances. The above-mentioned L. nigricans and L. orientalis possess morphological similarities to the cultivated L. culinaris. But only L. culinaris and L. culinaris subsp. orientalis are crossable and produce fully fertile seed. Between the different related species hybridisation barriers exist. According to their inter-crossability Lens species can be divided into three gene pools:
- Primary gene pool: L. culinaris (and L. culinaris subsp. orientalis) and L. odemensis
- Secondary gene pool: L. ervoides and L. nigricans
- Tertiary gene pool: L. lamottei and L. tomentosus
Crosses generally fail between members of different gene pools. However, plant growth regulators and/or embryo rescue allows the growth of viable hybrids between groups. Even if crosses are successful, many undesired genes may be introduced as well in addition to the desired ones. This can be resolved by using a backcrossing programme. Thus, mutagenesis is crucial to create new and desirable varieties. According to Yadav et al. other biotechnology techniques which may impact on lentil breeding are micro-propagation using meristamatic explants, callus culture and regeneration, protoplast culture and doubled haploid production.
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The primary progenitor of the cultigen is evidently L. orientalis native to the Near East; the other species may have contributed some genetic diversity to the crop. L. culinaris (L. esculentus), the domesticate, differs from the wild species in having indehiscent pods, due to a single recessive gene, and nondormant seeds.
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