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Lettuce (''Lactuca sativa'') is an
annual plant An annual plant is a plant that completes its life cycle, from germination to the production of seeds, within one growing season, and then dies. The length of growing seasons and period in which they take place vary according to geographical ...
of the family Asteraceae. It is most often grown as a leaf vegetable, but sometimes for its stem and seeds. Lettuce is most often used for salads, although it is also seen in other kinds of food, such as soups, sandwiches and wraps; it can also be grilled. One variety, celtuce (asparagus lettuce), is grown for its stems, which are eaten either raw or cooked. In addition to its main use as a leafy green, it has also gathered religious and medicinal significance over centuries of human consumption. Europe and North America originally dominated the market for lettuce, but by the late 20th century the consumption of lettuce had spread throughout the world. , world production of lettuce and chicory was 27 million tonnes, 56percent of which came from China. Lettuce was originally farmed by the ancient Egyptians, who transformed it from a plant whose seeds were used to obtain oil into an important food crop raised for its succulent leaves and oil-rich seeds. Lettuce spread to the Greeks and Romans; the latter gave it the name , from which the English ''lettuce'' is derived. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including several herbals. The 16th through 18th centuries saw the development of many varieties in Europe, and by the mid-18th century, cultivars were described that can still be found in modern gardens. Generally grown as a
hardy Hardy may refer to: People * Hardy (surname) * Hardy (given name) * Hardy (singer), American singer-songwriter Places Antarctica * Mount Hardy, Enderby Land * Hardy Cove, Greenwich Island * Hardy Rocks, Biscoe Islands Australia * Hardy, Sout ...
annual, lettuce is easily cultivated, although it requires relatively low temperatures to prevent it from flowering quickly. It can be plagued by numerous
nutrient deficiencies Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues ...
, as well as insect and mammal pests, and fungal and bacterial diseases. ''L. sativa''
crosses Crosses may refer to: * Cross, the symbol Geography * Crosses, Cher, a French municipality * Crosses, Arkansas, a small community located in the Ozarks of north west Arkansas Language * Crosses, a truce term used in East Anglia and Lincolnshire ...
easily within the species and with some other species within the genus ''Lactuca''. Although this trait can be a problem to home gardeners who attempt to save seeds, biologists have used it to broaden the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
of cultivated lettuce varieties. Lettuce is a rich source of vitamin K and vitamin A, and a moderate source of folate and iron. Contaminated lettuce is often a source of bacterial, viral, and parasitic outbreaks in humans, including ''
E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' and ''
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
''.


Taxonomy and etymology

''Lactuca sativa'' is a member of the '' Lactuca'' (lettuce) genus and the Asteraceae (
sunflower The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...
or
aster Aster or ASTER may refer to: Biology * ''Aster'' (genus), a genus of flowering plants ** List of ''Aster'' synonyms, other genera formerly included in ''Aster'' and still called asters in English * Aster (cell biology), a cellular structure shap ...
) family. The species was first described in 1753 by Carl Linnaeus in the second volume of his ''
Species Plantarum ' (Latin for "The Species of Plants") is a book by Carl Linnaeus, originally published in 1753, which lists every species of plant known at the time, classified into genera. It is the first work to consistently apply binomial names and was the ...
''. Synonyms for ''L. sativa'' include ''Lactuca scariola'' ''sativa'', ''L. scariola'' ''integrata'' and ''L. scariola'' ''integrifolia''. ''L. scariola'' is itself a
synonym A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
for '' L. serriola'', the common wild or prickly lettuce. ''L. sativa'' also has many identified taxonomic
groups A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...
,
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...
and varieties, which delineate the various
cultivar group A Group (previously cultivar-groupInternational Code of Nomenclature for Cultivated Plants, 4th edition (1969), 5th edition (1980) and 6th edition (1995)) is a formal category in the ''International Code of Nomenclature for Cultivated Plants'' ('' ...
s of domesticated lettuce. Lettuce is closely related to several ''Lactuca'' species from southwest Asia; the closest relationship is to ''L. serriola'', an aggressive weed common in temperate and subtropical zones in much of the world. The Romans referred to lettuce as ( meaning "dairy" in Latin), an allusion to the white substance, latex, exuded by cut stems. The name ''Lactuca'' has become the genus name, while (meaning "sown" or "cultivated") was added to create the species name.Katz and Weaver, p. 376. The current word ''lettuce'', originally from Middle English, came from the Old French or , which derived from the Roman name. The name ''romaine'' came from the variety of lettuce grown in the Roman papal gardens, while ''cos'', another term for romaine lettuce, came from the earliest European seeds of the type from the Greek island of Kos, a center of lettuce farming in the Byzantine period.Weaver, p. 172.


Description

Lettuce's native range spreads from the Mediterranean to Siberia, although it has been transported to almost all areas of the world. Plants generally have a height and spread of . The leaves are colorful, mainly in the green and red color spectrums, with some variegated varieties. There are also a few varieties with yellow, gold or blue-teal leaves. Lettuces have a wide range of shapes and textures, from the dense heads of the iceberg type to the notched, scalloped, frilly or ruffly leaves of leaf varieties. Lettuce plants have a root system that includes a main taproot and smaller secondary roots. Some varieties, especially those found in the United States and Western Europe, have long, narrow taproots and a small set of secondary roots. Longer taproots and more extensive secondary systems are found in varieties from Asia. Depending on the variety and time of year, lettuce generally lives 65–130 days from planting to harvesting. Because lettuce that flowers (through the process known as " bolting") becomes bitter and unsaleable, plants grown for consumption are rarely allowed to grow to maturity. Lettuce flowers more quickly in hot temperatures, while freezing temperatures cause slower growth and sometimes damage to outer leaves. Once plants move past the edible stage, they develop flower stalks up to high with small yellow blossoms. Like other members of the tribe Cichorieae, lettuce inflorescences (also known as
flower heads A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...
or capitula) are composed of multiple florets, each with a modified
calyx Calyx or calyce (plural "calyces"), from the Latin ''calix'' which itself comes from the Ancient Greek ''κάλυξ'' (''kálux'') meaning "husk" or "pod", may refer to: Biology * Calyx (anatomy), collective name for several cup-like structures ...
called a pappus (which becomes the feathery "parachute" of the fruit), a corolla of five petals fused into a
ligule A ligule (from "strap", variant of ''lingula'', from ''lingua'' "tongue") is a thin outgrowth at the junction of leaf and leafstalk of many grasses (Poaceae) and sedges. A ligule is also a strap-shaped extension of the corolla, such as that of a ...
or strap, and the reproductive parts. These include fused
anther The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filam ...
s that form a tube which surrounds a style and
bipartite Bipartite may refer to: * 2 (number) * Bipartite (theology), a philosophical term describing the human duality of body and soul * Bipartite graph, in mathematics, a graph in which the vertices are partitioned into two sets and every edge has an en ...
stigma. As the anthers shed
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
, the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube. The ovaries form compressed, obovate (teardrop-shaped) dry fruits that do not open at maturity, measuring 3 to 4 mm long. The fruits have 5–7 ribs on each side and are tipped by two rows of small white hairs. The pappus remains at the top of each fruit as a dispersal structure. Each fruit contains one seed, which can be white, yellow, gray or brown depending on the variety of lettuce. The domestication of lettuce over the centuries has resulted in several changes through selective breeding: delayed bolting, larger seeds, larger leaves and heads, better taste and texture, a lower latex content, and different leaf shapes and colors. Work in these areas continues through the present day.Davey, et al., pp. 222–225. Scientific research into the genetic modification of lettuce is ongoing, with over 85 field trials taking place between 1992 and 2005 in the European Union and the United States to test modifications allowing greater
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
tolerance, greater resistance to insects and fungi and slower bolting patterns. However, genetically modified lettuce is not currently used in commercial agriculture.


History

Lettuce was first cultivated in ancient Egypt for the production of oil from its seeds. The plant was probably selectively bred by the Egyptians into a plant grown for its edible leaves,Katz and Weaver, pp. 375–376. with evidence of its cultivation appearing as early as 2680 BC. Lettuce was considered a sacred plant of the reproduction god Min, and was carried during his festivals and placed near his images. The plant was thought to help the god "perform the sexual act untiringly". Its use in religious ceremonies resulted in the creation of many images in tombs and wall paintings. The cultivated variety appears to have been about tall and resembled a large version of the modern romaine lettuce. These upright lettuces were developed by the Egyptians and passed to the Greeks, who in turn shared them with the Romans. Around 50 AD, Roman agriculturalist Columella described several lettuce varieties – some of which may have been ancestors of today's lettuces.Weaver, pp. 170–172. Lettuce appears in many medieval writings, especially as a medicinal herb. Hildegard of Bingen mentioned it in her writings on medicinal herbs between 1098 and 1179, and many early herbals also describe its uses. In 1586,
Joachim Camerarius Joachim Camerarius (12 April 150017 April 1574), the Elder, was a Germans, German classical scholar. Life He was born in Bamberg, in the Prince-Bishopric of Bamberg. His family name was Liebhard, but he was generally called Kammermeister, previo ...
provided descriptions of the three basic modern lettuces – head lettuce, loose-leaf lettuce, and romaine (or cos) lettuce. Lettuce was first brought to the Americas from Europe by Christopher Columbus in the late 15th century. Between the late 16th century and the early 18th century, many varieties were developed in Europe, particularly Holland. Books published in the mid-18th and early 19th centuries describe several varieties found in gardens today.Weaver, pp. 172–173. Due to its short lifespan after harvest, lettuce was originally sold relatively close to where it was grown. The early 20th century saw the development of new packing, storage and shipping technologies that improved the lifespan and transportability of lettuce and resulted in a significant increase in availability. During the 1950s, lettuce production was revolutionized with the development of
vacuum cooling Vacuum cooling is a rapid cooling technique for any porous product that has free water and uses the principle of evaporative cooling. Vacuum cooling is generally used for cooling food products that have a high water content and large porosities, ...
, which allowed field cooling and packing of lettuce, replacing the previously used method of ice-cooling in packing houses outside the fields. Lettuce is very easy to grow, and as such has been a significant source of sales for many
seed companies Seed companies produce and sell seeds for flowers, fruits and vegetables to commercial growers and amateur gardeners. The production of seed is a multibillion-dollar business, which uses growing facilities and growing locations worldwide. While ...
. Tracing the history of many varieties is complicated by the practice of many companies, particularly in the US, of changing a variety's name from year to year. This practice is conducted for several reasons, the most prominent being to boost sales by promoting a "new" variety, or to prevent customers from knowing that the variety had been developed by a competing seed company. Documentation from the late 19th century shows between 65 and 140 distinct varieties of lettuce, depending on the amount of variation allowed between types – a distinct difference from the 1,100 named lettuce varieties on the market at the time. Names also often changed significantly from country to country. Although most lettuce grown today is used as a vegetable, a minor amount is used in the production of
tobacco-free cigarettes Herbal cigarettes (also called tobacco-free cigarettes or nicotine-free cigarettes) are cigarettes that usually do not contain any tobacco or nicotine, instead being composed of a mixture of various herbs and/or other plant material. However, Chi ...
; however, domestic lettuce's wild relatives produce a leaf that visually more closely resembles tobacco.


Cultivation

A
hardy Hardy may refer to: People * Hardy (surname) * Hardy (given name) * Hardy (singer), American singer-songwriter Places Antarctica * Mount Hardy, Enderby Land * Hardy Cove, Greenwich Island * Hardy Rocks, Biscoe Islands Australia * Hardy, Sout ...
annual, some varieties of lettuce can be overwintered even in relatively cold climates under a layer of straw, and older,
heirloom varieties An heirloom plant, heirloom variety, heritage fruit (Australia and New Zealand), or heirloom vegetable (especially in Ireland and the UK) is an old cultivar A cultivar is a type of cultivated plant that people have selected for desired trai ...
are often grown in cold frames. Lettuces meant for the cutting of individual leaves are generally planted straight into the garden in thick rows. Heading varieties of lettuces are commonly started in flats, then
transplant Transplant or Transplantation may refer to: Sciences *Transplanting a plant from one location to another *Organ transplantation, moving an organ from one body to another *Transplant thought experiment, an experiment similar to Trolley problem *Tra ...
ed to individual spots, usually apart, in the garden after developing several leaves. Lettuce spaced farther apart receives more sunlight, which improves color and nutrient quantities in the leaves. Pale to white lettuce, such as the centers in some iceberg lettuce, contain few nutrients. Lettuce grows best in full sun in loose, nitrogen-rich soils with a pH of between 6.0 and 6.8. Heat generally prompts lettuce to
bolt The BOLT Browser was a web browser for mobile phones including feature phones and smartphones that can run Java ME applications. The BOLT Browser was offered free of charge to consumers and by license to mobile network operators and handset manuf ...
, with most varieties growing poorly above ; cool temperatures prompt better performance, with being preferred and as low as being tolerated. Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Temperatures above will generally result in poor or non-existent germination of lettuce seeds. After harvest, lettuce lasts the longest when kept at and 96 percent humidity. The high water content of lettuce (94.9 percent) creates problems when attempting to preserve the plant – it cannot be successfully frozen, canned or dried and must be eaten fresh. In spite of its high water content, traditionally grown lettuce has a low water footprint, with 237 liters of water required for each kilogram of lettuce produced. Hydroponic growing methods can reduce this water consumption by nearly two orders of magnitude. Lettuce varieties will cross with each other, making spacing of between varieties necessary to prevent contamination when saving seeds. Lettuce will also cross with ''
Lactuca serriola ''Lactuca serriola'', also called prickly lettuce, milk thistle (not to be confused with ''Silybum marianum'', also called milk thistle), compass plant, and scarole, is an annual or biennial plant in the tribe Cichorieae within the family Asterac ...
'' (wild lettuce), with the resulting seeds often producing a plant with tough, bitter leaves. Celtuce, a lettuce variety grown primarily in Asia for its stems, crosses easily with lettuces grown for their leaves.Weaver, pp. 175–176. This propensity for crossing, however, has led to breeding programs using closely related species in ''Lactuca'', such as ''L. serriola'', '' L. saligna'', and '' L. virosa'', to broaden the available
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
. Starting in the 1990s, such programs began to include more distantly related species such as '' L. tatarica''. Seeds keep best when stored in cool conditions, and, unless stored
cryogenically In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
, remain viable the longest when stored at ; they are relatively short lived in storage. At room temperature, lettuce seeds remain viable for only a few months. However, when newly harvested lettuce seed is stored cryogenically, this life increases to a half-life of 500 years for vaporized nitrogen and 3,400 years for liquid nitrogen; this advantage is lost if seeds are not frozen promptly after harvesting.


Cultivars (varieties)

There are several types or cultivars of lettuce. Three types – leaf, head and cos or romaine – are the most common.Bradley, p. 129. There are seven main cultivar groups of lettuce, each including many varieties: * Leaf—Also known as looseleaf, cutting or bunching lettuce, this type has loosely bunched leaves and is the most widely planted. It is used mainly for salads. * Romaine/Cos—Used mainly for salads and sandwiches, this type forms long, upright heads. This is the most often used lettuce in
Caesar salad A Caesar salad (also spelled Cesar and Cesare) is a green salad of romaine lettuce and croutons dressed with lemon juice (or lime juice), olive oil, egg, Worcestershire sauce, anchovies, garlic, Dijon mustard, Parmesan cheese, and black pepper ...
s. ** Little Gem—a dwarf, compact romaine lettuce, popular in the UK. * Iceberg/Crisphead—The most popular type in the United States. Iceberg lettuce is very heat-sensitive and was originally developed in 1894 for growth in the northern United States by Burpee Seeds and Plants. It gets its name from the way it was transported in crushed ice, where the heads of lettuce looked like
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s. Today, it ships well, but is low in flavor and nutritional content, being composed of even more water than other lettuce types. * Butterhead—Also known as Boston or Bibb lettuce, and traditionally in the UK as "round lettuce", this type is a head lettuce with a loose arrangement of leaves, known for its sweet flavor and tender texture. * Summercrisp—Also called Batavian or French crisp, this lettuce is midway between the crisphead and leaf types. These lettuces tend to be larger, bolt-resistant and well-flavored. * Celtuce/Stem—This type is grown for its seedstalk, rather than its leaves, and is used in Asian cooking, primarily Chinese, as well as stewed and creamed dishes. * Oilseed—This type is grown for its seeds, which are pressed to extract an oil mainly used for cooking. It has few leaves, bolts quickly and produces seeds around 50 percent larger than other types of lettuce. *
Red leaf lettuce Red leaf lettuces are a group of lettuce cultivars A cultivar is a type of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and when Plant propagation, propagated retain those traits. Methods used ...
—A group of lettuce types with red leaves. The butterhead and crisphead types are sometimes known together as "cabbage" lettuce, because their heads are shorter, flatter, and more cabbage-like than romaine lettuces.


Cultivation problems

Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth. Many insects are attracted to lettuce, including cutworms, which cut seedlings off at the soil line; wireworms and
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s, which cause yellow, stunted plants; tarnished plant bugs and aphids, which cause yellow, distorted leaves;
leafhopper A leafhopper is the common name for any species from the family Cicadellidae. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or trees. Their hind legs are modified for jumping, and a ...
s, which cause stunted growth and pale leaves; thrips, which turn leaves gray-green or silver; leafminers, which create tunnels within the leaves; flea beetles, which cut small holes in leaves and
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sym ...
s,
slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a smal ...
s and snails, which cut large holes in leaves. For example, the larvae of the ghost moth is a common pest of lettuce plants. Mammals, including
rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit speci ...
s and groundhogs, also eat the plants.Bradley, pp. 129–132. Lettuce contains several defensive compounds, including
sesquiterpene lactone Sesquiterpene lactones (SLs) are a class of sesquiterpenoids that contain a lactone ring. They are most often found in plants of the family Asteraceae (daisies, asters). Other plant families with SLs are Umbelliferae (celery, parsley, carrots) an ...
s, and other natural phenolics such as flavonol and
glycosides In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
, which help to protect it against pests. Certain varieties contain more than others, and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance. Lettuce also suffers from several viral diseases, including big vein, which causes yellow, distorted leaves, and mosaic virus, which is spread by aphids and causes stunted plant growth and deformed leaves. Aster yellows are a disease-causing bacteria carried by leafhoppers, which causes deformed leaves. Fungal diseases include powdery mildew and downy mildew, which cause leaves to mold and die and bottom rot,
lettuce drop Lettuce (''Lactuca sativa'') is an annual plant of the family Asteraceae. It is most often grown as a leaf vegetable, but sometimes for its stem and seeds. Lettuce is most often used for salads, although it is also seen in other kinds of food, ...
and
gray mold ''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usually called "grey mould" or ...
, which cause entire plants to rot and collapse. Bacterial diseases include '' Botrytis cinerea'', for which UV-C treatments may be used: Vàsquez et al. 2017 find that phenylalanine ammonia-lyase activity, phenolic production, and ''B. cinerea''
resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...
are increased by UV-C. Crowding lettuce tends to attract pests and diseases. Weeds can also be an issue, as cultivated lettuce is generally not competitive with them, especially when directly seeded into the ground. Transplanted lettuce (started in flats and later moved to growing beds) is generally more competitive initially, but can still be crowded later in the season, causing misshapen lettuce and lower yields. Weeds also act as homes for insects and disease and can make harvesting more difficult. Herbicides are often used to control weeds in commercial production. However, this has led to the development of herbicide-resistant weeds in lettuce cultivation.


Production

In 2020, world production of lettuce (report combined with chicory) was 28 million tonnes, with
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
alone producing 14.3 million tonnes or 52% of the world total (see table). Lettuce is the only member of the genus ''Lactuca'' to be grown commercially. Although China is the top world producer of lettuce, the majority of the crop is consumed domestically. Spain is the world's largest exporter of lettuce, with the US ranking second. Western Europe and North America were the original major markets for large-scale lettuce production. By the late 1900s, Asia, South America, Australia and Africa became more substantial markets. Different locations tended to prefer different types of lettuce, with butterhead prevailing in northern Europe and Great Britain, romaine in the Mediterranean and stem lettuce in China and Egypt. By the late 20th century, the preferred types began to change, with crisphead, especially iceberg, lettuce becoming the dominant type in northern Europe and Great Britain and more popular in western Europe. In the US, no one type predominated until the early 20th century, when crisphead lettuces began gaining popularity. After the 1940s, with the development of iceberg lettuce, 95 percent of the lettuce grown and consumed in the US was crisphead lettuce. By the end of the century, other types began to regain popularity and eventually made up over 30 percent of production.Katz and Weaver, p. 378. Stem lettuce was first developed in China, where it remains primarily cultivated. In the early 21st century, bagged salad products increased in the lettuce market, especially in the US where innovative packaging and shipping methods prolonged freshness. In the United States in 2013, California (71%) and Arizona (29%) produced nearly all of the country's fresh head and leaf lettuce, with head lettuce yielding $9,400 of value per acre and leaf lettuce yielding $8,000 per acre.


Culinary use

As described around 50 AD, lettuce leaves were often cooked and served by the Romans with an oil-and-vinegar dressing; however, smaller leaves were sometimes eaten raw. During the 81–96 AD reign of Domitian, the tradition of serving a lettuce salad before a meal began. Post-Roman Europe continued the tradition of poaching lettuce, mainly with large romaine types, as well as the method of pouring a hot oil and vinegar mixture over the leaves. Today, the majority of lettuce is grown for its leaves, although one type is grown for its stem and one for its seeds, which are made into an oil. Most lettuce is used in salads, either alone or with other greens, vegetables, meats and cheeses. Romaine lettuce is often used for
Caesar salad A Caesar salad (also spelled Cesar and Cesare) is a green salad of romaine lettuce and croutons dressed with lemon juice (or lime juice), olive oil, egg, Worcestershire sauce, anchovies, garlic, Dijon mustard, Parmesan cheese, and black pepper ...
s. Lettuce leaves can also be found in soups, sandwiches and wraps, while the stems are eaten both raw and cooked. The consumption of lettuce in China developed differently from in Western countries, due to health risks and cultural aversion to eating raw leaves; Chinese "salads" are composed of cooked vegetables and are served hot or cold. Lettuce is also used in a larger variety of dishes than in Western countries, contributing to a range of dishes including
bean curd A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes thr ...
and meat dishes, soups and
stir-fry Stir frying () is a cooking technique in which ingredients are fried in a small amount of very hot oil while being stirred or tossed in a wok. The technique originated in China and in recent centuries has spread into other parts of Asia and th ...
s plain or with other vegetables. Stem lettuce, widely consumed in China, is eaten either raw or cooked, the latter primarily in soups and stir-frys. Lettuce is also used as a primary ingredient in the preparation of lettuce soup.


Nutritional content

Depending on the variety, lettuce is an excellent source (20% of the Daily Value, DV, or higher) of vitamin K (97% DV) and vitamin A (21% DV) (table), with higher concentrations of the
provitamin A A provitamin is a substance that may be converted within the body to a vitamin. The term previtamin is a synonym. The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converted ...
compound, beta-carotene, found in darker green lettuces, such as romaine. With the exception of the iceberg variety, lettuce is also a good source (10–19% DV) of folate and iron (table).


Food-borne illness

Food-borne pathogens that can survive on lettuce include '' Listeria monocytogenes'', the causative agent of listeriosis, which multiplies in storage. However, despite high levels of bacteria being found on ready-to-eat lettuce products, a 2008 study found no incidents of food-borne illness related to listeriosis, possibly due to the product's short shelf life, indigenous microflora competing with the ''Listeria'' bacteria or inhibition of bacteria to cause listeriosis. Other bacteria found on lettuce include ''
Aeromonas ''Aeromonas'' is a genus of Gram-negative, facultative anaerobic, rod-shaped bacteria that morphologically resemble members of the family Enterobacteriaceae. Most of the 14 described species have been associated with human diseases. The most imp ...
'' species, which have not been linked to any outbreaks; ''
Campylobacter ''Campylobacter'' (meaning "curved bacteria") is a genus of Gram-negative bacteria. ''Campylobacter'' typically appear comma- or s-shaped, and are motile. Some ''Campylobacter'' species can infect humans, sometimes causing campylobacteriosis, a d ...
'' species, which cause
campylobacteriosis Campylobacteriosis is an infection by the '' Campylobacter'' bacterium, most commonly ''C. jejuni''. It is among the most common bacterial infections of humans, often a foodborne illness. It produces an inflammatory, sometimes bloody, diarrhea or ...
; and ''
Yersinia intermedia ''Yersinia intermedia'' is a Gram-negative species of bacteria which uses rhamnose, melibiose, and raffinose. Its type strain is strain 3953 (=CIP 80-28 =ATCC 29909 =Bottone 48 =Chester 48). It has been found in fish, and contains several biotype ...
'' and ''
Yersinia kristensenii ''Yersinia kristensenii'' is a species of bacteria.Bercovier, Hervé, et al. "Yersinia kristensenii: A new species of Enterobacteriaceae composed of sucrose-negative strains (formerly called atypical Yersinia enterocolitica or Yersinia enterocoli ...
'' (species of '' Yersinia''), which have been found mainly in lettuce. Lettuce has been linked to numerous outbreaks of the bacteria '' E. coli O157:H7'' and '' Shigella''; the plants were most likely contaminated through contact with animal feces. A 2007 study determined that the vacuum cooling method, especially prevalent in the California lettuce industry, increased the uptake and survival rates of ''E. coli O157:H7''. ''
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is fur ...
'' bacteria, including the uncommon '' Salmonella braenderup'' type, have also caused outbreaks traced to contaminated lettuce. Viruses, including hepatitis A, calicivirus and a Norwalk-like strain, have been found in lettuce. The vegetable has also been linked to outbreaks of parasitic infestations, including '' Giardia lamblia''.


Religious symbolism and medicinal use

In addition to its usual purpose as an edible leafy vegetable, lettuce has had a number of uses in ancient (and even some more modern) folk medicine and
religious symbolism A religious symbol is an iconic representation intended to represent a specific religion, or a specific concept within a given religion. Religious symbols have been used in the military in many countries, such as the United States military chapl ...
. For example, ancient Egyptians thought lettuce to be a symbol of sexual prowess and a promoter of love and childbearing in women. The Romans likewise claimed that it increased sexual potency. In contrast, the ancient Greeks connected the plant with male impotency, and served it during funerals (probably due to its role in the myth of Adonis' death), and British women in the 19th century believed it would cause infertility and sterility. Lettuce has mild
narcotic The term narcotic (, from ancient Greek ναρκῶ ''narkō'', "to make numb") originally referred medically to any psychoactive compound with numbing or paralyzing properties. In the United States, it has since become associated with opiates ...
properties; it was called "sleepwort" by the Anglo-Saxons because of this attribute, although the cultivated ''L. sativa'' has lower levels of the narcotic than its wild cousins. This narcotic effect is a property of two
sesquiterpene lactone Sesquiterpene lactones (SLs) are a class of sesquiterpenoids that contain a lactone ring. They are most often found in plants of the family Asteraceae (daisies, asters). Other plant families with SLs are Umbelliferae (celery, parsley, carrots) an ...
s ( lactucin and lactucopicrin) which are found in the white liquid (latex) in the stems of lettuce,Katz and Weaver, p. 377. called lactucarium or "lettuce opium". Although the standard definition of lactucarium requires its production from '' Lactuca virosa'' or " wild lettuce", it was recognized that smaller quantities of lactucarium could be produced in a similar way from '' Lactuca sativa'' and ''
Lactuca canadensis ''Lactuca canadensis'' is a species of wild lettuce known by the common names Canada lettuce, Canada wild lettuce, tall lettuce, and Florida blue lettuce.Weakley, Alan S''. Flora of the Southern and Mid-Atlantic States.'' UNC Herbarium, North Car ...
'' var. ''elongata'', and that lettuce-opium obtained from ''
Lactuca serriola ''Lactuca serriola'', also called prickly lettuce, milk thistle (not to be confused with ''Silybum marianum'', also called milk thistle), compass plant, and scarole, is an annual or biennial plant in the tribe Cichorieae within the family Asterac ...
'' or ''
Lactuca quercina ''Lactuca quercina'' is a species of wild lettuce native to Europe and Asia. It is an annual or biennial herb in the tribe Cichorieae within the family Asteraceae growing from a taproot to maximum heights of or more. ''Lactuca quercina'' contai ...
'' was of superior quality. Romaine lettuce is one of the symbolic items on the Jewish
Passover Seder The Passover Seder (; he, סדר פסח , 'Passover order/arrangement'; yi, סדר ) is a ritual feast at the beginning of the Jewish holiday of Passover. It is conducted throughout the world on the eve of the 15th day of Nisan in the Hebrew c ...
plate as a bitter herb ( maror). Some American settlers claimed that smallpox could be prevented through the ingestion of lettuce, and an Iranian belief suggested consumption of the seeds when afflicted with typhoid. Scientific evidence of these benefits in humans has not been found. The religious ties of lettuce continue into the present day among the Yazidi people of northern Iraq, who have a religious prohibition against eating the plant.


References


Cited literature

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External links

* {{Authority control Leaf vegetables Crops Crops originating from Egypt
sativa Sativa, sativus, and sativum are Latin botanical adjectives meaning '' cultivated''. It is often associated botanically with plants that promote good health and used to designate certain seed-grown domestic crops. Usage ''Sativa'' (ending in -a) ...