Lettuce (Lactuca sativa) is anannual plant of the familyAsteraceae mostly grown as aleaf vegetable. The leaves are most often used raw ingreen salads, although lettuce is also seen in other kinds of food, such as sandwiches,wraps and soups; it can also be grilled.[3] Its stem and seeds are sometimes used;celtuce (asparagus lettuce) is one variety 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. As of 2021[update], world production of lettuce (andchicory) was 27 milliontonnes, 53percent of which came from China.[4]
Lettuce was originally farmed by theancient 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 namelactuca, from which the Englishlettuce is derived. By 50 AD, many types were described, and lettuce appeared often in medieval writings, including severalherbals. The 16th through 18th centuries saw the development of manyvarieties in Europe, and by the mid-18th century, cultivars were described that can still be found in modern gardens.
Generally grown as ahardyannual, lettuce is easily cultivated, although it requires relatively low temperatures to prevent it from flowering quickly. It can be plagued by numerousnutrient deficiencies, as well as insect and mammal pests, and fungal and bacterial diseases.L. sativacrosses easily within the species and with some other species within the genusLactuca. Although this trait can be a problem to home gardeners who attempt to save seeds, biologists have used it to broaden thegene pool of cultivated lettuce varieties.
Lettuce is a rich source ofvitamin K andvitamin A, and a moderate source offolate andiron. Contaminated lettuce is often a source of bacterial, viral, and parasitic outbreaks in humans, includingE. coli andSalmonella.
The Romans referred to lettuce aslactuca (lac meaning "milk" inLatin), an allusion to the white substance,latex, exuded by cut stems.[10] The nameLactuca has become the genus name, whilesativa (meaning "sown" or "cultivated") was added to create the species name.[11] The current wordlettuce, originally fromMiddle English, came from theOld Frenchletues orlaitues, which derived from the Roman name.[12] The nameromaine came from the variety of lettuce grown in the Roman papal gardens, whilecos, another term forromaine lettuce, came from the earliest European seeds of the type from the Greek island ofKos, a center of lettuce farming in theByzantine period.[13]
Description
Lettuce inflorescencesMature lettuceinflorescence in fruitLettuce seeds
Lettuce's native range spreads from theMediterranean toSiberia, although it has been transported to almost all areas of the world. Plants generally have a height and spread of 15 to 30 cm (6 to 12 in).[14] The leaves are colorful, mainly in the green and red color spectrums, with somevariegated varieties.[15] There are also a few varieties with yellow, gold or blue-teal leaves.[16]
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.[15] Lettuce plants have a root system that includes a maintaproot 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.[16]
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.[17]
Once plants move past the edible stage, they develop flower stalks up to 1 m (3 ft 3 in) high with small yellow blossoms.[18] Like other members of the tribeCichorieae, lettuceinflorescences (also known asflower heads or capitula) are composed of multipleflorets, each with a modifiedcalyx called apappus (which becomes the feathery "parachute" of the fruit), a corolla of five petals fused into aligule or strap, and the reproductive parts. These include fusedanthers that form a tube which surrounds astyle andbipartitestigma. As the anthers shedpollen, the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube.[16][19] 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 adispersal structure. Each fruit contains one seed, which can be white, yellow, gray or brown depending on the variety of lettuce.[1]
The domestication of lettuce over the centuries has resulted in several changes throughselective breeding: delayed bolting, larger seeds, larger leaves and heads, better taste and texture, a lowerlatex content, and different leaf shapes and colors. Work in these areas continues through the present day.[20] Scientific research into thegenetic 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 greaterherbicide tolerance, greater resistance to insects and fungi and slower bolting patterns. However, genetically modified lettuce is not currently used in commercial agriculture.[21]
History
Romaine lettuce, a descendant of some of the earliest cultivated lettuce
DNA analysis of 445 types of lettuce indicates that lettuce was first domesticated from itswild ancestor near theCaucasus, whereseed shattering was first selected out of thecultivar. At this time, the lettuce plant was only suitable for harvesting itsseeds, which could be pressed to extractoil, likely used for cooking, among other purposes.[22][23] From there, lettuce was likely transported to theNear East and then toancient Egypt,[22] where the first depictions of lettuce cultivation can be found as early as 2680 BC.[10] Like the early lettuce from the Caucasus, this lettuce was grown to produce cooking oil from its seeds.[22][24]
Lettuce was considered a sacred plant of the reproduction godMin, and was carried during his festivals and placed near his images. The plant was thought to help the god "perform the sexual act untiringly".[25] 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 75 cm (30 in) tall and resembled a large version of the modernromaine lettuce. These upright lettuces were developed by the Egyptians and passed to the Greeks, who in turn shared them with the Romans.[citation needed] Around 50 AD, Roman agriculturalistColumella described several lettuce varieties – some of which may have been ancestors of today's lettuces.[10]
The plant was eventually selectively bred into a plant grown for its edible leaves.[22][24] The long leaves in Egyptian depictions suggest that it may have been grown for its leaves, which would make it the first lettuce cultivar grown for this purpose.[24] However, genome wide analysis suggests the traits needed for cultivation as a leafy vegetable, like the loss of bitterness and thorns, evolved much later, from around 500 BC in Southern Europe. Lettuce cultivars radiated more rapidly from this point, withoilseed lettuce likely being brought by the ancient Greeks from Egypt to Italy, where it was modified intocos lettuce and cultivated for its leaves. From there, it was brought north to Central Europe, where it was modified intobutterhead lettuce and other varieties.[22][23]
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 earlyherbals also describe its uses. In 1586,Joachim Camerarius provided descriptions of the three basic modern lettuces – head lettuce, loose-leaf lettuce, and romaine (or cos) lettuce.[13] Lettuce was first brought to the Americas from Europe byChristopher Columbus in the late 15th century.[26][27] 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.[24]
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.[28][citation needed] During the 1950s, lettuce production was revolutionized with the development ofvacuum cooling, which allowed field cooling and packing of lettuce, replacing the previously used method of ice-cooling in packing houses outside the fields.[29]
Lettuce is easy to grow, and as such has been a significant source of sales for manyseed companies. 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.[30] Although most lettuce grown today is used as a vegetable, a minor amount is used in the production oftobacco-free cigarettes; however, domestic lettuce's wild relatives produce a leaf that visually more closely resembles tobacco.[31]
Cultivation
A lettuce farm in the United Kingdom
Ahardy annual, some varieties of lettuce can beoverwintered even in relatively cold climates under a layer of straw, and older,heirloom varieties are often grown incold frames.[32] 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, thentransplanted to individual spots, usually 20 to 36 cm (7.9 to 14.2 in) 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.[18]
Lettuce grows best in full sun in loose,nitrogen-rich soils with apH of between 6.0 and 6.8. Heat generally prompts lettuce tobolt, with most varieties growing poorly above 24 °C (75 °F); cool temperatures prompt better performance, with 16 to 18 °C (61 to 64 °F) being preferred and as low as 7 °C (45 °F) being tolerated.[33] Plants in hot areas that are provided partial shade during the hottest part of the day will bolt more slowly. Temperatures above 27 °C (81 °F) will generally result in poor or non-existent germination of lettuce seeds.[33] After harvest, lettuce lasts the longest when kept at 0 °C (32 °F) 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.[34] In spite of its high water content, traditionally grown lettuce has a lowwater footprint, with 237 liters (52 imp gal; 63 U.S. gal) of water required for each kilogram of lettuce produced.[35] Hydroponic growing methods can reduce this water consumption by nearly two orders of magnitude.
Lettuce varieties willcross with each other, making spacing of 1.5 to 6 m (60 to 240 in) between varieties necessary to prevent contamination when saving seeds. Lettuce will also cross withLactuca serriola (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.[18] This propensity for crossing, however, has led to breeding programs using closely related species inLactuca, such asL. serriola,L. saligna, andL. virosa, to broaden the availablegene pool. Starting in the 1990s, such programs began to include more distantly related species such asL. tatarica.[36]
A selection of lettuce cultivarsThe lettuce varietyceltuce is grown for its stem, used inChinese cooking.A butterhead lettucePrajzerica, a lettuce variety fromCroatia
Seeds keep best when stored in cool conditions, and, unless storedcryogenically, remain viable the longest when stored at −20 °C (−4 °F); they are relatively short lived in storage.[1] 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 ahalf-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.[37]
There are several types andcultivars of lettuce. Categorization may sometimes refer to "leaf" versus "head", but there are seven main cultivar groups of lettuce, each including many varieties:
Leaf—Also known as looseleaf, cutting or bunching lettuce,[38] this type has loosely bunched leaves and is the most widely planted. It is used mainly for salads.[34]
Romaine/Cos—Used mainly for salads and sandwiches, this type forms long, upright heads.[34] This is the most often used lettuce inCaesar salads.[26]
Little Gem—a dwarf, compact romaine lettuce, popular in the UK.[39]
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 byBurpee Seeds and Plants. It gets its name from the way it was transported in crushed ice, where the heads of lettuce looked likeicebergs.[40] Today, it ships well, but is low in flavor and nutritional content, being composed of even more water than other lettuce types.[34]
Butterhead—Also known as Boston or Bibb lettuce,[38] and traditionally in theUK as "round lettuce",[41] this type is a head lettuce with a loose arrangement of leaves, known for its sweet flavor and tender texture.[34]
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.[38]
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.[34]
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.[42]
The four main types in the Western world have been looseleaf, romaine, crisphead, and butterhead, with the others being intermediary or more exotic. 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.[43]
Cultivation problems
A lettuce surrounded by weeds, which have crowded it to the point ofbolting
Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth.[33] Many insects are attracted to lettuce, includingcutworms, which cut seedlings off at the soil line;wireworms andnematodes, which cause yellow, stunted plants;tarnished plant bugs andaphids, which cause yellow, distorted leaves;leafhoppers, 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 andcaterpillars,slugs andsnails, which cut large holes in leaves. For example, the larvae of theghost moth is a common pest of lettuce plants.[44] Mammals, includingrabbits andgroundhogs, also eat the plants.[45] Lettuce contains several defensive compounds, includingsesquiterpene lactones, and other naturalphenolics such asflavonol andglycosides, 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.[46]
Lettuce also suffers from severalviral diseases, includingbig vein, which causes yellow, distorted leaves, andmosaic 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 includepowdery mildew anddowny mildew, which cause leaves to mold and die andbottom rot,lettuce drop andgray mold, which cause entire plants to rot and collapse.[45] Bacterial diseases includeBotrytis cinerea, for whichUV-C treatments may be used: Vàsquez et al. 2017 find thatphenylalanine ammonia-lyase activity, phenolic production, andB. cinerearesistance are increased by UV-C.[47] Crowding lettuce tends to attract pests and diseases.[18] 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.[48] Herbicides are often used to control weeds in commercial production. However, this has led to the development ofherbicide-resistant weeds in lettuce cultivation.[20]
In 2022, world production of lettuce (report combined withchicory) was 27 million tonnes, withChina alone producing 55% of the total (table).
Lettuce is the only member of the genusLactuca to be grown commercially.[49] Although China is the top world producer of lettuce, the majority of the crop is consumed domestically.
Markets
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.[50] Stem lettuce was first developed in China, where it remains primarily cultivated.[51]
A lettuce plant that has bolted
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.[52][53][54]
In the United States in 2022, lettuce was the main vegetable ingredient in salads, and was the most consumed among leaf vegetables; its market was about 20% of all vegetables, with Romaine and iceberg having about equal sales.[53] Some 85% of the lettuce consumed in the United States in 2022 was produced domestically.[53]
Uses
Culinary
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 ofDomitian, the tradition of serving a lettucesalad before a meal began. Post-Roman Europe continued the tradition ofpoaching lettuce, mainly with large romaine types, as well as the method of pouring a hot oil and vinegar mixture over the leaves.[55]
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.[24] Most lettuce is used in salads, either alone or with other greens, vegetables, meats and cheeses. Romaine lettuce is often used forCaesar salads. Lettuce leaves can also be found in soups, sandwiches and wraps, while the stems are eaten both raw and cooked.[11]
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 includingbean curd and meat dishes, soups andstir-frys 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.[51] Lettuce is also used as a primary ingredient in the preparation oflettuce soup.
Nutrition
Raw iceberg lettuce is 96% water, 3%carbohydrates, and contains negligibleprotein andfat (table). In a reference amount of 100 grams (3.5 oz), iceberg lettuce supplies 14calories and is a rich source (20% or more of theDaily Value, DV) ofvitamin K (20% DV), with no othermicronutrients in significant content (table).
In lettuce varieties with dark green leaves, such asromaine (also calledcos),vitamin A contents are appreciable due to the presence of theprovitamin A compound,beta-carotene.[34] Dark green varieties of lettuce also contain moderate amounts ofcalcium andiron.[34] The edible spine and ribs of the lettuce plant supplydietary fiber, while micronutrients are contained in the leaf portion.[34]
Food-borne illness
Food-bornepathogens that can survive on lettuce includeListeria monocytogenes, the causative agent oflisteriosis, 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 shortshelf life, indigenous microflora competing with theListeria bacteria or inhibition of bacteria to cause listeriosis.[58]
Lettuce has been linked to numerous outbreaks of the bacteriaE.coli O157:H7 andShigella; the plants were most likely contaminated through contact with animal[61] or human feces.[62] A 2007 study determined that the vacuum cooling method, especially prevalent in the California lettuce industry, increased the uptake and survival rates ofE. coli O157:H7.[63] Scientific experiments using treatedmunicipal wastewater as irrigation for romaine lettuce have shown that the contamination levels of foliage, leachate, and soil withE. coli[62] andbacteriophage AP205 (used by researchers as a surrogate forenteric viruses), respectively, were directly correlated with the presence of these organisms in the irrigation water.[64]
Due to the increase in food demand, the use of treated wastewater effluent for irrigation and animal or human excreta (i.e., manure or biosolids) as soil amendments is increasing.[65] As such, so are the outbreaks of food-borne illnesses. Due to the overuse of antibiotics in farming, the number of pathogensresistant to antibiotics is increasing,[66] one of these beingAR E.coli, which has been found on lettuce irrigated with wastewater.[67]
Pathogens found on lettuce are not specific to lettuce (though someE. coli strains have affinity for Romaine). But, unlike other vegetables which tend to be cooked, lettuce is eaten raw, thus food-borne outbreaks associated with it are more frequent and affect a larger number of people.[68]
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