Lettuce
Lettuce is an annual plant of the family Asteraceae mostly grown as a leaf vegetable. The leaves are most often used raw in green salads, although lettuce is also seen in other kinds of food, such as sandwiches, wraps and soups; it can also be grilled. Its stem and seeds are sometimes used; celtuce 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. In 2023, world production of lettuce was 28 million tonnes, led by China with 53% of the total.
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 lactuca, 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 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, as well as insect and mammal pests, and fungal and bacterial diseases. L. sativa crosses 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 of cultivated lettuce varieties.
Contaminated lettuce is often a source of bacterial, viral, and parasitic outbreaks in humans, including E. coli and Salmonella.
Taxonomy and etymology
Lactuca sativa is a member of the Lactuca genus and the Asteraceae family. The species was first described in 1753 by Carl Linnaeus in the second volume of his Species Plantarum. Synonyms for L. sativa include Lactuca scariola ''sativa, L. scariola integrata and L. scariola integrifolia. L. scariola is itself a synonym for L. serriola, the common wild or prickly lettuce. L. sativa also has many identified taxonomic groups, subspecies and varieties, which delineate the various cultivar groups 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 lactuca, an allusion to the white substance, latex, exuded by cut stems. The name Lactuca has become the genus name, while sativa was added to create the species name. The current word lettuce, originally from Middle English, came from the Old French letues or laitues, 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.
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 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 are composed of multiple florets, each with a modified calyx called a pappus, a corolla of five petals fused into a ligule or strap, and the reproductive parts. These include fused anthers that form a tube which surrounds a style and bipartite stigma. As the anthers shed pollen, the style elongates to allow the stigmas, now coated with pollen, to emerge from the tube. The ovaries form compressed, obovate 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. 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 tolerance, greater resistance to insects and fungi and slower bolting patterns. However, genetically modified lettuce is not currently used in commercial agriculture.
History
DNA analysis of 445 types of lettuce indicates that lettuce was first domesticated from its wild ancestor near the Caucasus, where seed shattering was first selected out of the cultivar. At this time, the lettuce plant was only suitable for harvesting its seeds, which could be pressed to extract oil, likely used for cooking, among other purposes. From there, lettuce was likely transported to the Near East and then to ancient Egypt, where the first depictions of lettuce cultivation can be found as early as 2680 BC. Like the early lettuce from the Caucasus, this lettuce was grown to produce cooking oil from its seeds.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.
The plant was eventually selectively bred into a plant grown for its edible leaves. 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. 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, with oilseed lettuce likely being brought by the ancient Greeks from Egypt to Italy, where it was modified into cos lettuce and cultivated for its leaves. From there, it was brought north to Central Europe, where it was modified into butterhead lettuce and other varieties.
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 provided descriptions of the three basic modern lettuces – head lettuce, loose-leaf lettuce, and romaine 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.
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, which allowed field cooling and packing of lettuce, replacing the previously used method of ice-cooling in packing houses outside the fields.
Lettuce is easy to grow, and as such has been a significant source of sales for many seed 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. Although most lettuce grown today is used as a vegetable, a minor amount is used in the production of tobacco-free cigarettes; however, domestic lettuce's wild relatives produce a leaf that visually more closely resembles tobacco.