Lactuca sativa

The lettuce (Lactuca sativa) is an annual plant of the Asteraceae family. It is grown primarily as a leafy vegetable, but sometimes also for its stem and seeds. Lettuce is mostly used for salads, although it is also seen in other types of food, such as soups, sandwiches, sandwiches, and wraps; it can also be grilled. One variety, Chinese lettuce (asparagus lettuce), is grown for its stems, which are eaten raw or cooked. In addition to its primary use as a green leaf, it has also acquired religious and medicinal significance over centuries of human consumption. Originally, Europe and North America dominated the lettuce market, but by the end of the XX century, lettuce consumption had spread all over the world. In 2017, world production of lettuce and chicory was 27 million tons, 56% of which came from China.

Lettuce was originally cultivated by the ancient Egyptians, who transformed it from a plant whose seeds were used for oil into an important food crop for its succulent leaves and oil-rich seeds. Lettuce spread to the Greeks and the Romans; the latter gave it the name lactuca, from which "lettuce" derives in Spanish. Around the year 50 d. C. many types were described, and lettuce appeared frequently in medieval writings, including various herbaria. Between the centuries XVI and XVIII many varieties were developed in Europe, and by the middle of the century XVIII cultivars were described that can still be found in orchards.

Usually grown as a rustic annual, lettuce is easy to grow, although it requires relatively cool temperatures to prevent rapid flowering. It can suffer from 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 of the genus Lactuca. Although this trait can be a problem for home gardeners trying to save seed, biologists have used it to expand the gene pool 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, such as E. coli and Salmonella.

Taxonomy and etymology

Seeds L. sativa

Lactuca sativa is a member of the genus Lactuca of the family Asteraceae. The species was first described in 1753 by Charles Linnaeus in the second volume of his Species Plantarum. Synonyms of L. sativa are Lactuca scariola var. sativa, L. scariola var. integrata and L. scariola var. integrifolia.L. sativa also has many identified taxonomic groups, subspecies and varieties, which delineate the various cultivar groups of cultivated lettuce. Lettuce is closely related to several species of Lactuca from southwest Asia; closest related to L. serriola, an aggressive weed common in temperate and subtropical zones throughout much of the world.

The Romans referred to lettuce as lactuca (lac means 'dairy' or 'milk' in Latin), alluding to the white latex substance exuded by cut stems. The name Lactuca became the name of the genus, while that sativa (meaning 'sown' or 'cultivated') was added to create the species name. The actual word lettuce is a derivative of the Roman name. Instead, cos, another term for romaine lettuce, comes from the first European seeds of the type from the Greek island of Cos, central lettuce cultivation in the Byzantine period.

Description

Lettuce flowers

The native range of lettuce extends from the Mediterranean to Siberia. However, it has been transported to almost all areas of the world. The plants are typically 15–30cm tall and spread. The leaves are colourful, mainly in the green and red color spectrum, with some variegated varieties. There are also some varieties with yellow, gold or bluish leaves.

Fruit of lettuce with vines

Lettuces have a wide range of shapes and textures, from the dense heads of the iceberg type to the serrated, scalloped, ruffled leaves of the leafy varieties. Lettuces have a root system that includes a taproot and other smaller secondaries. Some varieties, especially those from the United States and Western Europe, have long, narrow tap roots and a small set of secondary roots. Asian varieties have longer tap roots and more extensive secondary systems.

Depending on the variety and the time of year, lettuce typically lives between 65 and 130 days from planting to harvest. Since lettuces that flower (through the process known as "heading") become bitter and unmarketable, plants grown for consumption are rarely allowed to grow to maturity. Lettuce flowers faster in warm temperatures, while freezing temperatures cause slower growth and sometimes damage to the outer leaves.

Once the plants are beyond the edible stage, they develop flower stalks up to 1m tall with small yellow flowers. Like other members of the Lactuceae tribe, lettuce inflorescences (also known as flower heads or heads) are composed of multiple florets, each with a modified calyx called a pappus (which becomes the feathery "parachute" of the fruit), a corolla of five petals fused into a ligule, and the reproductive parts. These include fused anthers that form a tube surrounding a style and a bipartite stigma. As the anthers shed pollen, the style elongates to allow the stigmas, now coated with pollen, to exit the tube. The ovaries form obovate achenes 3–4mm long. The fruits have 5 to 7 ribs on each side and are topped by two rows of small white hairs. The pappus remains at the top of each fruit as a dispersal structure. Each fruit contains a seed, which can be white, yellow, gray or brown, depending on the variety of lettuce.

Cultivation of lettuce over the centuries has led to several changes through selection: delayed regrowth stage, larger seeds, larger leaves and tops, better flavor and texture, less latex content and different shapes and colors of the leaves. Work in these areas continues to this day. Scientific research on the genetic modification of lettuce is ongoing, with more than 85 field trials conducted between 1992 and 2005 in the European Union and the United States to test the modifications. that allow a greater tolerance to herbicides, resistance to insects and fungi and the delay of the regrowth stage. However, genetically modified lettuce is not currently used in commercial agriculture.

History

Roman lettuce, descendant of some of the first cultivated lettuce

Lettuce was first cultivated in ancient Egypt to produce oil from its seeds. The Egyptians probably selectively bred it into a plant cultivated for its edible leaves, and evidence of its cultivation appears as early as 2680 BC. C. Lettuce was considered a sacred plant of the god of reproduction Min. It was carried during his festivals and placed near his images. The plant was thought to help the god "perform the sexual act tirelessly". Its use in religious ceremonies led to the creation of many images on tombs and wall paintings. The cultivated variety appears to have been about 75cm tall and resembled a large version of modern romaine lettuce. The Egyptians developed these upright lettuces and passed them on to the Greeks, who shared them with the Romans. Around the year 50, the Roman agronomist Columella described several varieties of lettuce, some of which could be the ancestors of today's lettuces.

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. Many early medieval herbaria also describe its uses. In 1586, Joaquín Camerarius described the three basic modern lettuces: head lettuce, loose-leaf lettuce, and romaine (or cos) lettuce. Lettuce was brought to the Americas from Europe by Christopher Columbus at the turn of the XV. Between the end of the 16th century and the beginning of the XVIII, wide varieties were developed in Europe, especially in Holland. Books published in the mid-18th century and early XIX describe several varieties found in orchards today.

Due to its short lifespan after harvest, lettuce was initially sold relatively close to where it was grown. In the early XX century, new packaging, storage and shipping technologies were developed that improved the shelf life and transportability of lettuce and led to a significant increase in its availability. In the 1950s, lettuce production was revolutionized with vacuum cooling, which allowed the cooling and packaging of lettuce in the field, replacing the previously used method of cooling with ice in the packaging warehouses outside the fields.

Lettuce is very easy to grow and as such has been a major source of sales for many seed companies. Tracing the history of broad varieties is complicated by the practice of many companies, particularly in the US, of changing a variety name from year to year. This practice is carried out for a number of reasons, the most prominent being to boost sales by promoting a 'new' variety or by preventing customers from knowing that a competing seed company has developed the variety. Documentation from the late 19th century shows between 65 and 140 varieties of lettuce, depending on the amount of variation allowed between types, a clear difference from the 1,100 named varieties of lettuce on the market at the time. The names also used to change significantly from country to country. Although most of the lettuce grown today is used as a vegetable, a small amount is used in the production of non-tobacco cigarettes; however, the wild relatives of domesticated lettuce produce a leaf that visually resembles more tobacco.

Varieties

Among the varieties of lettuce, the following stand out:

1. Beluga: of squeezed and dense buds, similar to cabbage; it lacks almost entirely of flavor, but enjoys wide use by its crisp texture and the ease to cut it finely. It is the most common variety in regions where lettuce is not naturally given, since it can be cultivated in hydroponic tanks;
2. Romana (also called) mule ear): long bud, with leaves approximately lanceolate, less thick than the iceberg but thick and crispy. It is known in Spain as mule ear;
3. Francesa or Troop: round bud, fine leaves and mantecous texture; it has a delicate but intense flavor. It is also known as Boston;
4. Batavia: similar to French, loose bud, curly leaves and mantecous texture;

Also:

• Tudela or baby;
• Mantecous or butterhead;
• Iceberg o Crunchy;
• Lollo Rossa;
• Red Salad Bowl;
• Cracarelle;
• Kagraner-Sommer.

Diseases

Some of the diseases that are common to members of the genus Lactuca are:

• Anthracnosis (Marssonina panattoniana)
• Botritis (Botrytis cinerea)
• Mildiu (Bremia lactucae)
• Sclerotinia (Sclerotinia sclerotiorum)
• Senorth (Septoria lactucae)
• Lechuga mosaic virus
• Tomato tanning virus (TSWV English Tomato Spotted Wilt Virus)

Pests

• Trips (Frankliniella Occidentalis)

This is one of the pests that causes the most damage to the crop, since it transmits the tomato wilt virus (TSWV). The importance of these direct damages (caused by bites and spawning cracks) depends on the population level of the insect (increasing from mid-spring to well into autumn).

Normally the main damage caused is not direct but indirect by transmitting the virus. The presence of this virus in plants begins by causing large leaf necrosis, and they quickly end up dying.

• Miners (Liriomyza trifolii and Liriomyza huidobrensis)

They form galleries in the leaves and if the pest attack is very strong the plant is weakened.

• White Mosca (Trialeurodes vaporariorum).

It produces a molasses that deteriorates the leaves, leading to a general weakening of the plant.

• Pulgones (Myzus persicae, Macrosiphum solani and Narsonovia ribisnigri)

It is a systematic pest, its incidence being variable according to climatic conditions. The attack usually occurs when the crop is close to harvesting. Although if the plant is young, and the attack is considerable, it can devastate the crop, in addition to being the entry of a virus that makes it unviable. Aphids colonize plants from the outer leaves and progress to the interior, except for the species Narsonovia ribisnigri, whose diffusion is centrifugal, that is, its colonization begins on the inner leaves, multiplying progressively and then moving to the outer parts.

Cultivation and uses

Lettuce supports high temperatures worse than low ones. It requires that there be a temperature difference between day and night. When it supports low temperatures for some time, its leaves take on a reddish coloration that can be confused with a deficiency.

The main problem it presents in a greenhouse is excessive humidity, so it is recommended to grow it outdoors, as long as the weather conditions allow it.

Prefers light, sandy-loamy soils with good drainage. The optimum pH is between 6.7 and 7.4. It grows well in humiferous soils, but if they are excessively acid it will be necessary to whitewash.

In no case does it admit drought, although it is convenient that the crust of the soil is dry to avoid as much as possible the appearance of neck rot.

The best irrigation systems are drip (when grown in a greenhouse) and exudation tapes (when grown outdoors). There are also other systems, such as gravity and sprinkler irrigation, but they are in recession.

This plant is very demanding on potassium and by consuming more potassium it will absorb more magnesium, so it is necessary to balance this possible deficiency when fertilizing the crop. It is also very demanding on molybdenum during the early stage of development.

Nutrition

Lettuce has very little nutritional value, with a high water content (90-95%) and very little sodium. The amount of its compounds varies depending on the type of lettuce. In general it contains antioxidants: vitamins A, C, E, B1, B2, B3, B9 and K; minerals: phosphorus, iron, calcium, potassium; fiber, folates and amino acids. The greener outer leaves are those with the highest content of vitamin C and iron.

Culinary

It is very common to use lettuce leaves as a base for serving salads. Indeed, in the countries of the Middle East it is customary to consume the tabbouleh inside lettuce leaves forming a package.

Similarly, American lettuce is used to make hamburgers.

Postharvest conservation

Lettuce has a very low rate of ethylene production but is highly sensitive to this phytohormone, so it should be kept in an ethylene-free environment. Its optimal storage conditions are: temperature of 0 °C and relative humidity of 98 to 100%. Under optimal storage conditions, it has a post-harvest shelf life of approximately 2 to 3 weeks.

In folk medicine

Lettuce has been and is used in infusion as a moderate anxiolytic that facilitates sleep. However, in remote Antiquity —especially in Egypt— deities considered patrons of the libido were worshiped by offering them lettuce plants. This cult seemed paradoxical until in 2006 it was discovered that a moderate dose of the alkaloids present in lettuce has slightly aphrodisiac effects, while a high one acts the other way around, as an anxiolytic.

Hygiene

In countries where irrigation water hygiene is deficient, or crops are even irrigated with sewage, lettuce represents an important source of infection for gastrointestinal diseases such as typhoid fever, cholera and salmonellosis, due to which is highly recommended to consume them well washed with drinking water and disinfected with a microbicidal solution.

Taxonomy

Lactuca sativa was described by Carlos Linnaeus and published in Species Plantarum 2: 795. 1753.

Etymology

Lactuca: generic name from the Latin lac, -tis (milk). This etymology refers to the milky liquid (that is, with a "dairy" appearance) that is the sap exuded by the stems of this plant when they are cut.

sativa. epithet that refers to its character as a cultivated species.

Sinonimia

• Lactuca capitata (L.) DC.
• Lactuca crispa (L.) Roth
• Lactuca dregeana DC.
• Laciniata lactuca Roth
• Lactuca palmata Willd.
• Lactuca sativa subsp. asparagine (L.H.Bailey) Janch.
• Lactuca sativa subsp. Captain (L.) Schübl. " G.Martens
• Lactuca sativa subsp. crispa (L.) Schübl. " G.Martens
• Lactuca sativa subsp. longifolia (Lam.) Alef.
• Lactuca sativa subsp. minii Hadidi
• Lactuca sativa subsp. Roman Schübl. " G.Martens
• Lactuca sativa var. sativa
• Lactuca sativa subsp. sativa
• Lactuca sativa subsp. secalina Alef.
• Lactuca scariola var. sativa (L.) Boiss.
• Lactuca scariola var. sativa Moris

Common name

Head, salad, farfalá, lettuce (29), tight lettuce, head lettuce, colored lettuce, common lettuce (3), crisp lettuce, heart lettuce, curly leaf lettuce, Nacla lettuce, ear lettuce mule lettuce, domestic lettuce, escarole lettuce from Madrid, flamenco lettuce (2), flamenco lettuce, long lettuce, full lettuce, molondra lettuce, moradita lettuce, moronda lettuce, motrileña lettuce, mulata lettuce, white ear lettuce, mule ear lettuce (2), purple ear lettuce, Seville pointed lettuce, cabbage lettuce, cabbage lettuce (2), curly lettuce (3), romaine lettuce (4), romanilla lettuce, lettuce (2), lettuce, bitter lettuce, lechuguinas, lechuguino, lettuce (3), mule ear. The number in parentheses indicates the frequency of use of the word in Spain.