Lactuca sativa (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Lactuca sativa L.

Protologue: Sp. pl.: 795 (1753).

Family: Compositae

Chromosome number: 2n= 18

Synonyms

• Lactuca serriola L. var. sativa Moris (1840-1843),
• L. scariola L. var. hortensis Bisch. (1851),
• L. scariola L. var. sativa Boiss. (1875).

Vernacular names

• Lettuce (En)
• Laitue (Fr)
• Indonesia: slada, salada bokor (Sundanese), selada, sladah (Javanese)
• Malaysia: salad, selada
• Papua New Guinea: letis
• Philippines: letsugas (Tagalog), tawa (Marinduque)
• Cambodia: sa-llaat
• Thailand: phakkat-hom, phaksalat (central), phakkat-kaeo (northern)
• Vietnam: rau diếp, xà lách, diếp xoăn.

Origin and geographic distribution

The origin of lettuce is believed to be in Asia Minor or the Middle East. It was known as a vegetable and for its medicinal properties as early as 4500 BC. It was a popular vegetable of the Greek and Romans. In western Europe, the headed types have been known since the 14th Century but the leafy types have been known for much longer. The cultivated lettuce is probably derived from the wild lettuce L. serriola. The chromosome number is the same and crosses are easily made. Numerous genetic differences in cultivated lettuce types, however, suggest a polyphyletic origin. Stem lettuce is very popular in China and Taiwan. Lettuce, especially the headed type, is currently the world's most important salad crop and a popular vegetable in almost all countries of the world.

Uses

Lettuce is grown for its leaves, which are usually eaten raw as a salad with a dressing of vinegar. Occasionally it is used as a cooked vegetable. Stem lettuce is grown in China and Taiwan for the fleshy stem, which is prepared by cooking; in South-East Asia (Indonesia, Malaysia and Vietnam in particular) it is grown for the leaves ("siomak", "yaomak"). Dried lettuce leaves are sometimes used in cigarettes as a substitute for tobacco. In Egypt, the oil extracted from the so-called oilseed lettuce (possibly a transitional type between L. sativa and L. serriola) is used as an aphrodisiac.

Production and international trade

There are no statistics on cultivated areas and production in South-East Asia, since lettuce is a small part of the group of leafy vegetables which also includes kangkong and amaranth. In temperate areas lettuce is one of the leading commercial vegetables, whereas in the tropics it is a secondary vegetable of small but increasing commercial importance. The total area of lettuce registered in European Community countries is about 90 000 ha with a production of 2.0 million t (22 t/ha); 16% of the area and 23% of the production is from greenhouses. The total annual world production is estimated at about 3 million t from a total area of 300 000 ha.

Lettuce is not traded much internationally, except in Europe. Since it is a very perishable product, it is mainly produced near big cities. Crisphead lettuce, which is less perishable, is produced in some South-East Asian countries as an export product, e.g. from Malaysia to Singapore and from Thailand and Vietnam (Dalat) to Hong Kong.

Properties

Per 100 g edible portion, the leaves contain: water 94 g, protein 1.2 g, fat 0.2 g, fibre 0.7 g, ash 0.7 g. Lettuce is quite low in carbohydrates, protein and fat, and the energy value (50 kJ/100 g) is low. There are considerable differences in nutritional properties among lettuce types. Headed types with a low chlorophyll content (light green leaves) have fewer micronutrients than leafy types; the dark green types have considerably more carotene, Fe and vitamin C. Butterhead lettuce contains about 30 mg Ca, 1 mg Fe, 1.5 mg β-carotene, 0.05 mg vitamin B₁, 0.08 mg vitamin B₂, 0.4 mg niacin and 10 mg vitamin C. For crisphead lettuce these values are lower.

In some western countries the presence of free nitrates in lettuce and other leafy vegetables is seen as a negative quality factor causing health problems. In the Netherlands the maximum content of NO₃ tolerated in summer lettuce is 2.5 mg per g fresh weight. The nitrate content strongly decreases with increasing light intensity and consequently it is no problem in tropical countries.

The 1000-seed weight is 0.8-1.2 g.

Description

• A very variable, glabrous, lactiferous, annual or biennial herb, 30-70(-100) cm tall, usually forming a dense basal rosette and later a tall, branched, flowering stem.
• Taproot slender at first, later thickening, reaching 1.5 m depth.
• Stem at first short with radical leaves arranged spirally, in cv. group Stem Lettuce developing into a 30-50 cm long, fleshy organ.
• Leaves variously arranged, depending on cultivar, in more or less compact heads or not in heads; shape, size and colour differing with cultivar; rosette leaves undivided to runcinate-pinnatifid, sometimes curly and fringed, shortly petiolate, green or sometimes with red anthocyanin pigment; stem leaves becoming progressively smaller, ovate to orbicular in outline, entire, cordate-amplexicaul, sessile, not held vertically.
• Inflorescence a dense, corymbose, flat-topped panicle with flowers arranged in heads; involucre 10-15 mm long, consisting of 3-4 rows of lanceolate or ovate bracts; head with 7-15(-35) florets, all ligulate and hermaphrodite, yellow, exserted above the involucre; stamens 5 with connate anthers; stigma bifid.
• Fruit a narrowly obovate achene, 3-8 mm long, compressed, 5-7-ribbed on each side, white, yellowish, grey or brown; tip constricted into a narrow beak, surmounted by a white pappus of 2 equal rows of soft hairs.

Growth and development

Germination is epigeal. The seed germinates within 4 days, viable seed even within one day, at temperatures from 15-25 °C. Lettuce seed often shows dormancy, especially when the seed has been stored at high temperatures and is sown at a soil temperature above 24 °C, which is the normal situation in tropical lowlands. The best remedy to break the dormancy is by storing the wetted seed in a refrigerator at 2-5 °C for 1-3 days. Growth of the young plants is exponential, slow at first and very fast in the last weeks before the harvest stage. The rosette formation becomes apparent in the third week after sowing, and the head formation in the headed types two weeks later. Depending on growing conditions and cultivar the head is fully formed and ready for harvesting about two months after sowing. Plants 2-3 months old start bolting. Flower stalk development of headed cultivars is stimulated by removal of the upper part of the head. The flowering stage may last 1-2 months. The flowers are generally open for 1-2 hours and do not open all at once. Lettuce is almost completely selfpollinated before the flowers open. Seed is produced abundantly. Seed matures in 9-13 days after anthesis, depending on e.g. temperature.

Other botanical information

The many hundreds of cultivars may be grouped into cultivar groups. However, intermediate types exist and a clear-cut distinction is difficult. Below a practical grouping is presented, mainly derived from the classification proposed by Rodenburg.

• Cv. group Butterhead Lettuce (synonyms: var. capitata L. p.p., var. capitata L. nidus tenerrima Helm): head lettuce. Soft solid heads of overlapping leaves; inner leaves thin, oily, buttery in texture. Originated in western Europe. The most popular lettuce of cool temperate areas; less popular in the tropics. Cultivar type: "May Queen"; numerous cultivars.

• Cv. group Crisp Lettuce (synonyms: var. capitata L. p.p., var. capitata L. nidus jaggeri Helm): iceberg lettuce, ice lettuce, cabbage lettuce. Thick crispy leaves with prominent flabellate veins and midribs; non-heading or slightly heading types occur next to cultivars with heavy firm cabbage-like heads. Originated in France, very popular in the United States. The most popular type in warm temperate and subtropical areas and in the cooler tropical areas (highlands, cool season in the lowlands). Cultivar type: "Great Lakes"; numerous cultivars.

• Cv. group Cos Lettuce (synonyms: var. romana hort. ex Bailey, var. longifolia Lamk): romaine lettuce. Long narrow leaves, forming a tall, loose, upright, cylindrical head. Eaten raw or cooked as spinach. Originated in southern Europe. Fairly common in the tropics. Cultivar type: "White Parish Cos"; many cultivars.

• Cv. group Bunching Lettuce (synonyms: ssp. acephala Alef. var. secalina Alef., var. crispa L.): leaf lettuce, loose-leaf lettuce, curled lettuce, cutting lettuce. Thin, broad, smooth or curled or crinkled, green or reddish leaves in a loose rosette or on a short stem, marketed by bunching of 3-10 plants. Very common in tropical areas. Cultivar types: "Salad Bowl", "Simpson", "Oakleaf"; many cultivars and landraces.

• Cv. group Stem Lettuce (synonyms: var. asparagina Bailey, var. angustana Irish ex Bremer): asparagus lettuce, stalk lettuce, celtuce. In the United states called "celtuce" because peeled stems are used as celery stalks and the leaves are used as lettuce. Grown for the fleshy 30-50 cm long, 3-6 cm thick stem which has a crisp texture and a faint lettuce taste. The stem bears many leaves with a rosette at the apex. Young leaves also edible and popular in South-East Asia. Originated in China, spreading to South-East Asia (Indonesia, Malaysia, Thailand). Cultivar type: "Celtus"; many cultivars and landraces.

• Cv. group Latin Lettuce: grasse. Small dark-green plants growing in a rosette or forming a loose head, with thick leathery leaves. Tolerant of high temperatures. Popular in France. Cultivar types: "Sucrine", "Creole".

Ecology

Lettuce grows best at moderate day temperatures of 15-20 °C and cool nights. In the tropics it thrives best in the highlands and during the coolest season in the lowlands. Headed cultivars usually form only a loose head at temperatures above 25 °C. Crisphead lettuce is more tolerant of high temperatures than butterhead lettuce. When day temperatures rise above 28 °C the heads will be very loose or will not form. For this reason, most lettuce grown in tropical lowlands is leaf lettuce. Lettuce shows a slight quantitative long-day reaction, but most modern cultivars are almost day-neutral. Bolting is strongly promoted by high temperatures.

Lettuce can be grown on any soil type with a good structure and high fertility. The water-holding capacity is important because the root system of lettuce is relatively small, which makes the crop very vulnerable to drought. Lettuce is often grown on slightly alkaline sandy-loam soils. It does not tolerate acid soils (pH < 6).

Propagation and planting

Many lettuce farmers in South-East Asia use seed of their own selection. If not properly stored, lettuce seed rapidly loses its viability. Plants of headed types and also stem lettuce are normally raised in a nursery. The seeds are sown in a shaded seed-bed and are pricked out one week later (in the two-leaf stage) in banana-leaf pots or soil blocks of 4 cm × 4 cm. The sowing rate is about 200 g/ha.

Leaf lettuce is usually sown directly in the field in drills 30 cm apart. For this cultivar group a regular plant density is less important. Seed requirement for direct sowing is about 0.5 kg per hectare. In heavily mechanized cultivation systems in western countries, headed lettuce cultivars are also sown directly in the field using precision drilling machines, but this practice is inappropriate for small farmers in the tropics. Crisphead lettuce is planted out in the field at 30 cm in the row and 50 cm between the rows (60 000 plants/ha) or at 35 cm × 35 cm (80 000 plants/ha). Butterhead lettuce may be planted more closely, depending on the mature head size of the cultivar, usually at 30 cm × 30 cm.

Husbandry

Young lettuce cannot compete with fast-growing weeds. Several weedings are needed in the first month when the soil surface is not yet covered by the lettuce plants. Further, the water supply must be very regular. The evapotranspiration increases fast, from 2-3 mm/day in the first weeks to 6-8 mm for the fully grown crop. Water shortage causes Ca deficiency in young leaves resulting in tip burn, an internal necrosis of the leaf margins in the head followed by bacterial rot.

Lettuce is a crop with a moderately high uptake of minerals. Depending on the soil conditions, a suitable fertilizer recommendation is 30 t/ha of farmyard manure combined with 50 kg N, 100 kg P₂O₅ and 80 kg K₂O before planting; a side dressing of 50 kg/ha N is given 3 weeks after planting and again 3 weeks later if needed. The mineral uptake (N, K) is low during the first month after sowing and highest in the last weeks before harvest. Too much nitrogen makes the crop susceptible to tip burn and diseases, and increases the content of free nitrates in the harvested product.

Diseases and pests

Apart from the physiological disorder tip burn, the most serious diseases of lettuce in the tropics are mosaic, bottom rot and downy mildew. Mosaic is caused by lettuce mosaic virus (LMV) and other viruses and may be controlled by the use of healthy seed, control of aphids and immediate removal of diseased plants. Bottom rot caused by Rhizoctonia solani commonly occurs under wet conditions. The symptoms are a slimy rotting of the underside of the plant, progressing into the head. Sclerotinia causes a wet rot of the entire plant, beginning at the stem base. Downy mildew caused by Bremia lactucae is the most serious disease of lettuce in temperate areas and in cooler locations in the tropics. The best control of bottom rot and Sclerotinia is good sanitation, crop rotation and drainage. Downy mildew is controlled by using cultivars with resistance to the relevant race of the fungus or by spraying with fungicides. Damping-off (Pythium), grey mould (Botrytis) and leaf-spot (Cercospora) are also reported on lettuce in the tropics.

The most serious pests are aphids, especially in headed lettuce, because they cannot easily be controlled by spraying with chemicals, which is, moreover, risky because of residues. Other pests are Agrotis cutworm, army worm (Spodoptera) and other caterpillars, leafhoppers, snails and slugs, and root knot nematodes. The insect pests are usually controlled by chemical sprayings. Nematodes in lettuce are kept under control by crop rotation, disinfection of the seed-bed or nursery soil by heating, and the use of ample manure.

Harvesting

The time to harvest depends upon the cultivar and purpose. Harvesting of headed lettuce is commenced when the heads are fully developed, usually 60-80 days after planting. Harvesting is done by cutting the plants at their base or, for bunching lettuce, by uprooting. Old outer leaves are trimmed off. Leaf lettuce can be harvested at any time from the young stage until bolting starts. The younger the lettuce, the more tender it will be, but also the lower the yield. Leaf lettuce is harvested between 30-50 days after sowing.

Yield

For headed lettuce a yield of 70% or more of the number of plants originally planted may be considered a satisfactory result. Successful farmers may reach 90%. The average world yield is 10 t/ha. A harvest of 50 000 heads/ha with an average weight of 300 g yields 15 t/ha. Yields above 20 t/ha are reported, but in the tropics the yield level usually reaches only 5-10 t/ha. Yields of leaf lettuce are lower than for headed lettuce (3-8 t/ha). Stem lettuce harvested at 80-100 days after planting may yield up to 20 t/ha.

Handling after harvest

Lettuce wilts easily. The most suitable packing of headed lettuce (butterhead, crisphead) is in open-topped polythene bags which are put in crates or boxes. Cooling or packing with ice greatly improves keepability. Headed lettuce is further trimmed if old or damaged outer leaves are still present. Plants of headed cultivars which have not produced a head of marketable size are often uprooted and bundled in bunches of 3-8 plants. Uprooted lettuce in street markets is kept fresh by putting the roots in a basin with water, as is done with kangkong and amaranth.

Genetic resources

Large germplasm collections of lettuce and wild Lactuca species are kept in the Netherlands (Centre for Genetic Resources, Wageningen), Commonwealth of Independent States (Vavilov Institute of Plant Industry, Petersburg), United Kingdom (Horticultural Research International, Wellesbourne), and the United States (USDA Agricultural Research Station, Salinas).

Lettuce was introduced into South-East Asia hundreds of years ago. Many local selections or landraces have developed which may contain valuable genes for disease resistances, heat tolerance, and other traits. There is a great risk of genetic erosion of this material, since seed companies promote improved cultivars.

Breeding

Many hundreds of cultivars have been bred in temperate countries (Europe, North America, Japan) with a large variation of very specific characters. Resistance to mosaic and downy mildew is common, but not to tip burn and bottom rot. Low nitrate content is a selection criterion in temperate countries. Some selection criteria for tropical headed lettuce cultivars are a short growing period, slow bolting, large compact heads which are not easily damaged during transport.

Prospects

Lettuce continues to increase in popularity in all tropical countries and will be universally grown for local markets and export. Research should focus on the selection of heat-tolerant cultivars and non-chemical control of diseases and pests.

Literature

• Bantoc Jr, G.B., 1967. Lettuce. In: Knott, J.E. & Deanon Jr, J.R. (Editors): Vegetable production in South-East Asia. University of the Philippines Press, Los Baños, the Philippines. pp. 329-341.
• Boukema, I.W., Hazekamp, Th. & van Hintum, Th.J.L., 1990. The CGN lettuce collection. Centre for Genetic Resources (CGN), Wageningen, the Netherlands. 53 pp.
• Feráková, V., 1977. The genus Lactuca L. in Europe. Univerzita Komenského, Bratislave, Slovenia. pp. 67-68.
• Kesseli, R., Ochoa, O. & Michelmore, R., 1991. Variation at RFLP loci in Lactuca spp. and origin of cultivated lettuce (L. sativa). Genome 34: 430-436.
• Mansfeld, R., 1986. Verzeichnis landwirtschaftlicher und gärtnerischer Kulturpflanzen (ohne Zierpflanzen) [Register of agricultural and horticultural plants in cultivation (without ornamentals)]. Schultze-Motel, J. et al., editors 2nd edition. Springer Verlag, Berlin, Germany. pp. 1330-1333.
• Purseglove, J.W., 1968. Tropical crops. Dicotyledons. Longman, London, United Kingdom. Vol. 1. pp. 74-77.
• Rodenburg, C.M., 1960. Salatsorten [Lettuce types]. Instituut voor de Veredeling van Tuinbouwgewassen, Wageningen, the Netherlands. 228 pp.
• Ryder, E.J., 1986. Lettuce breeding. In: Bassett, M.J. (Editor): Breeding vegetable crops. Avi Publishing Company, Westport, Connecticut, United States. pp. 443-474.
• Thompson, H.C. & Kelly, W., 1957. Vegetable crops. McGraw-Hill Book Company, London, United Kingdom. pp. 255-269.

Authors

• G.J.H. Grubben & S. Sukprakarn