Prota 2: Vegetables/Légumes
Sp. pl. 2: 795 (1753).
2n = 18
Lettuce (En). Laitue (Fr). Alface (Po). Saladi (Sw).
Origin and geographic distribution
The origin of lettuce is in Turkey and the Caucasus or the Middle East. The ancestor is probably the European prickly lettuce (Lactuca serriola L.), that crosses easily with the cultivated forms. Lettuce was known as a vegetable in the Mediterranean as early as 4500 BC; it was depicted in Egyptian tombs in 2500 BC and cultivated by the Greeks and Romans as a popular vegetable. In Western Europe, headed types have been known since the 14th century but leafy types have been known for much longer. At present lettuce, especially the headed types, is the world’s most important salad crop. Salads are traditionally more popular in temperate areas than in the tropics, but lettuce is increasingly important in Africa as an exotic, European type of vegetable, grown for the city markets, supermarkets, restaurants and hotels. It can be found in all African countries, most frequently at higher elevations and in the cooler season, and more often in francophone than in anglophone countries.
Lettuce is grown for its leaves, that are usually eaten raw as a salad with a dressing of oil and vinegar. Occasionally lettuce is used as a cooked vegetable, especially in lowland areas. In China, a form of lettuce with a thickened stem is eaten as a cooked vegetable.
Production and international trade
Worldwide, lettuce is one of the leading vegetables, with a registered area of about 880,000 ha producing some 20 million t of marketed product. With about 370,000 ha, China is the main producer followed by the United States with 125,000 ha. Other important lettuce producers are the European Union, Japan and India.
In tropical Africa salad vegetables are less popular and lettuce production is modest, although widespread. Lettuce consumption is concentrated in the urban centres, especially in francophone countries. Statistical data on cultivated areas and production in Africa are lacking. Since lettuce is a very perishable product destined for urban consumption, it is mainly produced in the proximity of the big cities. In Africa, it is rarely if ever traded internationally.
After removal of the outer leaves of fresh iceberg lettuce, the composition of the remaining edible portion (83% of the weight) per 100 g is: water 95.6 g, energy 53 kJ (13 kcal), protein 0.7 g, fat 0.3 g, carbohydrate 1.9 g, dietary fibre 0.6 g, Ca 19 mg, P 18 mg, Fe 0.4 mg, β-carotene 50 μg, thiamin 0.11 mg, riboflavin 0.01 mg, niacin 0.3 mg, folate 53 μg, ascorbic acid 3 mg. The carotene value of the pale green to white inner leaves is low, while the darker green outer leaves may contain 50 times as much carotene. In general headed types with a low chlorophyll content (pale green leaves) have fewer micronutrients than leafy types; the dark green types have considerably larger amounts of carotene, Fe and vitamin C. The composition of butterhead lettuce per 100 g edible portion (76%) is: water 94.4 g, energy 52 kJ (12 kcal), protein 0.9 g, fat 0.6 g, carbohydrate 1.2 g, dietary fibre 1.2 g, Ca 53 mg, Fe 1.5 mg, β-carotene 910 μg, thiamin 0.15 mg, riboflavin 0.03 mg, niacin 0.5 mg, folate 57 μg, ascorbic acid 7 mg (Holland, B., Unwin, I.D. & Buss, D.H., 1991).
The presence of free nitrates is seen as a negative quality factor causing health problems to susceptible individuals. In the Netherlands, the legally tolerated maximum content of NO3 is 2.5 mg per g fresh weight. The nitrate content decreases with increasing light intensity and is no problem in tropical countries. Lettuce contains a milky juice in which several lactones have been identified including lactucin and lactucopicrin, both with analgesic and sedative properties. Many cultivars contain anthocyanin.
Glabrous, annual herb up to 100 cm tall, containing latex, forming a dense basal rosette and later a tall, branched, flowering stem; root system shallow, but with a strong, fleshy taproot. Rosette leaves loose or arranged in more or less compact heads; petiole short; lamina undivided to sawtoothed or pinnatifid, sometimes curly and fringed, pale to dark green or with red or brown anthocyanin pigment; stem leaves arranged spirally, sessile, progressively smaller, ovate to orbicular in outline, entire, cordate-amplexicaul. Inflorescence a head; many heads arranged in a dense, corymbose, flat-topped panicle; involucre 10–15 mm long, consisting of 3–4 rows of lanceolate or ovate bracts. Flowers 7–35 per head, bisexual; corolla ligulate, yellow; stamens 5, with connate anthers; ovary inferior, 1-celled, style bifid. Fruit a narrowly obovate achene 3–8 mm long, compressed, ribbed, white, yellowish, grey, brown or black, with narrow beak, surmounted by a white pappus of 2 equal rows of soft hairs. Seedling with epigeal germination.
Other botanical information
The many hundreds of cultivars can be grouped in types or cultivar-groups. Cultivars of different types may easily be crossed among each other and spontaneous crossings occur. Intermediate types exist, making a completely clear distinction difficult. The following 4 types (defined as cultivar-groups in PROSEA 8: Vegetables) occur in tropical Africa, ordered by decreasing importance.
– Crisp lettuce, also named: iceberg lettuce, ice lettuce, cabbage lettuce. Thick crispy leaves, outer leaves dark green, with prominent flabellate veins and midribs; mostly cultivars with heavy firm cabbage-like heads weighing up to 1 kg, interior white to creamy yellow. Non-heading or slightly heading types occur. This is the most popular lettuce type in warm temperate and in subtropical areas, in tropical highlands and during the cool season in the lowland tropics. The standard cultivar type is ‘Great Lakes’, but there are many other cultivars, e.g. ‘Reine des Glaces’ (early, resistant to heat and bolting ), ‘Minetto’ (very early, small, heat resistant, resistant to tipburn) and ‘Trinity’ (compact, heavy, resistant to heat, tipburn, bolting, Cercospora, Sclerotinia). ‘Batavia’ is a crisp lettuce type originating from France, with thinner, undulated leaves, heads medium heavy, weighing up to 500 g, firm or loose. It is popular in francophone countries. Standard cultivar type is ‘Blonde de Paris’, and the traditional American cultivar called ‘Iceberg’ also belongs to this group. The African cultivar ‘Blonde de Yamoussoukro’ was developed in Côte d’Ivoire.
– Bunching lettuce: leaf lettuce, loose-leaf lettuce, curled lettuce, cutting lettuce. Leaves thin, broad, smooth, curled or crinkled, green or reddish, in a loose rosette or on a short stem; marketed in bunches of 3–10 plants. Common in African countries. Standard cultivar types are ‘Salad Bowl’, ‘Simpson’, ‘Oakleaf’, but many cultivars exist.
– Butterhead lettuce: head lettuce. Soft solid or loose heads of overlapping leaves, pale green, inner leaves thin, oily, buttery in texture, slightly undulate. Heads of up to 350 g. Originated in Western Europe. Butterhead is a popular lettuce type in temperate climates, but less common in the tropics. A standard cultivar type is ‘May Queen’, and many cultivars exist, e.g. the heat resistant ‘Kagraner’.
– Cos lettuce: romaine lettuce. Long narrow leaves, forming an upright, cylindrical head. Eaten raw or cooked. Originated in southern Europe. Fairly common in francophone countries and in Sudan. Standard cultivar type is ‘White Parish Cos’; few cultivars exist, e.g. ‘Verte Maraîchère’.
A fifth type, not reported from tropical Africa, is stem lettuce: asparagus lettuce, celtuce. This type is grown for the fleshy 30–50 cm long and 3–6 cm thick stem, that has a crisp texture and a faint lettuce taste. The stem bears many leaves with a rosette at the apex; the young leaves are also edible. Stem lettuce originated in China and is popular in East Asia and locally in South-East Asia. The standard cultivar type is ‘Celtus’. Oilseed lettuce is a primitive type with larger seeds, grown in Egypt for the cooking oil pressed from the seed.
Growth and development
The seed germinates within 1–4 days, at temperatures from 15–25°C. Young seed less than one month old shows some dormancy with retarded germination. Lettuce seed often shows dormancy when it has been stored at high temperature and is sown at a soil temperature above 25°C, which is the normal situation in tropical lowland. The best remedy is to store the wetted seed in a refrigerator at 2–5°C for 1–3 days. Growth of young lettuce plants is exponential, slow in the beginning and very fast in the last weeks before the harvest stage. Rosette formation becomes apparent in the third week after emergence, and head formation in 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 start bolting when 2–3 months old. Inflorescence 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 open for 1–2 hours in the morning and do not open all at once. Lettuce is an obligate self-pollinating crop. Spontaneous cross-fertilization rarely occurs (less than 1%). Seed matures in 9–13 days after anthesis, depending on temperature. Seed is produced abundantly, 10–30 g per plant.
Lettuce grows best at moderate day temperatures of 15–25°C and cool nights of 10–15°C. In the tropics it thrives best in the highlands and during the cool season. Above 25°C, headed cultivars usually form a loose head. When day temperatures rise above 28°C, the heads will be very loose or no head formation will take place. Heat tolerant cultivars have a better head formation at high temperatures. Crisp lettuce is better adapted to high temperatures than butterhead lettuce. Lettuce grown in tropical lowlands is either the bunching type, or densely grown crisp lettuce harvested as bunching 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 lettuce has a relatively small root system, which makes the crop vulnerable to drought. Lettuce is often grown on neutral sandy-loam soils (pH 6.5–7.2). It does not tolerate acid soils (pH < 5.5). In temperate countries lettuce is partially grown out of season and often in greenhouses.
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. In addition, the water supply must be very regular. The farmers normally sprinkle their lettuce beds twice daily, during the morning and in the evening, or during the day. The evapotranspiration increases fast, from 2–3 mm/day in the first weeks to 6–8 mm for a full-grown crop. Sprinkling the crop with water from a polluted source such as a local stream containing sewage or passing through dumping grounds should be avoided because of health risks.
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, 45 kg P and 65 kg K before planting. Many farmers only use poultry manure, applied freely and worked into the beds before planting. An N side dressing of 50 kg/ha 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 nitrate in the harvested product.
The physiological disorder tip burn is probably the most serious problem of lettuce in the tropics. The symptoms are an internal necrosis of the leaf margins in the head, often followed by bacterial rot. The necrosis is caused by water shortage in hot weather and fast growing conditions, which lead to Ca deficiency in the young leaves. It can be controlled by use of tolerant cultivars, liming before planting, slowing down of the growth by limiting the N-fertilizing, a light shading and especially by always keeping an ample and even supply of soil moisture.
Propagation and planting
Lettuce is propagated by seed; the 1000-seed weight is 0.8–1.2 g. Many lettuce farmers in Africa use seed of their own selection and only about half of the seed required in Africa is imported. Especially in semi-arid regions it is easy to produce good lettuce seed, but if not properly stored, lettuce seed loses its viability very fast.
Plants of headed types are normally raised in a nursery. Seeds are sown in a shaded seedbed, the seedlings pricked out 2–3 weeks after emergence, and planted in the field or in soil blocks of about 4 cm × 4 cm, which are planted out in the field 2–3 weeks later. Somewhat older plants are sturdier and easier to handle. With optimal cultural practices, the seed requirement is only 200 g/ha (1 g seed for 50 m2), but under suboptimal conditions the seed requirement is much higher. Crisp lettuce can be planted out in the field at 50 cm between rows and 30 cm within the row (66,000 plants/ha) or at 35–60 cm × 35 cm (50,000–90,000 plants/ha). Butterhead lettuce may be planted somewhat closer together, depending on the mature head size of the cultivar, at 25 cm × 20–25 cm. The spacing for cos lettuce can be even narrower, although farmers may prefer to leave a path of 45 cm between double rows spaced at 15 cm. Bunching lettuce is usually sown directly in the field in drills 20 cm apart, and thinned to 5–10 cm, 10–20 days after emergence. African farmers often plant very dense, at 7–9 cm × 7–9 cm. Crisp lettuce like ‘Great Lakes’ or ‘Blonde de Paris’ is also often planted densely to produce a bunching type lettuce. The seed requirement for direct sowing under optimal conditions is about 0.5 kg per ha, or 1 g seed per 20 m2. Heading lettuce is also sown directly in the field. In that case, plants should be thinned to 30 cm within the row. In strongly mechanized cultivation in western countries headed lettuce is sown directly in the field, using pelleted seed and precision drilling equipment.
Diseases and pests
Many fungal, bacterial and viral diseases infest lettuce in the tropics. Common diseases are mosaic virus, bottom rot and downy mildew. Mosaic is caused by lettuce mosaic virus (LMV). It 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, the most serious disease of lettuce in temperate areas, occurs in cooler parts of the tropics. The best control of bottom rot and Sclerotinia is good sanitation, crop rotation and drainage. It is recommended to plant on ridges instead of on flat land. Downy mildew is controlled by the use of 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. Cercospora longissima caused heavy losses on lettuce in Côte d’Ivoire. Infection takes place at high humidity by splashing from the soil, and control is possible by less dense planting and by spraying with benomyl.
The most serious pest are aphids, especially in headed lettuce, because they cannot be controlled easily by spraying with chemicals, which is, moreover, risky because of residues. Other pests are cut worm (Agrotis), army worm (Spodoptera) and other caterpillars, leaf hoppers, snails and slugs, and root-knot nematodes. Insect pests are usually controlled by spraying chemicals. Nematodes in lettuce can be kept under control by crop rotation, disinfection of the seedbed or nursery soil by heating, and fertilizing with ample organic matter, e.g. manure.
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. Bunching lettuce and densily planted heading cultivars are mostly uprooted for bunching. Old outer leaves are trimmed off. Bunching lettuce is usually harvested between 30–50 days after sowing, but can be harvested at any time from the young stage until bolting starts. The younger it is, the more tender the lettuce will be, but also the lower the 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 about 20 t/ha. Yields above 30 t/ha are reported from temperate areas, but in the tropics they are much lower. In tropical highland a harvest of 50,000 heads/ha with an average weight of 200–300 g yielding 10–15 t/ha is reachable. Crisp lettuce in the Kenyan highlands is reported to yield 15 t/ha with heads of 300–500 g. Yields of bunching lettuce reach only 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 is in open-topped polythene bags which are put in crates or boxes. Fast cooling to 0–2°C and packing with ice improve keepability. Commercial market gardeners may dip the cleaned heads in cold water and immediately put the produce in a cool place. Headed lettuce is further trimmed if old or damaged outer leaves are still present. In Nigeria, much lettuce is produced in the savanna area, packed in jute bags and transported over long distances to the cities in the south, resulting in high losses. Plants of headed cultivars which have not produced a head of marketable size (less than 150 g) are often uprooted or cut 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. It is advisable to wash lettuce carefully before serving to avoid intestinal diseases and parasites.
Large germplasm collections of lettuce and wild Lactuca species are kept at institutes in the Netherlands (Centre for Genetic Resources, Wageningen), Russia (Vavilov Institute of Plant Industry, Petersburg), United Kingdom (Institute of Horticultural Research, Wellesbourne) and the United States (National Seed Storage Laboratory, Fort Collins, Colorado).
Lettuce was introduced in Africa centuries ago. 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.
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, and cultivars with resistance to aphids, tip burn and bottom rot occur. Important selection criteria for tropical headed lettuce cultivars are heat resistance, a short growing period, slow bolting, and compact heads that are not easily damaged during transport. The seed company Tropicasem in Senegal performs lettuce breeding for African conditions.
The popularity of lettuce in tropical countries is increasing. Lettuce will be increasingly grown for city markets and export. Research should focus on breeding of tropical cultivars resistant to pests and diseases and on non-chemical control of diseases and pests.
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Sources of illustration
• Grubben, G.J.H. & Sukprakarn, S., 1993. Lactuca sativa L. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 186–190.
Correct citation of this article:
Grubben, G.J.H., 2004. Lactuca sativa L. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, habit (butterhead lettuce); 2, habit (crisp lettuce); 3, habit (bunching lettuce); 4, habit (stem lettuce); 5, habit (cos lettuce).
crisp (or iceberg) lettuce