Lactuca sativa L.
Asteraceae (sin. Compositae)
Origins and history
It is hypothesised that the cultivated species derives from the wild species Latuca serriola L. that was cultivated for pasture and oil dating back to Egyptian times. There is archeological evidence from pictures found in Egyptian tombs that it was cultivated before 4500 B.C. The Egyptians attributed an aphrodisiac effect to wild lettuce.
Cultivation spread throughout the Mediterranean with the Greeks and Romans and then into the rest of Europe. The Romans called it “lactuca” in reference to the milky liquid that came out of the cut stem; the milky liquid left to dry turns brown and was called “lattucario” or “lettuce opium” because it has a mild sleep-inducing effect.
In the New World its cultivation was first registered in 1494.centro primario di origine sembra il Medio-Oriente.
More specifically, beyond regulating the function of the intestines, fibre helps in reaching a sense of fullness during meals and so limiting the consumption of foods with high energy density; it also seems to have a positive role in controlling the level of glucose and cholesterol in the blood.
Additionally, lettuce contains numerous bioactive molecules (carotenoids, chlorophylls, polyphenols…) that have a positive effect on improving health and wellbeing and/or reducing the risk of disease: they slow cellular ageing by fighting free radicals; strengthen the immune system; improve intestinal function; protect from pathogens in the cardio-circulatory system.
Lettuce contains lutein and beta-cryptoxanthin (carotenoids that strengthen the immune system, protect from sun rays, free radicals, and some tumours) and flavonols (polyphenols with functions against oxidising agents, metal chelants, and have anti-inflammatory, antibacterial and antiviral actions).
Diffusion and importance
In Italy, around 17,000 ha are cultivated in open-air and 4,000 ha in greenhouses with a total production of about 500,000 t. Production is particularly widespread in Puglia (27%), Sicily (13%), Campania (10%), Emilia-Romagna (9%) and Latium (8%).
Lettuce is destined for the fresh market and for the industrial preparation of ‘ready-prepared salads’.
– var. capitata (L.) Janchen: headed lettuce with smooth leaves, entire margin that form central ball (head);
– var. crispa L.: headed lettuce with curly rumpled leaves, undulated shape, jagged or curly margin, that form a central ball (head);
– var. longifolia (Lam.) Janchen: Roman lettuce with long leaves, ribbed, rising that form an open or semi-closed tuft;
– var. acephala Dill.: leaf lettuce with smooth or curly leaves, ribbed;
– var. angustana Auct.: asparagus stem lettuce.
Biological type: Lettuce is an annual herbaceous plant.
Root system: Tap root, rather superficial (most of it radiates only within the first 30 cm of the soil), with numerous lateral branching.
Stem: Is very short.
Leaves: Are of differing form, dimension, number and behaviour depending on the botanical variety.
Inflorescence: Form in the second year, especially when stimulated by temperature and photoperiod: lettuce is a long day species even if the botanical varieties show different sensibilities to the length of day (L.s. var. capitata < longifoglia < crispa).
The flowers are small, hermaphrodite, with yellow or yellowish petals, grouped in a capitulum on a flower stem, with bracts, very branched, height of up to 150 cm. It is a self-pollinating species with about 6% of out-crossing.
Fruit: Is an achene (wrongly called “seed”), with thistledown, oval, slightly arched or almost straight, from light grey to dark brown in colour and varying in weight from 1,0-1,5g / 1000 achenes depending on the cultivar.
Climate and soil requirements
Lettuces have low thermal requirements (see table): they grow with a temperature of at least 5°C but they do not have a strong resistance to the cold (not lower than a few degrees below zero) even if genetic improvement has always selected more resistant varieties (destined especially for autumn and winter crops). Therefore in Italy, lettuce is cultivated in an open-field during the spring, summer and the first part of autumn in the center-north, and in the winter only in the south and in some areas of the centre.
Thanks to the diffusion of protected crops (tunnels and greenhouses), today lettuce is cultivated during the autumn and winter periods in many areas of the northern hemisphere, and so, with the opportune combination of variety, cultivation zone and means of protection, the production and consumption of lettuce is no longer seasonal.
Lettuce adapts to all different types of soil although it prefers free draining, with a good amount of organic matter and with a pH between 6 and 7. It has high water requirements. It is sensitive to salinity.
Varieties and/or commercial types
– cabbage lettuce: forms a compact head in the shape of a bowl, with smooth leaves, entire margin, generally tender, green colour varying in darkness;
– romaine: elongated leaves, with very evident central venation, crispy and robust;
– iceberg (also called “brasilian”): leaves with indented margin, forms a compact head, spherical (sold with the external leaves removed);
– summer crisp lettuce (Batavia): with green leaves (fair coloured) and with red leaves (known as the Canasta type): with a head more or less open, rumpled, with a slightly indented margin; the following types belong to this group gentilina (light green, open, finely rumpled, convex bottom), trentina (red), triestina, agostana…;
–loose leaf or oak leaf lettuce: green (fair coloured) and red: dense bowel-shaped head with leaf shape similar to that of an oak;
– lollo: green (fair coloured) and red: dense spherical head, with curly leaves of varying degree, very rustic;
– barba dei frati (known also as barbina or catalogna): green or red: erect head, pointy lobated leaves;
– lettuce for cutting.
The different cultivars, mostly F1 hybrids, are chosen for the following characteristics:
* dimensions and compactness of the head;
* response of the morphologic characteristics depending on the desired typology;
* resistance and/or tolerance to the following biotic and abiotic adversities:
* mildew (Bremia lactucae) strains 1-26;
* virus (es. LMV = Lettuce Mosaic Virus);
* other fungal diseases (es. Alternariosi…);
* tip burn (necrosis of the leaf margine): non parasitic physiopathy;
* adaptability to differing cultivation enrivonments;
* destination of the product (fresh market, ready-prepared).
The choice of the variety most adapted to the cultivation area and period is very important not only for resistance to the most common diseases (mildew, virosis, rot, aphids..) (more resistance, fewer treatments!) but also to avoid undesired early flowering (pre-flowering is favoured by high temperatures and long days).
Fundamentals of cultivation techniques
All lettuces, except for cutting lettuce, are generally transplanted. The plantlings should be compact and not discoloured due to lack of light nor too supple and watery; there should be a good equilibrium between the root system and the leaves. The plantlings should have 2-3 leaves when planting in spring-summer beginning of autumn (when the temperatures are opportune) and 4-5 leaves when planting in lower temperatures (late autumn-winter). It is also important that the plantlings are immune to diseases. The plantlings must be transplanted with all of the potting mix below the soil surface.
The distance between the rows and the number of plants per m2 depends on the size of the head: depending on the type, variety and season, the rows should be spaced about 30 cm apart and the density should vary from 7 (ex. romaine lettuce, iceberg) to 14 (ex. cabbage lettuce , summer crisp, lollo) plants per m2.
Lettuce needs soil that is not waterlogged: for better water drainage, cultivate on slightly raised rows.
It is recommended to use a black plastic film as a mulch to impede the growth of weeds. The black plastic film helps to heat up the soil, shorten the production time, and guarantee a cleaner lettuce head because it does not come into contact with the soil.
Protection from the cold
In outdoor open-air plantations, to protect the precocious transplanted plantlings from the cold in winter/beginning of spring, it is possible to cover them with non-woven fabrics that are very light and do not impede the plant from growing underneath; the fabrics are permeable to water but protect from the intense impact of water drops that can ruin the leaves and dirty them with soil.
Nitrogen fertilisation should not be overdone because lettuces tend to physiologically accumulate nitrates that if ingested contribute to the formation of toxic nirtrosamines.
Irrigate frequently with low volumes of water enough to maintain a moist soil (lettuce transpires a lot of water through its leaves but the root system is not well developed or efficient), being careful to avoid waterlogging; localised irrigation is preferred (rain irrigation wets the leaves and renders them liable to fungal diseases).