Lettuce thrives well when temperatures are low i.e.13°C–18°C. Seed germination is affected at very high as well as at very low temperatures. Seed germination may severely be affected when temperatures rise above 80 °F (27 °C). At very high temperatures, bolting occurs. Hence plants cultivated in hot areas may be provided with shading.
Lettuce is suitable for protected cultivation under glasshouses and greenhouses. Increased CO2 atmosphere (1,000–1,500ppm) within glasshouses favors plant growth and provides high yield. Lettuce is suitable for hydroponic growing also. Hydroponics in controlled environment is most suitable for growing lettuce.
Sandy loam soil (loose nitrogen-rich soils) is the most ideal soil for lettuce cultivation. It is highly sensitive to acidic soils. Optimum pH is 6.0–6.5. Propagation of lettuce is through seeds. 200-250 grams of seeds is sufficient to raise seedlings for planting one acre area. Chilling treatment by keeping seeds in moist sand at 4–6°C for 3–5 days is recommended to break seed dormancy. Instead of moist sand, seeds may be kept in refrigerators in moist cloth.
Seeds are normally sown during cool months i.e. October–November. Seedlings are raised in nursery beds. Sometimes direct sowing in the main field is also practiced. In case of nursery-raised seedlings, 5–6 week old seedlings are transplanted in the main fields. Recommended spacing is at 35-45 cm × 35-45 cm depending upon the variety.
For plant nutrition, 20 -25 tons of farmyard manure or compost is incorporated into the top soil for one hectare area. In addition to this, a basal dose of NPK fertilizers@ 25:90:25kg/ha must also be given to the plants. At the time of head formation or rosette formation in the plants a second dose of nitrogen is given @25–30kg N/ha.
Regular hoeing and weeding is recommended until plants get established and thereafter hoeing and weeding must be stopped. First hoeing is generally done after 3 – 4 weeks of planting seedlings. In total, 2–3 hoeing and weeding are recommended. Manual weeding is generally recommended.
Lettuce plants need lots of water. One pre-sowing irrigation is recommended before sowing seeds on the nursery beds or the main fields. Second irrigation is given soon after transplanting the seedlings on the fields. After that a light irrigation is given 4 days after transplanting. Since moisture stress results in bolting of the plants, keep the soil moist always.
Generally, a lettuce plant takes about 65–130 days from planting to harvesting depending upon the variety and growing conditions.
Correct maturity stage is when heads are fully developed and becomes compact. A compact head which can be compressed with moderate hand pressure is considered ideal maturity. A mature head has a good flavour. Avoid harvesting when there is rain, because the turgid leaves are crisper and therefore break easily on handling. A very loose head is immature and a very firm or hard head is over mature.
Under good cultural management conditions, growers may get an average yield of 10–15 tons/ha.
Grading: As of now, there are no national or international standards for grading lettuce heads. As a rule, large compact heads with crispy leaves are considered as the best grade. Diseased and injured heads and leaves are never acceptable.
Product Treatment to Enhance Shelf Life: Pre- and postharvest applications of BA (5–10ppm) help delay senescence in storage and improve the shelf-life.
Quality Indices: Major quality parameters are appearance of leaves and compactness of head. For the best quality product, appearance of leaves is of bright, light green color with good crispiness and turgidity. Head is large, and compact.
Precooling Requirements: Vacuum cooling and forced-air cooling are used successfully in crisphead lettuce.
Optimum Temperature and Relative Humidity: Optimum Temperature is 0°C (32°F) and Optimum Relative Humidity is more than 95% RH.
Product life: Shelf life at 0°C (32°F) and >95% RH is 21-28 days. Shelf life at 5°C (41°F) is 14 days (if no ethylene is present in the storage environment). Freezing injury may occur at <-0.2°C (31.7°F).
Ethylene Production and Sensitivity: Rates of Ethylene Production is very low in crisphead lettuce; it is <0.1 μL/kg·hr at 20°C (68°F). Crisphead lettuce is extremely sensitive to ethylene and a major symptom of ethylene injury is ‘Russet spotting’.
Controlled Atmosphere Storage (CA Storage): It is believed that CA storage at low O2 and high CO2 atmospheres at 0-5°C (32-41°F) enhances shelf life in lettuce heads. While low oxygen concentrations in CA reduce respiration process and ethylene-induced detrimental effects of the produce, low oxygen and high carbon di oxide atmosphere control browning on the cut surfaces in fresh-cut lettuce products.
Physiological Disorders: A major physiological disorder found in lettuce is tip burn. Tip burn i.e. burning of lateral margins of inner leaves of mature head which is caused by unfavourable climatic factors and calcium deficiency is a major physiological disorder in lettuce. This can be controlled by applying calcium chloride.
Russet Spotting is also a major disorder found in lettuce. This is due to exposure to low concentrations of ethylene. Here spots appear as dark brown spots on the midribs which make the lettuce unmarketable. Never store lettuce with ethylene-generating fruits such as apples, and pears.
Another major disorder is Brown Stain where stains appear as yellowish-reddish-brown spots on the midribs. It may also appear as reddish-brown streaks. This is caused by exposure to above 3% CO2 atmospheres at low temperatures.
Pink rib is a disorder where symptoms appear as a pinkish coloration on the midribs. It appears mostly on over mature heads and is normally caused by high storage temperature.
Pathological Disorders: Major pathological disorders of lettuce are bacterial soft rot, gray mold, and watery soft rot. A detailed account of these disorders is given below:
Bacterial soft-rots: It is caused by the infestations of numerous bacteria species, which subsequently result in a slimy breakdown of the infected tissue of the lettuce. Practices such as trimming of the outer leaves, rapid cooling of freshly harvested lettuce heads, and low temperature storage etc reduce bacterial soft-rots up to a great extent.
Gray Mold Rot or Botrytis Rot: It is caused by a fungus called Botrytis cinerea. Minimizing mechanical damage such as the bruises/wounds on the vegetable surface, field sanitation, and storage of lettuce heads at 0 to 2 °C (32 to 34 °F) reduces botrytis infection up to a great extent.
Watery Soft-Rot or Sclerotinia Rot: It is caused by a fungus called Sclerotinia sclerotiorum. Control measures are same as that of botrytis rot.
Economics of Lettuce Production: Cost of production varies depending upon locality, soil type, and other relevant parameters. Major production expenses include cost of seeds; labor cost involved in field preparation and planting process, and costs of cultural management; cost of fertilizers and pesticides, labor cost involved in irrigation and disease-pest management and weed management; costs of harvesting process and postharvest management. As a general rule labor cost may amount up to 50 % of total production costs. Major income obtained is through the sales of fresh and processed lettuce heads and leaves.
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