Lettuce (Lactuca sativa) is an economically important vegetable grown worldwide with top yields in the United States, Europe, and China. It is a member of Asteraceae. Consists of 23,000 to 30,000 species and is considered the largest botanical family.
Authors: Iqra Naeem, Fahd Rasul, Hassan Munir*, Hafsa Jannat, Sobia Shahzad
Lettuce leaves are rich in nutrients. Globally, the area under salad cultivation is about 1.23 million hectares with a yield of 21.89 tons per hectare. During the financial year 2020, the area under salad cultivation in Pakistan was 456 hectares and the production of salad leaves was 420 tons, most of which were in Sindh province. Lettuce is an important edible vegetable with edible leaves, which is considered desirable for salads.
There are following four popular varieties of lettuce are grown for general use:
i- Head lettuce (Capitata)
ii- Leaf lettuce (Crispa)
iii- Romaine lettuce (Longifolia)
iv- Celtuce lettuce (Augustana)
1- Head lettuce is named due to its round shape. This type includes iceberg and butterhead lettuces, both of which are usually sold in supermarkets.
2- Leaf lettuce (Crispa) does not form a head but is attached to a stem instead. Some of this lettuce is dark in color or has reddish leaves.
3- Romaine lettuce (Longifolia) is the type that is usually sold in stores. It is also added to Caesar salads.
4- Celtuce lettuce (Augustana) Also called asparagus lettuce or stem lettuce has a distinctive, large stem. This type of lettuce is common in China, but less common in the Western world.
Cultivation of Lettuce
Lettuce is an annual crop that grows best in nitrogen-rich, loose soil with a pH between 6.0 and 6.8. Temperatures above 24οC are not ideal for the growth of this crop because it produces better yield at lower temperatures. The optimum growth temperature for lettuce is 16 to 18οC and the plant can withstand temperatures up to 7οC. Chopped lettuce is best stored at 0οC and 96% humidity. It is difficult to store the plant because it contains a lot of water and therefore it is eaten fresh. Lettuce seeds are best stored at -20οC or lower.
Watering is stopped 48 hours before the nursery is moved to the field, but a good amount of irrigation should be done 30 minutes before transplanting. This will help in pulling the seedlings easily. Immediately after transplanting, the first irrigation takes place. The rest of the irrigation is done on light soils at intervals of 5-6 days whereas on heavy soils it is done at intervals of 8 to 10 days.
Benefits of lettuce for human health
The health benefits of lettuce vary depending on its nutritional value. There are many health benefits to using lettuce due to the presence of vitamin C, phenolic compounds and fiber content.
Icebergs are usually the salad leaf with the least amount of nutrients. Lettuce is a source of vitamin K, which helps strengthen bones and reduces your risk of fractures. Water is more than 95% in raw lettuce leaves. As a result, eating salad leaves can make up for the lack of water in the body. Lettuce is a source of vitamin A, which plays a role in eye health and can reduce a person’s risk of cataracts. It also helps prevent macular degeneration. Almost all types contain significant amounts of vitamin A and a small amount of vitamin C and iron. Eating it has no harmful effects on health as it is low in calories. Lettuce leaves are mainly eaten raw while one type is cultivated for seed preparation and oil and the other type is cultivated. Lettuce leaves are widely used in salads, soups, and sandwiches. Lettuce leaves are used in various dishes in different parts of the world.
Effects on vegetable production due to contaminated water
Vegetables especially leafy vegetables demand abundant irrigation water, which is difficult in populated and industrial areas where there is a large population burden in terms of housing and livelihood. However, the availability of large quantities of colonial wastewater and industrial wastewater attracts vegetable growers in such areas to meet the demand for crop irrigation.
Furthermore, the use of contaminated water which is rich in micro and macronutrients also causes the release of heavy metals in the water which is harmful to human health but is ignored due to commercial benefits. Wastewater irrigation is not only a way to save surface and groundwater for human consumption but also a way to use wastewater as a renewable resource for environmental and human health instead of wasting it may cause major risks. Crops grown from wastewater irrigation absorb heavy metals through their roots and transfer them to different parts of vegetables, which become harmful to human health after a certain amount. Vegetables grown in wastewater-irrigated soils have high levels of Cu, Zn, Pb, Cr, and Fe in their leaves and stems. Consumption of vegetables contaminated with heavy metals can lead to serious health problems. For example, Pb metal is associated with kidney disease. Diseases such as Alzheimer’s and “Itai-itai” are associated with Cu and Cd metals, respectively. Calcium deficiency causes Cd to build up in the bones and soften them. Furthermore, both Cd and Pb have long been classified as endocrine disruptors, although Zn is considered an essential micronutrient, certain amounts of these nutrients (e.g., Cu and Fe) can cause anemia, which in turn can lead to anemia, neutropenia and worsening of the immune system. Therefore, it is important to protect crops from the availability of heavy metals, especially root and tuber crops that come in direct contact with contaminated soil and water. Modern low-cost and simple methods need to be developed to reduce the risk of migration of heavy metals in food crops irrigated by wastewater. It contains biochar, widely used as an absorber to remove toxic metals, organic pollutants and nutrients from wastewater.
The use of Biochar
Biochar is a carbon-rich substance produced by thermochemical modification of plant biomass in an oxygen-deficient environment. Straw, nuts husks, rice husks, wood chips, tree bark, crop residues and forest and agricultural grasses are being used to make biochar. When we use Biochar before sowing or spraying by hand near the roots, it accumulates in the roots of the plants and protects the vegetables from various harmful heavy metals damage after irrigation with dirty water. There are the following benefits of using biochar.
- Biochar improves water quality.
- Reduces greenhouse gas emissions from the soil.
- Reduces nutrient excretion.
- Reduces soil acidity.
- Reduces the need for irrigation and fertilizer.
- Not only it is useful in reducing carbon emissions and greenhouse gas emissions, but it can also be a versatile product for reducing the transfer of contaminants from soil water media to food crops.
- Biochar improves soil fertility.
- Accelerates plant growth.
For example, laboratory and field-scale studies have shown the effectiveness of biochar in reducing the movement and bioavailability of heavy metals from soil and water sources.
Pests and their control and diseases
Lettuce is attacked by aphids, armyworms, imported cabbage worms, and cinnabar. Pest attack on summer and autumn crops is much higher than on spring crops. Leaf spot is a serious disease, while mildew and sclerotinia affect larger plants. Both pests and diseases can be controlled if chemicals are used properly.
-Imidacloprid 17.85-60 ml per acre should be sprayed in 150 liters of water to control slow oil.
Loose-leaf lettuce is harvested when they are fully grown or reaches marketable size. Soft leaves are cut regularly with an interval of one week. Leaf pruning is stopped in late March or early April. Because the plant reaches the milky stage. Full harvesting is done in May for seed. It gives an average seed yield of 50 kg per acre. Harvesting should be done early in the morning. It helps to keep the leaves fresh.