Lettuce (Lactuca sativa) is a member of the daisy family (Asteraceae). Ancient Egyptians grew lettuce for oil from its seeds over 4,500 years and then as an edible leaf crop. As a result, lettuce has been a part of the human diet for over 4,000 years (Elzebroek and Wind 2008). It is consumed mostly as a fresh leafy vegetable in butter head, loose leaf, head (iceberg), romaine (cos), bunching, and stem forms (Elzebroek and Wind 2008; Złotek et al. 2014). Lettuce spread throughout Europe as a leaf and medicinal crop and came to the Americas in the late 1400s (Elzebroek and Wind 2008). It is consumed mostly as a fresh vegetable, but can be cooked, and has been utilized as a medicinal, an ornamental, and as a model research crop.
Lettuce is grown as a cool weather annual in temperate regions, even tolerant of light frost. The most favorable temperature range for lettuce growth and development is reported between 59-70 degrees Fahrenheit, with a tolerance range of 43 to 83 degrees Fahrenheit, (Hanan 1998). Beyond these temperature ranges, developmental reactions begin to deteriorate, plants become stressed and growth slows and may eventually stop. The plant may not form heads in hot weather and some forms will begin to flower at high temperature during long days (Stern et al. 2008). High temperature and relative humidity.
Lettuce is one of the most commonly grown vegetables in the United States (USDA 2019). Historically, most lettuce has historically been grown in open fields, with some production in greenhouses and plastic tunnels (Elzebroek and Wind 2008). Currently it is grown outdoors, in greenhouses, and in plant factories. The earliest day for planting open field head lettuce in the temperate U.S. is from mid-March to early April, and mid-March to mid-May for leaf lettuce (Dana and Lerner 2009). The optimum soil temperature for soil germination of lettuce seeds in central Indiana is 75˚F (40-80˚F range) [35/85˚F – minimum/maximum] (Dana and Lerner 2009). The latest day for planting head lettuce in central Indiana is from June 15-August 11, and June 1-August 1 for leaf lettuce (Dana and Lerner 2009).
Lettuce is used for fresh consumption as well as a potential model leafy crop for botanical research. Demand for ready to eat leafy vegetables like lettuce has increased in recent years (Zhou et al. 2004). Consumers are typically to pay more for guaranteed fresh produce. Some farmer’s market customers have been reported to pay almost twice as much for as retail grocery shoppers for similar produce. Protected winter cultivation of lettuce in northern climates is a traditional alternative to importing fresh produce from warmer climates. Hilton et al. (2009) concluded the improved management of agronomic factors could extend the quality and shelf-life of field-grown lettuce. Its vegetative growth habit, short production cycle, and high proportion of edible biomass also make lettuce an excellent model crop for research purposes. Lettuce has also been proposed as a strong candidate crop for bioregenerative life-support system on space missions (Poulet et al. 2014). Harvesting hydroponically grown lettuce with roots attached which could extend the post-harvest storage life.
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