Lettuce (Lactuca Sativa ) is a hardy annual plant of the aster or sunflower family Asteraceae. Lettuce as we call it, is most often used for salads, although it is also seen in other kinds of food, such as soups, sandwiches and wraps. One type is grown for its stems, which are eaten either raw or cooked. Lettuce is a good source of vitamin A and potassium, as well as minor source for several other vitamins and nutrients. Despite its beneficial properties, contaminated lettuce is often the cause for bacterial, viral and parasitic outbreaks in humans.

Lettuce seeds are commercially grown to prevent the induction of thermo or photo-inhibition during the germination process. A secondary advantage of lettuce seed priming is a faster rate of radicle emergence across temperatures when seeds are planted. The combination of these advantages allows for more rapid and uniform field emergence, less variation in plant size, and thus a greater number of cartons per acre at harvest packout. It is most often grown as a leaf vegetable, but also sometimes for its stem and seeds. Lettuce was first cultivated by the ancient Egyptians, who turned it from a weed whose seeds were used to make oil into a plant grown for its leaves. It spread to the Greeks and Romans, who gave it the name "lactuca", from which the modern "lettuce" ultimately derives.

Unfamiliar Facts on lettuce:

• Lettuce is a member of the sunflower family.
• Americans eat about 30 pounds of lettuce every year. That's about five times more than what we ate in the early 1900s.
• In the United States, lettuce is the second most popular fresh vegetable.
• Darker green lettuce leaves are more nutritious than lighter green leaves.
• In 1988, more than 153,000 acres of lettuce were harvested in California.
• About 25 percent of all iceberg lettuce is made into fresh-cut salads.
• Almost all lettuce is packed right in the field.​      
• Where are the most lettuce grown?
• The United States is largest producer of lettuce in the world, with most production done in California and Arizona.

Types of lettuce:

• Butterhead/Bibb lettuce
• Chinese lettuce
• Crisphead/Iceberg (until 1920’s it was called the crisphead but was renamed when California began transporting large quantities under mounds of ice to keep it cool.
• Loose leaf/Green leaf lettuce
• Romaine/Cos

Summary of lettuce:

PLANT: Separate varieties flowering at the same time by at least 20 feet to ensure purity.

FLOWER: Lettuce produces perfect, self-pollinating flowers. Each flower ​produces one seed. Flowers are grouped in little heads of 10-25 flowers all of which open at once for as little as 30 minutes. Anthers are fused together into a little cone that completely surrounds stigma and style. Style is pushed up through anther cone and is coated with its own pollen. Mature head lettuce may ​need a slit (two or three inches deep) across the top to encourage flowering.

SELECTION TRAITS: Leaf color: red, Leaf color is controlled by at least two genes with a number of variations possible. Generally, hybrids produced by crossing red and green varieties result in red offspring. Leaf shape: no lobes; oak leaved, Seed stalk formation: bolt resistance.

SEED COLOR: white seeds,  black seeds

HARVEST: Some outside leaves can be harvested for eating without harming seed production. Allow seed heads to dry 2-3 weeks after flowering. Individual heads will ripen at different times making the harvest of large amounts of seed at one time nearly impossible. Wait until half the flowers on each plant has gone to seed. Cut entire top of plant and allow to it dry upside down in an open paper bag.

PROCESS: Small amounts of seed can be shaken daily from individual flowering heads. Rub with hands to remove remaining seeds. If necessary, separate seeds from chaff with screens.

• However, to be sure that there is no crossing between lettuce varieties, you should plant a different crop between the rows of lettuce.
• The small flowers will turn into feathery white tufts, and tucked inside are the tiny black (or white) seeds.
• They don’t ripen all at once, so you have to watch the pods and collect them as they are ready. Whenever you see some seeds turning dark, shake the pod over a paper bag so you can catch them.  Then, in newspaper, let them dry for two weeks, and when they are completely dry, store them in a Ziploc bag in your fridge. So that no moisture can penetrate and cause premature germination or lead to rotting of seeds.

Lettuce seed can remain viable for 3 years under cool and dry storage conditions.

Diseases and concerns:

Lettuce will wilt and rot in hot, humid weather. The plant will also bolt or go to seed stage in higher heat. Heading or bunching types are more susceptible to rotting and bolting. This happens when an elongated stalk with flowers shoots up from the main stem of the plant (lettuce seed stalk). Bolting usually occurs when temperatures are too high. There is nothing that can be done once bolting occurs. However, choosing good seeds that are resistant to high temperatures will help eliminate this problem. Leaf types grow and mature quickly, and have fewer disease problems.

Insects can become a real problem, too. Lettuce is delicate, and can absorb many insecticides. If you want or need to use insecticides, look for brands that ​are less harmful to you and the environment. You must avoid insecticides on leafy vegetables wherever possible. Organic sprays are suggested and a willingness to give up some of the harvest to insects versus using pesticides. It is ​not recommended insecticides at all for loose leaf lettuce varieties.

One of the worst diseases of lettuce is pythium, and the most common nutritional disorder is tip burn. The latter is caused by excessive water from the leaves accompanied by inadequate water uptake by the roots.

Slugs are a real problem for all types of lettuces, therefore the grower must insure that their lettuce is safeguarded from slugs.

Lettuce is mainly a cool season crop, but a few types will do reasonably well even in warmer weather with proper care.  Introduction of new, improved types is happening every year, and there are hundreds of individual lettuces with various cold and heat tolerances. Although lettuces differ widely in form and in growing season requirements, they all share the same gardening basics.  Loose, rich, well-drained soil, mulch, and a minimum of  half a day of sun are absolute musts for the best results. Vigorous growth is the goal, and continuous moisture is the key.

Reference

Retrieved from http://www.idph.state.ia.us/pickabettersnack/common/pdf/factsheets/lettuce.pdf           

How to collect and prepare lettuce seeds for propagation read more: http://gardenofeaden.blogspot.com/2009/01/how-to-collect-and-prepare-lettuce.html

How to grow lettuce. In Lettuce Seeds. The Gardeners Network. Retrieved from http://www.gardenersnet.com/vegetable/lettuce.htm

Primed lettuce seeds exhibit increased sensitivity to moisture during aging. In Lettuce Seeds. Seed Dynamics. Retrieved from http://seeddynamics.com/images/docs/PrimedLettuceAging.pdf

How to save lettuce seeds. In Lettuce seeds. eHow.com. Retrieved from http://www.ehow.com/how_5606942_save-lettuce-seeds.html

Ken, P., & Raid, R. (n.d.). 2006 florida plant disease management guide: Lettuce and endive. In Lettuce Seeds. EDIS. Retrieved from http://edis.ifas.ufl.edu/pg048

Lettuce. In Lettuce Seeds. Garden Blaze. Retrieved from http://www.gardensablaze.com/Vegetables/VegLettuce.htm