Avocado
2007 Schools Wikipedia Selection. Related subjects: Food and agriculture; Plants
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 Avocado fruit and foliage, Huntington Library, California
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| Persea americana Mill. |
Avocado (Persea americana) is a tree and the fruit of that tree, classified in the flowering plant family Lauraceae. It is native to Central America and Mexico. The tree grows to 20 m (65 ft), with alternately arranged, evergreen leaves, 12-25 cm long. The flowers are inconspicuous, greenish-yellow, 5-10 mm wide. The pear-shaped fruit is botanically a berry, from 7 to 20 cm long, and weighs between 100 to 1000 g. It has a large central seed, 3 to 5 cm in diameter.
An average avocado tree produces about 120 avocados annually. Commercial orchards produce an average of 7 tonnes per hectare each year, with some orchards achieving 20 tonnes per hectare (FAO statistics). Biennial bearing can be a problem, with heavy crops in one year being followed by poor yields the next. The fruit is sometimes called an avocado pear or alligator pear, due to its shape and rough green skin. The avocado tree does not tolerate freezing temperatures, and so can be grown only in subtropical and tropical climates.
Co-evolution theory
Barlow & Martin (2002) identify the avocado as a fruit adapted for ecological relationship with large mammals, now extinct (as for example the South American herbivorous giant ground sloths or the Gomphotheres). This fruit with its mildly toxic pit, co-evolved with those extinct mammals to be swallowed whole and excreted in dung, ready to sprout. The ecological partners have disappeared, and the avocado plant has not had time to evolve an alternative seed dispersal technique, aside from human cultivation.
Cultivation
This subtropical species needs a climate without frost and not too much wind. When a frost event does happen, the fruit drops from the tree, reducing the yield. The cultivar 'Hass' can tolerate temperatures down to −1 ° C. The trees also need well aerated soils, ideally more than 1 m deep. Yield is reduced when the irrigation water is highly saline. These soil and climate conditions are met only in a few areas of the world, particularly in southern Spain, Israel, South Africa, Peru, northern Chile, Vietnam, Indonesia, Australia, New Zealand, United States, The Philippines, Mexico and Central America, Malaysia, the centre of origin and diversity of this species. (In the U.S., avocados are produced commercially only in California and Florida, although the varieties used are different.)
Propagation and rootstocks
While an avocado propagated by seed can bear fruit, it will take 4-6 years to do so, and the offspring is unlikely to resemble the parent cultivar in fruit quality. Thus, commercial orchards are planted using grafted trees and rootstocks. Rootstocks are propagated by seed (seedling rootstocks) and also layering (clonal rootstocks). After about 1 year of growing the young plants in a greenhouse, they are ready to be grafted. Terminal and lateral grafting is normally used. The scion cultivar will then grow for another 6-12 months before the tree is ready to be sold. Clonal rootstocks have been selected for specific soil and disease conditions, such as poor soil aeration or resistance to the soil borne disease caused by Phytophthora root rot.
Breeding
The species is partially unable to self-pollinate, because of dichogamy in its flowering. The limitation, added to the long juvenile period, make it difficult to breed this species. Most cultivars are clonally propagated (via grafting), having originated from random seedling plants or minor mutations derived from cultivars. Modern breeding programs tend to use isolation plots where the chances of cross-pollination are reduced. That is the case of programs at the University of California-Riverside, as well as the Volcani Centre in Israel.
Harvest and post-harvest
The avocado fruit does not ripen on the tree, but will fall off or be picked in a hard, "green" state, then it will ripen quickly on the ground, but depending on the amount of oil that it has, the taste may be very different. Generally, the fruit is picked once it reaches a mature size, and will then ripen in a few days (faster if stored with other fruit such as bananas, because of the influence of ethylene gas). Premium supermarkets sell pre-softened avocados, treated with a special gas to stimulate ethylene synthesis in the fruit (the same process used to de-green lemons). The fruit can be left on the tree until required, rather than picked and stored, but for commercial reasons it must be picked as soon as possible. Growers can keep the fruit on the tree for about 4-6 months after fully developed; if the fruit stays on the tree for too long it will fall to the ground.
Introduction to Europe
The earliest known account of the avocado in Europe is that of Martin Fernandez De Encisco in 1519. The plant was first introduced to Indonesia by 1750, Brazil in 1809, Palestine in 1908, and South Africa and Australia in the late 19th century. (Source: indexfresh.com).
Cultivation in California
The avocado was introduced to the U.S. state of California in the 19th century, and it has become an extremely successful cash crop. 95% of United States avocado production is located in California, and 80% occurs in San Diego County . Approximately 59,000 acres (approximately 24,000 hectares) of avocados are grown in California. Fallbrook, California claims the title of "Avocado Capital of the World" and hosts an annual Avocado Festival.
While dozens of cultivars are grown in California, 'Hass' (commonly misspelled 'Haas') is most common, accounting for more than 80% of the crop. In appearance, Hass has a dark, rippled skin and rich, creamy flesh. All Hass avocado trees are related to a single "mother tree" that was purchased as a seedling by a mail carrier named Rudolph Hass. He purchased the tree as a seedling from A.R. Rideout of Whittier, California, in 1926. Hass planted the seedling in his front yard in La Habra Heights, California, and patented the tree in 1935. All Hass avocados can be traced back to grafts made from that tree. The "Mother Tree" died of root rot in 2002. Other avocado cultivars include 'Bacon', 'Fuerte' (pictured), 'Gwen', 'Pinkerton', 'Reed' and 'Zutano'. The fruit of the cultivar 'Florida', grown mostly outside of California, is larger and rounder, with a smooth, medium-green skin, and a less-fatty, firmer and fibrous flesh. These are occasionally marketed as low-calorie avocados.
The avocado is unusual in that the timing of the male and female phases differs among cultivars. There are two flowering types, referred to as "A" and "B" flower types. "A" cultivars open as female on the morning of the first day. The flower closes in late morning or early afternoon. The flower will remain closed until the afternoon of the second day when it opens as male. "B" varieties open as female on the afternoon of the first day, close in late afternoon and re-open in the male phase the following morning.
- "A" cultivars: 'Hass', 'Gwen', 'Lamb Hass', 'Pinkerton', 'Reed'.
- "B" cultivars: Fuerte, Sharwil, Zutano, Bacon, Ettinger, Sir Prize, Walter Hole. (ref: )
Certain cultivars, such as the 'Hass', have a tendency to bear well only in alternate years. After a season with a low yield, due to factors such as cold (which the avocado does not tolerate well), the trees tend to produce abundantly the next season. This heavy crop depletes stored carbohydrates, resulting in a reduced yield the following season, and thus the alternate bearing pattern becomes established.
As a houseplant
Avocado can be grown as a houseplant from seed. Although it will not normally bear fruit indoors, people enjoy it for its greenery. It can be germinated in normal soil in a large pot, or in a glass of water with a piece of charcoal for deodorizing, with the top half (the pointed end) held up by toothpicks.
Uses
The fruit of horticultural cultivars range from more or less round to egg or pear-shaped, typically the size of a temperate-zone pear or larger, on the outside bright green to green-brown (or almost black) in colour. Though the fruit does have a markedly higher fat content than most other fruits, most of the fat in avocados is monounsaturated fat, which is considered healthy in the human diet. A whole medium avocado contains approximately 25% of the recommended daily amount of saturated fat. Avocados also have 60% more potassium than bananas. They are also rich in B vitamins, as well as vitamin E and K.
A ripe avocado will yield to a gentle pressure when held in the palm of the hand and squeezed. The flesh is typically greenish yellow to golden yellow when ripe. The flesh oxidizes and turns brown quickly after exposure to air. To prevent this, lime or lemon juice can be added to avocados after they are peeled; vitamin C in the juice acts as an antioxidant. The avocado is very popular in vegetarian cuisine, making an excellent substitute for meats in sandwiches and salads because of its high fat content. The fruit is not sweet, but fatty, strongly flavored, and of smooth, almost creamy texture. It is used as the base for the Mexican dip known as guacamole, as well as a filling for several kinds of sushi, including California rolls. Avocado is popular in chicken dishes and as a spread on toast, served with salt and pepper. In Brazil and Vietnam, avocados are frequently used for milk-shakes and occasionally added to ice cream. In the Philippines and Indonesia, a dessert drink is made with sugar, milk, and pureed avocado. In Central America, avocados are served mixed with white rice. The fruit is also pressed for avocado oil production. In Chile it is often used in hamburgers, hot dogs and celery salads. Avocado flesh has also been used by some Native American tribes in the Southwestern United States in the mixing and application of adobe, a natural building material .
Toxicity
Feeding avocados to any animal should be totally avoided. There is documented evidence that animals such as cattle, horses, goats, rabbits, birds, dogs, cats, and even fish can be severely harmed or even killed when they consume the leaves, bark or fruit. Avocados contain a toxic fatty acid derivative known as persin and many animal organizations recommend total avoidance of all parts of the plant. The symptoms include gastrointestinal irriation, vomiting, diarrhea, respiratory distress, congestion, fluid accumulation around the tissues of the heart and even death. Birds seem to be particularly sensitive to this toxic compound.
Negative effects in humans seem to be primarily in allergic individuals.
Names
The English name for the avocado is derived from its usual Spanish language name, "aguacate", which in turn is derived from its Nahuatl name, 'ahuacatl', meaning testicle (due to its shape). In some countries of South America (such as Argentina, Bolivia, Chile, Peru, and Uruguay), the avocado is known by its Quechua name, 'palta'. In other Spanish-speaking countries it is called "aguacate", and in Portuguese it is "abacate". The name avocado pear is sometimes used in English, as is alligator pear. The Nahuatl ahuacatl can be compounded with other words, as in ahuacamolli, meaning "avocado soup or sauce", from which the Mexican-Spanish word guacamole derives. In Chinese, the avocado is evocatively called the "butter fruit" (牛油果 níuyóu gǔo), but also occasionally "alligator pear" (鳄鱼梨). In Taiwan, the word "alligator pear" is used.
Avocado related trade war
After the NAFTA treaty was signed, Mexico tried exporting avocados to the USA. The U.S. government resisted, claiming that the trade would introduce fruit flies that would destroy California's crops. The Mexican government responded by inviting U.S. Department of Agriculture inspectors to Mexico, but U.S. government declined, claiming fruit fly inspection is not feasible. The Mexican government then proposed to sell avocados only to the northeastern U.S. in the winter (fruit flies cannot withstand extreme cold). The U.S. government balked, but only gave in when the Mexican government started throwing up barriers to American maize.
Today avocados from Mexico are allowed in 47 states, excluding in Florida, California, and Hawaii. This is because USDA inspectors in Uruapan, Michoacan (the state where 90% of Hass avocados from Mexico are grown) have cut open and inspected millions of them, finding no problems. Imports from Mexico last season (2004-2005) exceeded 100 thousand tonnes.
Avocados are much more expensive in the USA than other countries due to the fact that they are grown almost exclusively in California and Florida, and the main potential competitor (Mexico) is banned from three states in the market - Florida, California, and Hawaii. Mexican farmers have argued against the ban, pointing out that not a single shipment has been found to contain pests since the U.S. Department of Agriculture began inspections in 1997 . Another argument is that the lower prices generated by the Mexican and Chilean imports would increase the popularity of avocados outside of California, thereby assuaging the loss of profits due to the new competition. In the year 2007, Mexican avocados will be permitted in all 50 U.S. States.