Avocado fruit and foliage, Huntington Library, California
The avocado (Persea americana) is a tree native to Mexico and Central America, classified in the flowering plant family Lauraceae and widely cultivated in subtropical regions for its large, edible fruit. The name "avocado" also refers to the fruit of the tree, which is characterized by an oval or pear-shape, with a rough or leathery skin, and a large seed; it is sometimes known as the avocado pear or alligator pear.
Rich in an assortment of vitamins, high in monounsaturated fat and potassium, and containing a unique fatty alcohol, avocadene, avocado fruits provide curative effects for a number of human ailments, from diarrhea to high blood pressure. However, their leaves are harmfully and even fatally poisonous, causing a number of illnesses in animals. Interestingly, those illnesses include gastrointestinal irritation, of which the fruit is known to help cure.
There is an important interdependency between avocados and people. The plant lacks a seed dispersal technique outside of humans. It is hypothesized that it originally co-evolved with large mammals that are now extinct, such as the giant ground sloth, with these ecological partners vital to seed dispersal. New mechanisms have not evolved, but the effectiveness of human intervention has allowed the plant to prosper. Of course, in exchange for this benefit, the avocado provides a nutritional and desirable fruit for people.
Avocados are part of the laurel family, Lauraceae, which comprises a group of flowering plants included in the order Laurales. The avocado, P. americana, is the best-known member of the genus Persea, which is comprised of about 150 species of evergreen trees. Members of Persea are typically medium-size trees, 15-30 meters tall at maturity, with leaves that are simple, lanceolate to broad lanceolate, and flowers arranged in short panicles, with six small greenish-yellow perianth segments 3-6 mm long, nine stamens, and an ovary with a single embryo.
The avocado, P. americana, grows to 20 meters (65 feet), with alternately arranged, evergreen leaves, 12–25 centimeters long. The greenish yellow flowers are an inconspicuous 5–10 millimeters wide. The pear-shaped fruit is botanically a berry. It typically measures 7 to 20 centimeters in length and weighs between 100 and 1000 grams. The avocado fruit also has one large central seed, 3 to 5 centimeters in diameter. The avocado is a climateric fruit, which means that it matures on the tree but ripens off the tree.
An average avocado tree produces about 120 avocados annually. Commercial orchards produce an average of seven metric tons per hectare each year, with some orchards achieving 20 ton per hectare (Whiley 2007). Biennial bearing can be a problem, with heavy crops in one year being followed by poor yields the next. The common names "avocado pear" or "alligator pear" for the fruit are due to its shape and rough green skin.
It is speculated that the avocado fruit's poisonous pit was once dispersed through the excretion of an animal with which it co-evolved. However, since the disappearance of its propagating partner, human cultivation seems to have unobliged further seed dispersal-driven evolution.
Previously, avocados had a long-standing stigma as a sexual stimulant and were not purchased or consumed by any person wishing to preserve a chaste image. Growers had to sponsor a public relations campaign to debunk the avocado's unsavory reputation before they eventually became popular. Avocados were known by the Aztecs as "the fertility vegetable."
The avocado tree does not tolerate freezing temperatures, and can be grown only in subtropical or tropical climates. It is crucial to cultivate most avocado species in climates without frost and little wind. Otherwise, the fruits will drop sporadically from the tree, and due to their climateric nature, ripen asynchronously, causing a decrease in yield. Additionally, due to the long four to six years period it takes for avocados to propagate, commercial orchards use grafted trees and rootstocks to expedite and increase production.
Contrary to the conventional wisdom, avocado trees cannot sustain frost. Even when mild frost does occur, the fruit drops from the tree, although the cultivar Hass can tolerate temperatures down to −1°C. Avocado farmers in California lost hundreds of millions of dollars in 2006 due to a temperature drop.
Avocado trees also need well aerated soils, ideally more than one meter 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, the Levant, South Africa, Peru, parts of central and northern Chile, Vietnam, Indonesia, Australia, New Zealand, parts of the United States, the Philippines, Malaysia, Mexico and Central America. Each region has different types of cultivars. Mexico is the largest producer of the Hass variety, with over one million metric tons produced annually.
Propagation and rootstocks
While an avocado propagated by seed can bear fruit, it takes four to six 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, which are clonal rootstocks. After about a year of growing 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.
The species is partially unable to self-pollinate, because of dichogamy in its flowering; that is, the asynchronous ripening of the stamen and pistil. The limitation, added to the long juvenile period, makes the species difficult to breed. Most cultivars are 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 for programs at the University of California, Riverside, as well as the Volcani Centre and the Instituto de Investigaciones Agropecuarias in Chile.
Harvest and post-harvest
As a climateric fruit, which matures on the tree but ripens off the tree, avocados used in commerce are picked hard and green and kept in coolers at 38 to 42°F (3.3 to 5.6°C) until they reach their final destination. Avocados must be mature to ripen properly. Avocados that fall off the tree ripen on the ground, and, depending on the amount of oil they contain, their taste and texture may vary greatly. Generally, the fruit is picked once it reaches maturity; Mexican growers pick Hass-variety avocados when they have more than 23 percent dry matter and other producing countries have similar standards.
Once picked, avocados ripen in a few days at room temperature (faster if stored with other fruits such as bananas, because of the influence of ethylene gas). Premium supermarkets sell pre-ripened avocados treated with synthetic ethylene to hasten the ripening process (Mindfully.org 2007). In some cases, avocados can be left on the tree for several months, which is an advantage to commercial growers who seek the greatest return for their crop; however, if the fruit stays on the tree for too long it will fall to the ground.
Introduction to Europe
The earliest known written account of the avocado in Europe is that of Martín Fernández de Encisco (c. 1470 – c. 1528) in 1519 in his book, Suma de Geografía que Trata de Todas las Partidas y Provincias del Mundo. The plant was introduced to Indonesia by 1750, Brazil in 1809, Palestine in 1908, and South Africa and Australia in the late nineteenth century (IFA 2007).
Cultivation in California
The avocado was introduced to the U.S. state of California in the nineteenth century, and has become an extremely successful cash crop. Ninety-five percent of United States avocado production is located in California, and 80 percent 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 percent of the crop. Hass avocado fruits have a dark, rippled skin and rich, creamy flesh. All Hass avocado trees are related to a single "mother tree" that was bought as a seedling by a mail carrier named Rudolph Hass. He bought the 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, Gwen, Pinkerton, Reed, and Zutano. The fruit of the cultivar Florida, grown mostly outside 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, "A" and "B." "A" cultivar flowers open as female on the morning of the first day and close in late morning or early afternoon. Then they open as male in the afternoon of the second day. "B" varieties open as female on the afternoon of the first day, close in late afternoon and reopen in the male the following morning.
- "A" cultivars: Hass, Gwen, Lamb Hass, Pinkerton, Reed.
- "B" cultivars: Fuerte, Sharwil, Zutano, Bacon, Ettinger, Sir Prize, Walter Hole (UCANR 2007; Crane et al. 2007).
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. It can germinate in normal soil in a large pot or by suspending a washed pit (generally using toothpicks embedded in the sides) pointed-side up and filling the glass until the bottom quarter of the pit is covered. The pit will crack as it absorbs water and germinates, and should sprout in four to six weeks. When the roots and stem emerge from the seed, it can be planted in soil. The young tree is amenable to pruning and training but will not normally bear fruit indoors without sufficient sunlight and a second plant to cross-pollinate.
The fruit of horticultural cultivars ranges from more or less round to egg- or pear-shaped, typically the size of a temperate-zone pear or larger. They tend to be bright green to green-brown (or almost black) in color on the outside. 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. The fruit has a markedly higher fat content than most other fruit, mostly monounsaturated fat.
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 considered sweet fruits, so are frequently used for milk-shakes and occasionally added to ice cream and other desserts. In Vietnam, 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. In Chile, its consumption is widespread and used as a puree in chicken, hamburgers and hot dogs, and in slices for celery or lettuce salads. The Chilean version of Caesar salad contains large slices of mature avocado.
Avocado flesh has also been used by some Native American tribes in the southwestern United States in the mixing and application of adobe.
Avocado is also thought to promote physical beauty and is used in cosmetics to this day. The Aztecs ate the fruit as an aphrodisiac.
A whole medium avocado contains approximately 55 percent of the United States FDA's recommended daily amount of fat, though they are high in monounsaturated fat. Avocados also have 60 percent more potassium than bananas. They are rich in B vitamins, as well as vitamin E and vitamin K (NutritionData 2007).
A fatty triol (fatty alcohol) with one double bond, avocadene (16-heptadecene-1,2,4-triol), is found in avocado and has been tested for anti-bacterial and anti-inflammatory properties. These properties are likely related with the curative effects of avocado described for a number of ailments (diarrhea, dysentery, abdominal pains and high blood pressure) (Cyberlipid Center 2007).
Toxicity to animals
There is documented evidence that animals such as cats, dogs, cattle, goats, rabbits, birds, parrots, fish, and particularly horses can be severely harmed or even killed when they consume the avocado leaves, bark, skin, or pit. The avocado fruit is poisonous to birds in some cases, so on a practical level feeding the fruit to birds should be avoided (Government of Canada 2006; Clipsham 2007). Avocado leaves contain a toxic fatty acid derivative known as persin, which in sufficient quantity can cause equine colic and with lack of veterinary treatment—death. The symptoms include gastrointestinal irritation, vomiting, diarrhea, respiratory distress, congestion, fluid accumulation around the tissues of the heart, and even death. Birds also seem to be particularly sensitive to this toxic compound.
Negative effects in humans seem to be primarily in allergic individuals.
The avocado may be an example of an "evolutionary anachronism," a fruit adapted for ecological relationship with now-extinct large mammals (such as the giant ground sloth or the Gomphothere). The fruit, with its mildly toxic pit, is believed to have co-evolved with megafauna mammals to be swallowed whole and excreted in their dung, ready to sprout. The avocado's hypothesized ecological partners have disappeared, and the avocado plant has not had time to evolve an alternative seed dispersal technique, aside from human cultivation (Barlow 2000). However, given the effectiveness of human intervention in dispersing the plant across the globe, there is no evolutionary pressure on the avocado that favors the development of alternative dispersal techniques.
After the North American Free Trade Agreement (NAFTA) treaty was signed in 1991, Mexico tried exporting avocados to the United States. The U.S. government resisted, claiming that the trade would introduce vegetable flies that would destroy California's crops. The Mexican government responded by inviting U.S. Department of Agriculture inspectors to Mexico, but the U.S. government declined, claiming vegetable 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 gave in when the Mexican government started throwing up barriers to American maize.
Legitimate pest invasion issues exist, as avocado pests originating in Mexico have made their way to California, including the persea mite and avocado thrips. These pests have increased pest control costs and made previously-relied-upon biological control less feasible. Other potentially disastrous pests, including a weevil, remain risks. 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.
Today, avocados from Mexico are allowed in all 50 states. This is because USDA inspectors in Uruapan, Michoacán (the state where 90 percent of Hass avocados from Mexico are grown), have cut open and inspected millions of fruit, finding no problems. Imports from Mexico in the 2005-2006 growing season exceeded 130,000 tons (Associated Press 2007).
Avocados are much more expensive in the USA than in other countries, due to the fact that those consumed in the USA are grown almost exclusively in California and Florida. California produces about 90 percent of the nation's avocado crop.
The word "avocado" comes from the Spanish word aguacate, which derives in turn from the Aztec, Nahuatl word, ahuacatl, meaning "testicle," because of 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 are "alligator pear" and "butter 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.
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- University of California Agricultural and Natural Resources (UCANR). 2007. Avocado handbook. Ventura County Cooperative Extension Retrieved October 28, 2007.
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