Avocado

Persea americana Miller

Synonyms - Laurus persea L., Persea gratissima Gaertn.

Family: Lauraceae

Names:

Avocado is from the Spanish aguacate which comes from the proto-Aztecan name for the tree and means testicle referring to the shape of the fruit.
Persea
Americana refers to its origin in central America.

Other Names:

Alligator pear
Criollo
Kiwi fruit - because it was widely grown in New Zealand.

Summary:

A slender, evergreen tree about 10 m tall with large, dark green leaves and green to dark, fleshy, oval fruit.

Description:

Cotyledons:

Two.

First leaves:

Leaves:

Alternate
Stipules -
Petiole - 20-50 mm, slightly sulcate adaxially, sparsely pubescent
Blade - Usually somewhat glaucous abaxially, green adaxially, narrowly elliptic, elliptic, ovate, or obovate, 80-250 × 50-120 mm, leathery, sparsely yellowish brown pubescent adaxially but very densely so abaxially when mature, midrib conspicuously elevated abaxially, impressed on lower part but plane on upper part adaxially, lateral veins 5-7 pairs, very elevated abaxially, slightly elevated adaxially, base cuneate or acute to subrounded, apex acute.

Stems:

Up to 20 m tall, typically around 10 m tall.
Grey green bark longitudinally fissured.

Flower head:

Cymose panicles 80-140 mm, most of them inserted on lower part of branchlet, pedunculate; peduncle 45-70 mm, peduncle and rachis densely yellowish brown pubescent; bracts and bracteoles filiform, ca. 2 mm, densely yellowish brown pubescent. Pedicels up to 6 mm, densely yellowish brown pubescent.

Flowers:

Inconspicuous, yellow-greenish, 5-10 mm. Perianth densely yellowish brown pubescent outside and inside;
Ovary - ovoid, ca. 1.5 mm, densely pilose;
style ca. 2.5 mm, densely pilose
stigma slightly dilated, discoid.
Perianth - perianth tube obconical, ca. 1 mm; perianth lobes 6, oblong, 4-5 mm, obtuse, outer 3 smaller, all dilated after anthesis and caducous.
Stamens - Fertile stamens 9, ca. 4 mm; filaments filiform, complanate, densely pilose, those of 3rd whorl each with 2 complanate ovate and orange glands at base, others glandless.
Staminodes - 3, of innermost whorl, sagittate-cordate, ca. 0.6 mm, glabrous, stalked; stalk ca. 1.4 mm, pilose.
Anthers - 4-celled; cells introrse in 1st and 2nd whorls, extrorse in 3rd whorl.

Fruit:

Large berry. Yellow-green or reddish brown, large, usually pear-shaped, sometimes ovoid or globose, 70-200 mm; exocarp corky; mesocarp fleshy and edible.
Weighs 100-1,000 grams with a single large central seed.
Some seedless fruit may appear on the trees and are known as "cukes" and are usually discarded commercially due to their small size.

Seeds:

Large, egg shaped and 50-64 mm long.

Roots:

Tap root.

Key Characters:

A slender, evergreen tree about 10 m tall with large, dark green leaves and green to dark, fleshy, oval fruit.
Leaves usually somewhat glaucous abaxially, green adaxially.
9 stamens and 3 staminodes.
Adapted from John Moore.

Biology:

Life cycle:

Perennial tree. Seeds germinate to form seedlings then juvenile trees that take four to six (or up to 10) years to bear fruit. The trees will produce fruit all year but often have a heavy harvest followed by a light harvest in the next season.
Prime quality varieties are propagated by grafting to rootstocks that are propagated by seed (seedling rootstocks) or by layering (clonal rootstocks). After about a year of growing in a greenhouse, the young rootstocks are ready to be grafted. Terminal and lateral grafting is normally used. The scion cultivar grows for another 6-12 months before the tree is ready to be sold.
Commercial varieties are usually grafted to maintain quality and quantity of fruit and produce 7-20 tons of fruit per hectare each year.
The avocado is a climacterid fruit like the banana which means it matures on the tree, but ripens off the tree. The fruit are picked when hard and green and kept in coolers at 3.3 to 5.6 °C until they reach their final destination. They must be mature to ripen properly. Fruit that falls off the tree ripens on the ground.
Fruit ripens in one to two weeks at room temperature. The ripening process can be sped up using ehtylene gas or storing with apples or bananas that release ethylene. Supermarkets often sell pre-ripened avocados which have been treated with synthetic ethylene to hasten ripening. Fruit can be left on the tree for several months without ripening and picked as required.

Physiology:

Does not tolerate frost or freezing temperatures.

Reproduction:

It is only able to partially self pollinate because of dichogamy in its flowering (i.e. the flowers change sex over time). This limitation, added to the long juvenile period, makes the species difficult to breed. Most cultivars are propagated by grafting.
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 as male the following morning.

Flowering times:

All year with a flush in autumn and spring.

Seed Biology and Germination:

Seed are normally pricked with toothpicks to encourage germination.

Vegetative Propagules:

Will layer.

Hybrids:

Hass is the most common cultivar. Others include Choquette, Gwen, Lula, Pinkerton and Reed as A cultivars and Bacon, Brogden, Ettinger, Fuerte, Monroe, Sharwil and Zutano as B cultivars.

Allelopathy:

Population Dynamics and Dispersal:

Not recorded as naturalised in Australia.

Origin and History:

It is native to Puebla in Mexico and Central America.
In 1982, the evolutionary biologist Daniel H. Janzen suggested that the Avocado may be an example of an 'evolutionary anachronism', a fruit adapted for ecological relationship with now-extinct large mammals (such as giant ground sloths or gomphotheres). Most large fleshy fruits serve the function of seed dispersal, accomplished by their consumption by large animals. There are some reasons to think that the fruit, with its mildly toxic pit, may have co-evolved with Pleistocene mega-fauna to be swallowed whole and excreted in their dung, ready to sprout. No extant native animal is large enough to effectively disperse avocado seeds in this fashion

Distribution:

NSW, QLD, SA, TAS, VIC, WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

Climate:

Mediterranean.
It needs a climate without frost and with little wind. High winds reduce the humidity, dehydrate the flowers, and affect pollination. When even a mild frost occurs, premature fruit drop may occur, although the Hass variety can tolerate temperatures down to -1°C.

Soil:

Prefers well-aerated soils more than 1 m deep.

Plant Associations:

Significance:

Beneficial:

Major commercial fruit crop with 59,000 ha in the USA. Mexico is the world's largest producer.
Avocados have diverse fats. For a typical avocado:
About 75% of an avocado's energy comes from fat, most of which (67% of total fat) is monounsaturated fat as oleic acid.
Other predominant fats include palmitic acid and linoleic acid.
The saturated fat content amounts to 14% of the total fat.
Typical total fat composition is roughly: 1% ù-3, 14% ù-6, 71% ù-9 (65% oleic and 6% palmitoleic), and 14% saturated fat (palmitic acid).
On a weight basis, avocados have 35% more potassium (485 mg/100 g) than bananas (358 mg/100 g). They are rich in folic acid and vitamin K, and are good dietary sources of vitamin B6, vitamin C, vitamin E and pantothenic acid.
Avocados have a high fiber content of 75% insoluble and 25% soluble fiber.
High avocado intake was shown in one preliminary study to lower blood cholesterol levels. After a seven-day diet rich in avocados, mild hypercholesterolemia patients showed a 17% decrease in total serum cholesterol levels. These subjects also showed a 22% decrease in both LDL (harmful cholesterol) and triglyceride levels and 11% increase in HDL (helpful cholesterol) levels.
Extracts of avocado have been studied in laboratory research to assess potential for lowering risk of diabetes mellitus.
A Japanese team synthesised the four chiral components of avocado, and identified (2R, 4R)-16-heptadecene-1, 2, 4-triol as a potential antibacterial component. Due to a combination of specific aliphatic acetogenins, avocado is under preliminary research for potential anti-cancer activity.

Detrimental:

Some people have allergic reactions to avocado. There are two main forms of allergy: those with a tree-pollen allergy develop local symptoms in the mouth and throat shortly after eating avocado. The second, known as latex-fruit syndrome, is related to latex allergy and symptoms include generalised urticaria, abdominal pain, and vomiting and can sometimes be life-threatening

Toxicity:

Avocado leaves, bark, skin, or pit are documented to be harmful to animals. Cats, dogs, cattle, goats, rabbits, rats, guinea pigs, birds, fish, and horses can be severely harmed or even killed when they consume them. The avocado fruit is poisonous to some birds.
Avocado leaves contain a toxic fatty acid derivative, persin, which in sufficient quantity can cause colic in horses and, without veterinary treatment, death. The symptoms include gastrointestinal irritation, vomiting, diarrhoea, 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. However, some dog and cat foods use oils and meal made from avocado meat as main ingredients without apparent ill effects.

Symptoms:

Treatment:

Legislation:

None.

Management and Control:


Thresholds:

Eradication strategies:

Usually controlled by mechanical removal.

Herbicide resistance:

None reported.

Biological Control:

Susceptible to phytophthora root rots.

Related plants:

Bay laurel (Laurus nobilis)
Camphorwood or Camphor Laurel (Cinnamomum camphora and Cinnamomum oliveri).
Cinnamon (Cinnamomum verum and Cinnamomum species)

Plants of similar appearance:

Trees
Rubber tree
Loquat

Notes:

Dalapon, amitrole and bromacil can be damaging to avocadoes as a directed spray (Jordan, Day & Russell 1968)

References:

Bodkin, F. (1986). Encyclopaedia Botanica. (Angus and Robertson, Australia).

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney).

Harden, Gwen J. (1991). Flora of NSW. (Royal Botanic Gardens, Sydney). Volume . P. Diagram.

Randall, J.M. and Marinelli, J. (1996) Invasive Plants. (Brooklyn Botanic Gardens Inc. Brooklyn). P. Photo.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.