Citrus trees are various species of trees in the genus Citrus, in the rue family, or Rutaceae. There are 60 species in the genus Citrus, of which about 10 are used in agriculture. The center of origin of most species of Citrus is southern and southeastern Asia . Citrus trees are widely cultivated for their edible fruits in sub-tropical and tropical countries around the world. The sweet orange (Citrus sinensis) is the most common species of citrus in cultivation, and is one of the most successful fruits in agriculture.
The rue family consists of about 1,500 species and 150 genera. Most species in this family are trees or shrubs. The greatest richness of species occurs in the tropics and subtropics, especially in South Africa and Australia . However, a few species occur in the temperate zone. Several species native to North America are the shrubs known as prickly ash (Zanthoxylum americanum), southern prickly ash (Z. clava-herculis), and three-leaved hop tree (Ptelea trifoliata).
Biology of citrus
Citrus trees are species of sub-tropical and tropical climates. They are intolerant of freezing, and their foliage and fruits will be damaged by even a relatively short exposure to freezing temperatures for just a few hours. Colder temperatures can kill the entire tree.
Species of citrus trees range in size from shrubs to trees. Most species have thorny twigs. The leaves are alternately arranged on the twigs, and the foliage is dark green, shiny, aromatic, leathery, and evergreen. The roots of citrus trees do not develop root hairs, and as a result citrus trees are highly dependent for their mineral nutrition on a mutualistic symbiosis with soil fungi called mycorrhizae.
Citrus trees have small white or purplish flowers, which are strongly scented and produce nectar . Both scent and nectar are adaptations for attracting insects , which are the pollinators of the flowers of citrus trees. Some species in the genus Citrus will easily hybridize with each other. This biological trait can make it easier for plant breeders to develop profitable agricultural varieties using controlled hybridization experiments to incorporate desirable traits from one species into another. However, the occurrence of hybrid Citrus plants makes it difficult for plant taxonomists to designate true species. As a result, there is some controversy over the validity of some Citrus species that have been named.
The ripe fruit of citrus trees is properly classified as a hesperidium, which is a type of berry, or a fleshy, multiseeded fruit. The fruits of citrus trees have a relatively leathery outer shell, with a more spongy rind on the inside. The rind of citrus fruits is very rich in glands containing aromatic oils, which can be clearly detected by smell when these fruits are being peeled. The interior of the fruit is divided into discrete segments that contain the seeds surrounded by a large-celled, juicy pulp. The seeds of citrus trees are sometimes called "pips." Edible fruits like those of citrus trees are an adaptation to achieve dispersal of their seeds. The attractive and nutritious fruits are sought out by many species of animals, who eat the pulp and seeds. However, the citrus seeds generally survive the passage through the gut of the animal , and are excreted with the feces. In the meantime, the animal has likely moved somewhere, and the seeds have been dispersed far from the parent tree, ready to germinate, and hopefully, develop into a new citrus plant.
Cultivation and economic products of citrus trees
The fruits of citrus trees contain large concentrations of sour-tasting citric acid . Nevertheless, the fruits of some species can be quite sweet because they contain large concentrations of fruit sugar. Plant breeders have developed various sorts of cultivated varieties, or cultivars, from the wild progenitors of various species of citrus trees. This has resulted in the selective breeding of varieties with especially sweet fruits, others which peel relatively easily, and yet others that are seedless and therefore easier to eat or process into juice.
Once plant breeders discover a desirable cultivar of a species of citrus tree, it is thereafter propagated by rooting stem cuttings or by grafting. The latter procedure involves taking a stem of the cultivar, and attaching it to the rootstock of some other, usually relatively hardy, variety. The graft is carefully wrapped until a protective callus tissue is formed. Because the attributes of the cultivar are genetically based, these methods of propagation avoid the loss of desirable genetic attributes of the new variety that would inevitably occur through sexual cross-breeding.
Citrus trees can also be propagated using relatively new techniques by which small quantities of cells can be grown and induced to develop into fully-formed plants through specific hormone treatments. These relatively new techniques, known as micro propagation or tissue culture, allow for the rapid and inexpensive production of large numbers of trees with identical genetic qualities.
The most important economic products of cultivated citrus trees are, of course, their fruits. In agriculture, the fruits of oranges and grapefruits are commonly picked when they are ripe or nearly so, while those of lemons and limes are usually picked while they are still unripened, or green.
The fruits of the sweet orange can be eaten directly after peeling, or they may be processed into a juice, which can be drunk fresh. It may also be concentrated by evaporating about three-quarters of its water content, and then frozen for transport to far-away markets. This is a more economical way of moving orange juice around, because removal of much of the water means that much less weight must be transported. In addition, juice concentrates can also be used to manufacture flavorings for various types of drinks.
The juice of citrus fruits is relatively rich in ascorbic acid, or vitamin C. For example, a typical orange contains about 40 mg of vitamin C, compared with only 5 mg in an apple. Vitamin C is an essential nutrient for proper nutrition of animals. However, animals cannot synthesize their own vitamin C and must obtain the micronutrient from their diet. In the absence of a sufficient dietary supply of vitamin C, a debilitating and eventually lethal disease known as scurvy develops. In past centuries scurvy often afflicted mariners on long oceanic voyages, during which foods rich in vitamin C or its biochemical precursors could not be readily obtained. Because they stored relatively well, lemons were an important means by which sailors could avoid scurvy, at least while the supply of those fruits lasted.
At one time, citric acid was commercially extracted from the fruits of citrus trees, mostly for use in flavoring drinks. Today, citric acid is used in enormous quantities to flavor carbonated soft drinks and other beverages. However, most of this industrial citric acid is synthesized by fungi in huge fermentation vats.
In addition, the shredded peel and juice of citrus fruits can be sweetened and jelled for use in such sweet spreads as marmalade.
The major economic value of oranges lies in their fruits, but several fragrant oils can also be extracted from their flowers, or, more commonly, their peel as a byproduct of the orange-juice industry. These essences can be used to manufacture so-called Neroli and Portugal oils. These fragrances were originally used in the manufacturing of perfumes and to scent potpourri, and they are still used for these purposes. In addition, many household products, such as liquid detergents, shampoos, and soaps, are pleasantly scented using the aromatic oils extracted from citrus trees.
Pomanders, which are oranges studded with cloves, are an archaic use of the fruit. Originating in Spain, pomanders were worn around the neck for several purposes-as perfumery, to ward off infections, or to attract a person of the opposite sex. Today pomanders are more commonly used to pleasantly scent closets and drawers.
In regions with a warm climate, citrus trees are sometimes grown as ornamental shrubs and trees. The citron was reputedly grown in the ancient Hanging Gardens of Babylon in what is now Iraq. During those times the citron was used in scenting toilet water and in making an aromatic ointment known as pomade.
The sweet orange
The sweet orange (Citrus sinensis) is a 16-46 ft (5-14 m) tall tree with evergreen foliage, white flowers, and spherical fruits. This species is originally from southern China or perhaps Southeast Asia. However, the original range is somewhat uncertain, because wild plants in natural habitats are not known. The sweet orange has been cultivated in China and elsewhere in southern Asia for thousands of years, being mentioned in dated Chinese scripts from 2200 b.c. The sweet orange reached Europe as a cultivated species sometime before the fourteenth century. Sweet oranges are now grown around the world wherever the climate is suitably subtropical or tropical.
The sweet orange tends to flower and fruit during periods of relatively abundant rainfall, and becomes dormant if a pronounced drier period occurs during the summer. The sweet orange is commonly cultivated in plantations or groves. These are widely established in subtropical parts of the southern United States, particularly in southern Florida and California. Oranges are also widely grown in Mexico, Spain, the Middle East, North Africa, and many other countries, for both local use and export. The global production of sweet oranges is more than 38.5 million tons (35 million metric tonnes) per year.
Orange fruits are very tasty and nutritious, containing 5-10% sugar, 1-2% citric acid, along with vitamin C and beneficial fiber and pulp. Most sweet oranges have an orange-colored rind when they are ripe as well as an orange interior and juice. However, some cultivated varieties of sweet oranges have a yellow or green rind, while still others have a deep-red interior and juice. Some varieties have been bred to be seedless, including navel, Jaffa, and Malta oranges.
Oranges were a scarce and expensive fruit in past centuries, and many children were delighted to find a precious orange in their Christmas stocking. Today, however, oranges are grown in enormous quantities and are readily available as an inexpensive fruit at any time of the year.
The tangerine or mandarin orange
The tangerine and mandarin (Citrus reticulata) are a species of small tree native to southern China. The fruits of this species are similar to those of the sweet orange, but they are generally smaller, their rind is much easier to separate from the interior pulp, and the segments separate more readily.
Compared with the sweet orange, the tangerine and mandarin do not store very well. As a result, these citrus fruits tend to be sold fairly close to where they are grown, and less of a long-distance export market exists for these crops . However, in some countries a tradition has developed of eating mandarin oranges at certain festive times of year. For example, North Americans and many western Europeans often eat mandarins around Christmas time. Because a premium price can be obtained for these fruits during the Christmas season, there is a well organized market that keys on about a one-or-two month export market for mandarins during that festive season.
The grapefruit or pomelo (Citrus paradisi) is a variety of cultivated citrus tree whose geographic origin is not known, but is likely native to Southeast Asia. The fruit of the grapefruit has a yellowish rind, and is relatively large, as much as 1 lb (0.5 kg) in weight. The pulp and juice of the grapefruit are rather bitter and acidic and are often sweetened with cane sugar before being eaten.
The lemon (C. limon) is an evergreen tree native to Indochina and cultivated there for thousands of years. The lemon was later imported to the basin of the Mediterranean Sea, where it has been cultivated for at least 2,000 years. Lemon trees are very attractive, especially when their fragrant white or yellow flowers are in bloom. However, the fruits of lemons are quite tart and bitter, containing about 5% citric acid, but only 0.5% sugar.
The fruits of lemons are picked when they are not yet ripe and their rinds are still green. This is done because lemon fruits deteriorate quickly if they are allowed to ripen on the tree. Commercial lemons develop their more familiar, yellow-colored rinds some time after they are harvested while they are being stored or transported to markets.
Although few people have the fortitude to eat raw lemons, the processed juice of this species can be used to flavor a wide range of sweetened drinks, including lemonade. Lemon flavoring is also used to manufacture many types of carbonated beverages, often in combination with the flavoring of lime. A bleaching agent and stain remover can also be made from lemon juice.
The lime is native to Southeast Asia and is very susceptible to frost. More sour than the lemon, the lime (C. aurantifolia) cannot be eaten raw. However, the lime can be used to make a sweetened beverage known as limeade, and an extract of its juice is widely used to flavor commercially prepared soft drinks.
Other citrus trees
The Seville, sour or bitter orange (C. media), is derived from a wild progenitor that grows in the foothills of the Himalayan Mountains of south Asia. The flowers of this species are exceedingly fragrant and have been used to produce aromatic oils for perfumery. The large orange-red fruits of the sour orange are rather bitter and acidic. These are not often eaten, but are used to make flavorings, marmalades, candied peels, aromatic oils, and to flavor a liquor known as curacao.
The citron (C. medica) is another species native to the southern Himalayas, probably in northern India. This may be the oldest of the cultivated citrus trees, perhaps going back as far as 6,000 years. The fruit of this species is very large in comparison to those of other citrus trees, weighing as much as 6.5 lb (3 kg). The rind of the citron is thick and has a lumpy surface, and the pulpy interior is bitter. The peel of the citron is soaked in salty water, which removes much of the bitter taste . It is then candied with sugar and used to flavor cakes, pastries, and candies.
The shaddock or pomelo (C. maxima) is probably native to Southeast Asia. This species is mostly used to manufacture candied rind. The pomelo develops a large, spherical, thick-rinded fruit, weighing as much as 13 lb (6 kg), and having a diameter of up to 6 in (16 cm). The name shaddock comes from the name of a sea captain who first introduced this species to the West Indies.
Other relatively minor species of citrus trees include the Panama orange or calamondin (C. mitis) and the bergamot (C. bergamia).
See also Mycorrhiza.
Hvass, E. Plants That Serve and Feed Us. New York: Hippocrene Books, 1975.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.
Klein, R. M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—A distinct variety of a plant that has been bred for particular agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered a subspecies.
—A section of a stem of a plant, which can be induced to root and can thereby be used to propagate a new plant that is genetically identical to the parent.
—A method of propagation of woody plants whereby a shoot, known as a scion, is taken from one plant and inserted into a rootstock of another plant. The desired traits of the scion for horticultural or agricultural purposes are genetically based. Through grafting, large numbers of plants with these characteristics can be readily and quickly developed.
—A disease of humans that is caused by an insufficient supply of ascorbic acid, or vitamin C, in the diet. The symptoms of scurvy include spongy, bleeding gums, loosening and loss of teeth, and subcutaneous bleeding. It can ultimately lead to death.
- Tissue culture
—This is a relatively recently developed method of growing large numbers of genetically identical plants. In tissue culture, small quantities of undifferentiated cells are grown on an artificial growth medium, and are then caused to develop into small plantlets by subjecting them to specific treatments with growth-regulating hormones.