The most recent research indicates an origin in Australia, New Caledonia and New Guinea. Some researchers believe that the origin is in the part of Southeast Asia bordered by Northeast India, Burma (Myanmar) and the Yunnan province of China, and it is in this region that some commercial species such as oranges, mandarins, and lemons originated. Citrus fruit has been cultivated in an ever-widening area since ancient times.
Citrus plants are native to subtropical and tropical regions of Asia and the Malay Archipelago and they were firstly domesticated in these areas. Some citrus species have been present in the Mediterranean basin for centuries. This group of species has reached great importance in some of the Mediterranean countries and, in the case of orange, mandarin and lemon trees, they found here soil and climatic conditions which allows them to achieve a high level of fruit quality, even better than in the regions where they came from.
The "native" oranges of Florida actually originated with the Spanish Conquistadors. The agronomists of classical Rome already made many references to the cultivation of citrus fruits within the limits of their empire. King Louis XIV of France housed citrus in orangeries, to protect the tropical fruit to be grown in the 1600s France.
The generic name originated from Latin, where it referred to either the plant now known as Citron (C. medica) or a conifer tree (Thuja). It is somehow related to the ancient Greek word for cedar, κέδρος (kédros). This may be due to perceived similarities in the smell of citrus leaves and fruit with that of cedar. Collectively, Citrus fruits and plants are also known by the Romance loanword agrumes (literally "sour fruits").
The large citrus fruit of today evolved originally from small, edible berries over millions of years. Citrus plants diverged from a common ancestor about 15 million years ago, which was about when it diverged from the closely related severinia, for example the Chinese box orange. About 7 million years ago, citrus plants diverged into two groups, the main citrus genus and the ancestors of the Trifoliate orange (poncirus), which is closely enough related that it can still be hybridized with all other citrus. These estimates are made using genetic mapping of plant chloroplasts. A DNA study published in Nature in 2018, concludes that citrus trees originated in the foothills of the Himalayas, in the area of Assam (India), western Yunnan (China), and northern Myanmar.
The three original species in the citrus genus that have been hybridized into most modern commercial citrus fruit are the mandarin orange, pummelo, and citron. Within the last few thousand years, all common citrus fruits (sweet oranges, lemons, grapefruit, limes, and so on) were created by crossing those original species. Something similar has occurred with the wide array of chili peppers originating in the hybridization of a few initial species.
The taxonomy and systematics of the genus are complex and the precise number of natural species is unclear, as many of the named species are hybrids clonally propagated through seeds (by apomixis), and there is genetic evidence that even some wild, true-breeding species are of hybrid origin.
Most cultivated Citrus seem to be natural or artificial hybrids of a small number of core ancestral species, including the citron, pummelo, mandarine, and papedas (see image). Natural and cultivated citrus hybrids include commercially important fruit such as oranges, grapefruit, lemons, limes, and some tangerines.
Apart from these core citrus species, there are Australian limes and the recently discovered Mangshanyegan. Kumquats and Clymenia sp. are now generally considered to belong within the citrus genus. Trifoliate orange, which is often used as commercial rootstock, is an outgroup and may or may not be categorized as a citrus.
These plants are large shrubs or small to moderate-sized trees, reaching 5–15 m (16–49 ft) tall, with spiny shoots and alternately arranged evergreen leaves with an entire margin. The flowers are solitary or in small corymbs, each flower 2–4 cm (0.79–1.57 in) diameter, with five (rarely four) white petals and numerous stamens; they are often very strongly scented.
The fruit is a hesperidium, a specialised berry, globose to elongated, 4–30 cm (1.6–11.8 in) long and 4–20 cm (1.6–7.9 in) diameter, with a leathery rind or "peel" called a pericarp. The outermost layer of the pericarp is an "exocarp" called the flavedo, commonly referred to as the zest. The middle layer of the pericarp is the mesocarp, which in citrus fruits consists of the white, spongy "albedo", or "pith". The innermost layer of the pericarp is the endocarp. The segments are also called "liths", and the space inside each lith is a locule filled with juice vesicles, or "pulp". From the endocarp, string-like "hairs" extend into the locules, which provide nourishment to the fruit as it develops.
Citrus fruits are notable for their fragrance, partly due to flavonoids and limonoids (which in turn are terpenes) contained in the rind, and most are juice-laden. The juice contains a high quantity of citric acid giving them their characteristic sharp flavour. The genus is commercially important as many species are cultivated for their fruit, which is eaten fresh, pressed for juice, or preserved in marmalades and pickles.
They are also good sources of vitamin C and flavonoids. The content of vitamin C in the fruit depends on the species, variety and mode of cultivation. Fruits produced with organic agriculture have been shown to contain more vitamin C than those produced with conventional agriculture in the Algarve, but results depended on the species and cultivar. The flavonoids include various flavanones and flavones.
Citrus trees hybridise very readily – depending on the pollen source, plants grown from a Persian lime's seeds can produce fruit similar to grapefruit. Thus all commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness.
The colour of citrus fruits only develops in climates with a (diurnal) cool winter. In tropical regions with no winter at all, citrus fruits remain green until maturity, hence the tropical "green oranges". The Persian lime in particular is extremely sensitive to cool conditions, thus it is not usually exposed to cool enough conditions to develop a mature colour. If they are left in a cool place over winter, the fruits will change colour to yellow.
The terms "ripe" and "mature" are usually used synonymously, but they mean different things. A mature fruit is one that has completed its growth phase. Ripening is the changes that occur within the fruit after it is mature to the beginning of decay. These changes usually involve starches converting to sugars, a decrease in acids and a softening and change in the fruit's colour.
Citrus fruits are non-climacteric and respiration slowly declines and the production and release of ethylene is gradual. The fruits do not go through a ripening process in the sense that they become "tree ripe." Some fruits, for example cherries, physically mature and then continue to ripen on the tree. Other fruits, like pears, are picked when mature but before they ripen, then continue to ripen off the tree. Citrus fruits pass from immaturity to maturity to over-maturity while still on the tree. Once they are separated from the tree, they will not increase in sweetness or continue to ripen. The only way change may happen after being picked is that they will eventually start to decay.
With oranges, colour cannot be used as an indicator of ripeness because sometimes the rinds turn orange long before the oranges are ready to eat. Tasting them is the only way to know whether or not they are ready to eat.
Citrus trees are not generally frost hardy. Mandarin oranges (C. reticulata) tend to be the hardiest of the common Citrus species and can withstand short periods down to as cold as −10 °C (14 °F), but realistically temperatures not falling below −2 °C (28 °F) are required for successful cultivation. Tangerines, tangors and yuzu can be grown outside even in regions with more marked sub-freezing temperatures in winter, although this may affect fruit quality. A few hardy hybrids can withstand temperatures well below freezing, but do not produce quality fruit. Lemons can be commercially grown in cooler-summer/moderate-winter, coastal Southern California, because sweetness is neither attained nor expected in retail lemon fruit. The related trifoliate orange (Citrus trifoliata) can survive below −20 °C (−4 °F); its fruit are astringent and inedible unless cooked but a few better-tasting cultivars and hybrids have been developed (see citranges).
The trees thrive in a consistently sunny, humid environment with fertile soil and adequate rainfall or irrigation. Abandoned trees in valleys may suffer, yet survive, the dry summer of Central California's Inner Coast Ranges. At any age citrus grows well enough with infrequent irrigation in partial shade, but the fruit crop is smaller. Being of tropical and sub-tropical origin, oranges, like all citrus, are broadleaved and evergreen. They do not drop leaves except when stressed. The stems of many varieties have large sharp thorns. The trees flower in the spring, and fruit is set shortly afterward. Fruit begins to ripen in fall or early winter months, depending on cultivar, and develops increasing sweetness afterward. Some cultivars of tangerines ripen by winter. Some, such as the grapefruit, may take up to eighteen months to ripen.
Major commercial citrus growing areas include southern China, the Mediterranean Basin (including southern Spain), South Africa, Australia, the southern United States, Mexico and parts of South America. In the United States, Florida, California, Arizona, and Texas are major producers, while smaller plantings are present in other Sun Belt states and in Hawaii.
Citrus trees grown in tubs and wintered under cover were a feature of Renaissance gardens, once glass-making technology enabled sufficient expanses of clear glass to be produced. An orangery was a feature of royal and aristocratic residences through the 17th and 18th centuries. The Orangerie at the Palace of the Louvre, 1617, inspired imitations that were not eclipsed until the development of the modern greenhouse in the 1840s. In the United States the earliest surviving orangery is at the Tayloe House, Mount Airy, Virginia. George Washington had an orangery at Mount Vernon.
Some modern hobbyists still grow dwarf citrus in containers or greenhouses in areas where it is too cold to grow it outdoors. Consistent climate, sufficient sunlight, and proper watering are crucial if the trees are to thrive and produce fruit. Compared to many of the usual "green shrubs", citrus trees better tolerate poor container care. For cooler winter areas, limes and lemons should not be grown, since they are more sensitive to winter cold than other citrus fruits. Hybrids with kumquats (× Citrofortunella) have good cold resistance. A citrus tree in a container may have to be re-potted every 5 years or so since the roots may form a thick "root-ball" on the bottom of the pot.
Citrus plants are very liable to infestation by aphids, whitefly and scale insects (e.g. California red scale). Also rather important are the viral infections to which some of these ectoparasites serve as vectors such as the aphid-transmitted Citrus tristeza virus which when unchecked by proper methods of control is devastating to citrine plantations. The newest threat to citrus groves in the United States is the Asian citrus psyllid.
The Asian citrus psyllid is an aphid-like insect that feeds on the leaves and stems of citrus trees and other citrus-like plants – but the real danger lies in that it can carry a deadly, bacterial tree disease called Huanglongbing (HLB), also known as citrus greening disease.
In August 2005 citrus greening disease was discovered in the south Florida region around Homestead and Florida City. It has since spread to every commercial citrus grove in Florida. In 2004–2005 USDA Ag statistics reported the total Florida citrus production to be 169.1 million boxes of fruit. The estimate for all Florida citrus production in the 2015–2016 season is 94.2 million boxes—a 44.3% drop.
In June 2008, the psyllid was spotted dangerously close to California – right across the international border in Tijuana, Mexico. Only a few months later, it was detected in San Diego and Imperial counties, and has since spread to Riverside, San Bernardino, Orange, Los Angeles and Ventura counties sparking quarantines in those areas. The Asian citrus psyllid has also been intercepted coming into California in packages of fruit and plants, including citrus, ornamentals, herbs and bouquets of cut flowers, shipped from other states and countries.
The foliage is also used as a food plant by the larvae of Lepidoptera (butterfly and moth) species such as the Geometridae common emerald (Hemithea aestivaria) and double-striped pug (Gymnoscelis rufifasciata), the Arctiidae giant leopard moth (Hypercompe scribonia), H. eridanus, H. icasia and H. indecisa, many species in the family Papilionidae (swallowtail butterflies), and the black-lyre leafroller moth ("Cnephasia" jactatana), a tortrix moth.
In eastern Australia, the bronze-orange bug (Musgraveia sulciventris) can be a major pest of citrus trees, particularly grapefruit. In heavy infestations it can cause flower and fruit drop and general tree stress.
Citrus plants can also develop a deficiency condition called chlorosis, characterized by yellowing leaves highlighted by contrasting leaf veins. The shriveling leaves eventually fall, and if the plant loses too many, it will slowly die. This condition is often caused by an excessively high pH (alkaline soil), which prevents the plant from absorbing iron, magnesium, zinc, or other nutrients it needs to produce chlorophyll. This condition can be cured by adding an appropriate acidic fertilizer formulated for citrus, which can sometimes revive a plant to produce new leaves and even flower buds within a few weeks under optimum conditions. A soil which is too acidic can also cause problems; citrus prefers neutral soil (pH between 6 and 8). Citrus plants are also sensitive to excessive salt in the soil. Soil testing may be necessary to properly diagnose nutrient deficiency diseases.
Many citrus fruits, such as oranges, tangerines, grapefruits, and clementines, are generally eaten fresh. They are typically peeled and can be easily split into segments. Grapefruit is more commonly halved and eaten out of the skin with a spoon. There are special spoons (grapefruit spoons) with serrated tips designed for this purpose. Orange and grapefruit juices are also very popular breakfast beverages. More acidic citrus, such as lemons and limes, are generally not eaten on their own. Meyer lemons can be eaten out of hand with the fragrant skin; they are both sweet and sour. Lemonade or limeade are popular beverages prepared by diluting the juices of these fruits and adding sugar. Lemons and limes are also used as garnishes or in cooked dishes. Their juice is used as an ingredient in a variety of dishes; it can commonly be found in salad dressings and squeezed over cooked meat or vegetables.
A variety of flavours can be derived from different parts and treatments of citrus fruits. The rind and oil of the fruit is generally very bitter, especially when cooked, and so is often combined with sugar. The fruit pulp can vary from sweet and tart to extremely sour. Marmalade, a condiment derived from cooked orange and lemon, can be especially bitter, but is usually sweetened to cut the bitterness and produce a jam-like result. Lemon or lime is commonly used as a garnish for water, soft drinks, or cocktails. Citrus juices, rinds, or slices are used in a variety of mixed drinks. The colourful outer skin of some citrus fruits, known as zest, is used as a flavouring in cooking; the white inner portion of the peel, the pith, is usually avoided due to its bitterness. The zest of a citrus fruit, typically lemon or an orange, can also be soaked in water in a coffee filter, and drunk.
Citrus fruit intake has been associated with a 10% reduction in odds of developing breast cancer.
Oranges were historically used for their high content of vitamin C, which prevents scurvy. Scurvy is caused by vitamin C deficiency, and can be prevented by having 10 milligrams of vitamin C a day. An early sign of scurvy is fatigue. If ignored, later symptoms are bleeding and bruising easily. British sailors were given a ration of citrus fruits on long voyages to prevent the onset of scurvy, hence the British nickname of Limey.
After consumption, the peel is sometimes used as a facial cleanser.
Citrus fruit intake is associated with a reduced risk of stomach cancer. Also, citrus fruit juices, such as orange, lime and lemon, may be useful for lowering the risk of specific types of kidney stones. Grapefruit is another fruit juice that can be used to lower blood pressure because it interferes with the metabolism of calcium channel blockers. Lemons have the highest concentration of citrate of any citrus fruit, and daily consumption of lemonade has been shown to decrease the rate of kidney stone formation.
The Moroccan professor Henri Chapot discovered that the acidity in the more common citrons or lemons is represented by red on the inner coat of seeds specifically on the chalazal spot, violet pigmentation on the outer side of the flower blossom, and also by the new buds that are reddish-purplish. The acidless varieties of citrus are completely lacking the red color in all the mentioned spots. This designation was cited by Herbert John Webber and Leon Dexter Batchelor, the editors of the fundamental treatise on citrus, namely The Citrus Industry, which was published by the University of California, Riverside in 1967.
A fossil leaf from the Pliocene of Valdarno (Italy) is described as †Citrus meletensis.  In China fossil leaf specimens of †Citrus linczangensis have been collected from coal-bearing strata of the Bangmai Formation in the Bangmai village, about 10 km northwest of Lincang City, Yunnan. The Bangmai Formation contains abundant fossil plants and is considered to be of late Miocene age, Citrus linczangensis and C. meletensis share some important characters, such as an intramarginal vein, an entire margin, and an articulated and distinctly winged petiole.
The genus Citrus has been suggested to originate in the eastern Himalayan foothills. Prior to human cultivation, it consisted of just a few species, though the status of some as distinct species has yet to be confirmed:
Sorted by parentage. As each hybrid is the product of (at least) two parent species, they are listed multiple times.
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