Renowned for their highly fragrant aromas and complex flavor, citrus are among the most popular fruits in cultivation. With a propensity to hybridize between species, making their taxonomy complicated, there are numerous varieties encompassing a wide range of appearance and fruit flavors.
The large citrus fruit of today evolved originally from small, edible berries over millions of years. Citrus species began to diverge from a common ancestor about 15 million years ago, at about the same time thatSeverinia (such as theChinese box orange) diverged from the same ancestor. About 7 million years ago, the ancestors ofCitrus split into the main genus,Citrus, and thePoncirus group (such as thetrifoliate orange), which some taxonomies consider a separate genus and others include inCitrus[4]Poncirusis closely enough related that it can still be hybridized with all other citrus and used as rootstock. These estimates are made using genetic mapping of plantchloroplasts.[5] A DNA study published inNature in 2018 concludes that the genusCitrus evolved in the foothills of theHimalayas, in the area ofAssam (India), westernYunnan (China), and northernMyanmar.[6]
Map of inferred original wild ranges of the mainCitrus cultivars, and selected relevant wild taxa[7]
The three ancestral species in the genusCitrus associated with modernCitrus cultivars are themandarin orange,pomelo, andcitron. Almost all of the common commercially important citrus fruits (sweet oranges,lemons, grapefruit, limes, and so on) are hybrids between these three species, their main progenies, and other wildCitrus species within the last few thousand years.[8][9][10]
Citrus plants are native to subtropical and tropical regions of Asia,Island Southeast Asia,Near Oceania, and northeastern and central Australia. Domestication of citrus species involved much hybridization andintrogression, leaving much uncertainty about when and where domestication first happened.[8] A genomic, phylogenic, and biogeographical analysis by Wuet al. (2018) has shown that the center of origin of the genusCitrus is likely the southeast foothills of theHimalayas, in a region stretching from easternAssam, northernMyanmar, to westernYunnan. It diverged from a common ancestor withPoncirus trifoliata. A change in climate conditions during theLate Miocene (11.63 to 5.33mya) resulted in asudden speciation event. The species resulting from this event include thecitrons (Citrus medica) of South Asia; thepomelos (C. maxima) ofMainland Southeast Asia; themandarins (C. reticulata),kumquats (C. japonica),mangshanyegan (C. mangshanensis), andichang papedas (C. cavaleriei) of southeastern China; thekaffir limes (C. hystrix) ofIsland Southeast Asia; and thebiasong andsamuyao (C. micrantha) of thePhilippines.[8][7]
Afossil leaf from thePliocene ofValdarno, Italy is described as †Citrus meletensis.[11]In China, fossil leaf specimens of †Citrus linczangensis have been collected from lateMiocene coal-bearing strata of the Bangmai Formation inYunnan province.C. linczangensis resemblesC. meletensis in having an intramarginal vein, an entire margin, and an articulated and distinctly wingedpetiole.[12]
Many cultivatedCitrus species arenatural orartificial hybrids of a small number of core ancestral species, including the citron, pomelo, and mandarin. Natural and cultivatedcitrus hybrids include commercially important fruit such as oranges,grapefruit, lemons, limes, and sometangerines. The multiple hybridisations have made the taxonomy ofCitrus complex.[13][14]
Kumquats andClymenia spp. are now generally considered to belong within the genusCitrus.[15] The false oranges,Oxanthera fromNew Caledonia, have been transferred to theCitrus genus onphylogenetic evidence.[16][17] A recent taxonomy reincorporates the trifoliate orange (Poncirus) into an enlargedCitrus, but recognizes that many botanists still follow Swingle in splitting it off.[4]
The citron (Citrus medica) was also introduced early into the Mediterranean basin from India and Southeast Asia, via two ancient trade routes: an overland route throughPersia, theLevant and the Mediterranean islands, and a maritime route through theArabian Peninsula andPtolemaic Egypt into North Africa. Although the exact date of the original introduction is unknown due to the sparseness of archaeobotanical remains, the earliest evidence is seeds recovered from theHala Sultan Tekke site ofCyprus, dated to around 1200 BCE. Other archaea botanical evidence includes pollen fromCarthage, dating back to the 4th century BCE, and carbonized seeds fromPompeii dated to around the 3rd to 2nd century BCE. The earliest complete description of the citron was written byTheophrastus,c. 310 BCE.[19][20][21]
Lemons, pomelos, and sour oranges were introduced to the Mediterranean by Arab traders around the 10th century CE. Sweet oranges were brought to Europe by theGenoese andPortuguese from Asia during the 15th to 16th century. Mandarins were not introduced until the 19th century.[19][20][21] Oranges were introduced to Florida by Spanish colonists.[22][23] In cooler parts of Europe, citrus fruit was grown inorangeries starting in the 17th century; many were as much status symbols as functional agricultural structures.[24]
The generic nameCitrus originates fromLatin, where it denoted either thecitron (C. medica) or a conifer tree (Thuja). The Latin word is related to theancient Greek word for thecedar of Lebanon,κέδρος (kédros), perhaps from a perceived similarity of the smell of citrus leaves and fruit with that of cedar.[25]
Citrus plants are large shrubs or small to moderate-sized trees, reaching 5–15 m (16–49 ft) tall, withspiny shoots and alternately arrangedevergreenleaves with an entire margin.[26] Theflowers are solitary or in smallcorymbs, 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, due to the presence of essential oil glands.[27]
The fruit is ahesperidium, a specialised berry with multiplecarpels, globose to elongated,[27][28] 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 apericarp. The outermost layer of the pericarp is an "exocarp" called theflavedo, commonly referred to as thezest. 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. This surrounds a variable number ofcarpels, shaped as radial segments. The seeds, if present, develop inside the carpels. The space inside each segment is alocule filled withjuice vesicles, or pulp. From the endocarp, string-like "hairs" extend into the locules, which provide nourishment to the fruit as it develops.[27][29] The genus is commercially important withcultivars of many species grown for their fruit. Some cultivars have been developed to be easy to peel and seedless, meaning they areparthenocarpic.[28]
The fragrance of citrus fruits is conferred byflavonoids andlimonoids in the rind. The flavonoids include variousflavanones andflavones.[30] The carpels are juicy; they contain a high quantity ofcitric acid, which with other organic acids includingascorbic acid (vitamin C) give them their characteristic sharp taste.[31]Citrus fruits are diverse in size and shape, as well as in color and flavor, reflecting their biochemistry;[32][33] for instance, grapefruit is made bitter-tasting by a flavanone,naringin.[31]
Citrus fruits are diverse in size, shape, color, and flavor.
Most commercial citrus cultivation uses trees produced bygrafting the desired fruitingcultivars ontorootstocks selected for disease resistance and hardiness.[34] The trees are not generallyfrost hardy. They thrive in a consistently sunny, humid environment with fertile soil and adequate water.[34]
The color 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".[35] The terms 'ripe' and 'mature' are widely used synonymously, but they mean different things. A mature fruit is one that has completed its growth phase.Ripening is the sequence of changes within the fruit from maturity to the beginning of decay. These changes involve the conversion of starches to sugars, a decrease in acids, softening, and a change in the fruit's color.[36]Citrus fruits are non-climacteric and respiration slowly declines and the production and release of ethylene is gradual.[37]
According to theUN Food and Agriculture Organization, world production of all citrus fruits in 2016 was 124 million tonnes, with about half of this production as oranges.[38] At US $15.2 billion equivalent in 2018, citrus trade[39] makes up nearly half of the world fruit trade, which was US$32.1 billion that year.[40] According to theUnited Nations Conference on Trade and Development, citrus production grew during the early 21st century mainly by the increase incultivation areas, improvements in transportation and packaging, rising incomes and consumerpreference for healthy foods.[38] In 2019–20, world production of oranges was estimated to be 47.5 million tonnes, led by Brazil, Mexico, the European Union, and China as the largest producers.[41]
Citrus plants can develop the deficiency conditionchlorosis, characterized by yellowing leaves.[50] The condition is often caused by an excessively highpH (alkaline soil), which prevents the plant from absorbing nutrients such as iron,magnesium, andzinc needed to producechlorophyll.[51]
SomeCitrus species contain significant amounts offuranocoumarins.[53][54] In humans, some of these act as strongphotosensitizers when applied topically to the skin, while others interact with medications when taken orally in thegrapefruit juice effect.[53] Due to the photosensitizing effects of certain furanocoumarins, someCitrus species causephytophotodermatitis,[55] a potentially severe skin inflammation resulting from contact with a light-sensitizing botanical agent followed by exposure toultraviolet light. InCitrus species, the primary photosensitizing agent appears to bebergapten,[52] a linear furanocoumarin derived frompsoralen. This claim has been confirmed for lime[56][57] andbergamot. In particular,bergamot essential oil has a higher concentration of bergapten (3–3.6 g/kg) than any otherCitrus-based essential oil.[58]
Asystematic review indicates that citrus fruit consumption is associated with a 10% reduction of risk for developing breast cancer.[59]
Many citrus fruits, such asoranges,tangerines,grapefruits, andclementines, are generally eaten fresh.[28] They are typically peeled and can be easily split into segments.[28] Grapefruit is more commonly halved and eaten out of the skin with a spoon.[60]Lemonade is a popular beverage prepared by diluting the juice and adding sugar.[61] Lemon juice is mixed in salad dressings[62] and squeezed over fruit salad to stop it from turning brown: its acidity suppressesoxidation bypolyphenol oxidase enzymes.[63]
A variety of flavors can be derived from different parts and treatments of citrus fruits.[28] The colorful outer skin of some citrus fruits, known aszest, is used as a flavoring in cooking.[64]The whole of thebitter orange (and sometimes other citrus fruits) including thepeel with itsessential oils is cooked with sugar to makemarmalade.[65]
By the 17th century,orangeries were added to great houses in Europe, both to enable the fruit to be grown locally and for prestige, as seen in theVersailles Orangerie.[66] Some modern hobbyists grow dwarf citrus in containers or greenhouses in areas where the weather is too cold to grow it outdoors;Citrofortunella hybrids have good cold resistance.[67]
Lemons appear in paintings, pop art, and novels.[68] A wall painting in thetomb of Nakht in 15th century BC Egypt depicts a woman in a festival, holding a lemon. In the 17th century,Giovanna Garzoni painted aStill Life with Bowl of Citrons, the fruits still attached to leafy flowering twigs, with a wasp on one of the fruits. TheimpressionistEdouard Manet depicted a lemon on a pewter plate. In modern art,Arshile Gorky paintedStill Life with Lemons in the 1930s.[68]
Citrus fruits "were the clear status symbols of the nobility in the ancient Mediterranean", according to thepaleoethnobotanist Dafna Langgut.[69] InLouisa May Alcott's 1868 novelLittle Women, the character Amy March states that "It's nothing but limes now, for everyone is sucking them in their desks in schooltime, and trading them off for pencils, bead rings, paper dolls, or something else… If one girl likes another, she gives her a lime; if she's mad with her, she eats one before her face, and doesn't offer even a suck."[69]
^Xie, Sanping Xie; Manchester, Steven R.; Liu, Kenan; Sun, Bainian (October 2013). "Citrus linczangensis sp. n., a Leaf Fossil of Rutaceae from the Late Miocene of Yunnan, China".International Journal of Plant Sciences.174 (8):1201–1207.Bibcode:2013IJPlS.174.1201X.doi:10.1086/671796.
^Klein, Joshua D. (2014). "Citron Cultivation, Production and Uses in the Mediterranean Region".Medicinal and Aromatic Plants of the Middle-East. Medicinal and Aromatic Plants of the World. Vol. 2. pp. 199–214.doi:10.1007/978-94-017-9276-9_10.ISBN978-94-017-9275-2.
^abZech-Matterne, Véronique; Fiorentino, Girolamo; Coubray, Sylvie; Luro, François (2017)."Introduction". In Zech-Matterne, Véronique; Fiorentino, Girolamo (eds.).AGRUMED: Archaeology and history of citrus fruit in the Mediterranean: Acclimatization, diversification, uses. Publications du Centre Jean Bérard.ISBN978-2-918887-77-5.
^Britz, Billie S. (May 1974). "Environmental Provisions for Plants in Seventeenth-Century Northern Europe".The Journal of the Society of Architectural Historians.33 (2):133–144.doi:10.2307/988906.JSTOR988906.
^Del Hotal, Tom."Citrus Pruning"(PDF).California Rare Fruit Growers.
^abcOrtiz, Jesus M. (2002). "Botany: taxonomy, morphology and physiology of fruits, leaves and flowers". In Di Giacomo, Angelo; Dugo, Giovanni (eds.).Citrus: The GenusCitrus. Taylor & Francis. pp. 16–35.ISBN978-0-2032-1661-3.
^abAbolore, Rasaq S.; Tsegaye, Bahiru; Jaiswal, Swarna; Jaiswal, Amit K. (2023). "An overview of industrial enzymes in beverage production and processing".Value-Addition in Beverages through Enzyme Technology. Elsevier. pp. 1–26 (section 1.2.3.2 Citrus juice).doi:10.1016/b978-0-323-85683-6.00013-2.ISBN978-0-323-85683-6.
^Luro, François; Curk, Franck; Froelicher, Yann; Ollitrault, Patrick (2017). "Recent insights on Citrus diversity and phylogeny".AGRUMED: Archaeology and history of citrus fruit in the Mediterranean. Publications du Centre Jean Bérard.doi:10.4000/books.pcjb.2169.ISBN978-2-918887-77-5.
^"Florida Citrus Statistics 2015–2016"(PDF).United States Department of Agriculture – National Agricultural Statistics Service. 3 October 2017. p. 62. Retrieved3 October 2017.Abandoned groves are a threat to the citrus industry and are a haven for psyllids carrying the bacterium that causes greening disease.
^A Sheath Nematode, Hemicycliophora arenaria raski, Pathogenic to Citrus, by D. E. Stokes, Florida Department of Agriculture and Consumer Services, July 1977[1]
^Wagner, A. M.; Wu, J. J.; Hansen, R. C.; Nigg, H. N.; Beiere, R. C. (2002). "Bullous phytophotodermatitis associated with high natural concentrations of furanocoumarins in limes".Am J Contact Dermat.13 (1):10–14.doi:10.1053/ajcd.2002.29948.ISSN0891-5849.PMID11887098.
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