Citrus

Citrusis agenusoffloweringtrees andshrubsin the familyRutaceae. Plants in the genus producecitrus fruits, including importantcropssuch asoranges,mandarins,lemons,grapefruits,pomelos, andlimes.
Citrusis native to South Asia, East Asia, Southeast Asia,Melanesia, and Australia. Indigenous people in these areas have used and domesticated various species since ancient times. Its cultivation first spread intoMicronesiaandPolynesiathrough theAustronesian expansion(c.3000–1500 BCE). Later, it was spread to the Middle East and theMediterranean(c.1200 BCE) via theincense trade route, and from Europe to the Americas.
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 ofCitrussplit into the main genus,Citrus, and thePoncirusgroup (such as thetrifoliate orange), which some taxonomies consider a separate genus and others include inCitrusPoncirusis 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.A DNA study published inNaturein 2018 concludes that the genusCitrusevolved in the foothills of theHimalayas, in the area ofAssam(India), westernYunnan(China), and northernMyanmar.
The three ancestral species in the genusCitrusassociated with modernCitruscultivars 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 wildCitrusspecies within the last few thousand years.
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.A genomic, phylogenic, and biogeographical analysis by Wuet al.(2018) has shown that the center of origin of the genusCitrusis 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 thebiasongandsamuyao(C. micrantha) of thePhilippines.
This was followed by the spread of citrus species intoTaiwanand Japan in theEarly Pliocene(5.33 to 3.6mya), resulting in thetachibana orange(C. tachibana); and beyond theWallace LineintoPapua New Guineaand Australia during theEarly Pleistocene(2.5 million to 800,000 years ago), where further speciation events created in theAustralian limes.
Afossilleaf from thePlioceneofValdarno, Italy is described as †Citrus meletensis.In China, fossil leaf specimens of †Citrus linczangensishave been collected from lateMiocenecoal-bearing strata of the Bangmai Formation inYunnanprovince.C. linczangensisresemblesC. meletensisin having an intramarginal vein, an entire margin, and an articulated and distinctly wingedpetiole.
Many cultivatedCitrusspecies arenaturalorartificial hybridsof a small number of core ancestral species, including the citron, pomelo, and mandarin. Natural and cultivatedcitrus hybridsinclude commercially important fruit such as oranges,grapefruit, lemons, limes, and sometangerines. The multiple hybridisations have made the taxonomy ofCitruscomplex.
Apart from these core species,Australian limesand the recently discoveredmangshanyeganare grown.KumquatsandClymeniaspp. are now generally considered to belong within the genusCitrus.The false oranges,OxantherafromNew Caledonia, have been transferred to theCitrusgenus onphylogeneticevidence.A recent taxonomy reincorporates the trifoliate orange (Poncirus) into an enlargedCitrus, but recognizes that many botanists still follow Swingle in splitting it off.
The earliest introductions of citrus species by human migrations was during theAustronesian expansion(c.3000–1500 BCE), whereCitrus hystrix,Citrus macroptera, andCitrus maximawere among thecanoe plantscarried byAustronesianvoyagers eastwards intoMicronesiaandPolynesia.
The citron (Citrus medica) was also introduced early into the Mediterranean basin from India and Southeast Asia. It was introduced via two ancient trade routes: an overland route throughPersia, theLevantand the Mediterranean islands; and a maritime route through theArabian PeninsulaandPtolemaic Egyptinto North Africa. Although the exact date of the original introduction is unknown due to the sparseness of archaeobotanical remains, the earliest evidence are seeds recovered from theHala Sultan Tekkesite ofCyprus, dated to around 1200 BCE. Other archaeobotanical evidence includes pollen fromCarthagedating back to the 4th century BCE; and carbonized seeds fromPompeiidated to around the 3rd to 2nd century BCE. The earliest complete description of the citron was written byTheophrastus,c.310 BCE.
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 theGenoeseandPortuguesefrom Asia during the 15th to 16th century. Mandarins were not introduced until the 19th century.Oranges were introduced to Florida by Spanish colonists.In cooler parts of Europe, citrus fruit was grown inorangeriesstarting in the 17th century; many were as much status symbols as functional agricultural structures.
The generic nameCitrusoriginates fromLatin, where it denoted either thecitron(C. medica) or a conifer tree (Thuja). The Latin word is related to theancient Greekword for thecedar of Lebanon,κέδρος(kédros), perhaps from a perceived similarity of the smell of citrus leaves and fruit with that of cedar.
Citrusplants are large shrubs or small to moderate-sized trees, reaching 5–15 m (16–49 ft) tall, withspinyshoots and alternately arrangedevergreenleaveswith an entire margin.Theflowersare 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.
The fruit is ahesperidium, a specialised berry with multiplecarpels, 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 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 aloculefilled withjuice vesicles, or pulp. From the endocarp, string-like "hairs" extend into the locules, which provide nourishment to the fruit as it develops.The genus is commercially important withcultivarsof many species grown for their fruit. Some cultivars have been developed to be easy to peel and seedless, meaning they areparthenocarpic.
The fragrance of citrus fruits is conferred byflavonoidsandlimonoidsin the rind. The flavonoids include variousflavanonesandflavones.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.Citrusfruits are diverse in size and shape, as well as in color and flavor, reflecting their biochemistry;for instance, grapefruit is made bitter-tasting by a flavanone,naringin.
Most commercial citrus cultivation uses trees produced bygraftingthe desired fruitingcultivarsontorootstocksselected for disease resistance and hardiness.The trees are not generallyfrosthardy. They thrive in a consistently sunny, humid environment with fertile soil and adequate water.
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 terms 'ripe' and 'mature' are widely used synonymously, but they mean different things. A mature fruit is one that has completed its growth phase.Ripeningis 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 colour.Citrusfruits are non-climactericand respiration slowly declines and the production and release of ethylene is gradual.
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.At US $15.2 billion equivalent in 2018, citrus trademakes up nearly half of the world fruit trade, which was US$32.1 billion that year.According to theUnited Nations Conference on Trade and Development, citrus production grew during the early 21st century mainly by the increase incultivationareas, improvements in transportation and packaging, rising incomes and consumerpreferencefor healthy foods.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.
Among the diseases of citrus plantations arecitrus black spot(a fungus),citrus canker(a bacterium),citrus greening(a bacterium, spread by an insect pest), and sweet orange scab (a fungus,Elsinöe australis).Citrus plants are liable to infestation byectoparasiteswhich act asvectorsto plant diseases: for example, aphids transmit the damagingcitrus tristeza virus,while the aphid-likeAsian citrus psyllidcan carry the bacterium which causes the seriouscitrus greening disease.This threatens production in Florida,California,and worldwide. Citrus groves are attacked by parasiticNematodesincluding citrus (Tylenchulus semipenetrans) and sheath nematodes (Hemicycliophoraspp.).
Citrus plants can develop the deficiency conditionchlorosis, characterized by yellowing leaves.The condition is often caused by an excessively highpH(alkaline soil), which prevents the plant from absorbing nutrients such as iron,magnesium, andzincneeded to producechlorophyll.
SomeCitrusspecies contain significant amounts offuranocoumarins.In humans, some of these act as strongphotosensitizerswhen applied topically to the skin, while others interact with medications when taken orally in thegrapefruit juice effect.Due to the photosensitizing effects of certain furanocoumarins, someCitrusspecies causephytophotodermatitis,a potentially severe skin inflammation resulting from contact with a light-sensitizing botanical agent followed by exposure toultravioletlight. InCitrusspecies, the primary photosensitizing agent appears to bebergapten,a linear furanocoumarin derived frompsoralen. This claim has been confirmed for limeandbergamot. In particular,bergamot essential oilhas a higher concentration of bergapten (3–3.6 g/kg) than any otherCitrus-based essential oil.
Asystematic reviewindicates that citrus fruit consumption is associated with a 10% reduction of risk for developing breast cancer.
Many citrus fruits, such asoranges,tangerines,grapefruits, andclementines, 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.Lemonadeis a popular beverage prepared by diluting the juice and adding sugar.Lemon juice is mixed in salad dressingsand squeezed over fruit salad to stop it from turning brown: its acidity suppressesoxidationbypolyphenol oxidaseenzymes.
A variety of flavours can be derived from different parts and treatments of citrus fruits.The colourful outer skin of some citrus fruits, known aszest, is used as a flavouring in cooking.The whole of thebitter orange(and sometimes other citrus fruits) including thepeelwith itsessential oilsis cooked with sugar to makemarmalade.
By the 17th century,orangerieswere added to great houses in Europe, both to enable the fruit to be grown locally and for prestige, as seen in theVersailles Orangerie.Some modern hobbyists grow dwarf citrus in containers or greenhouses in areas where the weather is too cold to grow it outdoors;Citrofortunellahybrids have good cold resistance.
Lemons appear in paintings, pop art, and novels.A wall painting in thetomb of Nakhtin 15th century BC Egypt depicts a woman in a festival, holding a lemon. In the 17th century,Giovanna Garzonipainted aStill Life with Bowl of Citrons, the fruits still attached to leafy flowering twigs, with a wasp on one of the fruits. TheimpressionistEdouard Manetdepicted a lemon on a pewter plate. In modern art,Arshile GorkypaintedStill Life with Lemonsin the 1930s.
Citrus fruits "were the clear status symbols of the nobility in the ancient Mediterranean", according to thepaleoethnobotanistDafna Langgut.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."