''Citrus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of flowering trees and shrubs in the family

Rutaceae The Rutaceae () is a family (biology), family, commonly known as the rueRUTACEAE
in Bo ...

. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is 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 into Micronesia and

Polynesia Polynesia ( , ) is a subregion of Oceania, made up of more than 1,000 islands scattered over the central and southern Pacific Ocean. The indigenous people who inhabit the islands of Polynesia are called Polynesians. They have many things in ...

through the Austronesian expansion (–1500 BCE). Later, it was spread to the Middle East and the

Mediterranean The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

() via the incense 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.

Evolution

Evolutionary history

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 that '' Severinia'' (such as the Chinese box orange) diverged from the same ancestor. About 7 million years ago, the ancestors of ''Citrus'' split into the main genus, ''Citrus'', and the ''Poncirus'' group (such as the trifoliate orange), which some taxonomies consider a separate genus and others include in ''Citrus'' ''Poncirus'' is 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 plant

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s. A DNA study published in ''

Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

'' in 2018 concludes that the genus ''Citrus'' evolved in the foothills of the Himalayas, in the area of Assam (India), western

Yunnan Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

(China), and northern

Myanmar Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and has ...

. The three ancestral species in the genus ''Citrus'' associated with modern ''Citrus'' cultivars are the mandarin orange, pomelo, and citron. 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 wild ''Citrus'' species 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 and introgression, leaving much uncertainty about when and where domestication first happened. A genomic, phylogenic, and biogeographical analysis by Wu ''et al.'' (2018) has shown that the center of origin of the genus ''Citrus'' is likely the southeast foothills of the

Himalayas The Himalayas, or Himalaya ( ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the Earth's highest peaks, including the highest, Mount Everest. More than list of h ...

, in a region stretching from eastern Assam, northern

, to western

. It diverged from a common ancestor with '' Poncirus trifoliata''. A change in climate conditions during the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

(11.63 to 5.33 mya) resulted in a sudden speciation event. The species resulting from this event include the citrons (''Citrus medica'') of South Asia; the pomelos (''C. maxima'') of Mainland Southeast Asia; the mandarins (''C. reticulata''), kumquats (''C. japonica''), mangshanyegan (''C. mangshanensis''), and ichang papedas (''C. cavaleriei'') of southeastern China; the kaffir limes (''C. hystrix'') of Island Southeast Asia; and the biasong and samuyao (''C. micrantha'') of the

Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...

. This was followed by the spread of citrus species into

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...

and Japan in the Early Pliocene (5.33 to 3.6 mya), resulting in the tachibana orange (''C. tachibana''); and beyond the

Wallace Line The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by the English biologist Thomas Henry Huxley. It separates the biogeographic realms of Asia and 'Wallacea', a ...

into

Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...

and Australia during the Early Pleistocene (2.5 million to 800,000 years ago), where further speciation events created the Australian limes.

Fossil record

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

leaf from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Valdarno, Italy is described as †''Citrus meletensis''. In China, fossil leaf specimens of †''Citrus linczangensis'' have been collected from late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

coal-bearing strata of the Bangmai Formation in

province. ''C. linczangensis'' resembles ''C. meletensis'' in having an intramarginal vein, an entire margin, and an articulated and distinctly winged petiole.

Taxonomy

Many cultivated ''Citrus'' species are natural or artificial hybrids of a small number of core ancestral species, including the citron, pomelo, and mandarin. Natural and cultivated citrus hybrids include commercially important fruit such as oranges, grapefruit, lemons, limes, and some tangerines. The multiple hybridisations have made the taxonomy of ''Citrus'' complex. and Supplement Hybrid origins of Citrus

Apart from these core species, Australian limes and the recently discovered mangshanyegan are grown. Kumquats and '' Clymenia'' spp. are now generally considered to belong within the genus ''Citrus''. The false oranges, '' Oxanthera'' from

New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...

, have been transferred to the ''Citrus'' genus on

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

evidence.Bayer, R. J., et al. (2009)
A molecular phylogeny of the orange subfamily (Rutaceae: Aurantioideae) using nine cpDNA sequences.
''American Journal of Botany'' 96(3), 668–685. A recent taxonomy reincorporates the trifoliate orange (''Poncirus'') into an enlarged ''Citrus'', but recognizes that many botanists still follow Swingle in splitting it off.

History

The earliest introductions of citrus species by human migrations was during the Austronesian expansion (–1500 BCE), where '' Citrus hystrix'', '' Citrus macroptera'', and '' Citrus maxima'' were among the canoe plants carried by Austronesian voyagers eastwards into Micronesia and

. 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 through Persia, the Levant and the Mediterranean islands; and a maritime route through the

Arabian Peninsula The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

and Ptolemaic Egypt into 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 the Hala Sultan Tekke site of

Cyprus Cyprus (), officially the Republic of Cyprus, is an island country in the eastern Mediterranean Sea. Situated in West Asia, its cultural identity and geopolitical orientation are overwhelmingly Southeast European. Cyprus is the List of isl ...

, dated to around 1200 BCE. Other archaeobotanical evidence includes pollen from

Carthage Carthage was an ancient city in Northern Africa, on the eastern side of the Lake of Tunis in what is now Tunisia. Carthage was one of the most important trading hubs of the Ancient Mediterranean and one of the most affluent cities of the classic ...

dating back to the 4th century BCE; and carbonized seeds from

Pompeii Pompeii ( ; ) was a city in what is now the municipality of Pompei, near Naples, in the Campania region of Italy. Along with Herculaneum, Stabiae, and Villa Boscoreale, many surrounding villas, the city was buried under of volcanic ash and p ...

dated to around the 3rd to 2nd century BCE. The earliest complete description of the citron was written by Theophrastus, . 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 the Genoese and Portuguese from 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 in orangeries starting in the 17th century; many were as much status symbols as functional agricultural structures.

Etymology

The generic name ''Citrus'' originates from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

, where it denoted either the citron (''C. medica'') or a conifer tree ('' Thuja''). The Latin word is related to the

ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

word for the cedar of Lebanon, (), perhaps from a perceived similarity of the smell of citrus leaves and fruit with that of cedar.

Description

Tree

''Citrus'' plants are large shrubs or small to moderate-sized trees, reaching tall, with spiny shoots and alternately arranged evergreen leaves with an entire margin. The flowers are solitary or in small corymbs, each flower diameter, with five (rarely four) white petals and numerous stamens; they are often very strongly scented, due to the presence of essential oil glands.

Fruit

The fruit is a hesperidium, a specialised berry with multiple carpels, globose to elongated, long and 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. This surrounds a variable number of carpels, shaped as radial segments. The seeds, if present, develop inside the carpels. The space inside each segment 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. The genus is commercially important with cultivars of many species grown for their fruit. Some cultivars have been developed to be easy to peel and seedless, meaning they are parthenocarpic. The fragrance of citrus fruits is conferred by flavonoids and limonoids in the rind. The flavonoids include various flavanones and flavones. The carpels are juicy; they contain a high quantity of citric acid, which with other organic acids including ascorbic acid (vitamin C) give them their characteristic sharp taste. ''Citrus'' fruits 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. File:Ichangfruit.jpg, Ichang papeda File:Quinotos.jpg, Kumquat, with a

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

inside one of the carpels File:Buddhas hand 1.jpg, Buddha's hand File:Citrus australasica red whole.jpg, Red finger lime File:Fortpolyandra.jpg, Clymenia

Cultivation

Most commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness. The trees are not generally frost hardy. They thrive in a consistently sunny, humid environment with fertile soil and adequate water. 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". 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. ''Citrus'' fruits are non- climacteric and respiration slowly declines and the production and release of ethylene is gradual.

Production

According to the UN 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 trade makes up nearly half of the world fruit trade, which was US$32.1 billion that year. According to the United Nations Conference on Trade and Development, citrus production grew during the early 21st century mainly by the increase in cultivation areas, improvements in transportation and packaging, rising incomes and consumer preference for 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.

Pests and diseases

Among the diseases of citrus plantations are citrus 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 by ectoparasites which act as vectors to plant diseases: for example, aphids transmit the damaging citrus tristeza virus, while the aphid-like Asian citrus psyllid can carry the bacterium which causes the serious citrus greening disease. This threatens production in Florida, California, and worldwide. Citrus groves are attacked by parasitic Nematodes including citrus ('' Tylenchulus semipenetrans'') and sheath nematodes ('' Hemicycliophora'' spp.). File:Citrus Black Spot on Valencia orange (detail).jpg, Citrus black spot on a Valencia orange File:CTV Leaf Chlorosis.jpg, Chlorosis caused by Citrus tristeza virus File:Citrus canker on fruit.jpg, Citrus canker, caused by the gammaproteobacterium ''Xanthomonas axonopodis'' File:Asian Citrus Psyllid adult (detail).jpg, Asian citrus psyllid, vector of citrus greening disease File:Huanglongbing (cropped).jpg, Citrus greening disease on mandarin orange File:Life stages of citrus nematode.svg, Life stages of the citrus nematode,'' Tylenchulus semipenetrans''

Deficiency diseases

Citrus plants can develop the deficiency condition chlorosis, characterized by yellowing leaves.Online at SumoGardener The condition is often caused by an excessively high pH ( alkaline soil), which prevents the plant from absorbing nutrients such as iron,

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

, and zinc needed to produce

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

Effects on humans

Some ''Citrus'' species contain significant amounts of furanocoumarins. In humans, some of these act as strong photosensitizers when applied topically to the skin, while others interact with medications when taken orally in the grapefruit juice effect. Due to the photosensitizing effects of certain furanocoumarins, some ''Citrus'' species cause phytophotodermatitis, a potentially severe skin inflammation resulting from contact with a light-sensitizing botanical agent followed by exposure to ultraviolet light. In ''Citrus'' species, the primary photosensitizing agent appears to be bergapten, a linear furanocoumarin derived from psoralen. This claim has been confirmed for lime and bergamot. In particular, bergamot essential oil has a higher concentration of bergapten (3–3.6 g/kg) than any other ''Citrus''-based essential oil. A systematic review indicates that citrus fruit consumption is associated with a 10% reduction of risk for developing breast cancer.

Uses

Culinary

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. Lemonade is a popular beverage prepared by diluting the juice and adding sugar. Lemon juice is mixed in salad dressings and squeezed over fruit salad to stop it from turning brown: its acidity suppresses oxidation by polyphenol oxidase enzymes. A variety of flavors can be derived from different parts and treatments of citrus fruits. The colorful outer skin of some citrus fruits, known as zest, is used as a flavoring in cooking. The whole of the bitter orange (and sometimes other citrus fruits) including the peel with its essential oils is cooked with sugar to make marmalade. File:Fried chicken legs!.JPG, Fried chicken garnished with lemon and onion File:Calamansi (sliced) - Philippines.jpg, Calamansi, ubiquitous in Philippine condiments File:Pomeranzen.jpg, Bitter oranges (''Citrus × aurantium'') are used for marmalade. File:Bottled Sunshine.jpg, Marmalade, with orange peel File:Lemonade 2.jpg, Lemonade

As ornamental plants

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 the Versailles Orangerie. 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.

In art and culture

Lemons appear in paintings, pop art, and novels. A wall painting in the tomb of Nakht in 15th century BC Egypt depicts a woman in a festival, holding a lemon. In the 17th century, Giovanna Garzoni painted a ''Still Life with Bowl of Citrons'', the fruits still attached to leafy flowering twigs, with a wasp on one of the fruits. The impressionist Edouard Manet depicted a lemon on a pewter plate. In modern art, Arshile Gorky painted ''Still Life with Lemons'' in the 1930s. Citrus fruits "were the clear status symbols of the nobility in the ancient Mediterranean", according to the paleoethnobotanist Dafna Langgut. In Louisa May Alcott's 1868 novel '' Little 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."

References

External links

Citrus Research and Education Center
of IFAS {{Authority control Cocktail garnishes Garden plants Citrus fruits Lists of plants Ornamental trees Aurantioideae genera Taxa named by Carl Linnaeus