Citrus

Citrus
Sweet orange (Citrus × sinensis cultivar)
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Rosids
Order:	Sapindales
Family:	Rutaceae
Subfamily:	Aurantioideae
Tribe:	Citreae
Genus:	Citrus L.
Species and hybrids
Important species: Citrus aurantiifolia – Key lime Citrus maxima – Pomelo Citrus medica – Citron Citrus reticulata – Mandarin orange Important hybrids: Citrus × aurantium – Bitter orange Citrus × latifolia – Persian lime Citrus × limon – Lemon Citrus × limonia – Rangpur Citrus × paradisi – Grapefruit Citrus × sinensis – Sweet orange Citrus × tangerina – Tangerine See also below for other species and hybrids.
Synonyms
Eremocitrus Microcitrus and see text

Citrus is a common term and genus (Citrus) of flowering plants in the rue family, Rutaceae. The most recent research indicates an origin in Australia, New Caledonia and New Guinea.^[1] 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,^[2][3][4] 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; the best-known examples are the oranges, lemons, grapefruit, and limes.

Name

The generic name originated in 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.^[5] Collectively, Citrus fruits and plants are also known by the Romance loanword agrumes (literally "sour fruits").

Taxonomy

Citrus fruits clustered by genetic similarity (PCA of SNP diversity). Citrus micrantha (top right) is a papeda.
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Hybrids are expected to plot between their parents. ML: ‘Mexican’ lime; A: ‘Alemow’; V: ‘Volkamer’ lemon; M: ‘Meyer’ lemon; L: Regular and ‘Sweet’ lemons; B: Bergamot orange; H: Haploid clementine; C: Clementines; S: Sour oranges; O: Sweet oranges; G: Grapefruits.

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.^[6]

Cultivated Citrus may be derived from as few as four ancestral species.^[7] Recent studies show that only four domesticated citrus fruit — the citron, pummelo, mandarine, and papeda — are actually original, and all the rest of citrus types developed through natural or artificial hybridization (see image).^[8] Natural and cultivated origin hybrids include commercially important fruit such as the oranges, grapefruit, lemons, limes, and some tangerines.

Description

Slices of various citrus fruits

Tree

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.

Fruit

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.^[9][10]

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 flavonoids include various flavanones and flavones.^[11]

Cultivation

Persian Limes in a grocery store.

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.^[12] 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.^[13]

Citrus fruits are non-climacteric and respiration slowly declines and the production and release of ethylene is gradual.^[14] 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.

Mediterranean Mandarin (Citrus ×deliciosa plantation, Son Carrió (Mallorca)

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).

Leaf of Citrus tree

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. Though broadleaved, they are evergreen and 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 southernmost 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.

As ornamental plants

Orangery of the Botanical Garden in Leuven (Belgium)

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.

Pests and diseases

Major producer regions

Citrus canker is caused by the gammaproteobacterium Xanthomonas axonopodis

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.^[15]

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.^[15]

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.

Since 2000, the citrus leafminer (Phyllocnistis citrella) has been a pest in California,^[16] boring meandering patterns through leaves.

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.

European brown snails (Helix aspersa) can be a problem in California, though laying female Khaki Campbell and other mallard-related ducks can be used for control.

Deficiency diseases

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.^[17]

Production

According to UN 2007 data, Brazil, China, the United States, Mexico, India, and Spain are the world's largest citrus-producing countries.

Uses

Culinary

Wedges of pink grapefruit, lime, and lemon, and a half orange (clockwise from top)

Citrus aurantifolia in Kerala

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.^[18] 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 fragant 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.

Medical

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.

Pectin is a structural heteropolysaccharide contained in the primary cell walls of plants. Limes and lemons as well as oranges and grapefruits are among the highest in this level.^[19]

After consumption, the peel is sometimes used as a facial cleanser.

Before the development of fermentation-based processes, lemons were the primary commercial source of citric acid.

Citrus fruit intake is associated with a reduced risk of stomach cancer.^[20] 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.^[21] 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.^[22]

List of citrus fruits

Citrons (Citrus medica) for sale in Germany

Red Finger Lime (Citrus australasica), a rare delicacy from Australia

The genus Citrus has been suggested to originate in Southeast Asia. Prior to human cultivation, it consisted of just a few species, namely:

• Citrus aurantiifolia – Key Lime, Omani Lime, from India
• Citrus crenatifolia – species name is unresolved, from Sri Lanka
• Citrus mangshanensis – species name is unresolved, from Hunan Province, China.
• Citrus maxima – Pomelo (pummelo, shaddock), from the Malay Archipelago
• Citrus medica – Citron, from India
• Citrus reticulata – Mandarin orange, from China
• Citrus trifoliata – Trifoliate orange, from Korea and adjacent China (often separated as Poncirus)
• Australian limes
  • Citrus australasica – Australian Finger Lime
  • Citrus australis – Australian Round lime
  • Citrus glauca – Australian Desert Lime
  • Citrus garrawayae – Mount White Lime
  • Citrus gracilis – Kakadu Lime or Humpty Doo Lime
  • Citrus inodora – Russel River Lime
  • Citrus warburgiana – New Guinea Wild Lime
  • Citrus wintersii – Brown River Finger Lime
• Citrus japonica – Kumquats, from East Asia ranging into Southeast Asia (sometimes separated in 4–5 Fortunella species)
• Papedas, including
  • Citrus halimii – limau kadangsa, limau kedut kera, from Thailand and Malaya
  • Citrus indica – Indian wild orange, from the Indian subcontinent^[23]
  • Citrus macroptera from Indochina and Melanesia^[23]
  • "Khasi Papeda" – Citrus latipes from Assam, Meghalaya, Burma^[23]

Hybrids and cultivars

Sweetie or Oroblanco is a pomelo-grapefruit hybrid.

The Etrog, or Citron, is central to the ritual of the Jewish Sukkot festival. Many varieties are used for this purpose (including the Yemenite variety pictured).

Clementines (Citrus ×clementina) have thinner skins than oranges.

Mikan (Citrus ×unshiu), also known as satsumas

Sweet oranges (Citrus ×sinensis) are used in many foods. Their ancestors were probably pomelos and mandarin oranges.

Sorted by parentage. As each hybrid is the product of (at least) two parent species, they are listed multiple times.

Citrus maxima-based

• Amanatsu, natsumikan – Citrus ×natsudaidai (C. maxima × unknown)
• Cam sành – (C. reticulata × C. ×sinensis)
• Grapefruit – Citrus ×paradisi (C. maxima × C. ×sinensis)
• Imperial lemon – (C. ×limon × C. ×paradisi)
• Kinnow – (C. ×nobilis × C. ×deliciosa)
• Kiyomi – (C. ×sinensis × C. ×unshiu)
• Lemon – (probably C. maxima × C. medica)
• Minneola tangelo – (C. reticulata × C. ×paradisi)
• Orangelo, Chironja – (C. ×paradisi × C. ×sinensis)
• Oroblanco, Sweetie – (C. maxima × C. ×paradisi)
• Sweet orange – Citrus ×sinensis (probably C. maxima × C. reticulata)
• Tangelo – Citrus ×tangelo (C. reticulata × C. maxima or C. ×paradisi)
• Tangor – Citrus ×nobilis (C. reticulata × C. ×sinensis)
• Ugli – (C. reticulata × C. maxima or C. ×paradisi)

Citrus medica-based

• Buddha's hand – Citrus medica var. sarcodactylus, a fingered citron.
• Citron varieties with sour pulp – Diamante citron, Florentine citron, Greek citron and Balady citron
• Citron varieties with sweet pulp – Corsican citron and Moroccan citron.
• Etrog, a group of citron cultivars that are traditionally used for a Jewish ritual. Etrog is Hebrew for citron in general.
• Fernandina – Citrus ×limonimedica (probably (C. medica × C. maxima) × C. medica)
• Ponderosa lemon – (probably (C. medica × C. maxima) × C. medica)
• Lemon – (probably C. medica × C. maxima)
• Lumia - a pear shaped lemon hybrid, (probably C. medica x C. limon)
• Rhobs el Arsa - bread of the garden, a Moroccan citron x lemon hybrid.
• Yemenite citron – a pulpless true citron.

Citrus reticulata-based

• Bergamot orange – Citrus ×aurantium ssp. bergamia or Citrus ×bergamia
• Bitter orange, Seville Orange – Citrus ×aurantium
• Blood orange – Citrus ×sinensis cultivars
• Calamondin, Calamansi – (Citrus reticulata × Citrus japonica)
• Cam sành – (C. reticulata × C. ×sinensis)
• Chinotto – Citrus ×aurantium var. myrtifolia or Citrus ×myrtifolia
• ChungGyun – Citrus reticulata cultivar
• Clementine – Citrus ×clementina
• Cleopatra Mandarin – Citrus ×reshni
• Siranui – Citrus reticulata cv. 'Dekopon' (ChungGyun × Ponkan)
• Daidai – Citrus ×aurantium var. daidai or Citrus ×daidai
• Grapefruit – Citrus ×paradisi (C. maxima × C. ×sinensis)
• Hermandina – Citrus reticulata cv. 'Hermandina'
• Imperial lemon – ((C. maxima × C. medica) × C. ×paradisi)
• Kinnow, Wilking – (C. ×nobilis × C. ×deliciosa)
• Kiyomi – (C. sinensis × C. ×unshiu)
• Laraha – ''C. ×aurantium ssp. currassuviencis
• Mediterranean mandarin, Willow Leaf – Citrus ×deliciosa
• Meyer lemon, Valley Lemon – Citrus ×meyeri ((C. maxima × C. medica) × C. ×paradisi or C. ×sinensis)
• Michal mandarin – Citrus reticulata cv. 'Michal'
• Mikan, Satsuma – Citrus ×unshiu
• Naartjie – (C. reticulata × C. nobilis)
• Nova mandarin, Clemenvilla
• Orangelo, Chironja – (C. ×paradisi × C. ×sinensis)
• Oroblanco, Sweetie – (C. maxima × C. ×paradisi)
• Ponkan – Citrus reticulata cv. 'Ponkan'
• Rangpur, Lemanderin, Mandarin Lime – Citrus ×limonia ((C. reticulata × C. maxima) × C. medica)
• Sweet orange – Citrus ×sinensis (probably C. maxima × C. reticulata)
• Tangelo – Citrus ×tangelo (C. reticulata × C. maxima or C. ×paradisi)
• Tangerine – Citrus ×tangerina
• Tangor – Citrus ×nobilis (C. reticulata × C. ×sinensis)
• Ugli – (C. reticulata × C. maxima or C. ×paradisi)
• Yuzu – Citrus ×junos (C. reticulata × C. ×ichangensis)

Cross section of Odichukuthi lime.

Other/Unresolved

• Alemow, Colo – Citrus ×macrophylla
• Djeruk limau – Citrus ×amblycarpa
• Gajanimma, Carabao Lime – Citrus ×pennivesiculata
• Hyuganatsu, Hyuganatsu pumelo – Citrus tamurana
• Ichang lemon, Ichang Papeda – Citrus ×ichangensis
• Imperial lemon – (C. ×limon × C. ×paradisi)
• Iyokan, anadomikan – Citrus ×iyo
• Kabosu – Citrus ×sphaerocarpa
• Kaffir lime, makrut – Citrus ×hystrix
• Limetta, Sweet Lemon, Sweet Lime, mosambi – Citrus ×limetta
• Palestine sweet lime – Citrus ×limettioides Tanaka

Odichukuthi fruit

• Odichukuthi – Citrus Odichukuthi from Malayalam
• Ougonkan – Citrus flaviculpus hort ex.Tanaka
• Persian lime, Tahiti Lime – Citrus ×latifolia

A pompia fruit.

• Pompia – Citrus monstruosa, a nomen nudum
• Rough lemon – Citrus ×jambhiri Lush.
• Sakurajima komikan orange
• Shekwasha, Hirami Lemon, Taiwan Tangerine – Citrus ×depressa
• Shonan gold – (Ougonkan) Citrus flaviculpus hort ex. Tanaka × (Imamura unshiu), Citrus unshiu Marc
• Sudachi – Citrus ×sudachi
• Sunki, Suenkat – Citrus ×sunki
• Tachibana orange – Citrus ×tachibana (Mak.) Tanaka
• Volkamer lemon – Citrus ×volkameriana

For hybrids with kumquats, see ×Citrofortunella. For hybrids with the Trifoliate Orange, see citrange.

See also

• Fruit anatomy
• Japanese citrus
• Juice vesicles
• Citric acid
• List of lemon dishes and beverages

Footnotes

References

• Andrews, A.C. (1961). "Acclimatization of citrus fruits in the Mediterranean region". Agricultural History 35 (1): 35–46. 
• Araújo, De; Freitas, E.; de Queiroz, L. Paganucci; Machado, M.A. (2003). "What is Citrus? Taxonomic implications from a study of cp-DNA evolution in the tribe Citreae (Rutaceae subfamily Aurantioideae)". Organisms Diversity & Evolution 3 (1): 55–62. doi:10.1078/1439-6092-00058. 
• Nicolosi, E.; Deng, Z.N.; Gentile, A.; La Malfa, S.; Continella, G.; Tribulato, E. (2000). "Citrus phylogeny and genetic origin of important species as investigated by molecular markers". Theoretical and Applied Genetics 100 (8): 1155–1166. doi:10.1007/s001220051419. 

Further reading

• Calabrese, Francesco (2002): Origin and history. In: Dugo, Giovanni & Di Giacomo, Angelo (eds.) (2002): Citrus. Taylor & Francis. ISBN 0-415-28491-0
• Ellis, R.H.; Hong, T.D. & Roberts, E.H. (1985): Chapter 64. Rutaceae. In: Handbook of Seed Technology for Genebanks (Volume II: Compendium of Specific Germination Information and Test Recommendations). International Board for Plant Genetic Resources, Rome, Italy. HTML fulltext
• Frison, E.A. & Taher, M.M. (eds.) (1991): FAO/IBPGR Technical Guidelines for the Safe Movement of Citrus Germplasm. FAO, IOCV, IPGRI. PDF fulltext
• International Plant Genetic Resources Institute (IPGRI) (1999): Descriptors for Citrus (Citrus spp.). PDF fulltext
• Janick, Jules (2005): Purdue University Tropical Horticulture Lecture 32: Citrus
• Luro, F.; Laigret, F.; Bové, J.M. & Ollitrault, P. (1995): RFLP analysis of cytoplasmic and nuclear genomes used for citrus taxonomy. In: Mandarines – développements scientifiques récents, résumés oraux et posters: 12–13. CIRAD-FLHOR, San Nicolao, France. HTML abstract
• Molina, A.B.; Roa, V.N.; Bay-Petersen, J.; Carpio, A.T. & Joven, J.E.A. (eds.) (2000): Citrus, Proceedings of a regional workshop on disease management of banana and citrus through the use of disease-free planting materials held in Davao City, Philippines, 14–16 October 1998. INIBAP. PDF fulltext
• Sackman. Douglas Cazaux (2005): Orange Empire: California and the Fruits of Eden.
• University of California Division of Agricultural Sciences (UC-DAS) (1967–1989): The Citrus Industry. HTML fulltext of Vol. 1, 2, & Vol. 5, Chapter 5

External links

• Citrus at the National Center for Biotechnology Information
• ITIS
• Citrus at the Encyclopedia of Life
• Effects of pollination on Citrus plants Pollination of Citrus by Honey Bees
• Citrus Research and Education Center of IFAS (largest citrus research center in world)
• Citrus Variety Collection by the University of California
• Citrus (Mark Rieger, Professor of Horticulture, University of Georgia)
• Porcher Michel H. et al. (1995 – 2020). "Multilingual Multiscript Plant Name Database (M.M.P.N.D)  – A Work in Progress". School of Agriculture and Food Systems, Faculty of Land & Food Resources, The University of Melbourne. Australia.  Check date values in: |date= (help)
• Fundecitrus – Fund for Citrus Plant Protection is an organization of citrus Brazilian producers and processors.
• Citrus – taxonomy, fruit anatomy at GeoChemBio
• Home Citrus Growers
• Citrus Variety Information Season of Maturity • Seeds / Fruit Information • Fruit Size • Fruit and Tree Insight