Citrus is a common term and genus of flowering plants in the family Rutaceae, originating in tropical and subtropical southeast Asia. The genus contains three species, and numerous natural and cultivated origin hybrids, including commercially important fruit such as the orange, lemon, grapefruit, lime, and tangerine. The taxonomy of the genus is complex, but recent genetic evidence (see e.g. external link cited below) supports the presence of only three species, C. maxima, C. medica and C. reticulata, with all the other taxa previously accepted as species being of hybrid origin between these three. They are large evergreen shrubs or small trees, reaching 5-15 m tall.

Citrus fruits are notable for their fragrance, and most are juice-laden. They contain a high proportion of citric acid giving them their characteristic astringent odor and flavor. They are also good sources of vitamin C, and apparently flavonoids. In botanical terms, "The fruit of all Citrus trees, in which the true fruit is the peel, [is] made up of an outer layer, brightly colored and rich in glands, a spongey whitish mesocarp, and a membraneous endocarp surrounding the segments. The succulent parts we eat is only a secondary tissue developed as a filler" - Paola Lanzara and Mariella Pizzetti Simon & Schuster's Guide to Trees, pp. 44.

Cultivation

As citrus trees hybridise very readily (e.g., seeds grown from limes can produce fruit similar to grapefruit), all commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness.

The color of citrus fruits only develops in climates with a cool winter. In tropical regions with no winter, citrus fruits remain green until maturity, hence the tropical "green orange". The lime plant in particular is extremely sensitive to cool conditions, thus it is usually never exposed to cool enough conditions to develop a colour. If they are left in a cool place over winter, the fruits will actually change to a yellow colour. Many citrus fruits are picked while still green, and ripened while in transit to supermarkets.

Citrus trees are not generally frost hardy. Citrus reticulata tends to be the hardiest of the common Citrus species and can withstand short periods down to as cold as −10 °C, but realistically temperatures not falling below −2 °C are required for successful cultivation [1]. A few hardy hybrids can withstand temperatures well below freezing, but do not produce quality fruit. A related plant, the Trifoliate orange (Poncirus trifoliata) can survive below −20 °C [2]; its fruit are astringent and inedible unless cooked.

The trees do best in a consistently sunny, humid environment with fertile soil and adequate rainfall or irrigation. Though broadleaves, they are evergreen and do not drop leaves except when stressed. 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 culivars 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 region, South Africa, Australia, the southernmost United States, and parts of South America. In the U.S., Florida, Texas, and California are major producers, while smaller plantings are present in other Sun Belt states.

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. 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. An orangery was a feature of royal and aristocratic residences through the 17th and 18th centuries. In the United States the earliest surviving orangery is at the Tayloe House, Mount Airy, Virginia.

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. For cooler areas, lime and lemon should not be grown, since they are more sensitive to cold than other citrus fruits. Tangerines, tangors and yuzu can be grown outside even in regions with sub-zero winters, although this may affect fruit quality. Hybrids with kumquats (citrofortunella) have good cold resistance.

Pests and diseases

Citrus plants are very liable to infestation by aphids, whitefly and scale insects. The foliage is also used as a food plant by the larvae of some Lepidoptera species including Common Emerald, Double-striped Pug, Giant Leopard Moth, Hypercompe eridanus, Hypercompe icasia and Hypercompe indecisa.

Uses

Citrus fruit are eaten both fresh, and cooked as marmalade.

List of citrus fruits

• Alemow, Colo, C. × macrophylla
• Amanatsu
• Bergamot orange C. × bergamia
• Bitter orange, Seville Orange
• Buddha's hand, C. medica
• Calamondin (Calamansi)
• Citron Citrus medica - Etrog
• Clementine
• Daidai, Seville, Sour Orange, Citrus aurantium
• Dekopon- hybrid between ChungGyun mandarins and Ponkan
• Djeruk limau, C. × amblycarpa, Indonesia
• Gajanimma, Carabao lime, C. × pennivesiculata
• Grapefruit, C. × paradisi
• Ichang Lemon Citrus × ichangensis
• Iyokan
• Kabosu
• Kaffir lime Citrus × hystrix
• Key lime Citrus × aurantifolia
• Khasi pepeda, C. × latipes
• Kumquat - in the related genus Fortunella, not Citrus; forms hybrids with Citrus (see Citrofortunella)
• Lemon Citrus × limon
• Lime Citrus aurantifolia
• limetta, Sweet Lemon C. × limetta
• Limequat Citrus × Fortunella hybrids
• Mandarin Lime C. × limonia
• Mandarin Orange, Dancy
• Meyer Lemon
• Mikan
• Natsumikan, Japan, C. × natsudaidai
• Orange Citrus sinensis
• Orangelo: Chironja
• Orangequat
• Oroblanco
• Persian lime, Tahiti lime Citrus × latifolia
• Pomelo, Pummelo, Shaddock, Citrus grandis
• Ponkan
• Rangpur, Lemanderin Citrus × limonia
• Rough Lemon C. × jambhiri
• Satsuma
• Shikwasa, Taiwan tangerine, Hirami lemon, C. × depressa
• Sudachi
• Sunki, Suenkat, C. × sunki
• Sweetie
• Sweet Lime, Sweet Lime, Central America, C. × limettioides
• Tachibana Orange
• Tangelo: Minneola tangelo Ugli
• Tangerine Citrus reticulata
• Tangor C. × nobilis
• Ugli fruit
• Yuzu C. × junos