Citrus
Citrus is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, such as citrons, mandarins, and pomelos. Many important citrus crops have been developed through extensive hybridization, including oranges, lemons, grapefruits, and limes, all of which have many cultivars.
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 through the Austronesian expansion. Later, it was spread to the Middle East and the Mediterranean via the incense trade route, and then 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, the genus has 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 diverged from the same ancestor. About 7 million years ago, the ancestors of Citrus split into the main genus, Citrus, and the Poncirus group, which some taxonomies consider a separate genus and others include in Citrus ''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 plant chloroplasts. A DNA study published in Nature in 2018 concludes that the genus Citrus evolved in the foothills of the Himalayas, in the area of Assam, western Yunnan, and northern Myanmar.
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 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. has shown that the center of origin of the genus Citrus is likely the southeast foothills of the Himalayas, in a region stretching from eastern Assam, northern Myanmar, to western Yunnan. It diverged from a common ancestor with Poncirus trifoliata''. A change in climate conditions during the Late Miocene resulted in a sudden speciation event. The species resulting from this event include the citrons of South Asia; the pomelos of Mainland Southeast Asia; the mandarins, kumquats, mangshanyegan, and ichang papedas of southeastern China; the kaffir limes of Island Southeast Asia; and the biasong and samuyao of the Philippines.
This was followed by the spread of citrus species into Taiwan and Japan in the Early Pliocene, resulting in the tachibana orange ; and beyond the Wallace Line into Papua New Guinea and Australia during the Early Pleistocene, where further speciation events created the Australian limes.
Fossil record
A fossil leaf from the Pliocene of Valdarno, Italy is described as †Citrus meletensis.In China, fossil leaf specimens of †Citrus linczangensis have been collected from late Miocene coal-bearing strata of the Bangmai Formation in Yunnan 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.File:Hybrid origins of Citrus.svg|thumb|center|upright=3|Many Citrus species are hybrids of citron, mandarin and pomelo.
Kumquats and Clymenia spp. are now generally considered to belong within the genus Citrus. The false oranges, Oxanthera from New Caledonia, have been transferred to the Citrus genus on phylogenetic evidence. A recent taxonomy reincorporates the trifoliate orange 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, where Citrus hystrix, Citrus macroptera, and Citrus maxima were among the canoe plants carried by Austronesian voyagers eastwards into Micronesia and Polynesia.The citron was also introduced early into the Mediterranean basin from India and Southeast Asia, via two ancient trade routes: an overland route through Persia, the Levant and the Mediterranean islands, and a maritime route through the Arabian Peninsula 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 is seeds recovered from the Hala Sultan Tekke site of Cyprus, dated to around 1200 BCE. Other archaea botanical evidence includes pollen from Carthage, dating back to the 4th century BCE, and carbonized seeds from Pompeii 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, where it denoted either the citron or a conifer tree. The Latin word is related to the ancient Greek word for the cedar of Lebanon, κέδρος, 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 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 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.
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 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.