Citrus

''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to South Asia, East Asia, Southeast Asia, Melanesia, and Australia. Various citrus species have been used and domesticated by indigenous cultures in these areas since ancient times. From there its cultivation spread into Micronesia and Polynesia by the Austronesian expansion (c. 3000–1500 BCE); and to the Middle East and the Mediterranean (c. 1200 BCE) via the incense trade route, and onwards to Europe and the Americas.

History

Citrus plants are native to subtropical and tropical regions of Asia, Island Southeast Asia, Near Oceania, and northeastern 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, 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 (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.

This was later followed by the spread of citrus species into Taiwan and Japan in the Early Pliocene (5.33 to 3.6 mya), resulting in the tachibana orange (''C. tachibana''); and beyond the Wallace Line into Papua New Guinea and Australia during the Early Pleistocene (2.5 million to 800,000 years ago), where further speciation events occurred resulting in the Australian limes.

The earliest introductions of citrus species by human migrations was during the Austronesian expansion (c. 3000–1500 BCE), where '' Citrus hystrix'', '' Citrus macroptera'', and '' Citrus maxima'' were among the canoe plants carried by Austronesian voyagers eastwards into Micronesia and Polynesia.

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 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, dated to around 1200 BCE. Other archaeobotanical evidence include 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 first attested from Theophrastus, c. 310 BCE. The agronomists of classical Rome made many references to the cultivation of citrus fruits within the limits of their empire.

Lemons, pomelos, and sour oranges are believed to have been introduced to the Mediterranean later by Arab traders at around the 10th century CE; and sweet oranges by the Genoese and Portuguese from Asia during the 15th to 16th century. Mandarins were not introduced until the 19th century. This group of species has reached great importance in some of the Mediterranean countries, and in the case of orange, mandarin, and lemon trees, they found here soil and climatic conditions which allow them to achieve a high level of fruit quality, even better than in the regions from where they came.

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 originated from Latin, where it referred to either the plant now known as citron (''C. medica'') or a conifer tree (''Thuja''). It is 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. Collectively, ''Citrus'' fruits and plants are also known by the Romance loanword ''agrumes'' (literally "sour fruits").

Evolution

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 genus ''Poncirus'' (such as the trifoliate orange), which 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 chloroplasts. A DNA study published in ''Nature'' in 2018 concludes that the genus ''Citrus'' first evolved in the foothills of the Himalayas, in the area of Assam (India), western Yunnan (China), and northern Myanmar.

The three ancestral (sometimes characterized as "original" or "fundamental") 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 involving these three species with each other, their main progenies, and other wild ''Citrus'' species within the last few thousand years.

Fossil record

A fossil leaf from the Pliocene

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