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Sweet Oranges: The Biogeography of Citrus sinensis

It is with difficulty that biogeographers have attempted to define the centers of origin and ancestors of citrus fruits. The multitude of natural hybrids and cultivated varieties, including spontaneous mutants, obscure the history of Citrus. The lack of sufficient descriptions and specimens, in addition to the destruction of the original habitats, contribute to the puzzlement as well (Spiegel-Roy and Goldschmidt 1996). In any case, citrus fruit trees originated in the region encompassing Southeast Asia and India (Zohary and Hapf 1993; Janick et. al. 1981; Speigel-Roy and Goldschmidt 1996). Citrus would have arose as a bitter fruit plant, possibly in what is now the Malay Archipelago over twenty million years ago (McPhee 1967). The modern fruit species probably evolved in China, where there is greater diversity in Citrus varieties and parasites than anywhere in the world (McPhee 1967). The hybridization of pummelos and mandarins, in environments such as mixed Chinese gardens, resulted in the creation of both C. sinensis and C. aurantium (Speigel-Roy and Goldschmidt 1996). However, the location of the origin of the sweet orange is controversial. China, India, Bhutan, Myanmar (Burma) and Malaysia are all candidates (The Orange History [1999]). Domestication of citrus species presumably began at several sites and by 4000 BCE (before the Common Era), the culture of lemons, limes and oranges was occurring in the Indus Valley (Solley 1997a).

Products

Today, oranges are primarily eaten fresh or prepared as frozen juice concentrate. The byproducts of juice making (the pulp, rind and seed) are utilized for cattle feed and molasses (for alcohol or feed) as well as flavorings, perfumes, pharmaceuticals and soap. The extracts of rinds and seeds include pectin and oil. When fermented, orange juices produce vinegar and liqueurs. Oranges are permitted to ripen on trees before harvesting. Most receive treatment with ethylene dye to enhance their orange color, and ship well to markets (Janick et. al. 1981). Originally, Citrus uses included beautification, embalming, mothballs, aphrodisiacs, protection from poisons and curing fever and colic (Solley 1997b). Before Europeans viewed oranges as a food, the people used their trees, flowers and fruits as ornaments, seasonings and for aromas (McPhee 1967).

Artificial Selection

For most of its existence, citriculture was a natural affair, involving random changes in color, size, aroma and flavor.

“Most present-day cultivars that are widely grown in the main commercial producing areas in the world were derived by selection, largely of sports [mutations], from the cultivars introduced into Europe and the Americas” (Soost and Roose 1996, p. 259).

After the rise of the Darwinian and Mendelian theories, however, research and experimentation began in Europe with the goal of improving orange characteristics. In 1893, the United States Department of Agriculture (USDA) initiated the systematic breeding of Citrus in Florida. The Universities of California and Florida have since embarked upon programs involving the propagation of sweet oranges. “Citrus breeding requires long-term commitment and few institutions have given it sustained support” (Cameron and Soost 1976). Difficulty arises from observations, which imply that some undesirable traits are dominant (Soost and Roose 1996). In addition, the lack of knowledge about the inheritance of major traits hinders biotechnology efforts (Soost and Roose 1996).

Varieties

Navel oranges constitute the most important C. sinensis varieties in the world for fresh fruit production. Their inclusion of a small “secondary” fruit embedded in the apex of the main fruit distinguishes them. The navel orange originated in Bahia (Baia), Brazil as a bud mutation (Speigel-Roy and Goldschmidt 1996; Janick et. al. 1981). Reverend Schneider, a missionary, discovered this nearly seedless (parthenocarpic) sweet orange, in 1869. The USDA received trees the following year and descendents of these ended up in Riverside, California in 1873 (Speigel-Roy and Goldschmidt 1996; Janick et. al. 1981; The Orange History [1999]). The Bahia Navel, later known as the Washington Navel, came into being when a Mrs. Luther Tibbits planted two of these trees. Since then, they have spread all over the world to be cultivated in both the north and south hemispheres. Numerous mutant strains with differing ripening periods allow them to be in season year-round. For instance, the Tule Gold and Skaggs Bonanza varieties, products of spontaneous mutation, are described in Soost and Roose (1996, p. 299) as “early ripening and heavy bearing.”Future Prospects
There is still much to discover concerning Citrus sinensis, in addition to its varieties, hybrids and relatives. Much of the monsoon region and tropical Africa has yet to host studies for wild Citrus relations (Speigel-Roy and Goldschmidt 1996) and the phylogeny (and taxonomy) of Citrus continues to be obscured by a history of repeated hybridizations and mutations.

“Future advances in citrus breeding and genetics will no doubt continue to be slow and will require long-range planning and effort, due to the quantitative nature of citrus inheritance, the long generation time, and the presence of nucellar embryony” (Soost and Roose 1996, p. 265).

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