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Citrus (Citrus spp.)

RUTACEAE, Rue Family

Citrus trees, which are usually evergreens, are planted in every tropical and subtropical region around the world. These tree crops had a humble origin in the forests of southeastern Asia, particularly in Malaysia. No early records exist on the first uses of citrus species, but in Asia citrus must have been mused for a long time before the species were transported westward to become part of ancient cultures in Mesopotamia (4000 B.C.), Egypt, and Greece.

One of the small, evergreen trees of this genus is the citron, Citrus medica var. ethrog, which is described in the Bible from 200 B.C. This is a fragrant golden, oblong fruit up to eight inches long, which has a knobby skin and a persistent style-stigma. The Jews were introduced to this fruit in Babylonia during their exile there in the fifth century B.C., and they carried it back to Palestine thereafter. This fruit then became one of the four elements of the (palm branch, willows, myrtle, and citron) in the religious Feast of Booths. For this festival, the citron had to be fresh and flawless--undamaged skin, stigma and style in place, and base attached. Hence, the plant had to be grown locally wherever Jews lived, so it had to be transported westward as they emigrated from the Middle East. Moreover, the citron was carried by the Moslem migration through northern Africa to Spain during the Middle Ages.

Many species of Citrus reached Spain following the end of the Middle Ages. In 1493 Columbus took seeds of limes (C. aurantifolia) to the West Indies. Limes were later taken on sailing ships to prevent scurvy, and this is the origin of the name "limeys" for British sailors. Citrus fruit, of course, contains high concentrations of vitamin C.

The fruit of the citrus is called a hesperidium. This is a fleshy berry with a peelable rind. A cross section of the fruit shows the regions we are interested in. The exocarp is the colored outer coat, the flavedo, which hides the white mesocarp, the albedo. In the exocarp and outermost mesocarp are located oil glands, which are located beneath the pits on the peel. In these are produced monoterpenes, which are released when the peel is squeezed. Citrus peel oil is used commercially for scents and for the manufacture of other compounds. Peels are also rich in pectin, which is used for jellies and orange marmalade and to thicken and stabilize foods, such as catsup and salad dressing. The albedo is a loose, airy tissue with many vascular bundles, the "strings" that can remain attached to the edible portions of the fruit. Each "section" of a citrus fruit is a chamber or locule of the ovary. The locule has been filled by plant hairs (trichomes) that grew from the endocarp into the locule. As they grow, cells of the trichome enlarge greatly with the uptake of water, to become juice sacs, which are loaded with sugars and organic acids. Seeds, if present, are produced on the inner margin of the locule, next to the central axis of the ovary (axile placentation).

Today there are 16 species of Citrus and many hybrids that are commonly cultivated. Another tree crop is the kumquat, Fortunella margarita, which is a very close relative. Differences between the species have to do mainly with the size, shape (another view), color, and flavor of the fruit. Flavors are controlled by the balance between sweet (sugar) and sour (acid).

Citrus is propagated exclusively by grafting. Breeders use a rootstock that is disease-resistant and that confers hardiness and good water uptake. A bud, called the scion, of the desired crop is then grafted on the stock in a way that the vascular cambium of both grow together. Once the bud forms a new shoot, the old shoot is pruned away so that the fruit formed on the plant comes only from the scion. However, if the grower is not careful, the scion can die or be overgrown by shoots of the original rootstock that arise below the graft. Much of the citrus rootstock in the United States has been sour orange or rough lemon, and all is now regulated to be virus free.

Through selection, many fine cultivars came into existence. The history of citrus cultivars is really the history of close observation. Someone would observe fruits on a particular branch that had a particular characteristic, and they would use buds from that branch to start a new crop. An example of this are the popular grapefruits of today. Grapefruit (C. paradisii) was obtained in the West Indies around 1700, apparently derived from the pumelo (C. pumilo) and brought to Tampa, Florida in 1823. In Lakeland, Florida, home of the Miss USA Pageant, a seedless fruit was observed by William Hancock (1862) and bud grafted by a nurseryman named Marsh, which became cv. Marsh seedless grapefruit. In 1913 a spontaneous pink fruit arose from Marsh to become the pink grapefruit. And in Texas in 1929, a new "bud sport" (mutation) for pink and seedless was saved as the cultivar Ruby.

Citrus arrived in Florida before 1565, but only became an important industry after Florida ceded from Spain in 1821. Florida became the center of production, because of its subtropical climate, but this state has had its problems with freezes, such as a major one in 1895-95, which took 15 years to recover from, and several since 1950.

The Navel orange was a bud sport from an orange in Bahia, Brazil, which was introduced into southern California in 1871, so that fruits could be shipped eastward via the railroad. In addition to California, much citrus is grown near Yuma, Arizona, and in southern Texas. The distinctive, peelable Minneola Tangelo, now one of the most popular hand fruits, was a hybrid between two species, a grapefruit and the peelable tangerine.

One interesting feature of citrus is polyembryony. This is a process by which a seed forms more than one embryo. In this case, one embryo can form via typical double fertilization, while others form from maternal tissue surrounding the embryo sac, in the region called the nucellus. It is believed that many forms of citrus within a single species may have arisen from mutant branches and fruits that formed nucellar embryos, i.e., clones of the branch.

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