Citrus Trees

The fruits of citrus trees contain large concentrations of sour-tasting citric acid. Nevertheless, the fruits of some species can be quite sweet because they contain large concentrations of fruit sugar. Plant breeders have developed various sorts of cultivated varieties, or cultivars, from the wild progenitors of various species of citrus trees. This has resulted in the selective breeding of varieties with especially sweet fruits, others which peel relatively easily, and yet others that are seedless and therefore easier to eat or process into juice.

Once plant breeders discover a desirable cultivar of a species of citrus tree, it is thereafter propagated by rooting stem cuttings or by grafting. The latter procedure involves taking a stem of the cultivar, and attaching it to the rootstock of some other, usually relatively hardy, variety. The graft is carefully wrapped until a protective callus tissue is formed. Because the attributes of the cultivar are genetically based, these methods of propagation avoid the loss of desirable genetic attributes of the new variety that would inevitably occur through sexual cross-breeding.

Citrus trees can also be propagated using relatively new techniques by which small quantities of cells can be grown and induced to develop into fully-formed plants through specific hormone treatments. These relatively new techniques, known as micro propagation or tissue culture, allow for the rapid and inexpensive production of large numbers of trees with identical genetic qualities.

The most important economic products of cultivated citrus trees are, of course, their fruits. In agriculture, the fruits of oranges and grapefruits are commonly picked when they are ripe or nearly so, while those of lemons and limes are usually picked while they are still unripened, or green.

The fruits of the sweet orange can be eaten directly after peeling, or they may be processed into a juice, which can be drunk fresh. It may also be concentrated by evaporating about three-quarters of its water content, and then frozen for transport to far-away markets. This is a more economical way of moving orange juice around, because removal of much of the water means that much less weight must be transported. In addition, juice concentrates can also be used to manufacture flavorings for various types of drinks.

The juice of citrus fruits is relatively rich in ascorbic acid, or vitamin C. For example, a typical orange contains about 40 mg of vitamin C, compared with only 5 mg in an apple. Vitamin C is an essential nutrient for proper nutrition of animals. However, animals cannot synthesize their own vitamin C and must obtain the micronutrient from their diet. In the absence of a sufficient dietary supply of vitamin C, a debilitating and eventually lethal disease known as scurvy develops. In past centuries scurvy often afflicted mariners on long oceanic voyages, during which foods rich in vitamin C or its biochemical precursors could not be readily obtained. Because they stored relatively well, lemons were an important means by which sailors could avoid scurvy, at least while the supply of those fruits lasted.

At one time, citric acid was commercially extracted from the fruits of citrus trees, mostly for use in flavoring drinks. Today, citric acid is used in enormous quantities to flavor carbonated soft drinks and other beverages. However, most of this industrial citric acid is synthesized by fungi in huge fermentation vats.

In addition, the shredded peel and juice of citrus fruits can be sweetened and jelled for use in such sweet spreads as marmalade.

Grapefruit ready to pick. © 1980 Ken Brate. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

The major economic value of oranges lies in their fruits, but several fragrant oils can also be extracted from their flowers, or, more commonly, their peel as a byproduct of the orange-juice industry. These essences can be used to manufacture so-called Neroli and Portugal oils. These fragrances were originally used in the manufacturing of perfumes and to scent potpourri, and they are still used for these purposes. In addition, many household products, such as liquid detergents, shampoos, and soaps, are pleasantly scented using the aromatic oils extracted from citrus trees.

Pomanders, which are oranges studded with cloves, are an archaic use of the fruit. Originating in Spain, pomanders were worn around the neck for several purposes-as perfumery, to ward off infections, or to attract a person of the opposite sex. Today pomanders are more commonly used to pleasantly scent closets and drawers.

In regions with a warm climate, citrus trees are sometimes grown as ornamental shrubs and trees. The citron was reputedly grown in the ancient Hanging Gardens of Babylon in what is now Iraq. During those times the citron was used in scenting toilet water and in making an aromatic ointment known as pomade.