Citrus fruits are the most valuable fruit crop in international trade.
All of the numerous Citrus varieties are cultivated in subtropical or tropical regions. They originate from different parts of the world. Oranges were already being used in China in 2000 BC, while the Seville orange (also known as the bitter or sour orange) was first grown in northern India. The lemon comes from the eastern Himalayas; mandarins, however, probably derive from the Mediterranean.
Today, citrus is cultivated in 140 countries; about 70% of fruits are produced in the northern hemisphere. Lemons, for example, are grown in Southern and Central America, Asia, around the Mediterranean and in parts of the USA, the most significant producers being Europe and Argentina. Under favorable conditions, three harvests per year are possible. Mandarins and kumquats are comparatively hardy and are therefore often cultivated in more northerly areas such as China.
More than two-thirds of global citrus production is done in Brazil, USA, China, Mexico and Spain. In Brazil and the USA, about 70% of the harvest is used for processing, whereas the Mediterranean countries produce mostly for fresh consumption, supplying primarily the European market. In Asia, most of the citrus produced is consumed domestically.
Citrus plants are typically evergreen shrubs or small- to medium-sized trees. They are heavily pruned during cultivation, and therefore do not reach their natural height.
Flowers are usually white and often fragrant. Many species bear spherical fruits with a green, yellow, or orange peel containing numerous oil glands. Another common characteristic of this genus are leaf thorns and feathered leaf stalks.
Individual trees of some species may live for up to 100 years.
Citrus fruits can make an important contribution to human nutrition, because they are a prominent source of vitamin C (L-ascorbic acid). The flesh of a lemon provides up to ~50 mg vitamin C per 100g. In the human body, ascorbic acid serves as a reducing agent, e.g. in hydroxylation during collagen synthesis, tyrosine degradation, catecholamine synthesis and the biosynthesis of bile. Vitamin C deficiency causes the disease scurvy, which leads to damage to connective tissues, internal bleeding, tooth loss, fever and eventually, death.
The economically most important varieties are lemon (Citrus limon), orange (Citrus sinensis), mandarin (Citrus reticulata, C. deliciosa), grapefruit (Citrus x paradisi), lime (Citrus aurantiifolia), Seville orange (Citrus aurantium), pummelo (Citrus maxima), citron (Citrus medica) and kumquat (Fortunella margarita/ Citrus margarita).