The orange is the fruit of various citrus species in the family Rutaceae (see list of plants known as orange); it primarily refers to Citrus × sinensis, which is also called sweet orange, to distinguish it from the related Citrus × aurantium, referred to as bitter orange. The sweet orange reproduces asexually (apomixis through nucellar embryony); varieties of sweet orange arise through mutations.
The orange is a hybrid between pomelo (Citrus maxima) and mandarin (Citrus reticulata). The chloroplast genome, and therefore the maternal line, is that of pomelo. The sweet orange has had its full genome sequenced.
The orange originated in a region comprising Southern China, Northeast India, and Myanmar, and the earliest mention of the sweet orange was in Chinese literature in 314 BC. As of 1987, orange trees were found to be the most cultivated fruit tree in the world. Orange trees are widely grown in tropical and subtropical climates for their sweet fruit. The fruit of the orange tree can be eaten fresh, or processed for its juice or fragrant peel. As of 2012, sweet oranges accounted for approximately 70% of citrus production.
In 2017, 73 million tonnes of oranges were grown worldwide, with Brazil producing 24% of the world total, followed by China and India.
All citrus trees belong to the single genus Citrus and remain almost entirely interfertile. This includes grapefruits, lemons, limes, oranges, and various other types and hybrids. As the interfertility of oranges and other citrus has produced numerous hybrids and cultivars, and bud mutations have also been selected, citrus taxonomy is fairly controversial, confusing or inconsistent. The fruit of any citrus tree is considered a hesperidium, a kind of modified berry; it is covered by a rind originated by a rugged thickening of the ovary wall.
Different names have been given to the many varieties of the species. Orange applies primarily to the sweet orange – Citrus sinensis (L.) Osbeck. The orange tree is an evergreen, flowering tree, with an average height of 9 to 10 m (30 to 33 ft), although some very old specimens can reach 15 m (49 ft). Its oval leaves, alternately arranged, are 4 to 10 cm (1.6 to 3.9 in) long and have crenulate margins. Sweet oranges grow in a range of different sizes, and shapes varying from spherical to oblong. Inside and attached to the rind is a porous white tissue, the white, bitter mesocarp or albedo (pith). The orange contains a number of distinct carpels (segments) inside, typically about ten, each delimited by a membrane, and containing many juice-filled vesicles and usually a few seeds (pips). When unripe, the fruit is green. The grainy irregular rind of the ripe fruit can range from bright orange to yellow-orange, but frequently retains green patches or, under warm climate conditions, remains entirely green. Like all other citrus fruits, the sweet orange is non-climacteric. The Citrus sinensis group is subdivided into four classes with distinct characteristics: common oranges, blood or pigmented oranges, navel oranges, and acidless oranges.
Other citrus groups also known as oranges are:
- Mandarin orange (Citrus reticulata) is an original species of citrus, and is a progenitor of the common orange.
- Bitter orange (Citrus aurantium), also known as Seville orange, sour orange (especially when used as rootstock for a sweet orange tree), bigarade orange and marmalade orange. Like the sweet orange, it is a pomelo x mandarin hybrid, but arose from a distinct hybridization event.
- Bergamot orange (Citrus bergamia Risso), grown mainly in Italy for its peel, producing a primary essence for perfumes, also used to flavor Earl Grey tea. It is a hybrid of bitter orange x lemon.
- Trifoliate orange (Poncirus trifoliata), sometimes included in the genus (classified as Citrus trifoliata). It often serves as a rootstock for sweet orange trees and other Citrus cultivars.
An enormous number of cultivars have, like the sweet orange, a mix of pomelo and mandarin ancestry. Some cultivars are mandarin-pomelo hybrids, bred from the same parents as the sweet orange (e.g. the tangor and ponkan tangerine). Other cultivars are sweet orange x mandarin hybrids (e.g. clementines). Mandarin traits generally include being smaller and oblate, easier to peel, and less acidic. Pomelo traits include a thick white albedo (rind pith, mesocarp) that is more closely attached to the segments.
Orange trees generally are grafted. The bottom of the tree, including the roots and trunk, is called rootstock, while the fruit-bearing top has two different names: budwood (when referring to the process of grafting) and scion (when mentioning the variety of orange).