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Citrus ID


Sour Oranges




Aurantium acre Mill.; Citrus florida Salisb.; C. vulgaris Risso; C. bigarradia Loiseleur-Deslongchamps; C. bigaradia Risso & Poiteau; C. amara Link.; C. karna Raf.; C. communis Le Maout. & Decaisn.; C. aurantium var. bigaradia Hook. f. (sec. Swingle and Reece 1967)



Swingle and Reece (1967) noted that:

“This species was introduced into the Mediterranean region from the East and for many centuries was the only orange known to Europeans. During this long period of culture it became very well known and much appreciated as a medicinal agent; the fruits were used for flavoring and for marmalade, and the flowers for perfumery. It was the orange of late medieval Europe. Good high-flavored varieties of the sweet orange, C. sinensis , did not reach Europe from southeastern Asia until the fifteenth century. From that time on there was more or less confusion over the name of the sour orange. The pharmacologists persisted in calling it C. aurantium and the citrologists, then the botanists, called it C. bigaradia. Many botanists considered both the sour and the sweet orange as merely varieties of a single species. As a matter of fact, the sour orange (C. aurantium) and the sweet orange (C. sinensis) are very distinct botanical species, not merely cultivated varieties of one species.”

Hodgson (1967) additionally noted that:

“Like most of the other citrus fruits of commercial importance, the sour, bitter or Seville orange is considered to have originated in the region of northeastern India and adjoining portions of China and Burma. Spreading northward to Japan and westward through India to the Mediterranean basin, it finally reached Europe sometime around the Christian era. The sour orange was among the first citrus to be taken to the New World. In such climatically favorable portions of the New World as Florida and Paraguay it escaped from cultivation and became feral. It is the naranja agria or amarga of Spain, melangolo or arancio amaro of Italy, bigarade or orange amére of France, khuskhash of Israel, khatta of West Pakistan and parts of India, and daidai of Japan.”



Crown compact or dense; not weeping. First-year twig surface glabrous or pubescent; second- or third-year twig surface striate; thorns absent or not persistent or straight; prickles absent or not persistent. Petiole glabrous or pubescent, length short, medium or very long; wings absent, if present, narrow, medium or wide, adjoining the blade or tucking beneath blade. Leaflets one, margin entire, crenate/crenulate, bluntly toothed or serrate/serrulate, shade leaflet blades flat or weakly conduplicate, sun leaflet blades weakly or strongly conduplicate. Scent of crushed leaflets sweetly orange-like, spicy, peppery, or somewhat to strongly malodorous. Fruit broader than long, as broad as long, or longer than broad; rind variegated, light green with some break to yellow (5), green-yellow (6), yellow (7-10), yellow-orange (11), orange (12), or red-orange (13); rind texture smooth (1-3), slightly rough (4-5), medium rough (6-7), or rough (8); firmness leathery; navel absent or present; flesh orange or yellow; taste acidic-sweet or sour.


Swingle and Reece (1967) provided the following additional notes on the species:


“A medium-sized tree up to 10 m high, with a rounded top; twigs angled when young, with single, slender spines, often short, or stout spines up to 5-8 cm long on rapidly growing shoots; leaves medium-sized, ovate, bluntly pointed at tip, broadly rounded to cuneate at base; petioles 2-3 cm long, rather broadly winged, often 1.2-1.8 cm wide at top, but sometimes narrower, 1 cm or less, narrowing rapidly to the wingless base; flowers large, very fragrant with oil of neroli; 5-12 per cent[sic] male (staminate only); fruits subglobose, usually slightly depressed at both base and top, peel thick, with a rather rough surface, becoming brilliant orange with a reddish tint at maturity; locules 10-12, filled with sharply acid pulp and numerous seeds; fruit becoming hollow at center as it matures, and then able to float in water. (See Hodgson's account of the cultivated varieties of the sour orange in chap. 4 of this work)”


“The chief morphological differences are as follows: In the sour orange the petioles are much more broadly winged than in the sweet, and the leaf blades are narrower and more acutely pointed at the apex, and less rounded and more cuneate at the base. Ruggieri has shown (1935) that the petioles of the sour orange are much the longer, averaging 25.89 mm, whereas those of the sweet orange average only 15.91 mm; in other words, the sour orange petioles average 63 per cent longer than those of the sweet orange. The fruits of the sour orange are of a brighter orange color and have a rougher peel; moreover, in the sour orange the oil glands are situated beneath minute sunken areas in the peel, whereas in the sweet orange the tissue covering the oil glands is often convex.


Uphof reported (1932, pp. 133-35, fig. 4) that he found no male flowers on several cultivated varieties of the sweet orange (C. sinensis), but that he did find from 5 to 12 per cent[sic] of male flowers on the sour seedling oranges of Florida (C. aurantium ). The sour orange, according to Uphof, "probably is very close to the sweet orange but from the standpoint of the production of male flowers constitutes a transition, so to speak, toward the lemons, limes and citrons." Uphof found no male flowers on cultivated varieties of the grapefruit (C. paradisi ). Many trees of the Dancy tangerine (C. reticulata) showed no male flowers (one small tree overloaded with a very heavy bloom had one single male flower among the thousands examined). Tangelos (hybrids of C. reticulata with C. paradisi) also showed no male flowers.


In view of these facts, it is clear that the occurrence in appreciable numbers of male flowers in C. aurantium constitutes an important differential character separating this species from C. sinensis.


The ethereal oil in the leaves, flowers, and fruits of the sour orange is of very different odor than that in the sweet (more agreeable and aromatic in the sour orange) and has a different composition. Also the oil recovered from the petals of the sour orange, neroli oil, finds a different use in perfumery and has a higher value than that of sweet orange flowers. This oil, considered by perfumery experts to be "indispensable to finer perfumery," is said to owe its high value to small amounts (only 0.4 to 1.0 per cent) of "a nitrogenous compound of exceeding fragrance," methyl anthranilate, NH2 · C6H4 · COOCH3 (see Gildemeister and Hoffmann, 1922, pp. 93 and 96; and Finnemore, 1926, pp. 436-41). This remarkable substance is not found in the oil extracted from the petals of the sweet orange (Theulier, 1902). The pulp of the sour orange is intensely sour, with a bitterish aftertaste, in contrast to the sweet, agreeable flavor of the sweet orange.”


“The sour orange also shows physiological differences from the sweet orange. It stands winter cold better and has almost complete immunity to the foot rot, or mal di gomma, so destructive to the sweet orange in some localities. However, the sour orange is severely attacked by the scab fungus, Elsinoë fawcetti, which does not attack the sweet orange.


Besides the morphological, chemical, and physiological differences…there are further anatomical difference[s] discovered by Ruggieri (1935) between the sour and the sweet orange in the separative layer of articulation that lies between the petiole and the leaf blade, which may be summed up as follows: (1) The pith in the lower articulation joint where the petiole joins the twig is much more flattened from the top to the bottom in the sweet orange than in the sour. In the sour orange the ratio of the pith to the woody cylinder averages 1:1.4, measured in a horizontal direction, and 1:2.2, in a vertical direction, whereas in the sweet orange the ratio of pith to woody cylinder is 1:1.5 horizontally (about the same as in the sour orange) and 1:4.5 in the vertical (dorsiventral) direction, or only half as thick as the pith of the sour orange petiole. (2) The cells of the interior layers of the cortical parenchyma of the upper articulation joint where the petiole joins the leaf blade are isodiametric and are 12 to 20.5 mu in diameter in the sour orange but are 20 to 27 mu in diameter in the sweet orange, or nearly one-half larger than in the sour orange. (3) The pericycle fibers that form a more or less interrupted sheath around the woody cylinder are strongly thickened in the sour orange but are little thickened, if any, in the sweet orange.


B. Miyazaba, S. Matsubara, and T. Kawaida (1928, p. 189, figs. 4, 5) have also found other anatomical characters that separate the sour orange from the sweet. Comparing the common sour orange (kaisei-to) of Japan with the Washington navel orange (tento) in leaf structure, they found that the sour orange leaf is thin (197 to 274 mu, the average of the ten measurements being 243.38 mu), whereas the sweet orange leaf is about 11.1 per cent thicker (247 to 334 mu, the average of the ten measurements being 270.33 mu ). The two layers of palisade tissue are about the same thickness in the two species, but the spongy tissue shows a wide variation, being only 148 to 189 mu thick in the sour orange leaf (the ten measurements averaging 167.28 mu) but ranging from 165 to 231 mu in the sweet orange leaf (the ten measurements averaging 197.51 mu), or nearly 17 per cent thicker. They also found that the number of stomata is somewhat greater in the sweet orange epidermis than in that of the sour orange, averaging 23.44 in the microscope field of vision for the sweet orange and 20 for the sour orange, or 17 per cent more for the sweet orange.


In view of this array of anatomical, physiological and chemical differences between the sour and the sweet orange it is obvious that they are distinct species even if the gross morphological differences between them are small.”


Hodgson (1967) provided the following additional notes on the species:


“While the sour and sweet oranges have close resemblances there are important differences which clearly justify their separation into different species. The sour orange leaf is somewhat darker in color and more taper-pointed and the petiole is longer and more broadly winged. The fruit is usually flatter and more deeply colored and the rind thicker and more loosely adherent. The rind surface is generally rougher and is minutely pitted with sunken oil glands. The core is normally hollow and the flavor sour with pronounced bitterness in both carpellary membranes and albedo. Most distinctive and easily recognizable differences relate to the odor of the oils in the leaves and rind. In the sour orange the leaf oil is agreeable and distinctive, whereas in the sweet orange it is merely pleasant. Sour orange rind oil is strong and somewhat disagreeable in contrast with the sweet and pleasant odor of sweet orange rind oil. Moreover, the chalazal spot is purple-tinted in the sour orange, reddish-brown in the sweet oranges in general, red in the deeply pigmented blood oranges, and cream-colored in the sugar or acidless oranges (Chapot and Praloran, 1955).


In comparison with the sweet orange, the sour orange tree is more upright and thorny and much more resistant to such unfavorable environmental conditions as frost, excess soil moisture, and neglect. However, the sour orange does not attain as large size as the sweet orange. It is also much more resistant to the widespread gummosis (mal di gomma) disease. In addition, it is susceptible to verrucosis (scab) and markedly intolerant to the tristeza virus when used as a rootstock, while the sweet orange is highly resistant to both diseases.”



Swingle and Reece (1967) additionally noted that: “Furthermore, the peel of the sour orange, which is official in the British Pharmacopœia (the peel of both the sour and the sweet orange is official in the U.S. Pharmacopœia), contains three glucosides, according to Tanret (1886, p. 518): (1) 4 to 30 parts per 1,000 of isohesperidin (= naringin ?), having the same percentage composition and the same rotary polarization as hesperidin but a bitter taste (hesperidin of the sweet orange is tasteless) and a very different solubility; (2) 15 to 25 parts per 1,000 of aurantamarin (to which, so Tanret asserts, most of the bitter taste of the sour orange peel is due), differing slightly in composition and in solubility from both naringin (?) and hesperidin; and (3) from a mere trace to six parts per 1,000 of hesperidin. In the sweet orange only this last glucoside, hesperidin, is found, but it is present in much larger quantities than in the sour orange.”

Hodgson (1967) additionally noted that:

“The fruit is too sour and bitter to be acceptable to most palates, although it can be used to make a distinctive and refreshing drink. Its principal use, however, is in the preparation of a distinctive marmalade, much in demand and appreciated in Europe and especially so in Great Britain, for which sweet orange marmalade is not an acceptable substitute. To meet this demand, sour oranges are grown to a limited extent in most of the Mediterranean countries. The principal producing area, however, centers around Seville in southern Spain, where commercial plantings are currently reported at approximately 4,000 acres. Great Britain comprises the principal sour orange market. Other products obtained or made from the fruits include rind oil and the liqueurs curaçao and Cointreau.

Oil of petit grain is distilled from the leaves and young shoots and oil of neroli from the flowers. For these products, however, which are much used in perfumery, special varieties selected primarily for flower production constitute the principal source of production. A byproduct of this process is orange flower-water, also used in perfumery, for flavoring cakes, and for medicinal purposes.

The importance of the sour orange in the citricultural world, however, arises from its use as a rootstock. Because of its marked resistance to the soil-inhabiting fungi principally involved in the gummosis and foot rot (mal di gomma) diseases, a century or more ago it became the leading rootstock. While its use as a rootstock has declined greatly in recent years because of marked intolerance to the tristeza virus disease, and it now seems destined to be abandoned for that purpose, sour orange remains a major rootstock in the Mediterranean basin and some other areas.

Because of the attractive appearance of both tree and fruit and their hardiness and resistance to unfavorable conditions in general, the sour oranges are also useful as ornamentals.

Three natural groups are distinguishable in the sour oranges, namely the common bitter orange, bittersweet orange, and the variant bitter oranges.”



Chapot, H. and J.C. Praloran. 1955. Les graines de Citrus. Report of the Fourteenth International Horticultural Congress [Neth.] 2: 1294–1323.

Finnemore, H. 1926. The essential oils. E. Benn, Ltd., London. 880 pp.

Gildemeister, E. and F. Hoffmann. 1913–22. The volatile oils. Second edition. Authorized translation by Edward Kremers. John Wiley & Sons, New York. 3 vol.

Hodgson, R.W. 1967. Horticultural varieties of Citrus. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The Citrus industry, rev. University of California Press.

Miyazaba, B., S. Matsubara, and T. Kawaida. 1928. Inner morphological studies on Citrus species and their varieties. Studia Citrologica 2: 185–205.

Ruggieri, G. 1935. La diversa resistanza alla defogliazione prodotta dal vento in alcune specie di "Citrus" in rapporto alla struttura anatomica del picciolo. Bolletino della Reale Stazione di Patologia Vegetale Nova Seria 15: 169–199.

Swingle, W.T. and P.C. Reece. 1967. The botany of Citrus and its wild relatives. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The Citrus industry. Ed. 2. Vol. I. University of California, Riverside.

Tanret, C. 1886. Sur quelques principes immédiats de 1'écorce d'orange amére. Comptes rendus de l'Academie des Sciences Paris 102: 518–520.

Theulier, E. 1902. Etude sur 1'essence de fleurs d'orangers douces ou néroli portugal. Bulletin Societe Chimique de France, Ser. 3, 27: 278–280.

Uphof, J.C.T. 1932. Wissenschaftliche Beobachtungen and [sic] Versuche an Agrumen. IV. Der polygamische Zustand einiger Citrusarten. Gartenbauwissenschaft 7: 121–141.



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Citrus ID Edition 2
October, 2011