Citrus amara Link

Tree, 3-10 m tall, much branched with a rounded crown. Young twigs angled and bearing slender short spines, older branches with stout spines up to 8 cm long. Leaves simple, alternate, subcoriaceous, dotted with glands, aromatic when bruised; petiole 2-3 cm long, upper half narrowly to broadly winged, wing triangular-obovate, up to 2.5 cm wide; blade broadly ovate to elliptical, 7-12 cm x 4-7 cm, base cuneate or obtuse, margin subentire to slightly crenulate, apex obtuse to bluntly pointed. Flowers axillary, single or in a fascicle of 2-3, very fragrant, white, usually bisexual but 5-12% male flowers occur; calyx cupular, 4-5 mm long, 3-5-lobed, lobes broadly ovate-triangular, glabrous to pubescent; petals 4-5, oblong, 1.5 cm x 4 mm; stamens 20-25, often in 4-5 groups, filaments 6-10 mm long, anthers oblong; pistil with glabrous ovary, stout style and capitate stigma. Fruit a depressed-globose hesperidium, 5-8 cm in diameter, with 8-12 segments, central core usually hollow, peel thick, smooth to warty, yellow-orange, strongly aromatic, pulp very acid and slightly bitter. Seeds numerous, polyembryonic, with a high number of nucellar embryos. Bouquetier cultivars are small, subspineless trees, flowering profusely with very large single flowers. The taxonomy of C. aurantium as species and at sub-specific level is very confused and in dire need of revision, as is the taxonomy of the entire genus Citrus L. It is possible that C. aurantium originated as a hybrid between the mandarin (C. reticulata Blanco) and the pummelo (C. maxima (Burm.) Merrill), which perpetuates chiefly apomictically. In Europe, sour orange was known long before the sweet orange, and for a long time sweet orange has been considered as a variety of sour orange. Now sweet orange (C. sinensis) is considered as a completely different species (main differences: sour orange has longer and broader winged petioles, leaf blades are narrower and more pointed, fruits are brighter orange and have a rougher peel and oil glands are sunken in the peel). Cultivars grown for the production of essential oils are often referred to as C. aurantium L. subsp. amara Engl., but a cultivar group classification is more appropriate for cultivated plants. Bouquetiers are a small group of cultivars grown mainly in France for the perfume industry. Some well known cultivars are: 'Bouquetier Ó Grandes Fleurs' (the most important perfume cultivar; synonym: 'Bouquetier Ó Peau Epaisse'), 'Bigaradier de Grasse', 'Bouquetier Ó Fruits Dur', 'Bouquetier Ó Fruits Mous', 'Bouquetier de Nice Ó Fleurs Double' (cultivar with double flowers; synonyms: 'Bouquetier de Nice Ó Fruits Plats', 'Bouquetier de Nice') and 'Bouquet' (a cultivar very well suited as ornamental hedge plant; synonym: 'Bouquet de Fleurs').

Flowering of C. aurantium is strongly influenced by weather conditions. Bright, sunny and dry weather with mild temperatures is most suitable for flower production and harvesting. Cloudy, misty or hot and humid conditions reduce flower numbers and oil content. In France and most of the Mediterranean flowering occurs in late spring (May-June). Bitter orange has a long life span; orchards 80-100 years old can still be productive.

If the climate is favourable, C. aurantium can be planted in almost any kind of soil that is not compacted, wet for long periods or contains excessive amounts of clay, lime or silica.

C. aurantium, including the Bouquetier cultivars, requires a warm climate with an optimum average annual temperature of 22-24 deg. C, but temperatures of up to 45 deg. C are tolerated, provided soil moisture is adequate. Plantations are mostly located below 500 m altitude and perform best up to 300 m. Cold weather may cause damage, especially to trees weakened by diseases or pests. Frost causes serious damage and a few days with minimum temperatures of -4 deg. C may kill the young, non-lignified branches, while severe frost or icing is often fatal. In some years, late spring frosts destroy a great part of the flowers in France and other Mediterranean countries. Growth and flowering are optimal under sunny conditions. Annual rainfall of 1000 mm falling mainly in spring and summer is the optimum, and irrigation is required where annual rainfall is below 750 mm. Very strong winds may cause tree damage, while dry hot winds in spring may reduce leaf size and number and cause extensive withering during flowering.

C. aurantium was one of the first citrus taken to South America in the 16th Century, where it soon escaped from cultivation and naturalized in many areas. It is now cultivated in many tropical and subtropical countries, but only rarely in South-East Asia.

Like other bitter oranges, Bouquetiers are propagated by seed or grafting. In France, seed is planted in seedbeds in spring and seedlings are transplanted into a nursery after 2 years at a spacing of 1 m x 30 cm. Grafting is usually performed at the beginning of summer, when the bark of the rootstock can be loosened easily from the wood.

Plantation: When planted out in the field, the spacing is 4 m x 4-5 m, but the spacing between rows may be reduced to 1-1.5 m if all cultivation is manual. Husbandry: During the first year after planting out, sour orange must be pruned to keep the foliage off the ground. Weed control is essential until the young plants are well established and growing vigorously. In France, the ground between trees is cultivated several times per year when applying fertilizer: in spring, twice during summer and after application of manure in September-October. In many other locations fertilizer is seldom used. Trees are commonly mulched with spent material from stills. Irrigation is applied when necessary and is particularly important during the first years after transplanting. Pruning to keep an open crown is done after the harvest of the flowers. The leaf material removed may be used to distill petitgrain bigarade. When prices are low, orchards are often neglected for long periods. When prices recover, trees can be coppiced without harm and resume normal growth and production.

Diseases: The principal disease of bitter orange in France is sooty mould. It often follows aphid attack and is caused by fungi living on honeydew and covering the leaves with a black dust, thus impeding their functioning. C. aurantium is resistant to gummosis, caused by Phytophthora spp., but susceptible to scab, caused by Elsinoe fawcetti, and intolerant of the tristeza virus. Pests: Scale insects are the main pests in the Mediterranean region.

Essential oils: The flowers of several citrus species yield essential oil called 'neroli oil' in the perfume trade. The flower oil of sweet orange (C. sinensis (L.) Osbeck) is called 'neroli Portugal', that of lemon (C. limon (L.) Burm.f.) 'neroli citronier'. Flowers of C. aurantium yield an essential oil called 'neroli bigarade oil'. The best quality is obtained from Bouquetier cultivars formerly from southern France and Italy and nowadays from Morocco and Tunisia. Neroli oil is a component of high quality perfumes and of the toilet water 'eau-de-Cologne'. Significant amounts (up to 25%) of aroma compounds remain in solution in the water left in the still after distillation of C. aurantium flowers. The oil obtained by extraction of these compounds is traded as 'orange flower water absolute' and is mainly used in the reconstitution of other essential oils and of the formerly popular 'orange flower water'. Medicine: C. aurantium has several medicinal uses, e.g. in the Caribbean against gall bladder problems; in the Philippines as a stimulant and against ringworm and as an antiseptic against Staphylococcus aureus. Bitter orange peel oil has stomachic, carminative and laxative properties. Dried bitter orange peel is used as tonic and carminative in treating dyspepsia. Preparations from unripe and nearly ripe fruits are used against stomach pains and indigestion in North Africa and China, and against colds and influenza in the Bahamas and China. In aromatherapy neroli bigarade oil is primarily a relaxing oil, but is also applied for its antiseptic, antispasmodic and carminative effect. Because of its sedative effect its use is incompatible with driving. Timber: The wood of sour orange is strong and takes a fine polish; it is sometimes used to make furniture. C. aurantium has been used extensively as a rootstock for other citrus species (particularly sweet orange, lemon and grapefruit) because it produces a well-developed root system and has a high degree of resistance to many important diseases (e.g. gummosis, root rot) and to cold. Its popularity as a rootstock has decreased, however, because of its susceptibility to the viral disease tristeza. Other products: Neroli bigarade oil is sometimes used as a flavour component in food products, including alcoholic and non-alcoholic beverages, especially in tea. 'Petitgrain oil' is produced by distilling the leaves and green twigs of several citrus species; those of C. aurantium, including the Bouquetiers, yield 'petitgrain bigarade oil'. Petitgrain oils are often used as a substitute for the much more costly neroli oils. Petitgrain water absolute or 'eau-de-brouts' is the equivalent of orange flower water absolute and is obtained as a by-product from petitgrain bigarade oil. It enhances the 'naturalness' of several other fragrances, e.g. jasmine, neroli, ylang-ylang and gardenia. Petitgrain bigarade oil is used as a flavouring in food products, including alcoholic and non-alcoholic beverages. It also yields the essential oil 'bitter orange oil' (also called 'bitter orange peel oil') mainly used as a flavouring, such as 'orange sec' and 'triple sec' liqueur flavours, and to modify or strengthen the flavour of sweet-orange soft drinks. The average maximum concentration used in food products and drinks is about 0.04%. Food: The peel of the fruits of C. aurantium is used extensively in the manufacture of citrus marmalade. The sour fruit juice can be used like vinegar.