Aurantium sinense Mill.

Citrus sinensis is a small, shallow-rooted evergreen shrub or tree about 6-13 m high with an enclosed conical top and mostly spiny branches. Twigs angled when young, often with thick spines. Leaves smooth, oval, 5-15 x 2-8 cm, dark green above, glossy, with a distinctive smell often similar to the fruit, petiole winged. Flowers small, waxy greenish-white, fragrant; calyx broad saucer-shaped, petals 5, white elliptic, 1.3-2.2 cm long. Fruits orange, reddish-green to yellowish-green, round, 4-12 cm, consist of a leathery peel 6 mm thick, tightly adherent, protecting the juicy inner pulp, which is divided into segments that may not contain seeds, depending on the cultivar. The specific epithet (sinensis) refers to its Chinese origin.

C. sinensis starts flowering and bearing fruit after 3-5 years. In Haiti, trees flower between March and May and fruits mature between November and April. Fruit maturation takes 9-12 months. Trees are hermaphroditic, and insect pollinated. Flower initiation occurs in mid-summer, before the late spring flush.

Trees will grow in almost any if well aerated. The optimum conditions for citrus orchards are fertile, light to medium, well-drained, deep, loose loams; soils with a high water table should be avoided. The species is sensitive to excess salts; pH range of 5-8 is preferred.

C. sinensis is a subtropical rather than a tropical species. It prefers a prominent change of seasons. Whereas most cultivars can be grown throughout the ecological range described for the genus, the requirements of commercial production are much more exacting, confining each cultivar to environments compatible with high yield and good fruit quality. Few cultivars do well in the humid tropical lowlands; the choice is much wider for areas with a monsoon climate and intermediate elevation. High constant humidity makes C. sinensis susceptible to pests and diseases, but it can tolerate a small amount of frost. Trees are intolerant of waterlogging.

C. sinensis is not known anywhere as a wild plant but must have originated near the border between China and Vietnam. Sweet oranges arrived in Europe in the 15th century. The Portuguese introduced C. sinensis to the forest regions of West Africa, where it is extensively cultivated for its sweet, refreshing fruits. In 1493, Columbus took C. sinensis seeds to the Americas, where they were planted. Oranges were well established in Florida, California and Louisiana by the mid-18th century. By the 16th century, the orange had reached England.

When harvesting the fruit, care should be taken not to scar the peel, as this will encourage fungal attack. C. sinensis is propagated artificially by seedlings, wildings and graftings. Most cultivars are propagated by budwood to avoid the long juvenile phase of seedlings. Rootstock of other species with a better root system, such as C. limon or C. aurantium, are frequently used to improve vigour and hardiness and increase resistance to diseases. Seed for the rootstock should be chosen from good, healthy trees when fruits are fully matured. They should be cleaned and only partially dried. No pretreatment is needed, and germination is best at temperatures between 20-30 deg. C. Three seeds per hole should be planted and later thinned out to 1 seedling. After about 8 months, the rootstock, or seedling, should be of pencil thickness, 15 cm high, and ready for budding. The time from budding to planting out may take from 6 months to 2 years, depending on the size of seedling required.

Seedlings should never be planted out deeper than they were in the nursery. Seedlings should be spaced about 5-6 cm apart; trees are planted at a spacing of 3-5 m squares, but a spacing of up to 1.3 m can be used, depending on the climate. Irrigation is necessary if rainfall is not adequate. If soil lacks nutrients, fertilizer containing nitrogen, phosphorus potash, magnesium, calcium, zinc, manganese and iron should be added. Pruning is practised to encourage branching and keep the tree low for easy harvesting of the fruit. Any growth below where budding took place should be removed. Pruning, sanitation, use of resistant varieties and spraying are used to control pests and diseases. Weeds are controlled using chemicals such as Diuron, Bromacil, Terbacil Simazine, Paraquat, Diquat, 2,2-DPA, Armitrol, MSMA and DSMA. A windbreak should be provided to protect trees. Trees aged 3-4 years produce 2.5-5 t/ha of fruit and 8-12 year old trees produce 20-40 t/ha of fruit. Single trees may live up to 100 years, but the economic life of an orchard seldom exceeds 30 years.

Pests include termites and nematodes of several species: Semipenetrans, Tylenchulus, Trichodirus and Xiphimena species. Other pests known to attack C. sinensis are Aleuroplatus samoanus, Aonidiella aurantii, Ceratitis capitata, Cryptoblades plagioleuca, Dacus dorsalis, D. melanotus, D. tyroni, Othreis fullonia, Parlatoria cinerea, P. pergandii, Phyllocoptruta oleivora and Toxoptera airantii. Disease causing organisms include Alternaria spp., Botrytis spp., Capnodium cotri, Colletotrichum spp., Diaporthe citri, Diplodia spp., Elsinoe fawcetti, Fusarium spp., Glomerella cingulata, Penicillium digitatum, P. italicum, Phytophthora parasitica, Psorosis spp., Sclerotinia and Xanthomonas citri. Tristeza is a viral disease that triggers yellowing and wilting of leaves, twig die-back and subsequent death of tree. Another die-back attacks citrus in India. Other important diseases include leaf mottle yellows in the Philippines and citrus vascular degeneration in Indonesia.

After removal from the fruit, seeds are cleaned and slowly dried to 7.2% mc then stored in a refrigerator. Desiccation from 45% to 20% reduces viability from 71% to 9%. Seeds tolerate desiccation to 10.6% at 70% r.h., but further desiccation reduces viability; only 10% of seeds survive desiccation to 4% mc at of 20% r.h. Storage behaviour is intermediate and seeds lose viability from 81% to 20% after 6 months storage. There are about 3500 seeds/kg.

Shade or shelter: C. sinensis is sometimes planted to provide shade. Ornamental: The species has the potential of being grown as an amenity tree in gardens.

Food: Fruits are a good source of vitamin C. They can be eaten fresh or made into juice, marmalade or jelly. In Brazil and Florida, juice the world’s largest producers, 90% of the production is converted to. Pectin, a setting agent, is made from the peel. Fodder: Pulp, molasses and residues from juice production are used as cattle feed. Apiculture: Trees are valued honey plants. Fuel: C. sinensis is a potential source of firewood. Timber: Wood can be used for boards and panelling. Essential oil: Peels, leaves and flowers contain fine essences of oils that may be used in manufacture of cosmetics and medicinal applications. Medicine: Leaf decoction with salt is taken orally for digestive tract ailments, nerve disorders, fever, asthma, blood pressure, general fatigue and vomiting. Crushed leaves or fruit juice is massaged into the skin to relieve itching. Macerated root, leaf or fruit mesoderm is taken orally for urethritis; macerated fruit mesoderm or bark decoction is taken orally for liver ailments. Fruit juice or leaf decoction with sugar is taken orally for cold and loss of appetite, while crushed leaf decoction as a bath relieves headache and rheumatism. Broken bones are massaged with roasted fruit. Leaf oil exhibits carminative properties and light antispasmodic and sedative properties.