1495

2: Edible fruits and nuts

Citrus sinensis (L.) Osbeck

Reise Ostind. China: 250 (1765).

RUTACEAE

2n = 18, x = 9; triploids and tetraploids occur.

Citrus aurantium L. (var.) sinensis L. (1753).

Sweet orange (En). Oranger doux (Fr). Indonesia: jeruk manis. Malaysia: limau manis, chula, choreng. Papua New Guinea: sava orens (Pidgin). Philippines: kahel. Burma: tungchin-thi. Cambodia: krôôch pôôsat'. Laos: kièngz. Thailand: somkliang (central), somtra (Bangkok), makhun (northern). Vietnam: cam.

Sweet orange is not known anywhere as a wild plant but must have originated near the border between China and Vietnam. It is cultivated everywhere in the subtropics and tropics; roughly between latitudes 40° North and South and, near the equator, up to 2000 m altitude.

Sweet orange is generally eaten fresh in South-East Asia, but 90% of the production is converted to juice in Brazil and Florida, the world's largest producers. Pulp and molasses are used as cattle feed; pectin and essential oils are made from the peel.

World production of sweet oranges was nearly 40 million t in 1984 and 44 million t in 1987. The total area exceeds 2 million ha. In 1987 the export price per tonne averaged US$ 825 and the import price came to US$ 1140. Indonesia produced 350 000 t in 1987, Cambodia 40 000, Laos 33 000, Malaysia 9000, the Philippines 20 000, Thailand 55 000 and Vietnam 116 000. Production of oranges in the region comes to about 2 kg per head per year, which is far below actual demand.

The edible portion of a sweet orange fruit takes up 40—50% and per 100 g it contains: water 80—90 g, protein 0.7—1.3 g, fat 0.1—0.3 g, carbohydrates (sugars) 12.0—12.7 g, fibre 0.5 g, ash 0.5—0.7 g, vitamin A 200 IU, ascorbic acid 45—61 mg, citric acid 0.5—2.0 g. The energy value is about 200 kJ/100 g. The glucoside hesperidin occurs in significant quantities; it is part of vitamin P (citrin) which activates vitamin C and has a curative action on blood vessels.

Evergreen tree, 6—15 m tall, with rounded crown; twigs angular when young, usually with rather blunt axillary spines (on seedlings). Leaves alternate, elliptic to ovate, 5—15 cm x 2—8 cm, rounded at base, margins undulate to crenate, apex pointed; petioles 1—3 cm long, narrowly winged, articulated at both ends. Flowers axillary, in few-flowered racemes or singly, 2—3 cm in diameter, bisexual, fragrant, pentamerous; calyx 5-lobed; petals white; stamens 20—25; ovary with 10—13 loculi. Fruit a subglobose berry, 4—12 cm in diameter, greenish-yellow to bright orange; peel up to 0.5 cm thick, dotted with glands; segments with juicy pulp, yellow to orange-red with solid central axis. Seeds few to several, cuneate-ovoid with rough-marginate plane surfaces, white inside, polyembryonic. Seedlings growing upright, very spiny, developing a long taproot.

The seed of sweet orange is very polyembryonic; it has no dormant period. Optimum temperature for germination is 32°C. Cotyledons stay underground and the shoot emerges after 3—4 weeks. Seedlings remain juvenile for up to 8 years, but budded trees flower in the second or third year. Bees, thrips and other insects visit the flowers and assist in self- or cross-pollination, but parthenocarpy also occurs (e.g. in navel oranges), due to male sterility. There are alternating cycles of growth between the roots and shoots; each cycle lasts 4—6 weeks. Flower initiation takes place in a dormant (dry or cool) period and bloom occurs 3 weeks after a sufficient amount of rain. Each developing fruit needs 45—50 leaves to sustain its growth (there are 150 000 or more leaves with a total area of 200 m² on a healthy and full-grown tree). During fruit growth acidity decreases from over 3 to less than 1% citric acid, while TSS (total soluble solids) increase to a level of 13°Brix or higher. Single trees may live 100 years, but the economic life of an orchard seldom exceeds 30 years.

The sweet orange is often confounded with the sour orange (Citrus aurantium L.), but the two can easily be distinguished: fruits of the sweet orange have a solid core, never becoming hollow like that of the sour orange; the petioles are narrowly winged in the sweet orange and broadly winged in the sour orange; the leaves and flowers of the two species have a very distinct odour.
Three groups of sweet orange cultivars are recognized: common oranges with normal fruit, blood oranges with red or red-streaked pulp, and navel oranges with a second row of carpels opening at the fruit apex in an umbilicus. The 'blood' characteristic does not develop in the lowland tropics since cool nights — below 17°C — are needed. The most important cultivar, also widespread in the tropics up to 1600 m elevation, is 'Valencia'; it is a 'late' cv. taking 8—9 months from bloom to maturity (but longer in the highlands). The fruit is medium-large, oblong to spherical, with few seeds (or none), abundant juice and good flavour. Nucellar (virus-free) budlines of 'Valencia' are 'Cutter', 'Frost' and 'Olinda'. Other well-known common cvs are 'Hamlin': early, high yielding, for warm and humid conditions; 'Pineapple': mid-season, high yielding, does well in the humid tropics; and 'Pera': late, the main cv. in Brazil. Characteristics of local cultivars in Java: 'Pacilan' from East Java has juicy flesh, but little citric acid (0.11%); 'Waturejo' from Central Java is productive, with excellent flavour and aroma; 'Punten', a seedling selection from Batu (East Java) with good flavour and aroma, is productive in high areas (above 700 m elevation). Average weights of the fruits are respectively 121, 185 and 188 g and the average number of seeds is 12, 18 and 24. Their °Brix/acid ratios are exceedingly high in comparison with subtropical standards, namely 89, 25 and 20.
The best known navel cv. is 'Washington', also called 'Bahia'. It is seedless, easy to peel, has crisp flesh and a rich flavour. As its juice becomes bitter in storage, this cv. is exclusively used as fresh fruit. The fruit is large, spherical to ellipsoid, with a thick rind and matures early (6 months). In the tropics it is grown at altitudes of 1000—2000 m.
Hybrids of sweet orange have been produced with most Citrus spp., Fortunella spp. and Poncirus trifoliata (L.) Raf. The best known are:
— Tangors (orange x mandarin); 'Jeruk Siem', one of the most widely grown cultivars of Indonesia, is probably a tangor;
— Citranges (orange x P. trifoliata);
— Citrangors (orange x citrange);
— Citrangequats (citrange x kumquat).

The sweet orange 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. There are few cultivars which do well in the humid tropical lowlands; the choice is much wider for areas with a monsoon climate and intermediate elevations. For the cultivars named in the previous section, the ecological niche has been briefly indicated.

Budding is the preferred propagation method. Important sweet orange rootstock cvs are 'Indian River',' Bessie' and 'Madam Vinous'; they are fairly resistant to foot rot. Sour orange cannot be used in South-East Asia because it is highly susceptible to tristeza virus in combination with sweet orange. Cleopatra mandarin, Rangpur lime (a lemon x mandarin hybrid, in Java formerly known as 'Japanse citroen' which is highly sensitive to exocortis) and the citranges 'Troyer' and 'Carrizo' are also good rootstocks. Rough lemon (Citrus jambhiri Lush.) is a fast grower, tolerant to tristeza but susceptible to foot rot; it is generally used on light soils; trees on this stock often bear coarse fruit with a low sugar and acid content.
Planting density varies from 200—400 trees/ha, e.g. a spacing of 5 m x 6 m (333 trees/ha). Holes of 30 cm x 30 cm x 30 cm are made if drainage is adequate; otherwise 40—50 cm cubic is better. The topsoil is kept apart and later returned to the top. One kg rock phosphate is put in the bottom of the hole. The trees are planted on a mound 10 cm high. After the soil has settled, the plant must still be above field level, to reduce the risk of foot rot. Irrigation is required during a dry period following planting; each plant should get 10 l water per week.

A cover crop is needed in wet areas to protect the soil against erosion. Pueraria phaseoloides (Roxb.) Benth. (tropical kudzu) is very suitable; it is a fast grower that fixes more nitrogen than is needed for good growth of the trees. One disadvantage is that it climbs; every month vines must be removed from the trees. Centrosema sp. and Flemingia sp. are also useful cover crops but fix less N than kudzu.
The need for fertilizers can be assessed from soil and leaf analysis. The grower must learn to discern deficiencies, of which zinc and magnesium are most likely to occur. Zinc deficiency is recognized by the appearance of yellow areas between veins of narrow young leaves. A spray of zinc sulphate (5 kg with 1.7 kg hydrated lime in 100 l water for low-volume spraying of 200 trees) quickly cures the mottling, but may have to be repeated twice per year to prevent recurrence. Magnesium deficiency causes bronzing: leaves, veins included, turn yellow from the top down, and then fall. In severe cases only a few leaves at branch tips are left. Dolomitic limestone and/or magnesium sulphate cure the disorder, but it takes 1 or 2 years before all symptoms disappear. No nitrogen is needed in the fertilizer where a good cover crop of kudzu is present. Little phosphate, about 50 kg/ha every 3 years, is needed; potassium must be supplemented unless the soil is rich in this element.
If the dry season lasts more than 3 months, irrigation (100 mm every 3 weeks) will be needed.

Sweet oranges mature 6—9 months after bloom. A sweet orange is at its best the moment it is picked; because the fruit contains no starch, there is no after-ripening as is the case with apples, bananas and such. Maturity can be judged by the proportion between TSS (total soluble solids) measured in degrees Brix (B) and the percentage of citric acid (A). In the subtropics a B/A ratio of 8 is generally used as minimum, but in the tropics 10—16 is very likely a better standard. At B/A above 20 the fruit has become too sweet for most consumers.
The fruit must not be pulled straight down, but with an upward rotating movement of the thumb (twisting). One person can pick up to 1500 kg fruit per day. Good ladders, picking bags and other equipment are needed.

Average yield of sweet orange is 40 t/ha per year in Florida. In tropical countries yields are much lower, e.g. 14 t/ha in Trinidad and in plantations in Surinam; in the latter country small farmers averaged only 7 t/ha, and from Java 5 t/ha is reported. Good planning and management is required to bring up the yield level to that of the subtropics.

Sweet orange is not very well represented in germplasm collections in South-East Asia, Thailand and Indonesia holding about 40 cvs each. Large collections are maintained elsewhere, especially in France (Corsica), Brazil, Japan, the United States and South Africa.

In South-East Asia the sweet orange plays a modest role. Production is too low to meet local demand and this can only change if the graft-transmissible diseases are controlled. A possible approach is to grow sweet oranges on a large scale in isolation, e.g. on small islands, under strict quarantine, following eradication of all diseased trees on these islands.

Bitters, W.P., 1968. Valencia orange rootstock trial at South Coast Field Station. California Citrograph 53: 172—174.
Reuther, W., Webber, H.J. & Batchelor, L.D. (Editors), 1967. The citrus industry. Revised edition. Vol. I. History, botany and varieties. University of California, Division of Agricultural Sciences, Berkeley. pp. 441—489.
Samson, J.A., 1986. Tropical fruits. 2nd ed. Longman, London. pp. 73—138.
Satsijati, 1989. Citrus growing conditions in Indonesia. In: Proceedings Asian Citrus Rehabilitation Conference, Malang, 4—14 July 1989 (in press).
Sugiyarto, M., 1989. Review of Citrus cultivars in Indonesia. In: Proceedings Asian Citrus Rehabilitation Conference, Malang, 4—14 July 1989 (in press).

J.A. Samson

Samson, J.A., 1991. Citrus sinensis (L.) Osbeck. In: Verheij, E.W.M. and Coronel, R.E. (Editors): Plant Resources of South-East Asia No 2: Edible fruits and nuts. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea