Citrus medica (PROSEA)

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Plant Resources of South-East Asia
Introduction
List of species


Citrus medica L.


Protologue: Sp. Pl. 2: 782 (1753).
Family: Rutaceae
Chromosome number: 2n= 18

Synonyms

  • Citrus aurantium L. var. medica Wight & Arnott (1834),
  • Citrus crassa Hasskarl (1844).

Vernacular names

  • Citron (En)
  • Cédratier (Fr)
  • Indonesia: jeruk sukade, sitrun
  • Malaysia: limau susu
  • Papua New Guinea: muli (Pidgin)
  • Philippines: bulid (Tagalog), sidris (Bisaya), sidras (Ilokano)
  • Burma: shouk-ta-kwah
  • Laos: vëëx
  • Thailand: manao-khwai (Yala-Pattani), somma-ngua (central), mawo-yao (Chiang Mai)
  • Vietnam: thanh yên.

Origin and geographic distribution

Citron probably originated in the sub-Himalayan region of north-eastern India and upper Burma. It spread westward to Persia, becoming the first citrus fruit brought under cultivation in the western world, and eastward into China. It has also been taken to most tropical countries, where it is of little importance. Commercial planting of citron is limited to certain islands in the Mediterranean region (belonging to Italy, Greece, France) and in the mountainous coffee regions of Puerto Rico; it is sparingly cultivated elsewhere.

Uses

The most important use of citron today is as a source of candied peel used in confections and cakes. Citron fruit has been used since Roman times as a perfume and moth repellent, and to flavour foods. The fresh shoots, leaves, flowers, fruits and seeds of citron have all entered into a number of traditional medicinal preparations for the treatment of asthma, arthritis, headache, stomachache, intestinal parasites and certain psychological disturbances (insanity, possession by evil spirits). Decoctions of the roots are reportedly used to treat respiratory problems and backache in China.

Properties

Per 100 g edible portion, citron fruits contain approximately: water 87 g, protein 0.08 g, fat 0.04 g, fibre 1.1 g, ash 0.4 g and ascorbic acid 368 mg.

Botany

  • A straggly shrub or small tree, up to 3 m tall, with light grey bark and relatively soft wood; twigs angular and purplish when young, turning terete, glabrous, with single axillary spines.
  • Leaves elliptic-ovate to ovate-lanceolate, 5-20 cm × 3-9 cm, cuneate or rounded at base, margins serrate, apex bluntly pointed or rounded; petiole short, wingless or nearly so, not clearly articulated with the blade.
  • Flower buds large, pinkish; flowers perfect or staminate, in axillary few-flowered racemes, 3-4 cm in diameter; petals 5, pinkish externally; stamens 30-40(-60); ovary 10-13-locular, style thick.
  • Fruit an ovoid to oblongoid berry, 10-20 cm long, slightly to considerably rough-tuberculate; peel very thick, yellow, fragrant; segments small, filled with pale green pulp-vesicles; juice acid to mildly acid.
  • Seeds numerous, ovoid, about 1 cm × 0.5 cm, acute, monoembryonic.

Citron flowers throughout the year, in the subtropics most abundantly in spring. The tree has a short life cycle compared with other citrus species. It starts producing when 3 years old, reaches a maximum in 15 years and dies in about 25 years. The fruit is dark green when young and takes about 3 months to turn yellow.

Citron cultivars are divided into two groups:

  • Acid cultivars, with pinkish floral buds and shoots, acid pulp and a dark inner seed coat and chalazal spot. Some well-known cultivars of this group are: "Diamante" - the leading cultivar in Italy, and "Etrog" - botanically also distinguished as Citrus medica L. var. ethrog Engler. It is the main cultivar in Israel, where it is used in the Jewish Feast of Tabernacles.
  • Non-acid cultivars, without pinkish floral buds and shoots, with non-acid pulp, a colourless inner seed coat and a pale-yellow chalazal spot. "Corsican" is a well-known cultivar, leading in Corsica (France) and in California (United States).

In China, Japan, Indo-China and India, Citrus medica L. var. sarcodactylis (Noot.) Swingle is widely cultivated. It is the fingered citron or Buddha's Hand Citron (Indonesia: jeruk tangan; Malaysia: limau jari; Thailand: som-mu; Vietnam: phât thu), whose fruit is split into a number of finger-like sections, without or with very scanty pulp. It is highly esteemed for its fragrance and beauty and used to perfume rooms and clothing. Grown as a dwarf plant it is also a valued ornamental throughout the Far East.

Numerous natural hybrids of citron are found in areas where it is indigenous. There are also a number of ill-defined, citron-like fruits of ancient and unknown origin which remain horticultural curiosities.

Ecology

In the tropics citron grows very well at elevations below 1300 m. The best citron locations are those without extreme temperatures. Citron is very sensitive to frost and to intense heat and drought; it is the tenderest of all Citrus species. The soil should be moist, well-drained, deep and fertile.

Agronomy

Citron is usually propagated by leafy cuttings taken from 2-4 year-old branches. Citron can be budded onto rough lemon, grapefruit, sour and sweet orange, but the fruits remain smaller than those produced from cuttings and citron tends to overgrow the rootstock.

Water sprouts should be eliminated. Low hanging branches are pruned to prevent the fruit touching the soil. In Italy branches are staked and spines are trimmed off to avoid damage to the fruit.

Branch knot, caused by the fungus Sphaeropsis tumefaciens, is a serious disease in Puerto Rico. Most citrus pests also attack citron; mites and scales are often troublesome.

Mature trees produce about 40 kg fruit per year, but yields up to 100 kg per year per tree have been obtained. "Etrog" fruits are wrapped in hemp fibre immediately after picking to preserve fruit quality. For the production of candied citron, fruit is picked green when it measures 12-15 cm × 7-10 cm. The fruits are sliced in half and allowed to ferment for a month or longer in barrels of salt brine, after which the halves are washed and put back into salt brine containing 4-5% sodium chloride. After partial desalting and boiling to soften the peel, it is candied in a strong sugar solution.

Genetic resources and breeding

Most germplasm collections of Citrus species also include citron. A rich collection of about 40 citron accessions is maintained at the USDA National Clonal Germplasm Repository for Citrus/Dates at Riverside, California, United States.

Prospects

Citron is a delicate plant. In the tropics, where a few fruits ripen throughout the year, the crop is more suited to home gardens than production in orchards. In South-East Asia the fruit plays a cultural rather than commercial role and this is not likely to change.

Literature

  • Hodgson, R.W., 1967. Horticultural varieties of Citrus. In: Reuther, W., Webber, H. & Batchelor, L. (Editors): The citrus industry. Revised edition. Vol. 1. University of California Press, Berkeley. pp. 552-557.
  • Morton, J.F., 1987. Fruits of warm climates. Creative Resource Systems, Winterville, N.C. pp. 179-182.
  • Ochse, J.J., Soule, M.J., Dijkman, M.J. & Wehlburg, C., 1961. Tropical and subtropical agriculture. Vol. 1. Macmillan, New York. pp. 495-496.
  • Swingle, W.T., 1967. The botany of citrus and its wild relatives. In: Reuther, W., Webber, H. & Batchelor, L. (Editors): The citrus industry. Revised edition. Vol. 1. University of California Press, Berkeley. pp. 370-372.

Authors

D.T. Jones