Daucus carota (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Daucus carota L.

Protologue: Sp. pl.: 242 (1753).

Family: Umbelliferae

Chromosome number: 2n= 18

Synonyms

• Daucus gingidium L. (1753).

Vernacular names

• Carrot (En)
• Carotte (Fr)
• Indonesia: wortel, bortol (Sundanese)
• Malaysia: boktel, lobak merah
• Papua New Guinea: karot, yalow pela sayor
• Philippines: karot
• Cambodia: karôt
• Thailand: khaerot
• Vietnam: cà rốt.

Origin and geographic distribution

It is generally assumed that the purple (anthocyanin-containing) carrot originated in Afghanistan in the region where the Himalayan and Hindu Kush mountains are confluent, and that it was domesticated also in Afghanistan and adjacent regions of Russia, Iran, India, Pakistan and Anatolia. Purple carrot, together with a yellow variant, spread to the Mediterranean area and Western Europe in the 11-14th Centuries, and to China, India and Japan in the 14-17th Centuries.

The orange (carotene-containing) carrot probably arose in Europe or in the western Mediterranean region through gradual selection within yellow carrot populations. The Dutch landraces Long Orange and the finer Horn types were the basis for the orange carrot cultivars grown at present all over the world. In Asia they have now largely replaced the purple and yellow types because of superior taste and nutritional value.

Uses

Carrot is the swollen taproot of D. carota and is an important market vegetable, even in tropical areas. The roots are consumed raw or cooked, alone or in combination with other vegetables (e.g. peas), as an ingredient of soups, sauces and in dietary compositions. Young leaves are sometimes eaten raw or used as fodder.

Production and international trade

World area under carrot cultivation is at least 600 000 ha producing yearly 13 million t: 250 000 ha in Asia, 110 000 in the former Soviet Union, 60 000 ha in Eastern Europe, 70 000 ha in the European Community, 70 000 ha in the Americas, and 30 000 ha in Africa. In Indonesia in 1988 there were 10 500 ha of carrot, all in highland regions. Carrot is an important vegetable in China (120 000 ha), India (60 000 ha), Japan (25 000 ha), South Korea (5000 ha) and Taiwan (3000 ha). All carrot production in Asia is for local markets.

Properties

Orange carrot has a good nutritional value, particularly a high carotene content. Per 100 g, fresh roots contain: water 87-91 g, protein 1 g, carbohydrates (sugar) 6-9 g, fibre 1 g, ß-carotene 6-20 mg, vitamin C 5-10 mg, Ca 40 mg, Fe 1 mg. The energy value is 120-170 kJ/100 g.

The seed in particular contains an essential oil, giving it a characteristic odour. Terpenoids and other volatile compounds influence the flavour of raw carrots. An astringent taste of carrots is caused by a high terpolene content in combination with a low percentage of sugars. The bitter taste of carrots after long storage is caused by the conversion of phenols into iso-coumarin under the influence of exogenous ethylene.

The 1000-seed weight is 0.6-2.2 g.

Description

• Annual or biennial erect herb, 20-50 cm tall at the mature vegetative stage and 120-150 cm tall when flowering. Taproot fleshy, straight, conical to cylindrical, 5-50 cm long and 2-5 cm in diameter at top, orange (most common), reddish-violet, yellow or white; the core (xylem) of mature roots is usually somewhat lighter in colour than the phloem, and the top of the root is often green.
• Leaves 8-12, growing in a rosette, glabrous, green, with long petiole often sheathed at its base; leaf-blade 2-3-pinnate, the segments divided into often linear ultimate lobes.
• Flowering stalks few to several, branched, each branch ending in a compound umbel (inflorescence); each umbel comprising 50 or more umbellets, each of which has up to ca. 50 flowers; involucral bracts more or less pinnatipartite; primary rays 2-25 cm, secondary rays 1-6 cm, pedicels 0.5-1.5 cm long; flowers mainly bisexual in primary umbels, in umbels of higher order an increasing number of male flowers may occur in addition to bisexual flowers; a few purple-red sterile flowers may be present in the central umbellets, especially in wild plants; flower small, 2 mm in diameter, epigynous, white, 5-merous but with 2 carpels and 2 styles.
• Fruit an oblong-ovoid schizocarp, 2-4 mm long, at maturity splitting into 2 mericarps, primary ridges ciliate, secondary ridges with hooked spines.
• Seed (inside the mericarp) with a long embryo embedded in endosperm.
• Seedling with long, thin taproot, cordate cotyledons and pinnate first true leaves.

Growth and development

Carrot seed will remain viable (70-80% germination) for 6-7 years when stored dry (moisture content 9%) at temperatures below 18 °C. Germination is epigeal with first appearance of seedlings 9-12 days after sowing. The first four true leaves are formed at 4 to 5-day intervals, starting 3-4 weeks after sowing, but then the interval increases gradually to 15-18 days for subsequent leaves. A thin taproot grows down vertically to 20-25 cm, and 30-40 days after germination it starts swelling and gradually turning orange (in carotene carrots) from the hypocotyl stem downwards. About 80% of all carbohydrates produced in the plant are diverted to the root during this stage of development.

The roots are mature 70-120 days after sowing according to the type of cultivar and growing conditions. The generative phase is induced by low temperatures. Carrot plants become sensitive to vernalization after the formation of at least eight leaves. The bolting-resistant cultivars of higher latitudes require 5-12 weeks at 2-6 °C to induce bolting. Local cultivars grown in the tropics show bolting when the night temperatures drop below ca. 16 °C. The generative phase is accelerated by long days after devernalization (20 °C). First a new rosette of leaves is formed followed by elongation of the flowering stalk and first flowering 3 months later. Flowers are arranged in spirals and development is centripetal: the first mature flowers are on the outer edges of the outer umbellets. Flowering may last for one month, starting with the primary umbel. Initially the umbels are flat and concave. At anthesis the umbellets turn downwards progressively from the outside towards the centre, so that by the time the central flowers are mature, the umbels are more or less convex to conical. After pollination the umbellets turn upwards again.

Carrot is predominantly outbreeding due to protandry. Insects such as bees and flies, attracted by abundant nectar, effect cross-pollination. The stigma becomes receptive 2-3 days after pollen dehiscence. Petals drop soon after fertilization and the seeds (mericarps) are mature 40-50 days later.

Other botanical information

D. carota is a complex, very variable species comprising wild and cultivated carrots. The variability has resulted in a confused taxonomy. At present the complex is subdivided into 13 subspecies, 12 for wild taxa and 1 for cultivated taxa (ssp. sativus (Hoffm.) Arc.). However, for cultivated taxa it is better to classify directly at cultivar level below the species level.

There are two main groups of cultivated carrot:

• the eastern (anthocyanin) carrot: roots branched, yellow, reddish-purple to purple-black, rarely yellowish-orange; leaves slightly dissected, greyish-green, pubescent; flowering in the first year;

• the western (carotene) carrot: roots unbranched, yellow, orange or red, occasionally white; leaves strongly dissected, bright green, sparsely hairy; normally biennial, but often annual in tropical regions.

At present the western carrot is by far the most important, although the eastern carrot is still cultivated in some Asian countries. Three main groups of western (carotene) carrot cultivars arose by selection in the 19th and early 20th Centuries in western Europe and the United States from the Dutch landraces "Long Orange" and "Horn":

• Early Short: 3-8 cm long globular-shaped roots, fine foliage; cultivars include "Grelot", "French Forcing", "Parisian Market";

• Early Half-Long: 10-20 cm long cylindrical to conical roots, fine to medium foliage; cultivars include "Amsterdam Forcing", "Nantes", "Vertou", "Touchon", "Sitan";

• Late Half-Long: 12-25 cm long conical and shouldered stump or pointed roots, medium to large foliage, productive; cultivars include "Chantenay" (short), "Royal Chantenay" (long), "Danver", "Autumn King", "Berlicum", "Imperator" (cross between "Chantenay" and "Nantes", United States, 1928); "Kuroda" is a long Chantenay type bred in Japan around 1950, combining good internal colour with heat tolerance; local selections in Indonesia are also of the late half-long type, such as "Cipanas", with cylindrical, smooth and dark orange carrots and strong foliage.

The cultivated carrot taxa cross readily with the wild carrot taxon (D. carota ssp. carota), which is very common in Europe and South-West Asia. It has to be rigorously removed from seed production fields to prevent white-rooted and prematurely bolting plants in a carrot field (white roots and annual habit are dominant over orange roots and biennial habit). There are several other wild carrot taxa and Daucus species occurring in the Mediterranean area and South-West Asia, most of which are crossable with the cultivated carrot.

Ecology

In their adaptation to the northern latitudes of Europe, carrots became biennial and tolerant to long days (non-bolting) during the vegetative phase. They require subsequent vernalization at low temperatures to induce flowering. Carrots adapted to tropical and subtropical latitudes respond to long days by bolting even before the roots have properly thickened.

Carrots are mostly cultivated as a cool season crop. High soil temperatures, in excess of 25°C, induce slow growth rates, fibrous roots and low carotene content. For economic yields, carrots should be grown in tropical regions at altitudes above 700 m. Early-maturing carrot cultivars may grow in the lowlands, but yields will be low and roots will have a poor colour. Optimum air temperatures are 16-24°C. Soils should be well-drained, fertile and of a sandy texture. Heavy clay soils may induce malformed and twisted roots and harvesting will be difficult. Optimum pH is 6.0-6.5. A regular supply of water is essential to obtain smooth and even roots. Flowering and seed set are successful only in climates with mean day temperatures below 20°C.

Propagation and planting

Seed multiplication at high latitudes with cold winters is based on stored and vernalized mature or young roots (stecklings) replanted in the field in spring (the root-seed method). In areas with mild winters and early snow cover, seeds are sown in late summer and the plants are left to overwinter in the field. These will bolt in spring and the seed-seed cycle is completed in 12-13 months. Carrot cultivars adapted to tropical regions have low vernalization requirements and are propagated in highland regions above 1200 m. Highland carrot growers often maintain their cultivars by selecting the best mature roots and replanting them at one end of the field. Bolting and seed set soon follow.

Seeds are sown, often mixed with sand, 1-2 cm deep in drills 10-15 cm apart in finely prepared soils previously cultivated to a depth of at least 30 cm. Seedlings are thinned to 5-8 cm in the rows. Seed requirements (200 plants/m² and 70% germination) for the dominant half-long carrot cultivars used in Asia, are 4-5 kg/ha. For bigger carrots, the density may be reduced to about 100 plants/m².

Husbandry

Crop rotation is essential to reduce soilborne diseases and pests. Mulching (rice straw) after sowing is recommended to encourage germination. Seedlings may be earthed-up when roots start swelling to keep them cool and prevent green tops. In hot weather, light overhead shade is beneficial. Irrigation during dry spells is necessary to prevent irregular root development.

Nutrient requirements of carrots are particularly high for K (200-300 kg/ha), low to medium for N (100-150 kg/ha), normal for P, Ca, Mg and other elements. Carrots are sensitive to high Cl concentrations and more susceptible to diseases at very high soil pH. Liming or the use of Ca-containing fertilizers is recommended when pH is below 5.5. Well-decomposed organic manures are beneficial when applied moderately (10-20 t/ha). Fresh organic matter, e.g. from a leguminous crop, can be detrimental to the carrot crop.

Diseases and pests

The major problems in tropical carrot production are leaf blights/spots (Alternaria dauci and Cercospora carotae) and root-knot nematodes (Meloidogyne hapla). Local Indonesian cultivars (e.g. "Cipanas") have strong foliage with a remarkable field tolerance to Alternaria leaf blight, which often completely destroys the foliage of cultivars introduced from Europe. Crop loss by root-knot nematodes may be kept under control by crop rotation, e.g. with cereals, and by the application of organic manure.

Other diseases are powdery mildew (Erysiphe polygoni and E. heraclei), white rust (Albugo candida), bacterial blight (Xanthomonas carotae), black spot on roots (Alternaria radicina), and purple root rot (Helicobasidium brebisonii). Various root rots occur before or during storage, often after mechanical damage or as secondary pathogens (Botrytis cinerea, Fusarium spp., Sclerotinia sclerotiorum, Pythium violae and other spp., Erwinia carotovora). Root diseases are more severe in heavy soils with a poor structure. A total of 14 virus diseases have been identified in carrots, the most important being red leaf. Aster yellows is a mycoplasma.

The most noxious pest of carrot in temperate areas is carrot root fly (Psila rosae) to which some degree of resistance has been found in the Nantes-type cultivar "Sitan" and in the wild species D. capillifolius Gilli. This pest has not been observed in South-East Asia. The lygus bug (Lygus hesperus and L. elisus) on seed crops, aphids (e.g. Cavariella aegopidii) as vectors of virus diseases, the leafhopper (Macrosteles fascifrons) as vector of aster yellows, carrot weevil ( Listronatus oregoneuses) and other foliage pests have all been reported in carrot, but probably the only pest which may cause serious crop losses in South-East Asia is army worm (Spodoptera spp.).

Harvesting

Carrot is mostly harvested manually by pulling up the roots at the leaves. This requires strong and healthy foliage. Mechanical harvesting (in Europe, United States) is also based on pulling up by the foliage, or first topping the leaves and then lifting the carrots as in potato harvesting. In Asia carrots are usually ready for harvesting 70-85 days after sowing. Mature roots should be orange-coloured internally down to the blunt tip.

Yield

In Asia yields vary from 8-20 t/ha; higher yields are possible above 800 m altitude. In Europe and the United States 30-120 t/ha can be reached, depending on the type of cultivar and culture. Marketable yield is much influenced by plant density and time of harvest. Root weight and uniformity are closely related to seed size and quality.

Seed yields are 200-500 kg/ha.

Handling after harvest

Carrots bunched with leaves will store up to 3 weeks in a cool place, but can remain in good condition for 100-150 days when topped (foliage removed) and stored at 1-4 °C with 95-100% relative humidity. Carrots should be stored separately from other vegetables to prevent a bitter flavour induced by ethylene. Generally carrots store better when the dry matter content is high, when they are grown on soils with low organic matter content, when they are mature and harvested under moist conditions, and undamaged and free of diseases and pests. Carrots may be graded according to weight A (< 50 g), B (50-200 g), C (200-400 g) and D (> 400 g).

Genetic resources

The genetic basis of modern orange carrot cultivars is rather narrow, considering that they are all derived from a few 18th Century Dutch cultivars. Exploitation of the genetic variation existing in wild Daucus L. germplasm in the Mediterranean and South-West Asian regions started only recently. Small working collections of D. carota and related species are available in Europe (United Kingdom, France, the Netherlands), United States and Japan.

Breeding

Before 1960 breeding methods were based on mass selection in open-pollinated populations, but F₁ hybrids with greater uniformity are now increasingly replacing the older cultivars, particularly in Europe, the United States and Japan. Seed production of F₁ hybrid cultivars is based on cytoplasmic male sterility (cms) of one of the parent inbred lines. Two types of cms are used: (1) the brown anther type, in which the anthers degenerate before anthesis, based on S-cytoplasm and at least two recessive genes with complementary action, and (2) the petaloid type, in which the anthers are replaced by five additional petals, based on S-cytoplasm and at least two dominant genes with complementary action. The development and maintenance of inbred lines are complicated by severe loss of plant vigour after a few generations of inbreeding.

Main breeding objectives are improvements in total yield, growth rate and earliness, uniformity of root size and shape (cylindrical), dark orange external and internal colour (uniform in xylem and phloem), smooth periderm, resistance to cracking and breaking of the root during harvesting and post-harvest handling, flavour, texture, carotene content, strong foliage, non-bolting, resistance to diseases and pests. The most popular cultivars are somewhat conical, as these break less easily during harvesting.

Prospects

Carrot will continue to be an important vegetable worldwide, but adaptation to hot climates will remain limited. Resistance to important diseases and pests is becoming an increasingly important aspect of carrot breeding.

Literature

• Banga, O., 1963. Main types of western carotene carrot and their origin. Tjeenk Willink, Zwolle, the Netherlands. 153 pp.
• Banga, O. & de Bruyn, J.W., 1968. Effect of temperature on the balance between protein synthesis and carotenogenesis in the roots of carrots. Euphytica 17: 168-172.
• Heywood, V.H., 1983. Relationships and evolution in the Daucus carota complex. Israel Journal of Botany 32: 51-65.
• Peterson, C.E. & Simon, P.W., 1986. Carrot breeding. In: Bassett, M.J. (Editor): Breeding vegetable crops. Avi Publishing Company, Westport, Connecticut, United States. pp. 321-356.
• Quagliotti, L., 1967. Effects of different temperatures on stalk development, flowering habit and sex expression in the carrot. Euphytica 16: 83-103.
• Shinohara, S. (Editor), 1984. Vegetable seed production technology of Japan. Shinohara's Authorized Agricultural Consulting Engineer Office, Tokyo, Japan. Vol. 1. 432 pp.
• Schoneveld, J.A. (Editor), 1991. Teelt van peen [The cultivation of carrot]. PAGV series No 36, Proefstation voor de Akkerbouw en de Groenteteelt in de Vollegrond (PAGV), Lelystad, the Netherlands. 151 pp.

Authors

• H.A.M. van der Vossen & E.N. Sambas