| Root crops |
RADISH, Chinese radish, Japanese radish, Oriental radish.
Raphanus sativus L.
Daikon (Asia, Haw., USA); Figal (Ar.); Figeli (Swah.); Hatsuka-daikon (Japan); LabaÃ±os (Philipp.); Lobak (Mal.); Lu Fu (China); Monla (Burma); Mourai (Trin.); Muli, Mulla, Mullangi, Mullanki, Mullong (Ind.); Radis (Fr.); Rettick (Ger.); Ripani (Cy.).
Rophanus sativus is an annual or biennial herb which exists in several different forms: the main distinction is between a small, short-season type of salad radish which is a cool climate plant, and a large type which has a wide range of temperature adaptation. Four botanical varieties are recognised within the species, R. sativus L., namely radicula, niger, mougri and oleifera, the first two of which are grown for their tuberous roots, while oleifera is grown primarily for the oil in its seeds. Numerous cultivars have been developed within each variety. All varieties intercross freely, and also hybridise with wild Raphanus spp.
The stems may be simple or branched, in the large types reaching as much as I m in height; the basal leaves are long, often pinnately lobed and coarsely toothed, but sometimes are not serrated, while the cauline leaves are simple and linear. The flowers are in long terminal racemes, usually white or lilac with purple veins. The fruit are narrow, indehiscent, 2.5-7.5 cm long and about 1.25 cm in diameter, with a long tapering beak. There are usually 6-12 globose seeds, yellow to chocolate-brown in colour. The tap root (except in var. mougri) is swollen, and varies from almost globular, about 1-2 cm in diameter in the salad types to as much as I m long and 15 cm in diameter, cylindrical or conical in shape, in the oriental types, and weighing up to 15 kg. The flesh is normally white, though in some may be pink to red. In the salad radish the skin is usually red (occasionally white); in the oriental radish it is normally white.
In this digest it is mainly oriental large-rooted types that are discussed.
Origin and distribution
There are several wild Raphanus, spp. particularly between the eastern Mediterranean and the Caspian Sea, and it is thought that R. sativus must have arisen in this region of Europe and Asia minor. Radish of the niger variety was an important food in Egypt probably as early as 2700 BC, and is thought to have spread to China by about 500 BC and to Japan by AD 700. The origin of the radicula variety is much more recent and it was first reported in the 16th century (from Europe). The globular forms of salad radish were developed from this variety in the 19th century. The large-rooted radishes are cultivars of the niger and radicula varieties.
Temperature - while the salad types of var. radicula are at their best in cool climates with maximum temperatures about 15°C, all types will tolerate tropical conditions and many do well under high temperatures, with maxima of 30-33°C and minima of 20-22°C. Even the cool climate types require temperatures of 10-13°C for germination and most cultivars are, in some degree, susceptible to frost damage.
Rainfall - a fairly evenly-distributed rainfall of at least 85-100 cm per year is required; with lower rainfall supplementary irrigation is needed.
Soil - radish is tolerant of a wide range of soils, though heavy clays may lead to mis-shapen roots. As the growing season is short, nutrients must be readily available: a general recommendation is for early application of a 6:10:8 complete (NPK) fertiliser at I 100-1 700 kg/ha. In India 40 t/ha of FYM is recommended before planting, followed by top dressing with chemical fertiliser at planting and subsequent application of foliar sprays during growth. Potash has been shown to improve the quality and storage life of the roots, and high potassium fertilisers are used in the Republic of South Africa where the crop is grown for livestock feeding. Deep ploughing is an essential preparation for large-rooted cultivars.
Altitude - in the tropics radish is grown from sea level to at least 1 800 m. In India it is grown as high as 2 700 m in the Himalayas, while var. oleifera has been found suitable for high mountain areas (2 500-3 000 m) in the Yunan Province of China. In Hawaii the Chinese half-long is adapted to year round production in lowland areas and is grown from April to August at elevations over 600 m, while Japanese long types are grown throughout the year at all elevations.
Day-length - the response to day-length varies with the cultivar: many of the red fleshed types require long days in order to produce flowers and seed.
Material - seed is used for propagation and since, as already noted, radish hybridises easily with wild or cultivated Raphanus spp. or cultivars only carefully selected seed should be used. Pre-treatment with 200 ppm naphthalene acetic acid or 10 ppm gibberellic acid has been reported to increase yields.
Method - the seed of oriental radish is normally sown in drills at a depth of about 2.5 cm: in the Republic of South Africa it is often mixed with about 100 kg/ha of finely-ground rock superphosphate and sown through the fertiliser hoppers of maize planters. The time of planting depends on local conditions: eg in parts of India radish is sown at the start of the rainy season in regions where rainfall is relatively light, but in regions where the monsoon rains are heavy, sowing is delayed until the end of the wet season. Unless the soil is moist, irrigation should be given immediately after planting and, unless there is rain, further irrigation is required at about weekly intervals. Regular weeding is necessary; hand-weeding is commonly practiced but nitrofen and diuron have proved to be effective in weed control without damaging the crop. Salad radishes are often sown by hand in nursery beds, often under glass in temperate climates, and then transplanted at the two-leaf stage.
Field spacing - practice differs widely. In the Republic of South Africa the spacing is frequently 30-37.5 cm in rows 90 cm apart (about 32 000/ha); densities as high as 30x 10 cm (300 000 plants/ha) or even more are reported from India and China. Early-maturing salad radishes may be planted at 2.5 x 25 cm (l 600 000/ha).
Seed rate - radish seeds are small, about 70-100/g: about 7-13 kg/ha are required depending upon the size and spacing used.
Pests and diseases
In most areas radishes are relatively free from serious attacks by pests and diseases, but sometimes aphids, particularly Aphis gossypii and the mustard seed aphid Lipaphis erysimi can be troublesome; the latter has been effectively controlled by dusting with an insecticide or by spraying with nicotine sulphate. In addition, flea beetles, Phyllotreta spp., and the cabbage root fly may damage the crop, which is also susceptible to attack by root-knot nematodes (Meloidogyne spp.). In India, severe losses to the crop are sometimes caused by the mustard sawfly, Athalia lugers, and control is either by hand-picking the larvae or by dusting with a safe insecticide.
Black rot, caused by Aphanomyces raphani, which produces blackening and deformity of the roots and is sometimes associated with boron deficiency, is found wherever white radishes are grown and is reported occasionally to cause serious losses to crops in the Federal Republic of Germany, Canada, Australia and New Zealand. In the Philippines, downy mildew, caused by Peronospora brassicae, sometimes infects the roots, and also a yellowing disease, the causal organism of which is Fusarium oxysporum f. raphani. In addition to some of the above, Indian reports include root periderm brown scorch, Pythium sp., radish mosaic virus, damping off, Rhizoctonia solani, and the seed-borne Alternaria alternata. Other reports include Fusarium spp. and Albugo candida, as well as common diseases of Cruciferae such as crown gall, caused by Agrobacterium tumefaciens, and club root, caused by Plasmodiophora brassicae, the latter in particular from the midwest USA.
Breeding for resistance to pests (eg cabbage root fly), diseases (eg Fusarium and Albugo), and viruses, is underway.
The early-maturing salad types can produce roots of marketable size 20-30 days after planting, and the so-called 'late-maturing' types require only 50-60 days. The large-rooted oriental types, however, require 45-100 days, according to cultivar (eg the Chinese half-long grown in Hawaii, matures in 40-50 days, but under similar conditions the Japanese long type requires 70-80 days).
Harvesting and storage
Early or salad radishes must be harvested as soon as they are mature, otherwise the roots become tough, pithy and unpalatable and the plants bolt; treatment of some 6 week old cultivars with 0.1 per cent aqueous maleic hydrazide is reported to prevent bolting and help the roots retain their flavour and texture. Winter and oriental radishes remain edible for longer periods and harvesting at the correct stage of maturity is not so critical. The small salad types are often pulled by hand, washed, sometimes topped, and tied in bunches of 6-12 for marketing. Hydrocooling to 4°C is recommended in order to extend their market-life which is very short. Bunched radishes can be kept for 1-2 weeks at 0°C and 90-95 per cent RH; topped radishes can usually be held for 3-4 weeks. Storage at low temperatures in a I per cent oxygen atmosphere has been reported to improve the storage life of salad radishes.
The large roots of late-maturing (winter) radishes and oriental types are either lifted by hand or mechanically. They store better than salad radishes and will keep for 2-4 months at 0°C and 90-95 per cent RH. At higher temperatures, storage life is often terminated by sprouting but it has been reported that dipping of appropriately trimmed roots in a suspension of campothecin (a naturally-occurring growth regulator) inhibits sprouting and permits storage at 10-20°C.
Roots - the enlarged tap roots show wide variation in colour and form according to the cultivar. The early-maturing radishes produce small roots, often globose, of approximately 1.25 cm diameter with a bright red skin, or red with a white tip, and (usually) white crisp flesh. The winter and oriental radish roots are often more conical in shape, usually about 25-40 cm in length and can weigh up to 2.3 kg, although certain oriental radishes can reach a length of 75-100 cm and weigh up to 5-15 kg. Most of these have white skin and flesh.
Yields of oriental radish are reported to be 15-20 t/ha in India, 12 t/ha in Hawaii and up to 60 t/ha for radishes grown for fodder in the Republic of South Africa. Yields for early-maturing (salad) radishes are lower, about 7.5 t/ha being quoted. Experimental work in Hawaii has yielded up to 50 t/ha of oriental radish suitable for human food. Yield is closely related to spacing; eg in experiments reported from China, at I million plants/ha, yields of 36-46 t/ha were obtained, but the roots were very small: at 500 000 plants/ha, yields were 35.5 t/ha, but the roots were still small. At 250 000 plants/ha, roots of more satisfactory size were obtained and yield was 27.7 t, while at 110 000 plants/ha, the yield was 18.7 t/ha.
The small early-maturing radishes are usually eaten raw in salads. The large winter radishes and oriental radishes are an important article of diet in many tropical and subtropical (and some temperate) countries, particularly in eastern Asia; the characteristic, somewhat pungent flavour is especially liked in Japan, the Philippines and Hawaii. They may be eaten raw in salads but are more often cooked and eaten as a vegetable (like, for example, turnip), or are made into 'takuwan' or 'sanbaizuke' (see Processing).
Radishes are grown in several countries for livestock feeding.
Secondary and waste products
The leaves and seed pods of some cultivars are boiled and eaten as a vegetable. It has also been suggested that the leaves could be utilised as a commercial source of leaf protein. In come countries the roots are used medicinally for the treatment of liver and gall-bladder complaints. The seeds contain a non-drying oil which is commercially extracted and is suitable for soapmaking and edible purposes, and is reported to be used in the manufacture of crayons in Japan. The seed cake remaining after oil extraction can be used as a fertiliser or, after the removal of isothiocyanates, as a feedingstuff.
Roots - average composition of the edible portion has been reported as: energy 86.7 kJ/100 g; water 93.5 per cent; protein 1.05 per cent; fat 0.15 per cent; carbohydrate 3.85 per cent; fibre 0.7 per cent; ash 0.75 per cent; boron 2.08 mg/100 g; calcium 33 mg/100 g; chlorine 19 mg/100g; copper 0.13 mg/100 g; iodine 8 mg/100 g; iron 0.8 mg/100 g; magnesium 15 mg/100 g; manganese 0.05 mg/100 g; phosphorus 29 mg/100 g; potassium 322 mg/100 g; sodium 18 mg/100 g; carotene 0.006 mg/100 g; thiamine 0.03 mg/100 g; riboflavin 0.03 mg/100 g; niacin 0.4 mg/100 g; pantothenic acid 0.8 mg/100 g; ascorbic acid 0.029 mg/100 g; glucose 640 mg/100 g; fructose 390 mg/100 g; campesterol 5 mg/100 g; ÃŸ-sistosterol 6 mg/100 g.
The characteristic pungent flavour of the roots is due to the presence of isothiocyanates, while the coloured cultivars contain anthocyanins which are reported to occur as naturally acylated, either with ferulic or p-coumaric acids. Catechol has been reported in the red cultivars and flavanols have been detected in minute quantities. A growth inhibitor, raphanusanol, has been isolated from radish seedlings.
Leaves - the leaves of oriental radishes are also nutritious; an analysis gives their approximate percentage composition as: water 87.4 per cent; protein 2.2 per cent; fat 0.4 per cent; carbohydrate 6.1 per cent; fibre 1.5 per cent; ash 2.4 per cent; calcium 400 mg/100 g; phosphorus 300 mg/100 g; ascorbic acid 17 mg/100 g; vitamin A 18 660 IU/100 g.
Seeds - radish seeds contain 30-50 per cent of oil with the following characteristics: SG (30°C) 0.9773; ND (30°C) 1.4704; acid val. 0.9; acet. val. 2.8; sap. val. 178.9; iod. val. 103.1. The fatty acid composition is: palmitic 1.3 per cent; stearic 1.4 per cent; arachidic 3 per cent; behenic 3.4 per cent; erucic 22 per cent; oleic 60.8 per cent; linoleic 4.5 per cent; linolenic 3.6 per cent.
After washing, trimming and salting, two types of pickled product are frequently prepared from the roots of oriental radish.
Takuwan - the salted roots are seasoned with sugar, vinegar, some colouring agent, and soaked for some time for flavouring. The soaked material is then bottled for distribution.
Sanbaizuke - is made by treating a mixture of dried radish roots and certain vegetables (eg lotus root, egg plant, cucumber, etc) with soy sauce, vinegar, pepper and other seasonings. The mixture is salted, compressed and packed for distribution.
Production and trade
Recent statistical information is scant. Published figures from Hawaii are:
A portion of the Hawaiian crop is pickled, averaging about 425 t for the years 1978-82.
There is reason to believe that the production of oriental radish is on the increase. Hawaiian figures show a ten-fold increase since the 1969-71 period, and the very large volume of recent literature from India, Japan, China, the Soviet Union and Central Europe dealing with this crop both as human food and animal feed, suggests that production must be rapidly growing in volume. The main supply to Europe is from Italy, Kenya and the Netherlands. The development of mechanised systems is likely to make the crop more attractive for large-scale operations.
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