Brassica juncea (PROSEA)

Plant Resources of South-East Asia
Introduction

List of species

Brassica juncea (L.) Czernjaew

Protologue: Consp. pl. Chark.: 8 (1859).

Family: Cruciferae

Chromosome number: 2n= 36

Synonyms

Sinapis juncea L. (1753),
S. timoriana DC. (1821),
Brassica integrifolia (West) Rupr. (1860).

Vernacular names

Indian mustard, Chinese mustard, vegetable mustard (En)
Moutarde indienne, moutarde de Chine (Fr)
Indonesia: sawi, sesawi
Malaysia: sawi pahit, kai choy
Philippines: mustasa (Tagalog)
Cambodia: khat naa
Laos: kaad khièw
Thailand: phakkat-khieo, phakkat-khieopli (central)
Vietnam: cải canh, cải bẹ xanh.

Origin and geographic distribution

B. juncea crops are grown worldwide, from India to northern Africa, to Central Asia (southern and south-eastern part of the former Soviet Union), to Europe and North America. The exact origin is unknown, but as an amphidiploid it seems logical that it originated in an area where the parental species, B. nigra (L.) Koch and B. rapa L., overlap in their distribution (e.g. Central Asia). It is generally agreed that the primary centre of diversity of B. juncea is Central Asia (North-West India, including the Punjab and Kashmir) with secondary centres in central and western China, Hindustan (East India and Burma) and Asia Minor (through Iran). In B. juncea two types of mustards with varying usage have evolved, i.e. oilseed types and vegetable types. The oilseed types (oilseed mustard) are particularly important in India, Bangladesh and China. The vegetable types comprise forms with edible leaves (leaf mustard), stems (stem mustard) and roots (root mustard).

The vegetable mustards are widely cultivated in Asian countries. The highest degree of variation occurs in China, which is regarded as the primary centre of varietal differentiation. The early Chinese traders might well have carried the crop into South-East Asia, whereas the appearance of B. juncea near European ports suggests a connection with grain imports. It has also been suggested that Indian contract labourers brought it to the West Indies. In South-East Asia it is the leaf mustards which are most common.

Uses

In eastern Asia, leaf mustard is consumed mainly after pickling. It is eaten in great quantity in China and Korea and many cultivars are available. In South-East Asia, it is used as fresh green, cooked or pickled, and is well-liked for its special flavour and pungent taste.

Production and international trade

Leaf mustard is not a significant product of commerce in international trade, except when the foreign market is close to the production areas. This is partly due to its highly perishable nature. Nearly 100% of the domestic production in many countries is consumed locally. In Indonesia leaf mustard is a minor vegetable and not separately accounted for in production statistics. Although no total production was reported, Malaysia had a total of 1250 ha under leaf mustard in 1985. Malaysia exported 2000 t to neighbouring Singapore in 1983, comprising 99% of the total leaf mustard imports of the latter. In the Philippines, 27 230 t were produced from an area of 2300 ha in 1986. In Thailand 43 000 t were produced from 4400 ha in 1988. The heading type is exported in cans from Thailand.

Properties

Per 100 g edible portion fresh leaf and stem contain approximately: water 92 g, protein 2.4 g, fat 0.4 g, carbohydrates 4 g, Ca 160 mg, Fe 2.7 mg, vitamin A 1.8 mg and vitamin C 73 mg. Cooking is known to decrease vitamin C; drying and pickling are known to decrease both vitamins A and C. The pungency is caused by the volatile mustard oil, present in all parts. The 1000-seed weight is about 2 g.

Description

Erect annual to biennial herb, 30-160 cm tall, normally unbranched, sometimes with long ascending branches in upper part, subglabrous, subglaucous.
Taproot sometimes enlarged (root mustard).
Leaves very variable in shape and size, pinnate or entire, petioled, pale to dark green, smooth or pubescent, heading or non-heading.
Inflorescence a corymbiform raceme, rather loose with numerous flowers, up to 60 cm long; flowers perfect; pedicel ascending, 5-12 mm long; sepals 4, oblong, 4-6 mm long, green; petals 4, clawed, blade obovate, 6-10 mm long, bright yellow; stamens 6, tetradynamous; stigma globose.
Fruit a silique (more than 3 times as long as broad), linear, somtimes inflated and often torulose, 25-75 mm × 2-3.5 mm, attenuate into a conical beak, dehiscent, containing 10-20 seeds.
Seed globose, 1-1.5 mm in diameter, finely reticulated, brown to grey-black.

Growth and development

Seeds of vegetable mustard exhibit slight dormancy, but when properly dried they may germinate soon. However, it is generally advisable to wait for at least two weeks after drying before sowing them. Germination requires about 3-5 days given optimum soil moisture and temperature (about 20-25 °C). In the warm tropical lowlands, leaf mustard grows fairly rapidly to develop succulent leaves. The growth and development period prior to harvest can be as short as three weeks from transplanting (the plant then has about 6-8 fully expanded leaves), but can be longer if larger plants are preferred in the market. Bolting and flowering generally do not require very low temperatures and can easily be induced by long days in the temperate zones. Some types are neutral to both daylength and temperature. Bolting is generally indicated by the elongation of the main stem as the flower buds initiate and grow. Axillary flowering branches develop in a similar manner. The bright yellow flowers attract bees (often honeybees) to effect natural cross pollination. After fertilization, the thin, slender siliques develop rapidly and reach full length some three weeks or so later and are ready for harvest in another two-week period.

Other botanical information

B. juncea evolved as an amphidiploid species (2n = 36), derived from the natural hybrid between black mustard (B. nigra, 2n = 16) and neep (B. rapa, 2n = 20).

It is a very polymorphic species and it has been classified and described under various specific and varietal names, which has resulted in a very confused taxonomy.

The variability of B. juncea can best be classified at cultivar level. "Indian mustard" should be maintained as the common name for the species B. juncea. Based on the morphotypes distinguished by Shi-ru Chen for China, the following cultivar groups are tentatively proposed to classify the vegetable mustards (the oilseed types are grouped in a separate cv. group Oilseed Mustard, and will be dealt with in the Prosea volume on "Vegetable oils and fats"): * (See Brassica (PROSEA Oils and fats).

cv. group Broad-leaf Mustard: large plants with broad and large, green or purple leaves; leaf margins entire, undulate or dentate; few leaves are slightly lobed; common in South-East Asia.
cv. group Dissected-leaf Mustard: plants having leaves which are variously dissected.
cv. group Head-leaf Mustard: plants with large leaves with broad petioles and midribs; the youngest leaves fold inward, the next overlapping each other, together forming a globose head; common in South-East Asia.
cv. group Strumiferous-leaf Mustard: plants with large leaves and well-developed petioles with tumour-like protuberances at the top.
cv. group Tillering Mustard: plants with many vigorous lateral buds on a shortened stem which grow into "tillers" before flowering; cultivars often referred to as pot-herb mustards.
cv. group Stem Mustard: biennial plants with swollen stems which are rod-shaped or variously enlarged with tumour-like protuberances just below the petioles; leaves large, green or purple, smooth or rugose, entire or dissected. Zacai is a processed product of this group, crisp and tender and very popular because of its flavour.
cv. group Shoot Mustard: erect annual or biennial plants; leaves oblong or ovate, entire to slightly dissected; shoots are swollen.
cv. group Root Mustard: plants with inflated fleshy cylindrical or conical taproots; leaves green or purple, entire or dissected.

Ecology

The leaf mustards have the best tolerance to high temperatures and humidity among the allied species, providing a good supply of leafy greens when the cool season cabbages could not. Its ecological complementarity with the cool season cabbages enabled mustard to develop as an important vegetable because it did not have to compete with the high-yielding crucifers such as Chinese cabbage. It has also basically determined the distribution of the two types. In the tropics, Chinese cabbage is commonly grown in the cooler highland areas whereas the leaf mustards are widely grown in the lowlands. The leaf mustards grow best in fertile, well-drained loamy soils that are relatively rich in organic matter. As vegetables, the leaf mustards have a wide variation in flowering behaviour. Seeds of the most commonly grown cultivars can be produced easily in the tropics, even under lowland conditions of the tropical fringes. Ordinarily, however, good seed development requires moderately cool and dry conditions which are often obtained only at medium to high elevations in the tropics.

Propagation and planting

The leaf mustards are propagated by seed and can be direct-seeded or transplanted. Direct seeding is labour-intensive because well-prepared field beds are necessary and close attention to thinning and weeding is required. Seedlings for transplanting are often nursed in special nurseries, consisting of well-prepared raised beds about 1 m wide (no large clods of soil and relatively free of weeds and disinfected with chemicals if necessary to kill harmful diseases and insects) or in specially prepared seed boxes.

Seeds are broadcast or sown in shallow furrows, covered lightly with soil or with a finely sieved soil-sand-compost mixture and then adequately watered. Seeds may be previously dusted with thiram to fend off attack by harmful diseases such as damping-off. In the tropics, the nursery is often covered with rice straw, banana leaves or palm fronds to prevent drying out through the intense sunlight. Watering twice to three times a day is necessary to keep the soil moist and the young plants growing vigorously. Sometimes seedlings are watered once a week with a 0.1-0.3% urea or ammonium sulphate solution, to enhance plant vigour. Seedlings are hardened by lightly withholding water about one week before transplanting. Three-week-old seedlings with about 3-4 true leaves are ready for transplanting. They must be planted out in such a way that the first true leaves are at about ground level when the hole is filled with soil. Transplanted seedlings do better if watered quickly after they have been planted.

Husbandry

Field beds should be well-prepared; they are often raised and about 1 m wide with a furrow space of 20-25 cm. Basal fertilizer plus compost are often incorporated into the soil during the preparation of the field beds. Leaf mustards respond well to compost and fertilizer amendments. Often, 10 t/ha of compost (decomposed plant material or manure) combined with 90-100 kg/ha of N and 90 kg/ha each of P₂O₅ and K₂O are more than adequate to sustain a good crop. Normally, split application of N is practised, half of the total applied as basal fertilizer and the other half often side-dressed two weeks or so later.

Between-plant spacing varies depending upon how large the plants are to be allowed to grow for consumption, but often this ranges from 10-20 cm. Spacing between the rows could be 30-40 cm but may be less with more rows planted per bed.

The leaf mustards grow relatively rapidly and obtain maximum growth and tenderness only if supplied with adequate moisture. The application of 25 mm of water every fourth day appears to be a suitable practice. Weeds must be hoed during the early growth stages until the plants shade the unoccupied spaces. The quick growth and close spacing of leaf mustards normally take care of the weed problem afterwards.

Diseases and pests

Important diseases are soft rot (Erwinia carotovora), downy mildew (Peronospora parasitica), turnip mosaic virus (TuMV), clubroot (Plasmodiophora brassicae) and Alternaria leaf-spot (A. brassicae or A. brassicicola). Soft rot is most damaging during the hot and humid season. No effective control measures have been developed; however, early cultivars or shortened growing schemes are known to enable crucifers to escape the disease. A number of fungicides (e.g. dithane, maneb, zineb, etc.) effectively control downy mildew and Alternaria leaf-spots. TuMV which is serious in the mustards during the dry season can be reduced by controlling aphids which act as the vector. Liming is known to minimize incidence of clubroot. Field sanitation to reduce the spread of clubroot to clean fields should be rigorously observed.

Foremost among the insect pests is diamond-back moth (Plutella xylostella), a globally important pest of crucifers. It is most prevalent during the cool dry period. A number of pesticides are used by farmers but invariably the insect develops resistance to chemicals quickly and pesticides generally degrade the environment. The efficacy of newly introduced pesticides often does not go beyond 2-3 years. Integrated pest management (IPM) using biological parasites (e.g. Diadegma eucerophaga and Apanteles plutellae) combined with selective microbial insecticides such as Bacillus thuringiensis may provide a satisfactory control. More importantly, IPM is safer for the consumer and more sustainable, but IPM technology still is seldom used by farmers in South-East Asia. Other pests such as webworm (Hellula undalis) and leaf webber (Crocidolomia binotalis), aphids (especially during the dry period) and striped flea beetle (Phyllotreta striolata) occasionally pose appreciable production problems.

Harvesting

In Asia, the entire young plants (about 20-30 cm tall with about 6-8 fully expanded leaves) are uprooted or cut at ground level with a knife, or single leaves may be snipped off when about 20 cm or so long. About five once-a-week harvests, starting about three weeks or so after transplanting, could complete the harvesting of the transplanted crop. For a direct-seeded crop, twice-a-week harvests for 5-7 weeks are normally necessary. Harvesting during the hottest part of the day should be avoided because the leaves or uprooted plants lose water and wilt very quickly.

Yield

The yield of leaf mustards has been estimated at 9-40 g per m² per day of growth. In South-East Asia, reported average yield was 3-10 t/ha in 1984-85. In Taiwan, average yield was 21 t/ha in 1989. Average yield was 11.6 t/ha in the Philippines in 1986 and 9.8 t/ha in Thailand in 1988.

Handling after harvest

Harvested plants should be covered quickly with wet materials such as burlap bags or other suitable materials or put in the shade quickly to minimize water loss. The harvested plants are then washed well and cleaned of old, decaying, injured or unsightly leaves to prepare them for the market. The harvested plants or leaves could be packed in suitable containers, often 10-kg type bamboo baskets in the Asian tropics but other materials such as plastic boxes or paper cartons with holes to allow air circulation are used if locally available and inexpensive.

Genetic resources

China is considered as the centre of varietal differentiation of the leaf mustards and is one of the richest sources of germplasm. Many institutional collections of B. juncea (oilseed and leaf type) exist throughout the world. Major collections are available in Australia, Canada, China (including Taiwan), Germany, India (especially landraces of the oilseed type), Japan, and the United States.

Breeding

Breeding of mustard has emphasized its improvement as an oilseed crop. No major breeding programme to improve the leaf mustard is known. Whatever improvements have been attained in commonly grown cultivars must have arisen from the efforts of farmers and seed producers to select only the superior plants for seed production. However, very little genetic improvement is expected from these efforts. Very little effort is expended by national programmes in selecting the best adapted cultivars, although a few trials of introduced cultivars have been reported occasionally.

Prospects

The potential to genetically improve the leaf mustards is great. Large collections, including those of the oilseed type, are available from various institutes. These resources will probably provide an adequate genetic reservoir for resistance to the major diseases and other desirable characters. Leaf mustards should thus be amenable to genetic improvement. Breeding could begin with more deliberate cultivar introduction and testing. Simple population breeding techniques such as mass selection, especially among genetically variable and highly adapted landraces, could lead to rapid improvement. Besides breeding, the control of pests, particularly insects such as diamond-back moth, through integrated pest management is a high priority to the national programmes of South-East Asia. The prospects for developing IPM technology to control insect pests in the hot and humid lowland areas are good.

Literature

Asandhi, A.A. & Sastrosiswojo, S., 1988. Research on vegetables in Indonesia. In: AVRDC, 1988a. Vegetable research in South-East Asia. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. pp. 95-102.
Bantoc Jr, G.B., 1967. Chinese cabbage and the mustards. In: Knott, J.E. & Deanon Jr, J.R. (Editors): Vegetable production in South-East Asia. University of the Philippines Press, Los Baños, the Philippines. pp. 276-284.
Herklots, G.A.C., 1972. Vegetables in South-East Asia. George Allen & Unwin, London, United Kingdom. pp. 211-224.
Nishi, S., 1980. Differentiation of Brassica crops in Asia and the breeding of "Hakuran", a newly synthesized leafy vegetable. In: Tsunoda, S., Hinata, K. & Gomez-Campo, C. (Editors): Brassica crops and wild allies: biology and breeding. Japan Scientific Societies Press, Tokyo, Japan. pp. 133-150.
Saharan, H.A. 1988. Research on vegetables in Malaysia. In: AVRDC, 1988a. Vegetable research in South-East Asia. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. pp. 65-72.
Shi-ru Chen, 1982. The origin and differentiation of mustard varieties in China. Cruciferae Newsletter No 7: 7-10.
Too, K.H. Christopher, 1970. Identification of some common Brassica species by their vegetative characters. The Malayan Agriculturist 9: 53-70.
Valmayor, R.V. & Tiamzon, M.F., 1988. Vegetable production and research policy in the Philippines. In: AVRDC, 1988a. Vegetable research in South-East Asia. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. pp. 17-30.

Authors

R.T. Opeña

See also the genus page
See Brassica (PROSEA Oils and fats) for the oil use.