Brassica rapa Pak Choi (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Brassica rapa L. cv. group Pak Choi

Protologue: Toxopeus, Oost & Reuling in Cruc. Newsl. 9: 58 (1984) ("Pak-Choi").

Family: Cruciferae

Chromosome number: 2n= 20

Synonyms

• Brassica chinensis L. (1759),
• B. campestris L. ssp. chinensis (L.) Makino (1912),
• B. rapa L. ssp. chinensis (L.) Hanelt (1986).

Vernacular names

(some in common with cv. group Chinese Cabbage).

• Pak choi, Chinese cabbage, celery cabbage (En)
• Chou de Chine, chou de Shangton (Fr)
• Indonesia: paksoi, pecai, sawi putih
• Malaysia: pak choi, sawi putih
• Papua New Guinea: kapis
• Philippines: pechay
• Cambodia: pe-chhaay, spéi khiew
• Laos: kaad chiin
• Thailand: phakkat-khaokwangtung, hong-tae, hong-how
• Vietnam: cải thià, cải ngọt, cải bẹ trắng.

Origin and geographic distribution

Pak choi evolved in China and its cultivation was recorded as far back as the 5th Century AD. It is widely cultivated in southern and central China, and Taiwan. This group is a relatively new introduction in Japan where it is still referred to as "Chinese vegetable". It was introduced to South-East Asia in the Malacca Straits Settlement in the 15th Century. At present, it is widely cultivated in the Philippines and Malaysia, and to a limited extent in Indonesia and Thailand. In recent years it has gained popularity in northern America, Europe and Australia where there are concentrations of east Asiatic emigrants.

Uses

Pak choi is mainly grown for its immature but fully expanded tender leaves, but all above-ground parts are edible. The succulent petioles are often the preferred part. It is used as the main ingredient for soup and stir-fried dishes. In Chinese cuisine, its green petioles and leaves are also used for decorating dishes. It is seldom eaten raw or pickled.

Production and international trade

In the Philippines, pak choi is one of the major leafy vegetables. From 1983-1986, the average annual production was 25 500 t from 3800 ha. Malaysia produced in 1986 50 000 t of leafy Brassica crops from 1250 ha, half of which could be pak choi; 2000 t were exported to Singapore. In Indonesia and Thailand, pak choi is a minor vegetable as it is a relatively new introduction. In China pak choi is one of the most important leafy vegetables and it represents 30-40% of the total vegetable production.

Properties

Trimming loss of harvested pak choi is about 14%. Per 100 g fresh edible portion pak choi contains: water 93 g, protein 1.7 g, fat 0.2 g, carbohydrates 3.1 g, fibre 0.7 g, ash 0.8 g. It is a good source of vitamins and minerals: β-carotene 2.3 mg, vitamin C 53 mg, Ca 102 mg, P 46 mg and Fe 2.6 mg. The energy value is 86 kJ/100 g. The 1000-seed weight is 3-5 g.

Description

• Erect biennial herb, cultivated as an annual, in vegetative state 15-30 cm tall, in generative stage up to 70 cm tall, glabrous, dull green.
• Leaves arranged spirally, not forming a compact head but spreading, 15-30; petioles enlarged, terete or flattened, 1.5-4 cm wide and 0.5-1 cm thick, growing upright forming a subcylindrical bundle, white, greenish-white to green; leaf-blade orbicular to obovate, 7-20 cm × 7-20 cm, entire, tender, smooth or blistering, green to dark green, shiny; stem-leaves auriculate-clasping.
• Inflorescence a raceme with pale yellow flowers about 1 cm long; flowers bisexual, perfect; petals 12 mm × 6 mm.
• Fruit a silique, 2.5-6 cm × 3.5-6.5 mm, slender, with slender beak up to 2 cm long, containing (10-)20(-30) seeds.
• Seed subglobose, ca. 1 mm in diameter, reddish to blackish-brown.
• Germination is epigeal; cotyledons notched, kidney-shaped.

Growth and development

At optimal temperatures (20-25 °C), germination takes place in 3-5 days. The first two true leaves are opposite, later leaves are arranged spirally. A 40 to 45-day cultivar with about 30 leaves at harvest has about 10 juvenile leaves each weighing 1-4 g, 14 fully grown and expanded leaves weighing 10-26 g, and 6 small inner leaves weighing 4-16 g. Pak choi is typically cross-pollinated. Two pairs of nectaries in the flower attract insects. It takes about 35-45 days from anthesis to pod maturation.

Other botanical information

Cv. group Pak Choi is defined as comprising cultivars that are non-headed, with conspicuous, fleshy, not winged petioles in contrast to cv. group Chinese Cabbage that comprises headed or semi-headed cultivars with winged petioles. In China, pak choi is the largest group of the non-headed leaf neeps. Cultivars are classified into autumn, winter, spring and summer types. Within each of these seasonal groupings, the cultivars are further subdivided according to their degree of erectness, petiole colour (white or green), plant height (short from 20-30 cm, medium, or tall from 45-60 cm), shape of the petiole (flat with thickness/width ratio of 0.4, intermediate with ratio of 0.4-0.7, or round with ratio of 0.7), leaf-blade colour (yellowish-green, green or dark green), and leaf-blade surface (smooth or blistered). The short plant type has a ratio of petiole length to leaf-blade length smaller than 1, the medium plant type about 1, and the tall plant type greater than 1. There are few summer cultivars, which reflects the lack of variation of the tropical type.

Compared with countries in East Asia, in South-East Asia pak choi is less well known than caisin (B. rapa cv. group Caisin), except for the Philippines. In Indonesia the local names of pak choi overlap with those of the caisin group as "sawi baso", "caisin", etc. In Thailand, the group is a recent introduction. It is called after the first introduced cultivars: "Hong-Tae" (meaning emperor in the Chinese Chaozhou dialect) for the green petiole and dark green leaf type, and "Hong-How" (meaning empress) for the white petiole and dark green leaf type. These two types have special roles in Chinese cuisine as the main vegetable dish and as garnish. They frequently feature in South-East Asian cookery books. They are clearly distinguished in the landrace purification and cultivar maintenance programmes of most South-East Asian seed companies. Another important type known as "Hsiao Pai Tsai" which is common in Taiwan and China has light green, thinner and slightly wavy leaf-blades, and flat light green and partially winged petioles. This type is probably more related to the cv. group Chinese Cabbage.

Ecology

Pak choi is a biennial but may behave as an annual depending on the cultivar and the environment. In South-East Asia, pak choi can be grown year-round in the lowlands. The seed crop is planted in the highlands to allow natural flower induction (vernalization). Towards the margins of the tropics, pak choi may seed during the relatively cool season. Vernalization at 5-12 °C for 10-40 days is effective immediately following germination. Usually tropical cultivars require minimal cold treatment but some cultivars may need more than 40 days. Over-vernalization induces premature flowering, sometimes even at the seedling stage.

In the tropics the required light intensity for maximum growth of pak choi is not known. However, etiolation due to overcrowding and shading should be avoided. Pak choi withstands wet weather relatively well if not flooded. Fertile alluvial sandy to clayey loam with pH between 5.5-7.0 is preferred for cultivation. However, other soil types such as peat and latosols are also suitable if well provided with organic manure and fertilizers.

Propagation and planting

Pak choi is propagated by seed. Seeds can be stored for at least a year at room temperature if dried to 5-7% moisture content and stored in moisture-resistant containers. Planting is by direct seeding or transplanting. Direct seeding at a rate of 1-5 kg of seed per ha is adopted mainly for the short (about 30-day) duration cultivars. It is carried out by broadcasting or by sowing in rows on ridges. Seeds are covered to a depth of about 1 cm by raking or spreading additional topsoil. Watering is done immediately after sowing. Mulching or a rain shelter is used to prevent washing off by heavy rain. The seed to be broadcast may be mixed with an equal amount of sawdust or sand with grain size the same as the seeds so that broadcasting can be done uniformly. Plant spacing should be 10-20 cm, which may be achieved by thinning and/or early harvesting of some plants.

In some countries transplants are preferred. Transplanting is done for the larger plant type and longer duration cultivars. For planting 1 ha, a 500 m² nursery bed is required at a seeding rate of 0.4-1.0 kg. The nursery bed is fertilized with 1 kg of farm compost, 10 g N, 10 g P, and 3 g K per m². Overcrowded patches should be thinned to prevent etiolation. Seedlings should be transplanted before or at the 5-leaf stage at a spacing of 15-20 cm, preferably in the evening, and watered immediately.

Husbandry

The field should have fine tilth and be well-levelled. During land preparation, 10-20 t of manure, and 55-75 kg N, 40-80 kg P and 80-110 kg K per ha should be incorporated before beds of 1.0-1.2 m width and 30 cm height are formed. The distance between beds should be about 30-40 cm. Two weeks after transplanting another side dressing of 55-75 kg/ha of N should be applied. Alternatively, the same amount can be applied in split dosages as a 1-2% solution at 3-4-day intervals starting 1-2 weeks after transplanting. Good timing of the split N applications is important for continuous vigorous growth.

Weeding is done manually during thinning until the canopy closes. The soil is maintained sufficiently moist all the time at 65-85% field capacity. During hot dry spells the crop must be watered daily, preferably in the morning or evening. When furrow irrigation is used the bed width should be narrower (30-50 cm) to ensure irrigation water penetrates into the middle of the bed. Good drainage has to be provided in the root layer by using high beds. In the rainy season, a temporary rain shelter built from palm or fern fronds, or nylon netting should be used to prevent lodging and damage to the leaves.

Commercial growers usually cultivate pak choi as a sole crop. In many market and home gardens, intercropping is used with a long duration vegetable of another plant family in order to maximize land use. Planting in rotation with crops of other families is important, to prevent build-up of cruciferous diseases and pests.

Diseases and pests

The heat- and wet-tolerant pak choi is relatively resistant to diseases. Due to its short growing period it often escapes soilborne diseases and pests. Seedling damping-off can be a problem if the soil and weather are too wet. It is controlled by treating seeds with fungicide such as thiram before sowing. If seedlings a few days old are found dying, watering must be reduced immediately and the seedlings must be drenched with metalaxyl or another suitable fungicide. Soft rot (Erwinia sp.) may become serious if the crop is left too long in the field. Pak choi is very susceptible to clubroot (Plasmodiophora brassicae).

Diamond-back moth (Plutella xylostella) and aphids are the major insect pests. Control of diamond-back moth is difficult because of its resistance to insecticides. At present, integrated pest management (IPM) networks are being set up by international agencies to control this pest. Aphids can be controlled by insecticides such as Pirimicarb.

Harvesting

Harvesting may take place as early as 3 weeks after planting but usually takes place between 30-45 days, depending on the cultivar and the planting method. It can be done in succession when incorporated with thinning. If pak choi is left growing too long in the field it deteriorates quickly. Diseases such as soft rot may become a problem and tropical cultivars may bolt, which reduces marketability. Harvesting at the hottest part of the day should be avoided.

Yield

The yields are 10-20 t/ha for the small plant type cultivars and 20-30 t/ha for the large cultivars.

Handling after harvest

The succulent tender plants easily suffer from mechanical damage, withering and post-harvest diseases. They therefore cannot withstand long-distance transportation and prolonged storage. Upon harvesting the plants are washed, the small outer and senescent leaves are trimmed and the roots are removed. The clean plants are graded according to size and quality, such as degree of pest damage, bolting, deformation and etiolation. They are then packed in strong rigid containers with holes at the sides to avoid squashing and transpiration heat (e.g. plastic baskets of 72 cm length × 47 cm width × 33 cm height to hold 30 kg). The plants are packed with their base to the sides of the baskets, and their turgidity is maintained by lining the basket with a layer of paper.

Genetic resources

Germplasm base collections of pak choi are maintained at the Institute of Horticultural Research, Wellesbourne, United Kingdom and the National Institute of Agrobiological Research, Tsukuba, Japan. The Asian Vegetable Research and Development Centre, Tainan, Taiwan, maintains a working collection with accessions collected from South-East Asia. A great amount of genetic diversity still occurs in southern and central China. The variation in South-East Asia is disappearing because only the few most popular cultivars are being contract-multiplied in Australia, China, Denmark, the Netherlands and the United States.

Breeding

Breeding is concentrated on the purification of the few remaining local landraces to obtain better uniformity, higher yield and early maturation. Another important objective is to breed for wider adaptation to the wet lowland tropics. In China, Japan and Taiwan, F₁ cultivars of the short green petiole type are being aimed at.

Prospects

The recent wave of East Asian emigrants to the West and Australasia has created a new demand for this group of vegetables. Non-oriental people are becoming acquainted with this vegetable too.

Interbreeding of the different cultivar groups will create new types to suit various requirements. Germplasm is currently being systematically collected in South-East Asia and China.

Literature

• Cao, Shou-chun & Li, Shi-jun, 1980. A preliminary study on the local variety of Chinese cabbage (Brassica chinensis L.) I. Morphological observation and study. Journal of the Nanjing Agricultural College 2: 1-13.
• Herklots, G.A.C., 1972. Vegetables in South-East Asia. George Allen & Unwin, London, United Kingdom. pp. 190-205.
• Li, Chia-wen, 1984. Chinese cabbages of China. Agricultural Publishing Company, Beijing, China. 234 pp.
• Lin, Weishen, 1980. A study on the classification of Chinese cabbages. Acta Horticulturae Sinica 7: 21-28.
• Tay, C.S., 1987. Characterization and evaluation work at AVRDC: an example with Brassica campestris. IBPGR Regional Committee for South-East Asia Newsletter, Special Issue: 55-65.

• See also the species page

Authors

• D.C.S. Tay & H. Toxopeus