Citrullus lanatus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Citrullus lanatus (Thunberg) Matsum. & Nakai

Protologue: Cat. sem. et spor. Horto bot. Univ. imp. Tokyo 30: No 854 (1916).

Family: Cucurbitaceae

Chromosome number: 2n= 22

Synonyms

• Momordica lanata Thunberg (1794),
• Citrullus vulgaris Schrader ex Ecklon & Zeyher (1836),
• Colocynthis citrullus (L.) O. Kuntze (1891).

Vernacular names

• Watermelon (En)
• Pastèque (Fr)
• Indonesia: semangka, cimangko (Minahasa)
• Malaysia: tembikai, mendikai
• Papua New Guinea: melon
• Philippines: pakwan (Tagalog), sandiya (Bicol), dagita (Marinduque)
• Cambodia: "öö'w llök
• Laos: môô, tèèng môô
• Thailand: taeng-mo (central), taeng-chin (peninsular), matao (northern)
• Vietnam: dưa hâú, dưa dỏ.

Origin and geographic distribution

Watermelon originated from the drier, open areas of tropical and subtropical Africa. Its cultivation became widespread in the Mediterranean region at least 3000 years ago. Introduction into India must also have occurred in ancient times and here a strong secondary centre of genetic diversity developed. Watermelon reached China around the 10th Century and Japan in the 16th Century. From India and China it spread to South-East Asia in the 15th Century. It was introduced to the Americas in post-Columbian times. Watermelon is now widespread in all tropical and subtropical regions of the world.

Uses

Most cultivars are grown for fresh consumption of the juicy and sweet flesh of mature fruits. In China and most countries in Asia large-seeded cultivars are grown for the oil- and protein-rich seeds, which are eaten after drying or roasting, with or without salt.

Other uses include sweet preserves and pickles from the rind of mature fruits (Philippines), fresh juice with salt and pepper (India), sweet syrup and beer (Russia), young fruits as ingredient in curries (India, Thailand). In Mediterranean countries, cultivars are grown as staple food for human and livestock consumption, while in Africa bitter-fruited watermelons are grown for the edible seeds. The seeds contain cucubicitrin, which has curative properties for kidney and urethral problems.

Production and international trade

Annual world production is about 30 million t from 2 million ha. At least 50% is produced in Asia, the most important producing countries being China 325 000 ha, India 250 000 ha, Thailand 40 000 ha, Japan 26 000 ha, Taiwan 24 000 ha, South Korea 20 000 ha, Vietnam 16 000 ha, Philippines 10 000 ha, Malaysia 6000 ha and Indonesia 3000 ha. Other major watermelon producing countries in the world are the Commonwealth of Independent States 500 000 ha, Turkey 250 000 ha, Iran 115 000 ha, United States 82 000 ha, Brazil 75 000 ha, Egypt 67 000 ha and Mexico 37 000 ha. Most watermelons are produced for local and urban markets, each country having its preferences for size and type. Production for export markets has developed in Spain and other Mediterranean countries, Japan and Taiwan, using smaller-fruited F₁ hybrid cultivars, including seedless types.

Properties

The nutritional value of watermelon is low. The composition per 100 g edible portion (50-70% of the mature fruit) is: water 90 g, protein 0.7 g, fat 0.1 g, carbohydrates 9 g, vitamin A 300 IU, vitamin B₁ 0.08 mg, vitamin B₂ 0.02 mg, niacin 0.2 mg, vitamin C 6 mg, Ca 8 mg, Fe 0.2 mg, Mg 10 mg, P 14 mg. The energy value is approximately 150 kJ/100 g. On the other hand, the seeds are rich in protein (40 g per 100 g edible portion) and fat (43 g per 100 g). The weight of 1000 seeds is 40-70 g (100-140 g for seedy watermelon).

Description

• Monoecious, occasionally andromonoecious, spreading, annual vine.
• Root system extensive but shallow, consisting of taproot and many lateral roots growing in the top 50-60 cm of the soil.
• Stem thin, angular and grooved, 1.5-5 m long, with soft, long, white hairs.
• Leaves simple, alternate, oblong-ovate in outline, cordate at base, 5-20 cm × 2-19 cm, palmately deeply 3-5(-7)-lobed; lobes elongated-ovate in outline, pinnately sinuate-lobulate, shallowly sinuate-toothed, rarely subentire, with the central lobe the largest; petiole 2-14 cm long; tendrils simple to 2(-4)-fid.
• Flowers solitary, axillary, on long hairy pedicels, pale yellow in colour, 2-3 cm in diameter, usually in cycles of 6 staminate flowers followed by 1 pistillate flower; calyx 5-lobed and corolla 5-partite; male flowers with 3 free anthers on short filaments; female flowers with inferior, ovoid, hairy ovary and a short style terminated by a 3-lobed stigma; nectaries present in male and female flowers.
• Fruit an indehiscent pepo, globular to oblongoid or ellipsoid, up to 60-70 cm in length, weighing 1.5-30 kg; fruit-wall glabrous to hairy, thin to thick, brittle to tough and flexible, colour varying from creamy, golden-yellow, light green to dark green, uniform or mottled or striped; flesh derived from the placenta, mostly red or yellow but also pink, orange or white; flesh texture from finely grained and "melting" to firm, coarse and fibrous.
• Seeds scattered throughout the flesh, numerous (200-900 per fruit), smooth, flattened, 6-15 mm × 5-7 mm × 2.5 mm, black, brown, red, yellow, rarely white, without endosperm.

Growth and development

Watermelon seed will remain viable for at least 8 years when stored dry at temperatures below 18 °C. Germination is epigeal with cotyledons unfolded within 10 days after sowing and the first true leaf appearing one week later. The first 2 to 3 true leaves are often not deeply lobed. Usually the main vine continues to grow for several nodes before the first lateral branch is formed. The first male flower appears on the 10-15th node and the first female flower some 7 nodes later. The first female flowers often have poorly developed ovaries and fail to set fruit. The flowering peak occurs 50-80 days after germination. Flowers open shortly after sunrise and remain open only one day. The pistillate flower and the staminate flower on the node 2-3 nodes behind it open on the same day. Pollination is effected by insects, mostly bees. Within 24 hours after pollination the pedicel starts to elongate and bends downward with the swelling ovary. The fruits are ready for harvesting 30-50 days after pollination, depending on climate and cultivar differences in fruit size and general earliness.

Other botanical information

C. lanatus is a very variable species of old cultivation in the warmer parts of the world. Although no strict separation lines can be drawn, the species is sometimes subdivided into 3 subspecies:

• ssp. lanatus with var. lanatus for the "tsamma" watermelon of South Africa and Namibia, important as water source for humans and animals in the semi-arid regions of the Kalahari and perhaps the ancestral form of the cultivated watermelon, and var. citroides (Bailey) Mansfeld for the fodder melon of South Africa, also cultivated in the United States and the Commonwealth of Independent States;

• ssp. mucosospermus Fursa, the "egusi" watermelon of West Africa, especially important for its large, protein-rich seeds;

• ssp. vulgaris (Schrader) Fursa, with var. cordophanus (Ter-Avan.) Fursa for forms in East Africa important as water source in dry regions and var. vulgaris for the most important cultivated watermelon from all over the world. Three ecological-geographical groups are distinguished within var. vulgaris : (1) the Russian group; (2) the Eastern group; and (3) the Asiatic group.

"Charleston Gray", "Crimson Sweet" and "Sugar Baby" are well-known cultivars in South-East Asia, but there are several hundreds of open-pollinated and F₁hybrid cultivars available to watermelon growers in the world. The following main types can be distinguished, using the name of a representative cultivar as type indicator:

• Fruits with sweet juicy flesh, containing seeds:

(1) "Charleston Gray": fruit oblongoid to cylindrical, 6-15 kg, rind light green with small veins, flesh pink-red;

(2) "Peacock": fruit oblongoid, 6-10 kg, rind dark green with thin, lighter stripes, flesh red;

(3) "Flower Mountain": fruit oblongoid, 10-15 kg, rind light to medium green with mottled dark green stripes, flesh red;

(4) "Crimson Sweet": fruit ovoid to globular, 1.5-10 kg, rind light green with mottled dark green stripes, flesh red or yellow;

(5) "Sugar Baby": fruit globular, 3-8 kg, rind dark green, flesh red;

(6) "Golden Crown": fruit globular to ovoid, 2-3 kg, rind yellow with narrow, light green stripes, flesh red;

(7) "Icebox": general indication for all small-sized globular watermelons, 1.5-3 kg, rind green or yellow, flesh red or yellow.

• Fruits with sweet juicy flesh, without seeds:

always F₁ hybrids (triploids by crosses between tetraploid femaleand diploid male parent lines): fruit generally ovoid to globular, 1.5-8 kg; rind dark green, green with dark stripes or green with dark green veins; flesh mostly red, sometimes yellow.

• Seedy watermelon:

grown for the edible seed: fruits with many, rather large, red or black seeds; rind often dark green; flesh spongy white.

Ecology

Watermelons are daylength neutral. A warm (day temperatures 25-30 °C, night temperatures > 18 °C), sunny and relatively dry climate is required for rapid growth and fruiting. Excessive rainfall and high humidity give excessive vegetative growth, affect flowering, induce leaf diseases and fruit rot. Market garden production is usually concentrated in the dry season, with furrow or drip irrigation. Soils should be well-drained, fertile loamy sands with high organic matter content and pH 6-7. At lower pH values, soilborne diseases (Fusarium) may become a serious problem.

Propagation and planting

There is no seed dormancy, but germination can be accelerated by pre-soaking for 24 hours in water after scarifying the seed at one end, especially for cultivars which have a hard seed-coat. Watermelons are seeded directly (2-3 seeds, sown 2-4 cm deep on mounds or ridges) or transplanted after raising seedlings in 9 cm diameter pots of polythene, paper or banana leaves. In Japan, China and Taiwan, watermelon is sometimes grafted at the cotyledon stage on a Fusarium-resistant rootstock. This can be "Citron", a bitter-tasting C. lanatus, bottlegourd (Lagenaria siceraria (Molina) Standley) or pumpkin (Cucurbita maxima Duchesne ex Lamk or C. moschata (Duchesne ex Lamk) Duchesne ex Poiret). Seedlings or grafted plants are transplanted to the field when they have 3-4 true leaves, some 5 weeks after sowing.

Planting distances are 0.9-1.2 m × 1.2-1.8 m giving a density of 5000-9000 plants/ha. Seed rates per ha are 1-2 kg for direct-seeded and 0.25-0.3 kg for transplanted watermelon.

Husbandry

Growing watermelon directly after paddy rice is an effective way of avoiding soilborne diseases, in particular Fusarium. Minimum tillage is recommended to preserve residual moisture, but planting holes are dug to which NPK fertilizer and organic manure are added. Upland soils are ploughed and harrowed, organic manure (25-30 t/ha) is added to the planting beds and a basal application of 200-250 kg/ha NPK fertilizer. This is followed by applications of fertilizers as two side dressings or regularly as a liquid nutrient solution, type and rates according to local requirements.

Irrigation under upland conditions should be frequent throughout the growing season, as the demand for water is high and the root system is rather shallow. Mulching with polythene sheets (black, transparent or silver-painted), rice straw and rice hulls is common practice to conserve moisture, raise or lower soil temperatures, to suppress weeds, and to prevent direct contact of the fruits with the soil. In the absence of mulch, frequent weeding will be necessary until the vines have covered the beds.

Vines are trained to prevent excessively dense vegetative growth and usually only two fruits per plant are left to mature, or 4-6 in small-fruited cultivars. Watermelon requires abundant insect pollinators to ensure sufficient fruit set and two beehives per ha are recommended to ensure maximum yield.

Diseases and pests

There are a number of important diseases. Fusarium wilt (Fusarium oxysporum f.sp. niveum) with three races 0, 1 and 2, can be prevented by wide crop rotation (preferably 1 : 8 years), planting after paddy rice, ensuring good drainage, grafting on resistant rootstocks or using tolerant/resistant cultivars. Anthracnose (Glomerella cingulata var. orbiculare, formerly Colletotrichum lagenarium) can be controlled by copper and organic fungicides, but cultivars resistant to some races are available. Gummy stem blight (Dydimella bryoniae, formerly Mycosphaerella citrullina) is also controllable with fungicides, and sources of resistance in wild Citrullus accessions have been identified. Powdery mildew (Erysiphe cichoracearum) occurs but more important in hot and humid climates is downy mildew (Pseudoperonospora cubensis). Bacterial rind necrosis (Erwinia carnegieana) may be serious but varietal differences in susceptibility exist. Watermelon blotch is a new bacterial disease (Acidovorax avenae spp. citrulli), reported in China and the United States since 1989. Watermelon mosaic virus (WMV-2), papaya ring spot virus (PRSV-W) and zucchini yellow mosaic virus (ZYMV) are all transmitted by aphids such as Aphis gossipii.

Common insect pests are thrips (Thrips spp.), mites (Tetranychus spp.), aphids (Aphis gossipii), fruit fly (Dacus ciliatus), cucumber beetles (Diabrotica spp.), red pumpkin beetle (Aulacophora sp.) and Epilachna beetles. There are many types of insecticides to control the various insect pests, but indiscriminate spraying usually aggravates the situation by destroying useful parasitic insects. Polythene mulch, especially when coated with reflective aluminium paint, repels thrips and aphids.

Root knot nematodes (Meloidogyne spp.), particularly serious on sandy soils, can be prevented by crop rotation, destruction of susceptible weed hosts, soil fumigation (expensive and environmentally hazardous) and grafting on resistant rootstocks.

Harvesting

The first fruits are ready for harvesting 65-90 days after transplanting to the field for most watermelon cultivars under tropical conditions. Indications of maturity are: the fruit gives a muffled sound when tapped, the light spot where the fruit rests on the ground has turned yellow, the fruit skin increases in lustre and loses trichomes, the tendrils directly opposite the fruit stalk are yellow and shrivelled. Watermelons do not ripen further after harvest. The fruit is cut from the vine with about 5 cm of stalk. Fruits harvested in the afternoon are less turgid and therefore less likely to crack during handling and transport.

Yield

Averaging worldwide about 15 t/ha, but varying from 5-60 t/ha, depending on cultivar and cultural practices. Seed yields are 150-400 kg/ha.

Handling after harvest

Watermelon fruits are rather fragile and susceptible to breakage and bruising. They should therefore be handled and shipped carefully. Fruits can be stored for more than 2 weeks at 10-15 °C and 85% relative humidity.

Genetic resources

Germplasm collections of watermelon are maintained at universities, horticultural institutes and gene banks in several countries, e.g. India, Japan, China, Taiwan (AVRDC), United States, Commonwealth of Independent States, Hungary, Bulgaria, Turkey, Iraq, Libya, Italy, South Africa. In the Philippines, about 60 accessions are maintained by the National Plant Genetic Resources Laboratory, Institute of Plant Breeding, Los Baños. There is a need to complement existing collections with additional germplasm of C. lanatus and related Citrullus species from the primary (central to southern Africa) and secondary (India, CIS, China) centres of genetic diversity.

Apart from the annual C. lanatus, which has bitter-fruited wild types from southern Africa as likely ancestors, three related African species form a valuable pool of germplasm for watermelon breeding because they all can be intercrossed successfully:

• C. colocynthis (L.) Schrader (synonym: Colocynthis vulgaris Schrader), the colocynth, a perennial species from northern Africa to Afghanistan and Pakistan, with small fruits and white, bitter flesh;

• Anthosicyos naudinianus (Sonder) C. Jeffrey (synonym: Citrullus naudinianus (Sonder) J.D. Hooker), a perennial species from south-western Africa, with thin-walled but spiny fruits, containing juicy flesh;

• C. ecirrhosus Cogn. (synonym: Colocynthis ecirrhosus (Cogn.) Chakrav.), a perennial species from south-western Africa, without tendrils and with very bitter fruits, adapted to the extreme climatic conditions of the Namib desert.

Breeding

Whereas in the United States all major watermelon breeding programmes until recently concentrated on developing open-pollinated cultivars with large oblong fruits, Japanese breeders started producing F₁ hybrid cultivars back in the 1930s and focused more on round-shaped, medium to small fruits. Also in Japan the technique of producing seedless (triploid) watermelon hybrids was developed in the early 1940s. However, seedless hybrids are difficult and expensive to produce (only 40-80 seeds per fruit), seeds are often difficult to germinate and fruits mature some 10 days later than comparable diploid cultivars. Seedless watermelon production therefore became more popular in Taiwan and other countries outside Japan with warmer climates and lower costs of seed production. In Asia, major watermelon breeding programmes are presently carried out by governmental institutes, universities and private breeders in India, Japan, mainland China and Taiwan.

Main breeding objectives include: compact plant types (short internodes), earliness (low number of days to first fruit set and from fruit set to maturity), fruit size and shape (small and round), fruit quality (thin but strong rind, high sugar content, finely grained flesh with small seeds, no hollow heart), disease and pest resistance (first priority Fusarium, anthracnose and virus), and improved seed production of seedless types. The most popular cultivars grown in South-East Asia originate from Taiwanese seed companies.

Prospects

The demand for watermelon, especially the smaller, ovoid to globular types, is rapidly increasing in many South-East Asian countries. Sources of resistance to the most important diseases (and pests) have been identified in wild accessions of C. lanatus and in related species. The longer term prospects of reducing dependence on pesticides when producing watermelon are therefore good. Male-sterile mutants have been found, giving prospects of seed production of F₁hybrids by bee pollination instead of hand pollination. New techniques from molecular biology (genetic transformation and DNA markers) can further increase breeding efficiency in watermelon, e.g. in resistance to virus diseases.

Literature

• Deanon Jr, J.R. & Villegas, G.G. 1984. Watermelon. In: Bondad, N.D. (Editor): Philippine science encyclopedia. National Research Council of the Philippines, Manila, the Philippines.
• Fursa, T.B., 1981. Intraspecific classification of water-melon under cultivation. Kulturpflanze 29: 297-300.
• Mohr, H.C., 1986. Watermelon breeding. In: Bassett, M.J. (Editor): Breeding vegetable crops. Avi Publishing Company, Westport, Connecticut, United States. pp. 37-66.
• Shinohara, S. (Editor), 1984. Vegetable seed production technology of Japan. Shinohara's Authorized Agricultural Consulting Engineer Office, Tokyo, Japan. Vol. 1. pp. 318-339.
• Tindall, H.D., 1983. Vegetables in the tropics. MacMillan, London, United Kingdom. pp. 150-152.
• Whitaker, T.W. & Davis, G.N., 1962. Cucurbits: botany, cultivation and utilization. Leonard Hill, London, United Kingdom. 250 pp.
• Yamaguchi, M., 1983. World vegetables: principles, production and nutritive values. Avi Publishing Company, Westport, Connecticut, United States. pp. 327-330.

Authors

• M.M. Paje & H.A.M. van der Vossen