Cucumis metuliferus (PROTA)

Cucumis metuliferus E.Mey. ex Naudin

Plant Resources of Tropical Africa Introduction

List of species

General importance
Geographic coverage Africa
Geographic coverage World
Fruit
Vegetable
Medicinal
Ornamental
Forage / feed
Food security

Protologue: Ann. Sci. Nat. Bot., sér. 4, 11: 10 (1859).

Family: Cucurbitaceae

Chromosome number: 2n = 24

Horned melon, African cucumber, horned cucumber, kiwano (En). Concombre cornu, métulon, kiwano (Fr). Maxije (Po).

Cucumis metuliferus occurs naturally throughout the tropical and subtropical sub-Saharan regions of Africa, from Senegal to Somalia and South Africa. It has also been recorded in Yemen. In Kenya, New Zealand, France and Israel the fruits of improved cultivars are commercially grown for export. Cucumis metuliferus has become naturalized in Australia, and is reported as adventive in Croatia.

The fruits of horned melon are mainly eaten, and in some parts of Africa the leaves are also used as a vegetable. The fruits are peeled and eaten in either the immature or the mature stages. Fruits in the unripe stages have the appearance and taste of cucumber. Mature fruits may have a sweet dessert-fruit flavour. Mature fruits may also be split open and dried in the sun for later use. In Botswana the Kalahari San people prepare the fruits by roasting. In Zimbabwe young leaves are stripped from the stems, washed and boiled as spinach, in the same way as musk pumpkin leaves (Cucurbita moschata Duchesne), adding peanut butter prior to serving.

Fruits from wild-growing plants are often bitter and inedible. Traditional medical practitioners in Zimbabwe consider the bitter wild fruits as poisonous if taken by mouth. The root is used in the Mutare area (Zimbabwe) for the relief of pain following childbirth. In Benin the fruit is said to possess medico-magical properties and is used to treat eruptive fevers in ‘Sakpata voodoo’ rituals. The decorticated fruit macerated in distilled palm wine or lemon juice is used to treat smallpox and skin rashes.

In Western countries Cucumis metuliferus is currently mostly marketed as an ornamental for its decorative fruit, with a unique appearance and extended keeping qualities.

In southern Africa horned melon is considered a traditional fruit vegetable. Cultivation has been on a small scale, e.g. in Zimbabwe it is cultivated in rural and peri-urban areas for sale in traditional markets and by street vendors. The development of the African horned melon into an international crop started in New Zealand where it has been commercially grown and exported since the 1980s. There it was given the name ‘kiwano’ in an attempt to promote the new fruit crop in Japan and the United States. Since then it has also been grown commercially to a limited extent in California for the United States market, and in Israel and Kenya from where the fruits are exported to markets in Europe. Recent efforts to grow the crop during the summer in southern France for the European market have been successful.

The nutrient content for fresh fruits of Cucumis metuliferus (per 100 g edible portion) is: water 91.0 g, energy 134 kJ (32 kcal), protein 1.1 g, fat 0.7 g, carbohydrate 5.2 g, crude fibre 1.1 g, Ca 11.9 mg, Mg 22.3 mg, P 25.5 mg, Fe 0.53 mg, thiamin 0.04 mg, riboflavin 0.02 mg, niacine 0.55 mg, ascorbic acid 19 mg (Wehmeyer, A.S., 1986). Some values may vary depending on fruit maturity, as the fruits are eaten at both immature and mature stages.

The leaf composition is approximately the same as other dark green leafy vegetables.

In wild Cucumis metuliferus, plants with bitter and non-bitter fruits occur, and the two types are morphologically indistinguishable. A significant proportion of wild-growing horned melon plants encountered in southern Africa are bitter-fruited and have caused poisoning. Bitter mature fruits may remain completely intact on the plants, as neither baboons nor other wildlife eat them. The amount of bitterness varies in immature and mature fruits on the same plant, with younger fruits having a less bitter taste. Bitterness is due primarily to the presence of cucurbitacins, bitter and toxic compounds occurring in Cucurbitaceae. Cucurbitacins can cause severe illness and death, due to their potent action as purgatives and laxatives. Cucumis metuliferus contains cucurbitacin B, a triterpene known to exhibit cytotoxic, antitumour and anti-inflammatory activities.

Vigorous annual herb with climbing or prostrate stems, having solitary, simple tendrils 4–10.5 cm long; root system strong, fibrous; stems reaching several m in length, grooved, with long stiff spreading hairs. Leaves alternate, simple; stipules absent; petiole 3–12 cm long, setose; blade ovate or pentagonal in outline, 3.5–14 cm × 3.5–13.5 cm, shallowly palmately 3–5-lobed, hispid setulose especially on veins below, becoming scabrid-punctate. Flowers unisexual, regular, 5-merous; sepals filiform, 2–4 mm long; petals united at base, 0.5–1.5 cm long, yellow; male flowers in 1–10-flowered fascicles, with pedicel 2–18 mm long, stamens 3; female flowers solitary, with pedicel 5–35 mm long, ovary inferior, ellipsoid, 1–2.5 cm long, covered with large soft spines, stigma 3-lobed. Fruit an oblong-cylindrical berry 6–16 cm × 3–9 cm, on a stalk 2–7 cm long, rounded at both ends and beset with stout, broad-based, spiny protuberances 1–1.5 cm long, dark mottled green, ripening through yellow to bright orange, many-seeded. Seeds narrowly ovoid, 5–8 mm long, compressed with rounded margins, sericeous hairy.

Most of the approximately 30 Cucumis species are native to Africa, but the cucumber (Cucumis sativus L.) probably originates from India. Cucumis metuliferus with its ‘horned fruits’ and hairy seeds is genetically more closely related to melon (Cucumis melo L.) than cucumber, but has proven to be cross-incompatible with other species. Based on meiotic and crossing studies, flavonoid patterns, chloroplast DNA data, and isozyme analyses, Cucumis metuliferus is isolated from the other species in the genus. Specimens with non-bitter fruits, totally lacking spiny protuberances, have been observed both in the wild and under semi-cultivated conditions near Bulawayo (Zimbabwe).

At optimum temperatures of 20–35°C seed germination takes place in 3–8 days. Below 8°C germination is completely inhibited. Vegetative stem growth, either climbing or sprawling, exhibits typical cucurbit exuberance and the plants are capable of smothering nearby plant growth. Flowering starts about 8 weeks after sowing, with male flowers appearing first, followed after several days by female flowers. Pollination is by insects. In experiments in Israel maximum fruit weight (on average 200 g) was reached 30–40 days after pollination, and the main period of fruit ripening on the plant in terms of changes in fruit constituents and colour (from green to yellow) occurred 37–51 days after pollination. Under field conditions, time from sowing to harvest was 3.5 months.

In Zimbabwe, sweet-fruited plants reseed themselves with little management and protection. Fruits continue to be sweet, barring the occasional pollination by bitter-fruited wild plants. Fruits of horned melon in southern Africa continue ripening after the cessation of the rainy season, long after the stems have died back.

The natural habitat of horned melon ranges from low-altitude riverine semi-evergreen forest to semi-arid highlands and Kalahari sands. Horned melon is a warm-season grower in tropical to subtropical regions, and does not tolerate cold conditions. It occurs at altitudes from near sea level to 1800 m. In southern Africa seeds germinate with the summer rains when night temperatures are above 12°C. A semi-arid climate with a warm-season rainfall regime appears to enhance the fruit ripening stage, allowing fruits to develop their full flavour. Plants tolerate a wide range of soil types throughout their natural distribution area.

Cucumis metuliferus is propagated by seed. The 1000-seed weight is about 14 g. Seed may be sown directly or seedlings are transplanted when they have two true leaves. The optimum time of planting is when soil and air temperatures are above 14°C. A planting density of 10,000 plants per ha produced good yields in Israel. In Spain direct planting of seed retarded production; the use of transplants with well-developed root systems was recommended.

Horned melon may be grown under field conditions similar to cantaloupe melon or cucumber. In Spain field-grown plants using supports did not produce as satisfactorily as those grown without, and this was thought to be due to climatic factors, principally the wind. Greenhouse planting is an option in which case pollinators must be introduced at the time of flowering. According to local soil conditions and soil test recommendations, compost, manure or inorganic fertilizers can be incorporated.

In southern Africa horned melon plants are seldom affected by diseases or pests in their natural habitat. Cucumis metuliferus is susceptible to cucumber mosaic virus, tobacco ringspot virus, tomato ringspot virus, watermelon mosaic virus 2, and a severe strain of bean yellow mosaic virus. Some accessions are susceptible to Fusarium wilt (Fusarium oxysporum). Plantings in Israel were affected by powdery mildew (Sphaerotheca fuliginea) and squash mosaic virus. Greenhouse plantings in Spain, with high temperatures and humidity, were affected by powdery mildew (Erysiphe cichoracearum) and the greenhouse white fly (Trialeurodes vaporariorum), but field plantings were unaffected. African horned melon is resistant to the musk melon yellow virus. Some accessions are highly resistant to watermelon mosaic virus 1, due to a single completely dominant gene, and hypersensitive-resistant to squash mosaic virus.

An orange and black cucurbit beetle, Sonchia pectoralis, has been observed damaging the leaves of young plants, but not to the extent of harming overall growth. The ubiquitous pumpkin fly, which ravages other cucurbit crops in southern Africa, does not attack horned melon.

Horned melon is highly resistant to root-knot nematodes (Meloidogyne spp.). Resistance to powdery mildew, melon aphid (Aphis gossypii), greenhouse white fly and Fusarium wilt has been recorded in several accessions.

Stems of horned melon die back at the end of the growing season while the fruits remain attached and continue ripening to a bright orange colour. They may be harvested over successive months. Immature fruits may be harvested at any time during the growing period. Care is needed during picking because the stiff sharp hairs on the stems and the spiny ‘horns’ on the fruits can easily puncture the skin; it is recommended that gloves be worn for harvesting. For home consumption leaves are picked from plantings or are collected from wild plants.

There are no records on yield from Africa. In New Zealand growers harvest up to 20 t/ha of horned melon fruits, in California about 8 t/ha. Growers in Israel harvested approximately 230,000 fruits/ha, fruits weighing on average 200 g, totalling 46 t/ha. Results of experiments conducted in Spain showed that each plant produced on average 66 fruits weighing 15 kg.

Fruits should not be stacked without protective covering; the sharp spines easily puncture the skin of other fruits, causing a dark-orange to reddish discharge from the wounds. The spines may be rendered less harmful by use of sandpaper or a file. Fruits have an exceptionally long keeping quality at room temperature and may be kept for many months without losing their decorative appeal. Fruits picked at the onset of ripening and kept at 24°C were undamaged after three months of storage, though they failed to develop the desired orange colour. Ethylene treatment resulted in fruit colour changing from green to yellow in three days, but had no effect on total soluble solids levels. There was a rise in reducing sugars during storage, unrelated to ethylene treatment. Fruit ripened on the stem showed higher values for total soluble solids and reducing sugars. At 20°C 30% spoilage among stored fruits occurred by day 37, and at storage temperatures below 12°C all ripe fruits spoiled within 55 days. Chilling symptoms in the form of opaque rind spots appeared on the fruit surface when fruits were stored at 4°C; cold storage is therefore not recommended.

Germplasm of horned melon is held at the National Plant Genetic Resources Centre in Windhoek, Namibia, and in the United States (Department of Agriculture, North Carolina Plant Introduction Station). In the wild Cucumis metuliferus is widespread and occurs in a variety of habitats, so there is no reason to consider it liable to genetic erosion.

Fruit quality in horned melon is measured by colour, size, taste, acidity and aroma. Original cultigens tested were found somewhat lacking in the taste factor, however these lines were being tested for plant vigour and pest and disease resistances. There is a need to identify sweet-fruited cultivars. More recent studies with germplasm from Botswana and Zimbabwe showed promising results for increased size and improved taste. Within the germplasm being grown in Zimbabwe and South Africa, cultivars are found with large (up to 18 cm) fruits; they are orange when ripe and have pleasantly tasting flesh, used as an attractive ingredient in fruit salads. Cucumis metuliferus possesses important genes for disease and pest resistance that would be of benefit to the gene pool of other commercially important Cucumis species, i.e. musk melon and cucumber, if they could be transferred. However, many attempts by various research groups to introduce these genes using traditional sexual hybridization methods have not been successful due to strong incompatibility barriers. Neither embryo culture nor somatic hybridization by protoplast fusion has produced successful results to date.

The prospects for horned melon as a subsistence leafy vegetable are rather poor, but as a cultivated cucumber-like vegetable or dessert fruit it has a bright future. The immature green fruits are highly prized in Zimbabwe. Because of the somewhat insipid taste of the cultivars being grown, it has not caught on as a dessert fruit or cucumber substitute in the United States and Europe to the extent that marketers had hoped. Until the flavour can be improved to the satisfaction of the consumer, the present marketing technique for horned melon is as a decorative ornamental fruit. Given its unique form and appearance, together with its appealing orange-ripe colour plus an extended shelf life, Cucumis metuliferus rightfully deserves centerpiece fruit-bowl attention. The preferred climate pattern of warm-season rain followed by a dry cool season could be an important consideration in any future successful commercial growing venture.

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• M.H. Wilkins-Ellert

4246 W. Flying Diamond, Tucson, Arizona 85742, United States

Wilkins-Ellert, M.H., 2004. Cucumis metuliferus E.Mey. ex Naudin. [Internet] Record from PROTA4U. Grubben, G.J.H. & Denton, O.A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.

Accessed 18 December 2024.

• See the Prota4U database.