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Lost Crops of Africa: Fruits, Volume III
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Lost Crops of Africa: Fruits, Volume III 10 WATERMELON Other than botanists, few people consider that watermelon (Citrullus lanatus) is African. Yet this is so: the crop’s wild ancestors occur abundantly in the dry zones of the continent’s southern region. The African origin may come as a surprise only because watermelon spread around the globe so long ago that for most people it has become part of the wallpaper of life. Today, this African fruit is cultivated throughout the warmer parts of the world—from the searing tropics to temperate latitudes and even beyond.1 Global annual production is approaching 100 million tons. Yet Africa scarcely registers in statistics: the largest producers are Turkey, Iran, and Egypt, the United States and Mexico, and–especially–China, which produces over two-thirds of the watermelon in the world. While consumption in the United States has been fairly stable over the past 25 years, elsewhere demand is increasing, and indications for the future suggest ever-greater global production. Given all this success, it is intriguing to consider that only a few watermelon types emigrated out of Africa. The descendents of those select few rose to a place among the best known of all the world’s fruits. With their colorful flesh and luscious sweet juice, they have been called the food of heaven. Mark Twain once wrote, “The true southern watermelon is a boon apart and not to be mentioned with commoner things. It is chief among this world’s luxuries, king by the grace of God over all the fruits of the earth. When one has tasted it, he knows what the angels eat. It was not a southern watermelon that Eve took; we know this because she repented.” The homebound watermelon types that remained in southern Africa’s arid regions are not at all like that. To naive outsiders they can look like miserable waifs, fully deserving their neglect. But that impression is false. Wild watermelons are useful in their own right, and always have been, their rinds, flesh, and moisture sustaining many inhabitants through waterless times. “The most surprising plant of the South African desert,” wrote David 1 It is abundantly grown, for instance, in parts of Java, providing farmers a substantial income. Major sites for production are also found in the semi-arid parts of coastal Peru and Ecuador. A state-record watermelon grown in Alaska, USA–located above 55° N–weighed 65 kg.
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Lost Crops of Africa: Fruits, Volume III Few people realize that watermelon is African. Yet this crop’s wild ancestors are scattered abundantly across the dry wastes of the continent’s semi-arid southern hinterland. In Africa’s Kalahari Desert the undomesticated watermelons are an important source of both food and water. In times of drought African farmers have traditionally relied on them for emergency use. (© Roland Bischoff / botanikfoto) Livingstone in 1857, “is the kengwe or keme, the watermelon. In years when more than the usual quantity of rain falls, vast tracts of the country are literally covered with these melons. Some are sweet and wholesome, and others so bitter that they are named by the Boers the ‘bitter watermelon.’” They are also plentiful. Wild watermelons still bespeckle millions of hectares of semi-arid African hinterland. Passersby see them alongside thousands of kilometers of roadsides and bushtracks, particularly in the Kalahari Desert of Botswana and Namibia. But few who spy the tiny nondescript orbs lying amongst scraggly vines beside the road or trail can appreciate that, botanically speaking, they are indistinguishable from the big beloved fruit in their own life. The difference is too great to be grasped at a glance. “This is a salutary lesson in the hidden potential of so many superficially unpromising wild fruits,” one of our insightful contributors wrote; “selection and domestication can improve them out of all recognition in a few [plant] generations.” Although their domesticated descendants have been cultivated for over 4,000 years and were old news to the ancient Egyptians, the progenitors that gave them life basically remain strangers to commerce and horticulture. Now is the time to better recognize these orphans of the wilderness. They have value for both direct and indirect use.
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Lost Crops of Africa: Fruits, Volume III Direct Use African watermelons deserve attention solely on the basis of their own qualities. Many are prolific and productive. They exist on the absolute outer edge of human settlement, where all life is tenuous. They are so important to some people as to literally stand between life and death. And they can be made even greater contributors to the African outback as well as to the world. Examples follow. Vegetable Watermelon Among all the variants within the species, types with small, hard, unsweetened fruits are among the least developed. In Africa they are grown exclusively as vegetables—cooked like pumpkin or squash. So far, these are unknown to most of the world. In India, though, one type has become a widely used vegetable.2 Its small, round fruits (up to 10 cm in diameter) look more like bloated cucumbers than watermelon. They are either pale or dark green in color, and are eaten fresh like cucumbers, pickled like gherkins, or candied like apples. The seeds may be dried and eaten like nuts. This type of variant appears to be a crop with considerable potential in its own right…and not just for one nation but for dozens. Edible-Seeded Watermelon Certain other types are grown solely for seeds. In parts of West Africa—particularly Senegal, but also Nigeria, Niger, Chad, and Cameroon—these are crucial articles of food and of commerce, and are sold in countless markets.3 The seeds are prepared in different ways. Most are dried, roasted, and eaten as nuts; some are pounded into a paste resembling peanut butter; others are ground and baked into bread, to which they add a nutty flavor; a number are used in soups or stews or parched and eaten with cereal products. The Yoruba of Nigeria ferment the kernel to produce a favorite food flavoring known as ogiri. Watermelon seeds appeal to more than just local taste preferences, and eating watermelon seeds is not restricted to Africa. West Africa already exports them to France for snack food, to be eaten out-of-hand. Sudan exports them as well. And this is not a new phenomenon: for centuries, Sahelian Africa shipped watermelon seed out of the deserts to populated areas far and wide, including Egypt and beyond. In certain Asian countries, they are also an important snack food. In China’s southern province of Guangdong, for example, toasted watermelon seeds are a common fare and an essential part of special occasions, including weddings, funerals, and New 2 Its common name is tinda, tensu, or tendi. Although botanically called Citrullus lanatus var. fistulosus, it may not even be a watermelon. Most botanists now claim that it more correctly belongs in a related genus, Praecitrullus fistulosus (Stocks) Pang, and say it is unlikely to exchange genes with C. lanatus. Even if they’re right and this proves not a true watermelon variant, it still is an interesting crop, and one with a bigger future. 3 These melons normally goes by the name “egusi.” The companion volume on African vegetables devotes an entire chapter to the use and potential of their edible seed.
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Lost Crops of Africa: Fruits, Volume III Year celebrations. Indeed, China is yet another nation exporting watermelon seeds, as are India and Pakistan. Pickling Watermelon In many localities, a hard, white-fleshed form of the fruit is used for pickling and jellies. Known as the citron or preserving melon,4 this watermelon variety is not edible in the fresh form. It is used exclusively for processing. Each year the United States, alone, employs over 3 million tons of this otherwise inedible fruit to prepare glacé and sweet preserves, both of which are major contributors to the culinary phenomenon called “Christmas cake.” Because of its high pectin content the citron is also used for making jams, jellies, and preserves. The white fleshy rind that occurs between the outer green skin and the inner edible portion of ordinary watermelons is used that way too. Even wild African watermelons are used for a similar purpose: flakes are boiled in sugar to make a tasty jam. Cooking Watermelon In Botswana the wild fruit is baked in the coals or cooked fresh as a relish. Some traditional types store so well they are still useful for cooking even a year after harvest. Food Security Such watermelons can be a foundation of the food system. It has been said, for example, that in olden times people couldn’t cross the Kalahari Desert except during a good melon season. These wild watermelons sustained not only the travelers, but also their livestock. In times of drought, traditional African farmers have long relied on them for emergency use as well. Sometimes the fruits become the sole source of water for their cattle—and even for themselves—for months on end. They also sustain the wild creatures in the Kalahari. Moreover, in addition to leaving the fruits in the desert, people also pile them up near their dwelling as a convenient store of food and water. The fruits keep a surprisingly long time—up to a year in some cases. In such areas, watermelon can be an important source of both food and water, as well as income. Indirect Use The ancestral genes to be found in the wild and cultivated African watermelons seem likely to provide tools for creating valuable new watermelons for use around the world, including forms that are presently inconceivable. Although the large green globe with the crimson flesh is likely to remain the main type, some of the changes that could occur in future decades include the following. Downsizing In a sense, most watermelons today are too cumbersome for convenience. Even the most affluent consumers can barely squeeze one into 4 Citrullus lanatus var. citroides.
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Lost Crops of Africa: Fruits, Volume III Though Africa is home, China grows more than two-thirds of the watermelon in the world, followed by the Middle East and warmer parts of North America. Yellow-fleshed varieties, once more common in Europe and the United States, are now surging in popularity throughout Asia, such as in this Chinese market in Yangon, capital of Myanmar. Although the varieties of flesh- and seed-type watermelons already seem overwhelming, scientists have really only begun to tap the genetic diversity available. (Greg Martinez and Francie Zant) their large refrigerator, some can barely lift the very large fruits, and it takes a big gathering to get one down at a single sitting. Small “icebox” melons–the smaller ones often called “palm” melons because they can fit in the palm of the hand–are becoming increasingly popular in the United States, and even smaller ones are available in Asia—in China, Taiwan, and Japan, for instance. Though watermelons grown around the world are getting smaller, most are still too large (2 to 3 kg) or have other shortcomings such as poor texture, flavor, or durability; vast new markets might open up worldwide if even smaller, very user-friendly types can be developed. This is not far-fetched, and Africa’s untapped watermelon resources could be a key to this. Those wild and feral watermelons can be as small as 2 cm in diameter, and likely contain “downsizing” genes that innovative horticulturists could employ to create fruits of less than 1 kg. The resulting “microwatermelons” might be small enough to be conveniently carried—maybe even in a lunchbox or backpack. They would be cut open only when
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Lost Crops of Africa: Fruits, Volume III eaten, a feature that would end the annoyance of juice dribbling into a lunch pail. They would also end the waste of having half a giant fruit sitting around softening and spoiling. And they could be consumed at a single sitting by an individual or small family—a feature particularly important in homes in hot regions with no refrigeration. Designer Colors Today’s watermelons are, in a manner of speaking, caught in a time warp. They are much the same color as in the era of the ancients: usually green on the outside and red on the inside. A returning Pharaoh would know them at a glance. But this is just one form out of dozens of possible color combinations: watermelons can be yellow or orange on the outside; white, yellow, pink, or orange on the inside; or any mix of those colors, including striped, speckled, or patterned. Yellow-fleshed types today are actually preferred in parts of Asia. They are gaining popularity in a number of nations and could soon have a global impact. In China, some varieties with orange rinds and golden flesh are already produced on a large scale. Their shiny bright skins and deep-yellow flesh make them extremely attractive. In the United States, too, golden watermelons are showing signs of catching on with consumers. Seedless Fruits Scattered as they are throughout the flesh, watermelon seeds are generally considered nuisances. They cannot (as in melons) be easily scraped out, and many rules of etiquette exist for spitting seeds. Seedless watermelons have of course been created, but the complex genetic manipulations required are often difficult to perform repeatedly.5 Another approach is searching among the array of Africa’s wild types, seeking variants with edible or inconspicuous seeds. Some of those might bear naked seeds (entirely lacking a seed coat), seeds with soft-coats that are chewable, and there might even be a few with seeds too small to bother anyone. West Africa already has a soft-seed type, known formally as subspecies mucospermus, so this path seems to merit exploration. Unusual Plant Forms Vines are always wasteful of space and awkward to cultivate. A dwarf, bush habit that still fruits on the ground could be extremely valuable to growers. This would be especially so if the plant were not too compact (because the shady dampness beneath a dense canopy tends to encourage diseases and pests). The possibility of such types occurring in the vast genetic wealth within Africa deserves exploration. Other horticulturally useful forms also probably await discovery. 5 Production generally involves crossing a tetraploid plant (four sets of chromosomes) with a normal diploid (two sets). The resulting triploid (three sets) is sterile and its ovules small and empty of seed. To complicate the procedure even more, a diploid plant must be present to pollinate, fertilize, and trigger fruit-set in the tetraploid pistillate flower.
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Lost Crops of Africa: Fruits, Volume III PROSPECTS Within Africa itself, as well as for other parts of the world, the watermelon’s unprepossessing ancestors have more potential than might be imagined. Considering the genetic wealth to be found in wild African types, even the places that supposedly know the crop best would have to concede they have hardly tapped its many potentials. Further, it is possible that–with or without incorporating genes from wild types–a watermelon rebirth will occur across Africa by returning home with types already known elsewhere. Within Africa As we’ve indicated, watermelon seems to have untapped promise in many parts of the continent, including, in this case, those parts that lie to the north of the Sahara. Humid Areas Fair. At first sight, the prospects in Africa’s lowland tropics are moderate because humidity enhances leaf diseases such as powdery mildew and root rot. On the other hand, though, cultivars bred to resist these fungal afflictions have made Florida and contiguous parts of the humid South the main watermelon-growing region of the United States. Such resistant types hold the key to greatly enhanced watermelon use in the even more fungus-challenged quarters of the globe, including Africa. Dry Areas Excellent. Because of its deep-roots, watermelon is a particularly good crop for dry season use or drought-prone areas.6 It is commonly planted as a relay crop in sorghum fields; the species being so drought tolerant that if the sorghum crop—or even the millet crop—fails for lack of water, there still can be a fair harvest of watermelons.7 Upland Areas Moderate; perhaps excellent. The prospects here depend on the elevation and the latitude. Watermelon requires a long growing period with high temperatures and much sunlight. For humid sites disease is a concern, and resistant cultivars should be employed. Beyond Africa Although many millions of tons of watermelons already are grown each year, prospects for boosting global consumption seem good. One indirect indication of their rising popularity is the observation that watermelon festivals continue to proliferate. The fruit ships well and has a reasonable shelf life. It has been suggested that sweet, small-sized, pink-fleshed 6 One contributor wrote, “I have seen it growing well in North Kordofan in the summer surrounded by pearl millet that was dead or dying.” 7 This is done in south-central Mali, to mention just one location out of many.
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Lost Crops of Africa: Fruits, Volume III A roadside watermelon vender holds market in Ghana. Watermelon fruits offer prospects for rural development almost anywhere they can be cultivated because they are generally easy to grow, easy to transport, easy to store, easy to sell, and easy to eat. (Scott Mori) specimens would find an almost limitless market in Europe. Dried or roasted seed, already popular in parts of Africa and Asia, also has promise for much greater use as a foodstuff. USES This is another crop offering multiple products. The edible parts are notably the crisp flesh of the fruits, but also the rinds, seed kernels, tender young leaves, and flowers. Fresh Fruits Watermelons are of course mostly eaten fresh as snacks or desserts. Many places make a refreshing drink from the juice, often incorporating a dash of lime or lemon juice. In Russia especially, the juice is boiled down into sugary syrup. In Namibia, the juice is fermented into a refreshing, lightly alcoholic drink. Rind In parts of Africa, the rinds are sliced and dried. The resulting brown circlets are sweet, and are eaten cooked. The fresh rinds are also often carved as table decorations—typically, the fruits are cut in half, scooped out,
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Lost Crops of Africa: Fruits, Volume III and then heaped with colorful mixtures of edible treats. Pickled watermelon rind is widely eaten in parts of the southern United States. In much of Asia, watermelon chunks are similarly preserved in brine. Seeds In parts of West Africa, particularly Senegal, seeds are used in food preparation. Usually after roasting or frying (which imparts a nutty flavor), the small dried kernels are cracked between the fingers to extract the oily white seeds. These are ground into a smooth meal that supplements cereal flours (such as for use in couscous), or mixed as a pasty thickener to form soup broth. Roasted, pounded seeds are also wrapped in green leaf, steamed, fried, and marketed as a snack with red pepper sauce. In Nigeria, seeds are squeezed into melon-seed balls that are fried (robo) or steamed (monu). Vegetable Oil The oil in the seeds is yellowish, edible, and semi-drying. It is suitable for cooking and salad oil. It is extracted locally in West Africa by boiling a mass of pounded seeds and decanting the oil off the top, or by applying pressure to seeds that have been pounded and steamed. In Israel, where seedless fruits are sold in the markets, the fruits of the male pollen donors are collected and their seed extracted for snacks. Leaves The young leaves and shoots are widely cooked as vegetables, added to soups, or used as a relish. In East and southern Asia they are often a component of salads. Flowers These are edible and said to be delicious battered and deep-dried (like pumpkin flowers). Animal Feed Wild watermelons are often the only source of moisture for animals, both wild and domestic. Moreover, the seedcake left after the oil has been extracted can be used as a livestock feed. NUTRITION At first sight, watermelon fruits are hardly nutritious; no one eats them for their health.8 The edible portion, which typically constitutes about 60 percent in the modern types, is about 90 percent water and 8 percent carbohydrate, essentially sugar. Watermelon, on a dry-weight basis, could be considered a significant source of provitamin A (more than 300 RE per 100 g for some red types) and vitamin C (nearly 100 mg per 100 g). However, since watermelon is mostly water, not everyone will sit down and eat their quota … though many 8 “Lets face it, it is not broccoli!” a contributor wrote. “It is nothing more than a sack of water with a high sugar content that is a gift from the gods. No one really cares what vitamins or minerals it has. They appreciate it for what it is, and are thankful.”
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Lost Crops of Africa: Fruits, Volume III people do. Nonetheless, a “normal” serving provides a good nutritional boost, up to a quarter of daily needs. Red types may have more provitamin A activity than yellow or white forms, as the red color comes from the pigment lycopene, a carotenoid of special interest these days as a potent antioxidant. At about 80-100 mg lycopene per 100 g dry weight, the red flesh can contain half-again the lycopene of fresh tomatoes. As for minerals, watermelon is a fair source of potassium (about 120 mg per 100g), but not as good as banana (about 400 mg) or tomato (about 220 mg). It is low in sodium, high in fiber. Unlike the fruit, the seeds seem very nutritious even at first sight. They contain 20-50 percent or more oil and 20-40 percent protein as well as good quantities of minerals and B vitamins. More detail is given in the Egusi chapter of the companion volume on African vegetables. HORTICULTURE The watermelon’s cultivation is too well known to require great detail here. The plant is grown from seed. On dry sites, planting is done as the rains begin; on moist sites the crop is sown as the rains end. For good germination, soil temperatures must be at least 20°C. Seeds, in groups of 1-3, are sown 2-4 cm deep in trenches, on mounds, or in widely spaced planting holes. Later, the seedlings are thinned to one per station. Alternatively, seedlings may be raised in containers and transplanted when 10-14 cm high. Ideally, the fruits should be matured on a pad of grass or straw for protection against soil pests and blemishing. Pollination is by insects, notably honeybees, though other bees may be more effective. The viney crop is often inserted into a rotation following the harvest of a staple such as maize, millet, or sorghum; in many Asian and African countries, it is also commonly grown as an intercrop, directly with the main crop. In Africa, some watermelons are never planted at all; after a field is cultivated, some are left to sow themselves the following season. However, the selected types used for cooking or eating fresh are always planted, also usually as an intercrop. The wild strains used as emergency food and for their seeds are left to sow themselves on the edges of cultivated fields and in unfenced areas. Pests of the growing plants include melon fly (Daucus spp.)—the most serious pest in Africa, but some wild strains apparently remain unaffected. Root knot nematodes are also problematic. Major diseases include bacterial fruit blotch, damping off, anthracnose, powdery and downy mildew, Fusarium wilt, gummy stem blight, and various viruses (e.g., watermelon mosaic virus). The viruses are transmitted by aphids and cucumber beetles, which must be controlled. A number of watermelon varieties show resistance to Fusarium wilt and these should be employed where soils are infected with this fungus. As in tomato, calcium deficiency can result in a disorder called “blossom-end-rot.”
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Lost Crops of Africa: Fruits, Volume III HARVESTING AND HANDLING Yields are usually extremely high—at least when measured on the basis of wet weight. Each plant of the cultivated types generally produces 5-10 fruits—some of which can be very large. Production ranges from 5,000 to 7,500 fruit per hectare for standard varieties and commercial cultivation methods. This usually equals about 37 to 75 tons per hectare. With the adoption of newer production practices, yields of 120 to 175 tons per hectare are achieved using plastic mulch, row covers, drip irrigation, and hybrids.9 Watermelons reach maturity approximately 45 days after blooming, although the timing is highly dependent upon cultivar. They typically are harvested when the tendril nearest the melon is wilting and the “ground spot” on the bottom of the fruits turns from white to yellow. They are hand harvested and should be cut cleanly from the vine to avoid stem damage and prevent stem-end rot. Because of their minimal starch content, watermelon ripens very little after harvesting. Optimum storage temperatures are reported to be 7-10°C.10 Below that, they are subject to chilling injury and loss of quality. The fruits are generally consumed within 2 to 3 weeks after harvest, primarily because of the gradual loss of crispness. Quality is determined largely by sugar content, color, and texture, all of which depend on maturity, cultivar, and handling methods. Commercial melons for distant markets are usually harvested when slightly less than fully mature. These details all of course relate to watermelon cultivated for its sweet flesh. No figures are available on the yields or handling of the wild plants, but in Botswana it has been noted that they produce around 8 fruits per plant. Wild strains are harvested after other crops, and long after the vines have died. Some traditional types will store well for several months and still be edible for a year, and sometimes longer. NEXT STEPS In advancing the watermelon to new heights, there is an excellent role for African leadership. As the primary center of genetic diversity, Africa could become the seedbed for change. Many opportunities for improving the strains remain. Two factors are favorable: 1) the cultivars are very heterozygous; 2) inbreeding does not seem to reduce vigor. For the adventurous African plant breeder looking for a challenge, the search for unusual watermelons offers opportunities for satisfaction, perhaps profit, maybe the chance to open up new markets, and possibly enhancing the security of many people. Even individual growers could set up test plots, 9 About half of China’s crop is cultivated using plastic film covering the ground. This is said to increase yields 50-200 percent, raise sugar content 0.7-1.5 percent, and reduce the maturing period 10-15 days. 10 Nonnecke, I.L. 1989. Vegetable Production. Van Nostrand Reinhold, New York.
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Lost Crops of Africa: Fruits, Volume III Small watermelons–often called “palm” melons because they fit into the palm of the hand–are becoming increasingly popular in the United States, and even smaller ones are available in Asia—in China, Taiwan, and Japan for instance. At 2-3 kg, however, most are still too large (or have other shortcomings such as texture, flavor, seediness, or durability) for them to reach their full market potential as “single-use” fruits. Africa’s wild watermelons can be as small as 2 cm in diameter, and likely contain “downsizing” genes that innovative horticulturists could employ to create handy fruits of less than 1 kg. (George Boyhan) challenge types from around the world to local diseases and harshness of all kinds, indeed, become the global go-to experts for the future of the crop. Examples follow. Upgrading Watermelon In addition to direct use, there is the possibility of employing genes from Africa to influence the watermelons used around the world, so now seems a good time to pay more attention to the cultivated watermelon. In cost-per-kilo it is usually the most economical fruit, and it is already changing its spots in industrialized nations, where new varieties with potential to help farmers and consumers in less fortunate regions have also been developed. Further, much has been learned in recent years about the genetics of drought tolerance, disease resistance, and other desirable traits, making “new” genes even more valuable. Today, watermelons are taking on whole new looks, as yellow, seedless, and small-sized types grab increasing market share. Public demand for greater variety has led to more intensive breeding efforts, which in turn greatly increase the possibilities that totally unexpected qualities will manifest themselves to the alert breeder.
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Lost Crops of Africa: Fruits, Volume III Food Security In particular, watermelon can become a more productive and more reliable source of both sustenance and water for the people, livestock, and wildlife eking out life under some of the world’s most tenuous circumstances. While improved types can help, in appropriate drought-stressed parts of Africa the wild watermelon areas might also be protected from commercial over-exploitation; they are primary means of survival for desert peoples and wildlife during periods of drought. Genetic improvement Genetic improvement might include such things as the following: Plants with vigor, earliness, high yield, exceptional sugar content, and resistance to disease (Fusarium wilt or anthracnose, for instance); Fruits whose rinds are thin and yet strong enough to withstand damage during handling and storage; Fruits whose flesh is crisp, sweet, and free from stringiness; Fruits with few or no seeds; and Plants producing high yields of seed and seed oil. An intriguing possibility is hybridization with other species in the genus, especially Citrullus ecirrhosus and C. colocynthis (see below). Both contain useful genes, especially for drought tolerance and perhaps for disease resistance as well. Watermelon landraces in Niger already appear to contain genes from the latter, presumably from natural hybridization. Nutrition Although food value is not a paramount virtue of this crop, nutritional analyses should nonetheless be done on each of the cultivars. The carotene and lycopene contents should especially be compared. This would help to encourage higher production of the more nutritious types and a search for even better ones. Genetic diversity There’s no telling what strange types might be found from the wilds of the Kalahari as well as the Sahel. The future is wide open to discovery. These are also excellent opportunities for sharing the benefits of such biodiversity with the communities living in these areas. Vegetable Types Many Africans eat watermelon seeds as an important source of protein and vegetable fat, so large-seeded varieties are also useful. The seeds of these special types are promising sources of oil and protein, nutrients that are especially valuable in drought situations when watermelons are often the only crops left growing. Investigations to enhance the “vegetable” characteristics of the flesh might also seem in order.
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Lost Crops of Africa: Fruits, Volume III SPECIES INFORMATION Botanical Name Citrullus lanatus (Thunb.) Matsumara & Nakai Family Cucurbitaceae Synonyms Citrullus vulgaris Schrad. ex Eckl. & Zeyh.; Colocynthis citrullus (L.) O. Ktze. Common Names Afrikaans: wartlemoen, waatlemoen Botswana: marotse/makatane (an orange-fleshed type for cooking only), mmamonwaane (a white-fleshed type eaten raw), mokgatse (a yellow/white type cultivated for stock feed), tsamma (wild type) English: watermelon, edible-seed melon Ethiopia: hab-hab (Am/O) French: pasteque German: Wassermelone Kenya: mtikiti, masindi Malawi: chimwela/o, mavwende (Ch), litichiti (Y), chimwamaji (Tu) Mali: zéré, zere (bambar) Mauritius: melon d’eau Namibia: oontanga (Oshiwambo) Nigeria: ibara, bara, egusi ibara Somalia: Kare (Som) Russian: arbuz Sudan: khujar, bateech, buttiku (Arabic) Tanzania: mtikiti, maji/mkubwa, masindi Zambia: chimwanyanza, chivwembe, ntanga (Ny), ntanga, chitatakunda (B) Zimbabwe: muvembe, mugibe, munwisi, munwiwa, muvise/i (C), inkhabe (Nd), makavatya (H), budzi (W) Description The plant is an annual climbing or trailing herb with long runners. Some have a fetid musky odor. Its pinnately lobed leaves distinguish it from melon and cucumber (Cucumis) and pumpkin and squash (Cucurbita). Watermelon is monoecious, with the pale yellow male (staminate) flowers blooming first. Insects, especially bees, transfer the pollen from male flowers to female flowers, making fruit set possible. The vines carry anything from 2 to 15 fruits weighing up to 50 kg or more. Seeds may be white, green, yellow, brown, red, or black in color. Wild watermelons look like conventional watermelons in size and shape,
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Lost Crops of Africa: Fruits, Volume III except for the wild tsama, which is small and round (average diameter 15 cm or less), with flesh varying from orange to white. Distribution This native of southern tropical Africa is now widely spread throughout the tropics, subtropics, and warm-temperate zones. Horticultural Varieties There are hundreds of varieties, and most countries have local cultivars; numerous websites list their merits. Environmental Requirements Watermelons are a warm-temperature crop requiring a relatively long, hot growing season (usually about 4 months of frost-free weather). Although drought-tolerant, they require a steady supply of water for best fruit production. A committed grower needs the right kind of soil, long warm summers, and not too much rainfall. Rainfall The plant may require only a small amount of rainfall (250-500 mm), since the root system can usually exploit deep soil moisture. Excessive rainfall and relative humidity reduce flowering, and encourage development of leaf diseases. Waterlogging kills the plants. Altitude Watermelons grow well up to 1,000 m in the subtropics, and may reach 1,500 m above sea level at tropical latitudes. Low Temperature For growing watermelon the optimum temperature range is 23-27°C. Growth stops below about 18°C and the plants are very susceptible to frost. This limits their production in regions with cool summers or sharp nights. For germination of seeds, the minimum soil temperature at 5 cm depth is 15°C. High Temperature Wild melons of southern African deserts grow where the temperature is often 36°C. While temperatures over 30°C during blooming may reduce fertilization in many types, most plants tolerate higher temperatures for short periods. Temperatures of 40°C and above have been measured in Botswana, though extremes can also damage ripening fruit. Soil Watermelons grow on any type of soil, but do best on well-drained, sandy loams, with good moisture-retaining capacity and high organic matter. They grow successfully on soil of low fertility. Soil depth should be at least 10 cm. They tolerate both acidity (pH as low as 5.0) and alkalinity (up to 8.0); the optimum range, however, is pH 5.5-7.0.
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Lost Crops of Africa: Fruits, Volume III Related Species Watermelon belongs to the family of climbing plants that includes gourds, melons, gherkins, cucumbers, and loofahs. The genus Citrullus contains three or four species, native to tropical and subtropical Africa, one (C. colocynthus) being also native to South Asia and perhaps Australia. In Namibia, the word tsama (or tsamma) is used not only for the wild watermelon, but also for Citrullus ecirrhosus Cogn., or “bitter apple.” This desert perennial lives where rainfall is almost nonexistent, and it has a tremendous capacity to survive drought. Not only does it employ water from the very occasional rainfall, it has a remarkable ability to reach moisture deep underground and possibly to employ morning fogs as a moisture source. Its fruit is inedible unless reboiled many times, but its genes might be useful in helping the watermelon crop survive drought even better. Though cucurbitacins are in the seed (as well as the fruit), the oil can be decanted from crushed seed to remove these bitter substances in times of dearth. It has been successfully crossed with watermelon, opening the possibility for new genetic advances in one of the world’s special crops. Another “bitter apple,” native from Africa to India, is Citrullus colocynthus (L.) Schrad., also called the “vine of Sodom.” Growing at the edge of rainfall, this perennial has edible, even nutritious, seeds, which are often found in archeological digs. However, the unripe flesh contains an exceptionally bitter alkaloid and resin that combined creates one the most violent purgatives known. For this reason, it has been studied in some detail, but its potential contributions to watermelon–in the future and perhaps even in the past–still await the curious to discover. Both species have great bitterness, something they share with the ancestor of watermelon. The fact that our ancestors worked with such a raw ingredient to create today’s watermelon, the epitome of sweet summer juiciness, bodes well for the future of many such “unpromising” fruits.
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