Lost Crops of Africa: Volume II: Vegetables (2006)

The plant known internationally as yambean is arguably one of the most interesting of all the world’s new food crops.¹² This species, which hails from the Americas, looks like a bean plant above ground but is actually grown for the swollen roots it produces below ground. Since the dawn of history these yam-like tubers have fed tropical Americans. Then Spanish galleons³ carried the seed across the Pacific, and this productive, palatable, and nutritious legume subsequently spread through Asia and became a market-garden favorite from China all the way to India.

In recent decades yambean has taken on renewed momentum and is now among the world’s fastest rising new crops. Already it is the top selling specialty vegetable in the United States, its tubers being sold in many (if not most) U.S. supermarkets under its Mexican name, jicama. Americans buy the round, squat tubers for use in salads, for replacing scarce water chestnut in Chinese cooking, and for a low-calorie snack food. Demand has risen to such an extent that Mexico now exports half a million tons annually.⁴

In Europe, this food is catching on, too. Part of Thailand’s large yambean output, for instance, is now shipped to many Asian stores in major European cities. This American crop has even entered production trials in Portugal, where, under conditions seemingly so different from its native tropical habitat, it has demonstrated very impressive yields: 54 tons per hectare, with up to 24 percent dry matter containing 10 percent crude protein.⁵

Seeds of three varieties of African yambean from farmers’ fields in Nigeria (soybean lower-right for comparison). The seeds are notably nutritious, and experiments in Nigeria have yielded of 2 tons per hectare, an exceptional amount for an undeveloped legume. Starting from such a base, the African yambean—given good research support—would seem capable of ultimately yielding as much as the best bean crops. (Thomas Betsche)

More to the point, though, is the fact that during the last few years yambean has also arrived on Africa’s shores. Here, too, it has been a smash hit, creating what a researcher describes as “remarkable success in a number of West African countries.” Again, yield was the major draw. In trials carried out in Benin, for example, two genotypes produced around 80 tons of tubers per hectare.⁶ At separate locations in Senegal, the per-hectare production was 40 and 100 tons.⁷

That is an enormous quantity of food for such a small area to produce and, although the tuber has a rather exotic texture and taste, West Africans have taken to it. Indeed, thanks to local media coverage, it has generated such intense public interest that the biggest problem, so an observer reports, is finding enough seed to meet the need. Stemming from this has come one unforeseen consequence: “A peculiar situation has arisen,” the observer writes, exhibiting great circumspection, “with several of the field trials subjected to ‘unauthorized testing and sampling’ at night by local farmers!”

Given all these developments across the various continents, it seems little wonder that this species is steadily climbing upward toward becoming a truly global resource. But a seldom-recognized fact is that Africa has its own counterpart. The so-called African yambean (Sphenostylis stenocarpa) is not uncommon in central and western Africa, especially southern Nigeria. Outside tropical Africa, however, no one has seen it and few heard of it. Indeed, not only has the African yambean not taken even a baby step toward being a global resource, it actually is moving in the opposite direction…toward extinction.

Although classified as belonging to a different genus, the African yambean is closely related to the American version and also is grown for its fleshy swollen roots. Its tubers, however, are elongated and look more like sweet potatoes than yams. In nutritional terms, they are a class above the mainline root crops, containing more than twice the protein of sweet potatoes, yams, or potatoes and more than ten times that of cassava. Moreover, the protein is of exceptional nutritional quality, superbly complementing the proteins of maize, sorghum and the other staples. Eating African yambean together with those major foods helps provide the body a “complete protein.” The combination, in other words, closely matches the chemical requirement for constructing the thousands of separate proteins human bodies need to make constantly.

And that is just one of the surprises this minor and almost lost crop conceals. Another is that this almost unknown African resource surpasses its transatlantic counterpart by producing both edible seeds and leaves in addition to the edible tubers.

All yambeans are unusual in that they are legumes, a plant family renowned for peas, beans, soybean, peanut, and other nutritious seeds, but not for edible roots. Yet the yambeans’ swollen underground stems are succulent, white, sweet, mildly flavored, and crisp as a fresh-picked apple. They can be eaten out of hand. They can be used to add crunch to green salads and fruit salads. They can be steamed or boiled, and have the unusual property of retaining their crispness even under conditions that convert potato to mash. In cooked form they taste like potato, but whereas it averages 5 percent protein, African yambean tubers have from 11 to 19 percent protein (on a dry-weight basis).

And good taste and good nutrition are just the start of the appeal. African yambean appears to combine qualities that make it attractive to the farmer, the processor, the consumer, AND the environment: good adaptability to a wide range of climates and soils, reliable yield, a good balance between protein and starch, agreeable taste, good shelf-life, biological nitrogen fixation, and energy efficiency.⁸

All this makes one wonder why a species combining the reliability of a root crop with the protein content and high sustainability of a legume is not better known. Certainly, Africans familiar with the plant regard it highly. Trouble is, only a few of the continent’s 600 million inhabitants have heard of it, let alone tasted it. It is mainly restricted to pockets of West and Central Africa, where smallholders grow it exclusively for their own use. Thus, even within the region that knows it best much of the populace is unaware of the botanical gem in their midst.

That botanical gem remains unpolished, however. Perhaps because it seldom enters commerce to any degree, it has received little formal recognition from the agricultural authorities. Indeed, there is so little understanding of it that only sketchy summaries and a scattering of research papers highlighting specific technical features can be found in the international literature.

Yet African yambean is not unimportant in people’s lives. Cultivated in Nigeria mainly for seed, it is also grown for tubers in Côte d’Ivoire, Ghana, Togo, Cameroon, Gabon, Democratic Republic of Congo, Ethiopia, and parts of East Africa, notably Malawi and Zimbabwe.

Moreover, the plant is adaptable, and capable of growing anywhere that common yam is cultivated.⁹ Indeed, it could be grown in many more locations than that, not to mention grown much more successfully in those locations where it already exists. A vigorous vine that climbs and twines to heights over 3 m, the plant thrives in weathered soils where the rainfall can be extremely high. It tolerates even the acid, infertile, highly leached sites that are the humid tropical lowlands’ special curse.

Part of the reason for this adaptability to bad substrates is that like other members of the legume family, the African yambean enjoys a symbiosis with bacteria that fix nitrogen from the air. The invisible bacterial microbes inhabiting its roots relieve the farmer of the necessity to supply additional nitrogenous fertilizer. They also make the plant a fine candidate for sustainable development purposes. This is, in other words, a food source that supports itself while helping both the soils under it and the species that succeed it.

And the African yambean is no slouch in the yield department, either. It produces its seeds, tubers, and biomass in abundance:

As far as the seeds are concerned, experiments in Nigeria have demonstrated yields of 2 tons per hectare,¹⁰ an exceptional amount for an undeveloped legume. Starting from such a base, the African yambean, given good research support, would seem capable of ultimately yielding as much as the best bean crops. As mentioned, the seeds are notably nutritious, with crude-protein levels ranging from 20 to as high as 29 percent. Although this is less than the amount in soybeans (38 percent), the protein contains levels of essential amino acids likely to make African yambean seed the soybean’s nutritional equal. Lysine, for example, comprises up to 8 percent of the protein, and methionine and cysteine together may comprise 2.4 percent.

But it is the below-ground product that is of greatest immediate interest. The tuber yield varies between cultivars and has not been pinned down with precision. Nonetheless, it is generally high. If grown in pure stands, 100,000 plants per hectare is a reasonable field density, and each produces up to 500g of tubers. This already good figure can undoubtedly be raised—possibly dramatically—merely by preliminary research attention.

Of the biological nitrogen fixation, only a few details have been reported. Hopefully, the actual amount fixed can eventually be raised to a level similar to that of its American counterpart: 200 kg of nitrogen per hectare.¹¹ In that transatlantic species, about half the nitrogen, or more than 600 kg protein per hectare, accumulates in the tubers. This is an amazing amount for a hectare of root food. It approaches or exceeds the stellar seeds of soybean and peanut, which are considered world protein-production leaders.

Any root crop even vaguely capable of delivering record quantities of protein from soils normally considered marginal would seem to deserve intense global attention. For all its potential, though, African yambean remains a neglected, even primitive, resource. There has never been a concerted steady effort to advance it, despite the fact that throughout the tropics root foods are increasingly sought. Because of this neglect, the farmers who know it best are switching to crops for which more help is available, and sadly their heritage of seeds and age-old wisdom with the crop are slowly fading away.

African yambean tubers, with their succulence and crunchy texture, could appeal to millions of palates. Moreover, it grows easily and is well suited to the difficult environment of the hot wet tropics where the climate and conditions now constrain local diets to far less nutritious foods. (International Grain Legumes Information Centre)

This trend must be changed. A plant that perhaps could benefit millions of the malnourished deserves urgent attention. The highly efficient way in which it absorbs nitrogen makes it an especially attractive tool for helping farmers whose land is worn out. Moreover, it grows easily and is well suited to the difficult environment of the hot wet tropics where the climate and conditions now constrain local diets to far less nutritious foods. And the tubers, with their succulence and crunchy texture, could appeal to millions of palates. Possibly the African yambean could do much more than just help subsistence farmers eat better…it might make a valuable cash crop across regions that desperately need a biological fulcrum for leveraging rural development upwards.

Given the fact that little has yet been done to advance this neglected vegetable it is hard to project how far it might ultimately go in helping humankind. Nonetheless, some sense of the promise can be grasped by a quick comparison with its transatlantic counterpart, which as noted has become one of the fastest rising new food crops and is already a megamillion-dollar resource.

Humid Areas Excellent. The plant is found growing through much of tropical Africa and is fully at home in lowland areas where the rainfall is high and nutritious crops hard to come by.

Dry Areas Unknown. In Nigeria the species is cultivated from the tropical forests to at least the savanna’s edge. The issue of growing it in dry areas is less one of biology than of economics; root crops typically are drought resistant but they need copious water to yield profitably.

Upland Areas Excellent. Although often classified as a low-altitude legume, it seems little affected by height above sea level, and flourishes at elevations up to at least 2,300 m.

In other tropical regions of the world this plant would likely thrive, but no one has tried to find out.

Seeds and tubers represent the primary food, although the plant also has potential utility producing feed for livestock and green manure for soil restoration.

Seeds The seeds may be eaten alone or in soups, and are commonly served with yam, maize, or rice. They are said to be delicious and to be “often preferred over other types of beans.”¹² The leathery or slightly woody

pods are typically bundled together and hung over the fireplace, where they stay safe until consumed or sold. The beans extracted from the dried pods are prepared in several ways. Many are partially roasted over a fire and eaten together with palm kernel. Others are soaked several hours, boiled 4 to 6 hours, and eaten like common beans. In another procedure, they are squeezed with palm oil and chili pepper as well as various spices and vegetables to form a paste, which is wrapped in banana leaves and heated over the coals. In every case, so it is said, African yambean seeds make a nutritious, filling, and tasty meal.

Roots The tubers are eaten either raw or cooked. The exact details are still unclear, but some idea of their eventual utility can be inferred from the yambean in Mexico, where:

• Raw tubers are cut into sticks and sprinkled with lime juice and chili;¹³

• Fresh tuber slices are added to salads of both the dinner and dessert kinds;

• Cooked tubers are used on their own or with other vegetables to prepare soup;

• Chopped tuber is added to Oriental stirfries;

• Tubers are grated and boiled in milk to create a tasty drink;

• Sliced or diced tubers are pickled with onion and chili to form a popular snack food; or

• Tuber segments and green beans are preserved in vinegar as a sort of three-bean salad.

Leaves Although the leaves are said to be edible, nothing more than those few words are on record, so the extent, mode, and safety of eating them remain uncertain. The leafy vegetation remaining from the harvest provides useful fodder. Likely, it is very beneficial to livestock, due both to its protein content and the massive quantities produced.

Sustainable Agriculture Of all the 17,000 nitrogen fixing species in the plant kingdom, yambean seems to be among the most efficient. On a per-hectare basis, the American species has been recorded as regularly producing 120-150 kg of nitrogen.¹⁴ This is higher than that recorded in other grain-legume residues, and gives yambean an important role in crop

rotations, a fact known for centuries in tropical America. In many traditional farming systems in Mexico, for instance, the crop is employed to restore soil fertility after years of cropping maize and cassava and cotton and other nutrient-draining species. African yambean could well prove to have a comparable soil-restorative capacity. Already, there is preliminary evidence that African yambean could make an excellent species for crop rotations, for ground cover, for binding soil and related purposes.¹⁵

Other Uses With its prolific spattering of large, colorful, dangling flower clusters—pink, purple, or greenish white, among other hues—this vine makes a vivid ornamental, reminiscent of wisteria (although the flowers are much smaller and the inflorescences are upright, rather than hanging).

African yambean is a nicely rounded non-fat food: from 50-75 percent carbohydrate, 20-25 percent protein, around 1 percent oil, and 5 or 6 percent fiber, all providing nearly 400 calories per 100 g dry-weight. Although the protein is produced in copious quantities, that is not necessarily the ultimate measure of a proteinaceous foodstuff. More important is the protein’s nutritional quality, because a protein lacking certain minor components is nutritionally next to useless. In a protein, quality is judged by the incidence of a few amino acids, and many plant proteins, being deficient in lysine and methionine, are low in nutritional quality. On the other hand, African-yambean seed protein contains those two in abundance: lysine 7-8 percent and methionine 1-2 percent. And the levels of the other essential amino acids enhance that already exceptional condition. In one seed sample, for instance, the overall complement was: cysteine 1.9, leucine 6.6, lysine 8.3, methionine 1.2, phenylalanine 4.8, threonine 3.3, and valine 4.1 percent.¹⁶

As mentioned, the tubers are nutritious too. The raw protein in those swollen root tissues amounts to 11 to 19 percent, which puts the plant into a nutritional class above the major root crops. The tuber protein is also of high quality: cysteine 1.8, isoleucine 4.5, leucine 7.7, lysine 7.6, methionine 1.7, phenylalanine 4.5, threonine 4.3, and valine 5.5 percent. Adding to the tubers’ nutritional contribution is their 63-73 percent content of carbohydrate and their 3-6 percent fiber. The starch alone has been put at 65 to 70 percent and about 370 overall calories.¹⁷

Chimaliro Forest, Kasungu Province, Malawi. African yambean is a legume whose fleshy swollen roots look something like sweet potatoes but are succulent, sweet, and crisp as a fresh-picked apple. In nutritional terms, they are a class above the mainline root crops, containing more than twice the protein of sweet potatoes, yams, or potatoes and more than ten times that of cassava. Moreover, the protein is of exceptional nutritional quality, superbly complementing the proteins of maize, sorghum and the other staples. In addition, both seeds and leaves are edible. And the African yambean is no slouch in the yield department, either. It produces its edibles in abundance, and seems capable of delivering record quantities of protein from soils normally considered marginal. (Søren Døygaard)

The true importance of the quality proteins from both beans and tubers lies in their ability to complement cereals such as maize and sorghum as well as roots such as yam, cassava, and sweet potato. When measured against the uncompromising necessities of human nutrition, those staple foods are deficient in essential amino acids. This fact of nature creates a nutritional crack in the foundations of Africa’s food supply because people living primarily on cereals and roots (which includes many of the poor, the sick,

and the very young) can run out of a dietary essential amino acid. When that happens their bodies stop producing all protein—brain, muscle, hair, antibodies, enzymes, blood cells, skin, and the rest. In such situations, the addition of even a small amount of the missing essential amino acid from, say, yambean raises the overall protein effectiveness out of all proportion and restores the bodily processes to normal operation.

For children, especially, yambean may be valuable in this regard. In normal situations, adults eat cereals with sauces that contain small amounts of protein and vitamins. But children typically find the sauces too peppery and are served cereals without that special nutrient supply. Adding some yambean would make up for the loss. In addition, children can nibble on the raw tubers to get quality protein.

Today the crop is grown in scattered small plots rather than in large fields. Although it can be cultivated alone, it is mostly grown with yam or maize. At the beginning of the wet season farmers plant it using seeds, small tubers, or pieces of root. Like yam, it is normally planted on ridges or little hills. It is also normally supported on trellises or stakes—often the same ones supporting the yam vines. However, tests in Nigeria suggest that it grows and yields tubers satisfactorily even unsupported.

If the maximum yield of tubers is desired, the above-ground parts should be severely pruned back. We don’t know how widespread this “reproductive pruning” process is in Africa, but in the Americas it is considered the key to achieving big yambean tuber crops. There, both the flowers and flower buds are pruned as many as four times a season, with the first cut being made about 2 months after planting. The target is the reproductive parts; a few flowers may be left for seed production, but the rest, and the main growing shoots, are rigorously removed. The plant responds by pumping its energy into the parts underground.

Normally no special irrigation is applied. Nor is nitrogenous fertilizer needed, although experience in Mexico suggests that a shot of phosphorus could be helpful.

Pests are seldom serious, and typically they are the same ones affecting other legumes in the area. One report lists the African yambean’s major insect attackers as bean pod borer (Maruca testulalis), stem-boring beetle (Sagra adonis), and the variegated grasshopper (Zonocerus variegatus). The exact mix of pests, however, is likely to vary with location. Controls that work on other legume crops should prove equally effective on African yambean.

Diseases are also similar to those of other local legumes. The main fungal threat is downy mildew (Phytophthora phaseoli). An unidentified yellow mosaic virus has been observed attacking the plant in Nigeria.

Like most legumes, this one is highly susceptible to nematodes.

The African yambean typically begins flowering 80-120 days after planting. Pods start maturing at about 150 days after sowing. From then on, flowering and pod formation continues for as long as the climate remains conducive. As a practical matter, though, harvesting usually ceases after 60 days.

The tubers develop more slowly than the flowers, normally taking 5 to 8 months to swell to harvestable size. Most are dug up toward end of the rainy season. They can, however, be harvested early (if local preferences encompass small or medium sized tubers) or they may be left in the soil for a time after the rains cease.

As to handling the tubers, methods developed for the related species in Mexico probably apply. There, the ridges or little hills of soil are loosened with a hoe or with a crossbar mounted on a tractor. Each tuber is then lifted by hand, and the vegetative top is removed with a pair of shears and left in the field for later use as forage or organic fertilizer. Finally, the tubers are collected, sacked, and stacked for shipment.¹⁸

The optimal storage temperature for the Mexican tubers is given as 12.5-17.5°C. And the only treatment given them is washing, trimming (to remove roots and stem parts), and dipping in hypochlorite solution to sterilize and bleach the surface. The tubers then remain usable for about a month, provided they are kept indoors and well ventilated. The African yambean tubers can probably be treated similarly.

A number of limitations need be kept in mind:

• The crop is slow to mature and is considered sensitive to daylength.

• As in many beans (and other foods), the raw seeds contain trypsin and alpha-amylase inhibitors, tannins, oxalates, saponins, and phytic acids, as well as “potentially very toxic” cyanogenic glycosides (especially in white seed) and flatulence-causing alpha-galactosides.¹⁹ These levels were improved somewhat by thorough cooking (see below), and the levels likely vary significantly among different genotypes (as in common and lima bean). An alternate route suggested by the authors was Rhizopus- and lactic-acid fermentation, as used with soybean, which greatly reduces most of these antinutritional factors with much less energy cost, while yielding additional fermented foods for this part of the world, where such delicacies are already

so popular. Others have shown it makes good dawadawa paste.²⁰

• The dry beans need lengthy cooking, traditionally 4-6 hours (like most beans) but much longer (up to 12) to apparently have significant impact on undesirable compounds.²¹

• Certain peoples prefer certain seeds. Southern Nigerians, for instance, like darker types, while northerners want brightly colored seeds.

• High moisture content makes yambeans shrivel and lose condition more quickly than the mainstream root crops.

• Bean weevils (bruchids) can attack the seeds in storage, as they do other beans.

• The beans reputedly “sit heavy in the stomach” and are said to cause thirst and wind.

Clearly, this crop deserves the attention of modern science. There is little doubt that both basic and applied sciences can dramatically increase its productivity and usage. As of now, though, this foodstuff is known only in Africa, so most initiatives must be local.

Many research avenues can help make the crop more productive and more useful. Some are presented under the headings below.

Surveying the Scene A first priority should be the development of baseline African-yambean knowledge, including such things as geographic limits, traditional uses, and standard cultivation practices. Such basic expeditions can be simple, inexpensive, and enlightening.²² An initial survey in Ghana, for example, found “[African yambean] is used extensively in various dietary preparations and has potential for supplementing the protein requirements of many families throughout the year.” Important actions include:

• Initiating countrywide searches to locate the crop and its farmers;

• Documenting the methods traditionally employed for growing it;

• Publicizing the methods traditionally employed for cooking the different parts (including leaves);

• Assessing natural genetic differences throughout its range; and

• Collecting and evaluating representative germplasm.

The pool of knowledge resulting from such initiatives can provide insight into the African yambean’s existing limits, possibilities, and extension priorities. It will also point out ways by which farmers can be helped to manage the plant more intensively, more securely, and more productively. It will, in other words, provide preliminary guidance on the present status and the future steps, including best practices for planting, cultivating, controlling pests and diseases, harvesting, handling, and cooking the crop.

Support for Farmers Preliminary surveys should be quickly converted into advice to help those who are the keepers of Africa’s age-old yambean heritage. They should become, in other words, part of the extension agents’ advisory tasks throughout the yambean zone. The goal is to stop any more farmers abandoning the crop if they don’t really want to. Toward that end, other actions that can be mounted include:

• Marketing initiatives. African yambean should be quickly tested as a cash crop. In Mexico, the tubers sell for three times as much as cassava, and provide profit to farmers small and large.

• Taste tests and other popularizing activities.

• Demonstration plots. Local agronomists should undertake trials to optimize production. These would best be done in the local fields with the owner’s participation—perhaps incorporating a measure of competition among, and financial reward for, the best local yambean growers. The psychology of getting the growers involved may be as important as any technical advances achieved.

Food Technology

Almost nothing is known about the basic properties of the various African-yambean foodstuffs. Investigations should now be conducted into basic features, including:

• Digestibility and Antinutritional Factors Trials on the actual digestibility of seeds, tubers, and forage are required. For example, alpha-amylase inhibitors in the seed could reduce caloric uptake while increasing the intestinal gases for which many beans are renowned. The challenge posed by other antinutritional factors should also be examined. In particular, the presence and fate of potential cyanogenic glycosides in both raw and cooked portions should be traced from plant to plate, just as they have been in soybean, cassava, sorghum, and other staples carrying these compounds.

• Culinary Issues The long cooking time is a barrier to the plant’s wider use as a bean crop. Needed now is a search for the cause as well as for quicker-cooking types. As a first step, the starch in the seeds as well as the skin around them should be compared with those of other beans, such as

cowpea, common bean, and bambara bean, and correlated with the cooking time.²³

• Recipes Home economics groups should involve themselves in the rescue of this ancient native. The development of dishes for both seeds and tubers is one need. Yambean tubers, for example, may well prove an attractive addition to various traditional dishes, as well as a popular snack.

• Pods In India, young American yambean pods are eaten like French beans. Now is the moment to test African yambean counterparts to see if they have any value as “tropical snow peas.” The key is to search for and to measure the occurrence and fate of antinutritional factors such as rotenone.

Leaves The safety of eating the leaves (presumably as a boiled vegetable) deserves assessment.

Starch The chemistry of the tuber starch deserves investigation. Although it is said to be “comparable to that of cassava flour,” the granules are supposedly distinctly different.²⁴

Curing Tests on the storage of African yambean tubers are needed. Once the vegetative top has been removed, smallholders tend to leave the tubers in the ground until eaten or sold. The fate of the starch, protein, and other components under such treatment needs investigation. In the American species the tubers get much sweeter and more valuable when “aged.”²⁵

Plant Physiology This is the moment for an all-points inspection of the species itself. A plant as promising and yet as little understood as this one needs almost everything looked at. Topics needing exposure include:

• Nitrogen Fixation The efficiency of biological nitrogen fixation under both greenhouse and field conditions should be examined. Field collections of some indigenous strains of Rhizobium and Bradyrhizobium have been carried out, but the selection of genotypes and strains with high potential for nitrogen fixation still awaits.

• Protein Chemistry The biochemical, topochemical, and structural characterization of protein bodies of seeds and tubers need investigation.

• Seed Features The issues of seed set, seed size, and seed components deserve study.²⁶ One especial need is to check the content of rotenone in the mature and maturing seeds.²⁷ The different genotypes’ ability to retain germination capacity during long-term storage also needs analysis.

• Soil Requirements Tests should be run on soils of different pH, density, and fertility levels.

Genetic Development In order for this crop to remain attractive to local farmers, improved cultivars are needed. High priority should be given to those benefiting the traditional smallholders of West and Central Africa.

The plant’s dual propagation capability is a feature that will speed the process along. This is a rare crop that can be sexually propagated by seed as well as clonally propagated by tuber or root tissue. For one thing, it can be bred in the normal way using crosspollination and the resulting progeny can be multiplied clonally. This allows for flexibility, speed, and efficiency.

Genetic improvement targets of opportunity include:

• Higher yields;

• Bigger tubers;

• Seed of select colors;

• Bush-type plants that stand by themselves without staking;

• Daylength-insensitive plants that make the crop more reliable across seasons and latitudes;

• Fast maturity. Early maturing cultivars are known in yambean, and likely exist in the African form as well;

• Quick-cooking seeds;

• Lower levels of potential antinutritional factors;

• Thick-skinned tubers that don’t bruise during shipping and have a long shelf life; and

• Tubers of good size and taste for consumer acceptance.²⁸

Although the above research initiatives are individually important, the ultimate target should be drought-tolerant, photothermally neutral, and pest-and pathogen-resistant cultivars capable of producing high yields and nutritious, tasty food over a wide range of climates and soils.

Exploring the Wild Resource The genetic characteristics for horticultural development highlighted above are likely present in the wild ancestor from which the cultivated form developed. They may also be present in the crop’s wild relatives: Sphenostylis angustifolia (endemic to South Africa), Sphenostylis briartii (native to Congo), Sphenostylis erecta subsp. erecta (Central Africa, East Africa), Sphenostylis erecta subsp. obtusifolia (southern Africa), Sphenostylis marginata (southern Africa), Sphenostylis schweinfurtii (Central and West Africa), and Sphenostylis zimbabweënsis (Zimbabwe).

Sphenostylis schweinfurthii is an especially interesting species, characterized by having hairy stems and hairy leaflets that making it more resistant to drought—a feature of potential interest in breeding.

These wild relatives deserve genetic evaluation, too. Such investigations could help pin down the African yambean’s ancestry. They could expose the cross-pollination success ratio, and thereby provide insight into the closeness of the genetic relationships. They could also uncover qualities that might be usefully bred into the cultivated crop.

Agronomic Exploration Although little is known about its environmental tolerances, the plant is recommended for small-scale cultivation trials in tropical regions outside its native habitat. These should be collaborative tests conducted at different altitudes, latitudes, soils, and climatic conditions, notably humid and semiarid ones. One outcome will be clarifying the plant’s suspected daylength sensitivity.

Comparisons among the yambeans of commerce would also be instructive. In this regard, the African one should be put into head-to-head trials with the three New World species (and perhaps with the yam itself). The differences and similarities will teach much, to the benefit of them all.

With all the emphasis on tuber production, there has been little work on optimizing seed production. Means that have successfully increased seed yield of better-known bean crops deserve to be also applied to this species

With any viney crop the issue of plant support is paramount. It is therefore critical to rapidly test whether the African yambean really does yield tubers satisfactorily when unsupported. And if it indeed does need supports, to find the cheapest, simplest, and most effective ones.

Reproductive pruning. Because of the obvious competition between tuber

growth and seed production, there is a need for field studies to clarify the effects of pruning in the more promising landraces.

Propagation In-vitro techniques are a possibility for rapidly multiplying rare genotypes for conservation purposes, or of hybrids or new material from field collections or field trials. If biotechnology can make available such genotypes in quantity, it would mean field evaluations could be made almost immediately afterwards, and especially good material could be quickly moved to growers.

Industrial Development Although today strictly a subsistence crop, the African yambean might well prove a valuable provider of industrial resources. Several products are possible on a local or sizable scale:

• Flour The large tuberous roots can undoubtedly be dried and ground to form a top-quality flour, useful for cakes, desserts, and other culinary purposes. The properties of this flour should be comparable to those of cassava flour, but perhaps more nutritious and profitable. In India, tubers from the Mexican yambean are ground this way and the flour is considered to be high grade.

• Sugar African yambean tubers need testing as a sugar source. Whether any possess practical quantities of sugar is unknown, but their cousin from the Andes (Pachyrhizus ahipa) has proven to contain more than twice the sugar found in sugar beet.²⁹

• Protein Even though the protein content of the tuber is lower than that of grain-legume seeds, the total protein yield per hectare for the tubers exceeds that of soybean, the world’s premier protein crop. African yambean may therefore have a future as a source of protein for people, pets, livestock, laboratory animals, and industrial processes.

Sustainable Development With its high biological nitrogen fixation, yambeans may return a substantial amount of nitrogen to the soil. The crop therefore could form an integral part of land-use systems, benefiting land and people. Programs developing sustainable African agriculture should include African yambean in appropriate trials.

In parts of tropical America, yambean plays an important role in crop rotation systems. It is grown in the same field for two consecutive seasons, producing a higher yield in the second than the first. Starting in the third season maize, beans onions, or other crops are planted there instead. Then, after a break of 3 or 4 years, yambean is returned to the field again to restore the fertility lost to the interim harvests. In Africa’s generally worn-down

soils, a rotation like this might prove even more effective at sustaining land productivity.

Botanical Name Sphenostylis stenocarpa (Hochst. ex A. Rich.) Harms.

Synonyms Dolichos stenocarpus, Vigna katangensis, Vigna ornata,Vigna ornata var. latifolia, Sphenostylis congensis, Sphenostylis ornata, Sphenostylis stenocarpa var. latifolia

Family Leguminosae. Subfamily: Papilionoideae (Faboideae)—Pea family

Common Names

The species is a perennial that is usually grown as an annual. The cultivated type is a twining, herbaceous vine 1 to 3 m long. The stems are strongly branched and are often reddish in color. They bear trifoliate leaves up to 14 cm long. The individual leaflets are oval with pointed tips and smooth edges.

The butterfly flowers, borne in racemes up to 30 cm long, have twisted petals 2.5cm long, and are probably insect-pollinated. The color is variable and not only includes pink, purple, and greenish white, but also yellowish white, red, magenta, lilac, and blue.

Most blooms develop into narrow pods, 20 to 30 cm long and about 1 cm wide. They are pointed and are subdivided inside by fine transverse walls. The ripe ones are brownish in color and up to 30 cm long. They contain 20-30 seeds. The seeds themselves are large (up to a cm long) and can vary from white to brown and black. Some are speckled or marbled in brown and white, and there veined seeds are known as well. The hilum has a brown

border.

The root system typically branches vigorously. However, some roots thicken into the storage organs. These spindle-shaped tubers outwardly resemble sweet potatoes but taste more like potatoes. In general, they are from 5 to 25 cm in length and weigh 50-300 g (average 250 g). The smallest are normally kept aside and used for planting the next crop.

Within Africa Although African yambean is obscure in a general sense, it exists in a number of countries. In broad terms, the cultivation area extends from tropical West Africa to Sudan, Eritrea and northern Ethiopia. From there it extends southward to Congo in the west and Zanzibar in the east. However the greatest importance, without a doubt, is in West Africa, primarily Nigeria and its immediate neighbors.³⁰

Beyond Africa At the time we write this, the plant seems unknown outside Africa.

Only local landraces are grown, though some accessions are named.

Although little is known about its needs, a humid tropical climate with well-drained soil seems necessary or at least best.

Rainfall According to claims in the literature, the crop requires between 900 and 1,400 mm annual precipitation.

Altitude Seemingly little affected by altitude, it flourishes at elevations from sea level to 1,800 m.

Low Temperature Almost certainly sensitive to frost.

High Temperature The limit is unreported. Good growth is possible between 19 and 27°C.

Soil Type The crop is mostly cultivated on poor soils. Its optimal

substrates are said to be weakly acid, with pH 5 to 6. The site should be well drained. Fertile sandy soils are said to be “highly suitable.”³¹

The African yambean is so closely related to Vigna species that a botanist once classified it as Vigna ornata. This genus—an important one for food in the tropics—includes moth bean, mung bean, bambara bean, rice bean, cowpea, and adzuki bean. Modern techniques, including embryo rescue, might allow the production of interspecific hybrid combinations between it and the other two cultivated species. This suggestion is a stretch, but if it proves possible the resultant plants could be particularly valuable in developing cultivars that are early maturing, heat- and day-length neutral, bushy, and better adapted to new areas.

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