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TROPICAL YAMS AND THEIR POTENTIAL Part 2. bulbifera

Agriculture Handbook No. 466: ^-^

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Agricultural Research Service UNITED STATES DEPARTMENT OF in cooperation with U.S. Agency for International Development This publication reports research involving pes- ticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate State and/or Federal agencies before they can be recommended.

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CAUTION: Pesticides can be injurious to humans, domestic animals, beneficial insects, de- sirable , and fish or other wildlife—if they are not handled or applied properly. Use all pesti- cides selectively and carefully. Follow recom- mended practices for the disposal of surplus pesti- cides and pesticide containers.

Washington, D.C. Issued October 1974

For sale fay the Superintendent of Documents, U.S. Government Printing Office Washington, D.C, 20402 - Price 40 cents Stock Number 0100-03349 The feeding of future generations requires a knowledge of the individual crop plants of the world and their potentials. Crops can be recommended for use in particular regions only on the basis of potential yield, the costs of production, the food and feed value of the crop, and the way the crop can be processed or otherwise used. For most of the major food crops of the world, a body of information is already available. However, tropical roots and , which are widely used as staple foods, have been largely neglected. Only in recent years has an awareness been growing of the potential of these crops to supply large amounts of food in relatively small amounts of space. Yams are the second most important tropical root, or , crop. The annual production, perhaps 25 million tons, places them second in importance to . But yams are better food than cassava, and while they are usually thought to be more difficult to grow, under some conditions yams out- produce cassava. Yams fill an important role in the diet of many areas of the Tropics—a role that can increase in im- portance. That role and its potential are not, however, well understood. The is not a single species. Perhaps 60 species have edible tubers; of these about 10 species can be considered crop plants. The literature concerning these species is widespread but fragmentary. This is the second in a series of Agriculture Handbooks dealing with the major species of yams. It is part of a research effort cosponsored by the Agricultural Research Service of the U. S. Department of Agriculture and the U. S. Agency for International Development to introduce, evaluate, and distribute better yam varieties.

Also in 'Tropical Yams and Their PotentiaF* series— Part 1. , USD A Agriculture Handbook No. 457. CONTENTS Page Preface ..- .- iii Introduction - .- 1 History and origin 1 Geographic distribution —- — 1 Botany 2 Classification -. 2 Morphology 2 Cytology 10 Varieties 12 Culture 13 Environmental requirements 13 Land preparation and planting 14 Fertilization -.. — 14 Staking 15 Pests and diseases --. 15 Harvesting 15 Culinary characteristics .- 15 Composition 16 Poisonous or obnoxious contents 18 Folk medicine 19 Potential uses . 19 Literature cited 19

IV TROPICAL YAMS AND THEIR POTENTIAL Part 2. Dioscorea hulhifera

By FRANKLIN W. MARTIN, geneticist^ Federal Experiment Station^ Southern Region, Agricultural Research Service, JJ. S, Department of Agriculture, Mayaguez, P,R, INTRODUCTION and . The African varieties are so distinct from the Asian that One of the most common and their distribution must have taken widespread yams of the Tropics is place in prehistoric times. Perhaps Dioscorea hulhifera L., the air- the species had two centers of di- yam, so called because it is versity, one in Indochina, Malaya, grown chiefly for its edible aerial or Indonesia, and the other in Af- tubers. Known in both Asia and rica. Although Coursey (4)^ Africa for centuries, it has been speaks only of the former center, widely distributed through the ac- two closely related species in East tivities of man and has gone wild Africa, D, asteriscus Burk. and D, in many tropical regions. The spe- paleata Burk., suggest an African cies is best known in India, where center of diversity also. Burkill there are superior edible varieties, (1) agrees with this hypothesis but it is also common in West Af- and suggests that the wide diver- rica. D. hulhifera probably is only sity of African varieties is due to a poor fourth or fifth in worldwide the hand of man. Certainly the production of yams, for it is sur- range of variation in West Africa passed by D. alata L., the Asian is sufficient to indicate independ- greater yam; D. rotundata Poir. ent evolution of Asian and African and possibly D. cayenensis Lam., forms. Furthermore, a major dif- the West African yams; and D. es- ference of shape is usually suffi- culenta (Lour.) Burk., the Asian cient to distinguish Asian from lesser yam. Although not one of African types. Whereas the former the best yams, some varieties pro- are more or less spherical, the lat- duce large numbers of aerial tu- ter are angular. bers over long periods of time, making them especially suited to Geographic Distribution the home garden. In addition, some Probably D, hulhifera can be also produce underground tubers. found in every hot, humid, tropical The better varieties are not well region. Burkill (1) speaks of D, distributed, but merit introduction hulhifera as having extended to and trial. the most remote islands of the Pacific. Wild forms, usually bitter History and Origin D. hulhifera is the only edible 1 Italic numbers in parentheses refer yam species native to both Asia to items in "Literature Cited," p. 19. AGRICULTURE HANDBOOK 466, U.S. DEPT, OF AGRICULTURE and often poisonous, are the most esculenta (Lour.) Burk.], airpo- common. The more ennobled vari- tato yam, air potato, aerial yam, eties usually do not survive in the bulbil-bearing yam, and, in Span- wild for long periods. ish, ñame de gunda (bulbil-bear- D, bulbifera is not native to the ing yam). Western Hemisphere. Neverthe- D. bulbifera belongs to the sec- less, it is so widespread that it is tion Opsophyton, an Old World noted in most floras of the tropical section, with two other African countries of Central and South species, D. asteriscus Burk. and D. America. Because it is common, it paleata Burk., which are neither has frequently been given colorful edible nor commercially valuable. names 5ueh as mata gallina (kills In common with other yams, the hens). The more desirable forms airpotato yam belongs to the fam- of Z). bulbifera are not widely dis- ily , of which Dio- tributed in the Western Hemi- scorea is the principal genus. The sphere, and the species is not an family is usually classified among important food anywhere within the , although this region. some evidence of a second cotyle- don has been found (7). The fam- ily is characterized by rhizomes, BOTANY usually reduced to a nodeless struc- Classification ture. The male and female , D. bulbifera L., because of its usually on separate plants, are wide distribution, has many syn- small and often inconspicuous. The onyms. The African forms have inferior ovary becomes quite been distinguished as D, latifolia prominent after fertilization. The Benth,, a name not regarded floral pattern is based on sets of with favor now. Other synonyms three. include D, anthropophagorum A. Chev., D. hoff a Cordemog, JD. hof- Morphology ika Jumelle & Perrier de la Bathie, D. bulbifera is a glabrous D. longepetiolata Baudon, D. op- that climbs by twining to heights positif olia Campbell, Z>. perrieri R. of 12 meters or more (fig. 1). The Knuth, D, sylvestris de Wild., D, stems range from thin (1-milli- violácea Baudon, and Helmia bul- meter diameter) to thick (8-milli- bifera Kunth. The name D. sativa meter diameter) and twist to the Thumb, is also sometimes seen in left in twining. The , often the literature, specifically with re- quite large, are alternate and usu- spect to the edible Asian varieties. ally orbicular, but with well-devel- This name has also been applied to oped acuminate tails. They are other species, but it is botanically cordate at the base. Details of the invalid. Among the common names sinus between the lobes, of the are potato yam [used also for D, tail, and of the degree of rugosity, TROPICAL YAMS AND THEIR POTENTIAL, PART 2

PN—3671 FIGURE 1.—Mature D. bulbifera climbing bamboo poles for support. folding, cupping, and reflexing of surface of the tuber itself in some, the are varietal traits making but not all, varieties. identification possible (fig. 2). The small flowers are frequent- The leaves are usually glabrous, ly seen during summer. They are but in some varieties they are cov- sessile and appressed to the ped- ered with a bluish bloom. Another uncle in long racemes produced in trait useful for identification is the axils of the leaves (fig. 4). The the resistance of the leaf to leaf- male and female flowers superfi- spot disease, common in all yam- cially appear alike, but the female growing regions. The petiole, is easily recognized from its in- which is enlarged at the base, has ferior ovary. The perianth ranges earlike projections that often en- from green to white to slightly circle the stem (fig. 3). pink. The flowers produce a pleas- Only small seedlings have true ant odor, which attracts bees and roots, and these are short-lived. other insects. The principal adventitious roots The seed capsules are trilocular, arise from the crown, the region 2 to 5 centimeters long, and rise of the stem immediately above the vertically from the racemes (fig. tuber. Finer roots arise from the 5). At maturity they dry and open A(JRICUI.TURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE

PN—3672 FIGURE 2.—Typical leaves of five D. bulbifera varieties. Cultivated varieties are shown above, wild varieties below. (Two-fifths actual size.) longitudinally, beginning at the one end of the wing. There are 70 distal tip. The seeds are eventually to 110 seeds per gram. Fresh seeds dislodged by the wind and may be germinate readily in soil or on wet carried some distance from the filter paper in 2 to 3 weeks. Seed- plant. The seed is surrounded by a lings grow to mature size and flow- membranous falcate wing, which er in 1 year (unusual among is hooked at its attachment to the yams). placenta. The flattened seed is at In common with many yam spe- TROPICAL YAMS AND THEIR POTENTIAL, PART 2

produced until the plant reaches suitable maturity, but once pro- duction begins, it continues until the end of the season. Aerial tubers eventually fall from the vine. The time of fall depends on the variety. In some varieties tubers of all sizes fall, but in the better varieties, the tubers reach a suitable size and maturity before falling. The aerial tubers of Asian vari- eties, which occasionally weigh a kilogram, are usually spherical to ellipsoid, but occasionally may be slightly flattened. The surface is characterized by many lenticels, which may be prominent or sub- dued. Otherwise, the surface is PN—3673 relatively smooth, with no adventi- FIGURE 3.—Petioles of five yam species. tious roots. The exterior of im- Left to right: D. alata, D. rotund- proved varieties is usually a light ata, D. esculenta, D. bulbifera, and D. trífida. Note the earlike projec- tan or gray, but when anthocyan- tions, small aerial tubers, and flow- ins are present, it may be very ering spikes of D. bulbifera. (One- dark. half actual size.) The aerial tubers of the African races are sharply angled and thus cies, D. bulbifera produces both very different in appearance from underground and aerial tubers the tubers of the Asian races. They (fig. 6). The aerial tubers, or bul- are often characterized by depres- bils, such as those of D. alata, are sions where preformed buds have often eaten, even though the un- developed. Color varies from gray derground tuber is the principal or tan to dark brown. The dark food part. Apparently man early tubers are often said to be poison- selected the aerial tuber of D. bul- ous, but in fact no correlation ex- bifera as the chief edible part, ists between color and alkaloid since some Asian and African va- content. rieties have evolved without sig- The underground tuber, pro- nificant underground tubers. duced at an early stage in the Aerial tubers begin to develop seedling, arises from a swelling of in the axil between leaf and stem the young stem and enlarges rap- (fig. 7) about the time that the idly as a . It is quite leaf itself begins to unfold, and distinct in external morphology they grow rapidly. They are not from the root. Neither the true nor AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE

PN—3674 FIGURE 4.—Male (right) and female (center) flowers of D. bulbifera, and an old inflorescence with two young seed capsules (left). (One-third actual size.) adventitious roots ever give rise to bers, can be found in distinct vari- tubers; rather, all tubers, aerial eties. The underground tuber re- and underground, arise from stem sembles the aerial tuber to some tissues. extent. The Asian races produce The characteristics of under- spherical or ellipsoidal under- ground tubers vary from variety ground tubers (fig. 9), whereas to variety also. No true tuber those of the African races are very forms in some varieties, but roots much more branched and irregu- and stems originate from a fibrous lar. Although the tubers of some tuber remnant (fig. 8). From this varieties are covered with fibrous extreme all degrees of develop- roots, others are completely ment, from small to very large tu- smooth. TROPICAL YAMS AND THEIR POTENTIAL, PART 2

PN—3675 FIGURE 5.—Mature capsules and seeds of D. bulbifera. (Actual size.)

The flesh (parenchymatous tis- considerably among varieties. sue) of the aerial and underground Many varieties contain anthocy- tubers is crisp, fine grained, and anins, which in small quantities almost free of fibers. Color varies and combined with yellow pig- 8 AGRICULTURE HANDBOOK 466. U.S. DEPT. OF AGRICULTURE ments produce pinkish or grayish below the cork is colored bright flesh. In some cases the flesh has green by chlorophyll. A small purple blotches. The flesh of most amount of chlorophyll is also found varieties is yellow owing to the in the fleshy parenchyma. presence of carotenoids. The flesh of the aerial tubers is usually the When a tuber is cut, mucilagin- same color as that of the under- ous gums, or glycoproteins, exude ground tubers, but the area just from the cut surface. The surface

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PN—3676 FIGURE 6.—Aerial tubers of several African and Asian (three bulbils in lower left corner) varieties of D. bulbifera. (Two-fifths actual size.) TROPICAL YAMS AND THEIR POTENTIAL, PART 2 9

PN—3677 FIGURE 7.—Aerial tubers and flowering spikes of D. bulbifera developing in leaf axils and terminal inflorescence. (Two-fifths actual size.) oxidizes and turns brown within a layers of cells below are impreg- short time after being cut. In this nated with suberin, which effec- species of yam cuts and wounds tively seals the wound and pre- heal naturally. After initial drying vents further moisture loss and in- of the uppermost layers, the intact fection. 10 AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE

Cytology numbers are multiples of 20, be- Numerous chromosome counts ginning with 40 (40, 60, 80, and have been made of D. bulbifera. In 100). These numbers can be in- Asian varieties the chromosome terpreted as polyploid numbers

PN—3678 FIGURE 8.—The fibrous underground tubers of some African varieties of D. bulbi- fera alongside the smooth aerial tubers of the same variety. (Two- fifths actual size.) TROPICAL YAMS AND THEIR POTENTIAL, PART 2 11

PN—3679 FIGURE 9.—Large underground tuber of D. bulbifera 'Sativa', an Asian variety. (Two-fifths actual size.) based on a set of 10 (8). In West freely, it should be possible to African varieties the chromosome breed new varieties systematical- numbers are 36, 40, 54, and 60, ly, although there are no records of which are based on multiples of its having been done. Where male either 9 or 10. Evidently in the and female flowers of different very early evolution of the species, plants are not too far apart, polli- a chromosome was lost in the nation occurs readily, and capsules progenitor of the 36- and 54- with up to six seeds are produced. chromosome varieties. When large On the other hand, the failure of numbers of chromosomes occur, some combinations suggests that some multivalent figures are nor- male or female sterility may occur. mally seen. Poor chromosome dis- Selection must have been prac- tribution and resulting genetic in- ticed by early man. The many vari- balance probably account for the eties in both Africa and Asia sug- sterility of some varieties. gest that breeding, probably by Because many varieties selection for forms with large 12 AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE aerial tubers, has been quite suc- eties of the species, especially with cessful. Natural crossing among respect to the size and characteris- selected large-tubered males and tics of the aerial tuber. This is al- females could lead to an increase ways spherical to elongate, some- in variation with respect to tuber times rather irregular in shape or size and to a gradual evolution of lumpy, but never more than slight- varieties with larger aerial tubers. ly flattened. The surface of the Hill selection of large-tubered, tuber may be very smooth or be vigorous plants should be useful in roughened by the prominence of maintaining healthy, productive the lenticels. Four botanical vari- stock, but must be continued each eties have been distinguished. The generation to be effective. wild plant, with abundant, small aerial tubers and cordate leaves is VARIETIES varietas bulbifera. Another form with longer leaves has been named Two important studies have heterophylla. The cultivated forms been made of D, bulbifera vari- have been distinguished as vari- eties. The African varieties have etas sativa, with large, gray, been treated in depth by Chevalier smooth bulbils, and varietas siiav- (2), and the Asian species, by ioT, with smaller, dark gray-brown Prain and Burkill (10,11 ), Where- bulbils having prominent lenticels. as the latter study is conservative, Varieties can be considered from in the former the African varieties other standpoints, of course. Thus, are considered to be of a separate they can be classified by their species {D. latifolia Benth.), and horticultural value. In this respect, in fact are divided into several bo- the cultivated varieties of India tanical varieties, as follows: seTie- and Southeast Asia are far superi- gambica^ sylvestris, anthropopha- or in flavor and yield to any of the gorum, contralatrones, longipetio- African varieties. A list of the lata, and violácea. The differences principal varieties tested by the among these varieties are real author is found in table 1. enough, but undoubtedly some have been given too much taxono- The following characteristics mic weight. For example, varietas are most useful in distinguishing contralatrones differs from vari- varieties: size and shape of aerial etas anthropophagorum chiefly on tuber, size and shape of under- the basis of the presence of poisons ground tuber, pattern and promi- in one and not the other. The an- nence of lenticels on aerial tuber,' thocyanin coloration in varietas size and shape of leaves, diameter violácea is probably not sufficient of mature stem, minor character- to distinguish it as a separate bo- istics of leaf, color of flesh, and bit- tanical variety. terness of cooked flesh. Prain and Burkill (11) found The characteristics desired in a great diversity in the Asian vari- good variety include large, abun- TROPICAL YAMS AND THEIR POTENTIAL, PART 2 13

TABLE 1.—Yields of D. bulbifera varieties from various regions Yield Acces- (metric tons per hectare) sion Source Varietal name Under- No.i or characteristic Aerial ground Total tubers tubers 14861 Puerto Rico cv. 'Sharp Angled' 4.90 0.00 4.90 15216 --. do cv.'Round' 9.65 18.71 28.36 15330 India cv. 'Sativa' 16.90 13.35 30.25 15338 Puerto Rico cv. 'Smooth Angled'.. . 2.00 1.12 3.32 15472 cv. 'Poison' -.. 10.60 22.09 32.69 15492 cv. 'Thuma' 19.50 2.47 21.99 15500 Ivory Coast.... Wild 1.70 .97 2.67 15501 do Cultivated 1.87 3.74 5.61 15772 do Very small tubers .16 25.34 25.50 15773 Nigeria cv. 'Olode' 3.00 .30 3.30 15774 do Extremely small tubers - . I.OO .93 1.93 15775 do Purple flesh 2.50 1.83 4.33 15776 do Small tubers 2.75 1.18 3.93 15777 do Large lobes 1.49 1.93 3.42 15778 Sierra Leone. Wild .05 .15 .20 15779 do Cultivated 8.00 3.47 11.47 ' Specimens in the living collection at the Federal Experiment Station, Maya- guez, P.R. dant, well-shaped aerial tubers, the case of a wooded area, prevents some sweetness but no bitterness the loss of soil moisture. of flesh, and resistance to foliage Most varieties of D. bulbifera diseases. require long rainy seasons. The CULTURE bulbils of some varieties fall throughout the growing season Environmental Requirements and may germinate within a few Judging from the wide distribu- weeks on the ground. In other vari- tion of wild D. bulbifera varieties, eties the bulbils are seasonally the species grows in many kinds of dormant. This behavior, accom- soil. However, loams or loose clays panied by the dieback of foliage with good drainage are preferred. and its replacement 1 to 4 months A high level of organic material later, is independent of rainfall promotes vegetative and tuberous conditions and may be an effect of growth. The wild varieties require photoperiod. The foliage of all some protection from dry condi- varieties is annual; it eventually tions, since the aerial tubers fall to dies back and is replaced. Varieties the ground where they need mois- differ in the length of the time be- ture to germinate. An accumula- tween dieback of old and produc- tion of leaves on the ground, as in tion of new foliage. 14 AGRICULTURE HANDBOOK 466, U.S, DEPT. OF AGRICULTURE

Land Preparation and Planting at the beginning of the rainy sea- son, during the time of lengthen- D, bulbifera is seldom planted ing days. on a commercial scale. The tech- The best plant spacing has not niques for cultivating it are not been investigated. In Puerto Rico much different from those used good yields have been obtained by for other species of yam. Usually spacing the rows at 1.6 meters and the soil is heaped in hills or ridges the plants at 0.7 meter in the row. to provide drainage and aeration, Bulbils are planted 8 to 12 centi- but this may not be necessary for meters below the surface of the this species. Special plowing is ground with a short hoe. Preemer- probably never done. gence herbicides, such as atrazine The bulbils, or aerial tubers, at 3 kilograms per hectare, may are used as seed. They have polar- prove useful in preliminary weed ity (1) and thus germinate from control. Later, contact herbicides certain eyes, where buds are can be used with caution. formed at a very early stage. How- ever, polarity is destroyed by cut- Fertilization ting the bulbils. Large ones may be Since the culture of D. bulbifera conveniently cut into several has not been studied systematic- pieces. New buds can then be pro- ally, experience with other yam duced from the meristem of the species must serve as a guide. D, cortex. Larger bulbil pieces pro- bulbifera grows best in a loamy duce healthier plants. After 2 or 3 soil, preferably high in organic days of drying and healing in a material. The mineral require- shady place, the cut pieces are ments have not been investigated. ready for planting. Yams in general live from the nu- The bulbils pass through a defi- tritional reserves of their tubers nite period of dormancy unique for 4 to 6 weeks after germination. for each variety. It is difficult to In the early months of the life cycle change this period significantly, they benefit from good quantities for bulbils will not germinate un- of nitrogen, which promotes vege- til they are ready. The appropri- tative growth. Later, when tuber ate conditions (damp soil) can production begins, they require stimulate germination to some ex- much potassium. Yams remove tent, but on the other hand, the phosphorus from the soil efficient- bulbils will eventually germinate ly; thus, low levels seldom limit whether or not conditions are ap- growth. propriate. In addition, although In Trinidad, the following fer- cool, dry storage is useful in inhib- tilization schedule has been recom- iting germination, the character- mended for Z). esculenta: apply istics of the variety determine either 200 kilograms ammonium optimum planting time. The bul- sulfate per hectare 3 months after bils should be ready to germinate planting or 400 kilograms com- TROPICAL YAMS AND THEIR POTENTIAL, PART 2 15 pound fertilizer high in potassium any time, but immature tubers (11:11:13) per hectare 6 to 8 contain less starch and are less ap- weeks after planting. Local trials petizing than mature tubers. Fully with fertilizers are always desir- mature tubers fall to the ground, able. from which they can be removed Staking for home use. On the other hand, Yams are forest plants that mature tubers are easily dislodged climb by twining. Although some from the stem with a slight twist. species produce satisfactory yields Since mature tubers are produced without staking, D, bulbifera for several months, they may be plants require support. Primitive harvested every week or two. supports include bamboo stakes When the vines die back, a few and branches. Sometimes the vines immature tubers are left. are allowed to climb trees or even The underground tubers are banana plants, but such support usually not harvested until the puts the developing aerial tubers foliage dies back. They may then out of reach. Short supports (5 to be harvested with handtools. Since 6 feet high), with lateral rods to they are usually compact and facilitate coverage, permit easy small, they are easy to harvest. harvest. The underground tubers can be Pests and Diseases left in the soil until needed. Some varieties of D, bulbifera Typical yields of 16 varieties are are somewhat susceptible to the given in table 1. The yields of the leaf-spot diseases (especially Cer- better varieties have been tested oospora) seen in other species. various times. Improved cultural Usually such diseases are seen only practices should double any of late in the season, when the foliage these yields. is beginning to die back normally. Both aerial and underground Another disease, stem rot, some- tubers are resistant to fungal in- times occurs, but it is not a serious fections. Harvest wounds usually condition. Premature drop of par- heal promptly, especially if cut tially developed bulbils is frequent tubers are exposed to dry air a few and may be associated with minor days. On the other hand, exposure fungal infections during rainy to sunlight rapidly damages the weather. The underground tubers tuber, and weight loss, internal are sometimes attacked by nema- breakdown, and rot result. Tubers todes and beetles, but they appear for planting should be stored un- to be more resistant than tubers of der cool, dry conditions. other species. CULINARY Harvesting Edible aerial tubers may be pro- CHARACTERISTICS duced in as little as 3 months after The aerial tubers of some vari- planting. Tubers may be picked at eties are small and hard to peel, 16 AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE but the better varieties are ideal ways bitter, but the palate is easily and convenient for the kitchen. accustomed to it, and somewhat Moreover, the aerial tubers can be bitter yams may be eaten with rel- boiled or baked without peeling, al- ish. Asian varieties have a softer, though the peel may increase the more succulent flesh, a smooth bitterness of the adjacent flesh. texture, and often a pronounced The flesh of the tuber is compact sweetness. Their flavor does not, and hard. The large-tubered Asian however, have the richness of some varieties have softer flesh. When varieties of Z?. alata and D. rotun- the tubers are boiled, the African data. varieties tend to remain firm, but the Asian varieties can easily be COMPOSITION overcooked. The yellow color is The composition of the aerial usually darkened by cooking, and and underground tubers of an the resulting color may not be ap- edible African variety were given petizing. The yellow color of the by Wildeman (îâ) and are report- flesh makes the tubers unsuitable ed in table 2. The dry weight of for fried chips and french fries, the tuber ranges from about 12 to products in which the color must 35 percent of the whole-tuber be appealing. weight. Immature tubers contain The flavor of each D, bulbifera relatively less starch. The aerial variety is distinctive. The edible and underground tubers differ African varieties are almost al- very little in dry weight. HOW-

TABLE 2,—Composition of tubers of an African variety of D. bulbifera [Percent of whole-tuber weight]

Aerial Underground Substance tubers tubers

Water 73.400 69.081 Starch 14.860 16.833 Cellulose __._ . .. ^ 3.645 6.744 Sugars -. .470 .451 Chlorophyll .,_. . . .031 Pats _.. .230 .104 Brown resin (phenolics) .304 Yellow resin (carotenoids) 1.660 .461 Albumins (protein) __ _ .530 .890 Glutinous substances . .. .. 2.304 .525 Tannins ...... 052 Soluble substances 1.442 Mucilage, pectins, dextrines 2.870 3.113

Source: Wildeman (IS), TROPICAL YAMS AND THEIR POTENTIAL, PART 2 17

0) . ever, African varieties are usually 0 rji to tí 10 0 more compact and dense than 5.S T-í iH TH i O) a >. Asian varieties. Of the dry weight, 0 80 to 95 percent is starch. U t> 0 TJI .s UO rH The bulk of the starch in D. bul- U l> 0Ö t> tí ■^ *5J) l> ri i¿ 1 bífera consists of fairly large par- Tí «H ticles, from 3 to 6.5 microns long. 0/ 0 tí 4¿ The particles are flat and triangu- *s Tí» IÄ 0 1 g rí «^ TjÍ 'S 0 I> 00 g lar because of the buildup of size H CO oí oí A '•g S, by successive layers at only one H s s end. A large starch particle is not s tí Tij i> r> as easily digested as a small par- rji CO ^ í 1 CO 0 00 •a ticle. In most varieties the amylose ^ 1 nî CO CO .a content of the starch is moderately o I •a ■§■ V 1 low, 15 to 18 percent, but in one Co .s Oi 00 t- tí variety a high amylose content OÎ l> OS § CO t> CD tí (28 percent) was found. D. bulbi- 1 CO r!< high gelatinization temperature. "^ CO ^ The characteristics of the starch 'S ^CJ :S 0> àO »s? o> t> U TS ^. «H tain sufficient protein to contrib- .s c

The yellow pigments of D, bul- Ï Î Q) 1 •43 %i 'S 1 0 bífera unfortunately have no nu- tí Ö QQ ¿ < ' 0) ^.'. .V tí tritional value. The carotenoids ce Í7 tí > 'S are entirely xanthophylls and their rt tí '43 03 tí > rtí 0 Cí A 0 c« i-( esters (^). These include, in de- 50 gd D: OQ Pi M 18 AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE creasing order of importance, of D. bulbifera is apparently dio- lutein, neoxanthin, violaxanthin, scorine, an alkaloid found in many auroxanthin, and cryptoxanthin. poisonous yams and sometimes Such pigments are normally found used as a heart stimulant. On the in leaves and stems, and their oc- other hand, a saponin has some- currence in tubers is suggestive of times been reported. Up to 4.5 per- the origin of these tissues from cent of the saponin dioscine has stem tissue. Aerial tubers also con- been reported (^), but this con- tain chlorophyll. The occurrence of tent seems excessive, and other ac- anthocyanins in tubers of D. buU counts estimate the steroidal sapo- bifera is common. genin as nil. Tubers of the poisonous and Poisonous or Obnoxious nonpalatable varieties of D, bulbi- Contents fera are often used as food after special processing (5). Treat- That the tubers and bulbils of ments include prolonged or re- some varieties of D, bulbifera are peated boiling, often with wood poisonous is well known and often ashes, and steeping the tuber slices put to use (5). On the island of in running water. Another tech- Java the aerial tubers are used to nique is to pound the tubers with make fish poison. In Africa poison- lime or sand and then roast them ous varieties are planted among slowly. Treatments may include all the nonpoisonous in order to con- of these steps. Perhaps in part be- found and discourage thieves. Pre- cause of the complexity of the op- sumably the farmer himself dis- eration, the tubers are often used tinguishes the poisonous yams by only as an emergency food. remembering where he planted them or by minor morphological Tubers of D. bulbifera are often differences. Chevalier (2) distin- bitter. In a search for sapogenins, guished his varietas contrala- Kawasaki et al. (6) discovered a trones on the basis of its poisonous new series of bitter substances, content only. Karnick (5) report- furanoid diterpenes, but Telek et ed cases of death from ingestion al. (12) showed that these sub- of the tubers, and Wildeman (18) stances also occur in the sweet reported the death of 38 soldiers varieties. Other related substances who had eaten poisonous varieties. are not bitter. Probably all D, bul- Poisonous yams have been used in bifera varieties contain small Africa to control criminals (S), amounts of bitter substances in Perhaps from their poisonous concentrations often below the qualities, the belief in the power level of detection by taste only. The of the plants to impede intruders, whether human or animal, is com- bitter substances are most soluble monpla<îe in Africa. in methanol, but they can be re- One of the poisonous substances moved to some extent by prolonged TROPICAL YAMS AND THEIR POTENTIAL, PART 2 19 boiling or soaking in water. The bulbifera merit collection, trial, nutritional significance of the and widespread distribution. Be- bitter substances is not known, but cause of their potentially high there is no reason to believe they yields, they should be magnificent are poisonous. producers of edible flesh and starch. Their commercial potential Folk Medicine has not been well tested. Process- The aerial and underground ing techniques comparable to those tubers have been used many ways used for potatoes need to be tested in folk medicine (5). In India the and developed for this species. bulbils have been used externally The necessary research does not for sores and internally for hem- seem to be in progress anywhere. orrhoids. Among the Santals of Thus their potential is still undis- central India a paste from the covered. tuber is used for swellings and as a cure for snakebite. In Burma, tubers are used as galactagogue. In LITERATURE CITED Jamaica the tuber is used for the treatment of scorpion stings and (1) BURKILL, I. H. ulcers. In Africa extracts are used 1911. The polarity of the bulbils of in toddy to stimulate excessive Linn. J. Proc. Asiat. Soc. drinking. Bengal (n.s.) 7: 467-469. (2) CHEVALIER, A. 1936. Contribution à Tétude de POTENTIAL USES quelques espèces afri- caines du genre Dioscorea. The potential of D. btilbifera is Bull. Mus. Nati. Hist. Nat., undoubtedly in the home garden, Paris, 2d Ser. 8(6): 520- where its health and vigor com- 551. mend it for growth under uncon- (3) trolled conditions. The fact that an 1947. Une igname d'Afrique em- ployée dans les empoison- edible crop is borne over 4 to 5 nements criminels. Rev. months makes this yam particu- Int. Bot. Appl. Agrie. Trop. larly desirable. No other edible 27(291-292): 56-57. species is equal in this respect. The (4) COURSEY, D. G. spherical or ellipsoid shapes of the 1967. Yams. An account of the nature, origins, cultivation better varieties make preparation and utilisation of the use- in the kitchen simple. The well- ful members of the Dio- textured flesh of the African vari- scoreaceae. 230 pp. Long- eties is easy to appreciate, but the mans, London. varieties are usually bitter. In ad- (5) KARNICK, C. R. dition, they are irregular in shape. 1969. Dioscorea (yams), the food of the slaves with poten- The Asian varieties have a softer tials for newer drugs. Q. flesh that needs less cooking. J. Crude Drug Res. 9: The best Asian varieties of D, 1372-1391. 20 AGRICULTURE HANDBOOK 466, U.S. DEPT. OF AGRICULTURE

(6) KAWASAKI, T., KOMORI, T., and (10) PRAIN, D., and BURKILL, I. H. SETOGUCHI, S. 1914. A synopsis of the Dio- 1968. Furanoid norditerpenes scoreas of the Old World, from Dioscorea plants. I. Africa excluded, with de- Diosbulbins a, b, & c from scription of new species Dioscorea bulbifera forma and varieties. J. Proc. spontanea makino et ne- Asiat. Soc. Bengal (n.s.) moto. Chem. Pharm. Bull. 10:5-41. (Tokyo) 15: 2430--2435. (11) (7) LAWTON, J. R. C, and LAWTON, 1936. An account of the genus JUNE R. Dioscorea. Part I. Species 1967. The morphology of the which turn to the left. dormant embryo and young Annu. Rev. Bot. Gdn. (Cal- seedling of five species of cutta) 14: 1-210. Dioscorea from Nigeria. (12) TELEK, L., MARTIN, F. W., and Proc. Linn. Soc. Lond, 178 : RUBERTé, R. 153-159. 1974. Bitter substances in tubers (8) MARTIN, F. W., and ORTIZ, S. of Dioscorea hidbifera L. 1963. Chromosome numbers and J. Agrie. Food Chem. 22: behavior in some species of 332-334. Dioscorea. Cytologîa 28 : (13) WiLDEMAN, E. DE. 96-101. 1938. Dioscorea alimentaires et (9) TELEK, L., and RUBERTé, toxiques (morphologie et R. biologie). Espèces et vari- 1974. The yellow pigments of étés congolaises, Inst. R. Dioscorea bulbifera. J. Colon. Belge, Sect. Sei. Agrie. Food Chem. 22: Nat. Med., Collect. Octavo, 335-337. Mem., vol. 7, pt. 2, 262 pp.

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