44 TROPICAL ROOT CROPS SYMPOSIUM

Seed germination was tested monthly during this can be recommended, work should be con­ the first 8 months of storage at six conditions. ducted to find suitable conditions for storing Cold-storage treatments especially, when seed, to develop methods to break seed dor­ combined with desiccation, reduced the per­ mancy, and to present strong evidence that centage of germination and increased the disease and pests are not seed-borne. number of days to the onset and to 50% These are some of the problems that need germination. Storage at 25 DC without silica urgent attention to maximize opportunities to gel resulted in the highest germination rate and improve yams through hybridization and sex­ the least number of days to the onset and to ual propagation. 50% germination, whereas storage at 25 DC over silica gel resulted in opposite results. Al­ Coursey, D. G. Yams. Longmans, London, 1967. though germination of seed stored at 25 DC for Doku, E. V. Sexuality and reproductive biology in Ghanaian yam (Dioscorea spp.J cultivars. 1. 8 months is superior to that at cold storage, Preliminary studies. Paper presented at the the long-term effect of cold storage is not yet Third International Symposium on Tropical known. Root Crops, Ibadan, Nigeria, Dec. 2-9 (in press), 1973. Henry V. C. R. Studies on botananical and agro­ Research Needs nomic characteristics of cush-cush (Dioscorea The opportunities to improve yams through trifida L.F.J. Ph.D. thesis, McGill University, hybridization have been greatly enhanced by Montreal, Canada, 1967. increasing flowering and by achieving seed Okoli, O. O. Yam production from seeds, pros­ pects and problems. Paper presented at the 11th germination. However, many problems remain Annual Conference of the Agricultural Society before further advancements can be made. Al­ of Nigeria, Enugu, Nigeria, July 1-15 (in though flowering has been improved quanti­ press), 1975. tatively and qualitatively, methods for inducing Pitkin, B. R. Larothrips dentipes (Thysanoptera, flowering in nonflowering plants and species thripidaeJ, a new genus and species of thrips must be found before the genetic resources of from yam flowers in Nigeria. Bull. Ent. Res. 62, such plants can be utilized. Studies of the bar­ 1973,415-418. riers that prevent inter- and intra-specific Rao, V. R., Bammi, R. K., and Randhawa, G. S. Interspecific hybridization in the genus Dio­ hybridization are also urgently needed. scorea. Ann. Bot. 37, 1973,395-401. Conserving yam germ plasm in tuber form Sadik, Sidki. llTA Annual Report. IITA, Ibadan, is difficult and undesirable because of the great Nigeria (in press), 1975. bulk, poor storability, and the possibility of Sadik, Sidki, and Okereke, O. U. A new approach disease- and pest-transmission from one crop to improvement of yam Dioscorea rotundata. to another. Because of these factors, quarantine Nature, 254(5496), 1973,34-135. regulations restrict the movement and ex­ Flowering, pollen grain germination, fruiting, change of germ plasm among research workers. germination and seedling development of white yam, Dioscorea rotundata Poir. Ann. Bot. 39, Germ plasm can be conserved and exchanged 1975,597-604. through seeds that are less bulky, not restricted Waitt, A. W. Report on Agricultural Research in by quarantine regulations, and contain more Nigeria 1957-58. Federal Government Printer, genetic diversity for selection. However, before Lagos, Nigeria, 18-19, 1959.

Selected Yam Varieties for the Tropics Franklin W. MartinI

This 8-year program for selecting better yam (Dioscorea) varieties for the tropics includes: a worldwide collection of varieties of the principal species; the development of techniques to evaluate varieties agronomically, and for culinary and processing char­ acteristics; the selection and testing of varieties; and the distribution of selected varieties throughout the tropics. In addition, composition with respect to proteins and starch was

IMayaguez Institute of Tropical Agriculture, Box 70, Mayaguez, Puerto Rico. MARTIN: SELECTED YAM VARIETIES 45 determined, and the nature of the yellow pigments and the bitter substances was elucidated. Taxonomic relations were clarified by numerical computerized techniques. Dioscorea alata was judged the most flexible and useful species, and 17 selected varieties were obtained. Dioscorea esculenta was found to be much more variable than expected, and 8 varieties were selected for their high potential. On the other hand, persistent problems with viruses prevented the distribution of Dioscorea rotundala - D. cayenensis. In addition, this species complex appears to be narrowly adapted.

The edible yams of the tropics, of which to the different species. Criteria for selection there are 50 or more species, have hardly left were developed. Preliminary selections were their original homes to serve mankind. To be tested for yields in replicated field trials. sure, edible yams are found in every part of This paper represents a summary of activi­ the tropics where they can be grown. Never­ ties and findings, and includes plans up to the theless, distribution has been accomplished expected closing of the program in June 1977. haphazardly. The cultivars present in any particular region are often inferior to the D. alata better ones known elsewhere. Furthermore, the introduction of better cultivars has been im­ Of the various species of Dioscorea col­ peded by a lack of published information. The lected, the greater yam (Dioscorea alata) result has been that yams are not as widely quickly became our favorite. Under the con­ utilized as their potential merits. ditions where we have seen D. alata grown in To remedy this situation the U.S. Agency for Puerto Rico, it is the most dependable species, International Development suggested a pro­ although some of its varieties are unreliable gram of collection and distribution of yam due to their susceptibility to Colleotrichum, varieties. Collections were begun in 1969 at Cercosporum, and other foliar diseases. D. the Mayaguez Institute of Tropical Agricul­ a/ala often yields exceptionally well. Its tubers, ture. It became evident at that time that varie­ if kept free of damage at harvest, can be stored ties easily available in the Caribbean were not about 5 months. We have stored tubers for 8 representative of the full range of germ plasm, months or more by regularly eliminating and that progress could only be made by col­ shoots. lecting yams extensively on an international All varieties of the greater yam have certain basis. It was also evident that we did not know traits in common. Their growing season is enough about yams to define a good variety. long (8-10 months). All require support for Therefore, while USAID funded the collection the vines. All can be established easily from and distribution phases of the program, MITA any piece of the tuber. Regardless of planting funded supplementary investigations as neces­ time, all mature about the same time, with no sary. more than a month of difference between the The collection of yams was made over a earliest and the latest varieties. I believe all are period of several years through correspondence susceptible to virus disease, but the majority and through collecting trips. A visit to West can be maintained symptom free by rogueing Africa was financed by FAa, and one to South­ and planting only from superior tubers. east Asia was financed by USAID. During During the study of the greater yam we these trips about 800 accessions of yams were found some unexpected differences. Yellow acquired. I am indebted to many persons fleshed varieties were shown to contain nu­ throughout the tropics for help in obtaining tritionally valuable amounts of carotene. Un­ new varieties. Without qualified professional usual varieties from Papua New Guinea help in each location, the collecting expeditions showed the most fascinating shapes, consisting would have been unsuccessful. of a series of intersecting vertical planes. In Puerto Rico, newly introduced varieties Varieties were found that were almost free of were grown for a year or more in isolation wings on stems and petioles, and in some cases from the principal collection, and were ob­ leaves were principally alternate rather than served for pests and diseases. A system of opposite. Tuber characteristics were highly evaluation based on morphological, agronom­ variable. Varieties that had reverted to the wild ic, physiological, culinary, and processing were found in Africa and in the West Indies. characteristics was developed, and was adapted These were propagated exclusively from aerial 46 TROPICAL ROOT CROPS SYMPOSIUM tubers. Very few varieties flowered, and fertile varieties generally reach this stage rapidly. The seeds were never produced during the 7-year cooking water should remain free of gray or study period. pink pigments. The flesh of the boiled tuber The within-species relationships of the may be white, cream, or light yellow, appe­ varieties were studied by numerical taxonomic tizing in appearance, and free from gray techniques. About 235 of the 350 varieties we colour. It should appear to be smooth, and that collected were used for this study, and 100 appearance should be verified when tasted. The characteristics were noted. It was possible to cooked tuber should be moist in the mouth; classify varieties into 13 groups that were de­ not dry and difficult to swallow. The taste fined on the basis of character means, These should be rich and distinctive, neither too groups were related to, but not identical with, bitter nor too sweet. certain geographical areas. Based on the In addition, the variety should produce good amount and nature of variation seen, we con­ yields (20 t/ ha or more), the tuber should cluded that Papua New Guinea was a centre of store well and resist fungi, and the seed pieces variation, and possibly the centre of origin of should germinate readily when planted. the species. The sympatric species Dioscorea No varieties were seen that combined all nummularia resembles some of the D. alata desirable characteristics. Selection of new cultivars found there. An unexpected finding varieties thus became a matter of compromise. was that the Caribbean and African varieties Seventeen excellent varieties were selected are somewhat related to each other, and that for distribution. I believe that some of these will their affinities in Southeast Asia were not replace traditional varieties wherever they are traceable with our materials. grown. These 17 were obtained from widely Tubers of D. alata were found especially use­ diverse regions, including Fiji, the Philippines, ful for processed products, including fries, Java, Malaysia, Papua New Guinea, India, chips, instant flakes, and flour. They were not Nigeria, Sierra Leone, and Puerto Rico. suitable for the production of fufu. The tubers were usually rich in protein compared to roots D. cayenensis - D. rotundata and tubers of other species, and some ex­ ceptionally rich varieties were found. The Complex proteins contained sufficient lysine but were al­ In contrast to D. alata, the collection of ways short of methionine. cultivars of D. cayenensis and D. rotundata A good variety of D. alata is resistant to leaf was the most difficult to manage. We started spot diseases and viruses, and is not affected with an excellent group of 220 introductions adversely by excessive moisture. The tubers collected from Sierra Leone to Nigeria as well are borne in pairs or in threes, are spherical or as local varieties found in the Caribbean. From cylindrical in shape, are not often branched, the beginning many plants showed strong virus­ and have smooth but thickened skin that resists like symptoms. This condition was the same as abrasion. The surface may have some fine that called the shoestring disease in West roots, but the principal roots should be concen­ Africa. There was no doubt that introductions trated in the fibrous upper extreme of the tuber. showing such symptoms had been collected in Resistance to insects, diseases, and nematodes all of the principal yam growing regions of is desirable. Yields must be high and depend­ Africa, and in the Caribbean. Our experience able, even when the crop is produced without with this disease in Puerto Rico is that it reduces support for the climbing vines. yields, sometimes drastically. The use of small Some special characteristics related to cook­ tubers for replanting results in a more rapid ing are desirable. The parenchyma storage flesh spread of the disease, possibly because such should be white or cream coloured, and free of tubers are found more frequently on diseased anthocyanin pigments. The "grain" of the tuber plants. Plantations grown under optimum con­ caused by starch accumulation around the ditions and rogued free of symptoms for 2 or 3 vascular bundles should be fine, giving a com­ years become almost symptom-free. Constant pact and uniform appearance. After being cut vigilance appears necessary to maintain rea­ or injured, or after prolonged storage, the sonably healthy materials. flesh should not discolour readily by oxidation. Since most introductions were based on a The boiling time necessary to reach an ac­ single small tuber, it is no surprise that virus­ ceptable softness may vary, but the better like symptoms were often severe. Even in the MARTIN: SELECTED YAM VARIETIES 47 first planting, symptoms were found in all in­ fully evaluated before our program terminates. troductions. In an effort to preserve the germ Nevertheless, we believe the breeding work will plasm, all introductions were multiplied for 1 continue at another experiment station in or 2 years before rogueing was begun. Pro­ Puerto Rico. Obtaining virus-free varieties will longed heat treatment of tubers, sometimes not be enough. If resistance is not found, the found useful in reducing symptoms in D. alata, job of keeping varieties free of virus will be was not useful in the case of the African formidable. species. When virus symptoms were not eliminated, D. esculenta a program of severe rogueing was begun. During the 2 years of rogueing, the majority of Of the species of Dioscorea we have worked the varieties were eliminated. Furthermore, with, perhaps D. esculenta is the least known. with time even the more resistant varieties have Nevertheless, we believe that it might have a shown intolerable levels of the disease. real potential in the tropics. The habit of pro­ Given the circumstances, we have taken an ducing mUltiple tubers not unlike potatoes is extremely difficult decision. The African yam useful when machine harvest is contemplated. collection will not be distributed but will be The small tubers are useful at the household eliminated. Three varieties that appear to be level and are easy to handle if replanted. Some completely resistant to the virus disease will be varieties, if handled carefully, store well. tested in isolation for 2 more years, but it is Furthermore, D. esculenta is probably more unlikely that these will be used anywhere ex­ resistant to viruses than even D. alata. The cept on the island of Puerto Rico. chief disadvantage with respect to D. esculenta While working with this collection, we have is that the growing season is long, almost 12 written a production bulletin, analyzed the months. I believe that it might be possible to carotenoid pigments, and identified a bitter plant and harvest D. esculenta at any time of substance as leucoanthocyanidin. In addi­ the year in some regions ncar the equator. tion, we have finished a study of the relation­ Nevertheless, D. esculenta is fussy about ships of 97 cultivars using the techniques of season. Its period of dormancy is not very numerical taxonomy. In that study the species flexible, and indeed we have had problems in complex is divided into nine groups that adjusting varieties from the southern hemi­ coalesce to form two principal trunks. One of sphere to 18 ON in Puerto Rico. The wide varia­ these represents chiefly yellow tubered culti­ tion among cultivars of D. esculenta has never vars, the other represents chiefly white ones. been described, but has at least been hinted at, It is interesting to note that the two trunks in the literature. Varieties from Papua New anastamose with respect to two groups that Guinea, the presumed centre of origin, have show similarities even though they differ in been carried to some islands of the Pacific but tuber colour. The conclusion from our study apparently in a random fashion. Although I is that the two names D. cayenensis and D. was able to collect a wide variety of cultivars rotundata reflect artificial classification of what from the regions, I have no doubt that much is an extremely variable complex. remains to be done to entirely describe this With respect to the work of the future with species. this species, we are convinced that, in the The West African and Caribbean cultivars Caribbean, D. alata is better adapted and more of D. esculenta bear about 20-50 relatively reliable. But in West Africa even the best D. small tubers that range from mere swellings to alata varieties may not be competitive with the about 400 g in size. Their flesh is white and African cultivars. In most parts of the world, their eating qualities are high. In contrast, these African yams have never had a real try. some varieties of D. esculenta in Papua New It appears to us that the breakthrough in seed Guinea bear 6 or 8 tubers up to 2 kg in weight. production achieved at lITA by Sadik and The largest tuber I have seen was the shape and others opens the door for a more extensive use size of a small watermelon, and weighed about of the African yams throughout the tropics. It 5 kg. In general the flesh of these large tubers would be highly desirable to introduce African is somewhat coarse and subject to polyphenolic yams as seed and to develop varieties for local oxidation. Between the extremes there are adaptation. Seedlings that we have established many interesting varieties that bear inter­ from seeds sent to us by Sadik may not be mediate sized tubers of good quality. Some of 48 TROPICAL ROOT CROPS SYMPOSIUM these have been selected for distribution in spect to their nutritive values. The composi­ January or February of 1977. tion of the flesh was analyzed with re­ In Papua New Guinea one also encounters spect to carotenoids, polyphenols that oxidize, many primitive varieties, often with small, and bitter substances. Certain physiological branched, or irregular tubers. Flesh colour may characteristics were studied: the curing period, be white, purple, or yellow. Thorniness of the and how it affects storage life; the extension of roots surrounding the crown varies. An un­ storage life with chemical treatment; and the expected characteristic sometimes seen is the stimulation of sprouting. To develop standards curling of the tuber toward the surface, and at of selection, fried chips, instant flakes, and times, the penetration of the surface by the flours were developed and tested. Superior tuber. selections were described. Plans call for a The starch grain of D. esculenta is fine com­ thorough review of written contributions and pared to that of the principal species D. alata recommendations before terminating the pro­ and D. rotundata. The amylose content of the gram. starch is low, and the protein content of the Reprints and planting pieces are available tuber is extremely high in some cuItivars. free to investigators throughout the tropics. Re­ D. esculenta merits a wider trial throughout prints, already in short supply, are sent out at the tropics, but only where growing season is time of request. Tubers are shipped each Jan­ long, and rainfall is abundant. uary or February. Shipments are furnished with phytosanitary certificates. Nevertheless, Other Species we recommend that tubers we distribute be quarantined for a year until shown to be During the course of our studies we have disease and insect free. had the opportunity to work with six other edible species. While finding some merit in all Manuscripts Concerning Yams of them, and realizing that we have not seen all of the germ plasm available, we have re­ Published jected some of the species, as follows: Diosco­ Martin, F. W., and Thompson, H. E. Crude protein rea bulbi/era, about 100 collections. The tubers content of yams. Hort. Sci. 6. 1971,545-546. of this species are always bitter; Dioscorea Martin, F. W. Yam conservation problems. Plant hispida, about 15 collections. This species is Genetic Resources Newsletter 28, 1972, 35-36. poisonous and needs special treatment to Martin, F. W. Yam Production Methods. USDA render it harmless; Dioscorea dumetorum, Agricultural Research Service Production Re­ about 12 collections. This is sometimes poison­ search Report No. 147, 1972, 17 p. Martin, F. W., and Ruberte, R. Yams (Dioscorea ous, and the tuber is very irregular; Dioscorea spp.) for production of chips and french fries. J. nummularia and D. pentaphylla. Tubers are Agric. Univ. Puerto Rico 56, 1972, 228~234. large, irregular, sometimes mUltiple, and cook­ Rhodes, A. M., and Martin, F. W. Multivariate ing characteristics are below par; and Diosco­ studies of variations in yams (Dioscorea alata rea trifida, which is very susceptible to viruses. L.). J. Amer. Soc. Hort Sci. 97, 1972, 685-688. Yields decline rapidly when the virus appears. Rodriguez-Sosa, E. 1., Gonzalcz, M. A., and Martin, F. W. Evaluation of ten varieties of yam (Dioscorea spp.) for production of instant Literature Produced flakes. J. Agric. Univ. Puerto Rico 56, 1972, 235-243. As part of the research and development Martin, F. W. Yam (Dioscorea spp.) in West program 37 manuscripts were prepared for Africa. In Frankel, O. H., ed., Survey of crop publication, and a few more are planned. The genetic resources in their centres of diversity. titles of these manuscripts are appended to this Food and Agriculture Organization of the paper. Perhaps the major task was a review of United Nations, and International Biological modern production techniques, followed by Program, 1973,65-67. detailed handbooks on the principal species. Martin, F. W. Review of: Anatomy of the mono­ Within-species taxonomic relationships were cotyledons. VI Dioscoreales. By Ayensu, E. S., and edited by Metcalfe, G. R. Carendon Press, studied in the case of the principal species. Oxford, Quarterly Review of Biology 48, 1973, Nutritive values, especially protein and amino 37-38. acid contents, were determined and yams were Martin, F. W., and Rhodes, A. M. Correlations compared to other roots and tubers with re- among greater yam (Dioscorea alata L.) varie- MARTIN: SELECTED Y AM VARIETIES 49

ties. Tropical Agric. (Trinidad), 50, 1973, 183- Martin, F. W. The storage of germ plasm of 192. tropical roots and tubers in the vegetative form. Martin, F. W., and Thompson, A E. Protein In Frankel, O. H., and Hawkes, 1. G., eds., Crop content and amino acid balance of yams. 1. genetic resources for today and tomorrow. Agric. Univ. Puerto Rico, 57,1973,78-83. Cambridge, London, 1975,369-377. Rodriguez, E. J., Cruz-Cay, J. R., Gonzalez, Martin, F. W., and Ruberte, R. Carotenoid pig­ M. A, and Martin, F. W. Shelf-life study of ments of Dioscorea cayenensis. Ann. Appl. Farm Lisbon (Dioscorea alata L.) yam chips. BioI. 30,1975,317-322. J. Agric. Univ. Puerto Rico, 58, 1973, 196-202. Martin, F. W. The edible yams. Roles for the Splittstoesser, W. E., Martin, F. W., and Rhodes, future. J. Root Crops (India), 1, 1975,51-54. A. M. Amino acid composition of yams as Martin, F. W., and Ruberte, R. The flours of related to species and cultivars. 1. Amer. Soc. edible yams. J. Agric. Univ. Puerto Rico, 59, Hort Sci. 98, 1973,563-567. 1975, 255-263. Splittstoesser, W. E., Martin, F. W., and Rhodes, Martin, F. W. Tropical yams and their potential. A. M. The nutritional value of some tropical III. D. alata. USDA Agricultural Handbook root crops. Proc. Trop. Reg. Amer. Soc. Hort. 495, 1975,40 p. Sci. 17, 1973, 31-34. Martin, F. W. Southeast Asian yams and their Telek, L., Martin, F. W., and Ruberte, R. M. potential. In Williams, 1. T., and Lamoureux, Bitter compounds in tubers of Dioscorea bulbi­ C. H., eds., Plant genetic resources of Southeast fera L. J. Agric. Food Chern. 22, 1974, 332-334. Asia. Bogor, Indonesia, 1975, 83-90. Martin, F. W., Telek, L., and Ruberte, R. Yellow Martin, F. W., and Ruberte, R. The bitterness of pigments of Dioscorea bulbifera. J. Agric. Food Dioscorea cayenensis. J. Agric. Food Sci. 23, Chern. 22, 1974,335-337. 1975, 1218-1219. Martin, F. W. Tropical yams and their potential I. Martin, F. W., and Ruberte, R. The polyphenol of Dioscorea esculenta. Agric. Res. Ser. U.S. De­ D. alata that oxidizes to brown. J. Agric. Food partment of Agriculture, Agricultural Hand­ Chern. 24, 1976,67-70. book 457, 1974, 18 p. Womack, M., Martin, F. W., and Vaughan, D. A Martin, F. W., Koo, F. K. S., and Cuevas, J. Toxicity of some edible yams to rats. Tropical Stimulation of yam (Dioscorea) tuber develop­ Root and Tuber Crops Newsletter, 9, 1976, ment by gamma irradiation. J. Amer. Soc. 27-32. Hort. Sci. 99,1974,282-284. Martin, F. W. Curing yams before storage I. Pending Publications Wound types, species, and humidity effects. J. Martin, F. W. A collection of West African yams. Agric. Univ. Puerto Rico, 58,1974,211-218. Martin, F. W. The edible yams. Martin, F. W. Tropical yams and their potential, Martin, F. W., and Sadik, S. Tropical yams and II. Dioscorea bulbifera L. USDA Agricultural their potential. IV. Dioscorea rotundata and D. Handbook 466, 1974,20 p. cayenensis. Splittstoesser, W. E., and Martin, F. W. The Martin, F. W. Stimulating the germination of yam tryptophan content of tropical roots and tubers, tubers with ethephon. Hort. Sci. 10, 1975,23-24. Martin, F. W., and Splittstoesser, W. E. Protein Martin, F. W., and Rhodes, A M. The classifica­ and amino acid contents of tropical roots and tion of Dioscorea alata varieties. tubers. Root and Tuber Crops Newsletter, 8, Cabanillas, E., and Martin, F. W. Propagation of 1975,7-15. edible yams from stem cuttings. Martin, F. W., Cabanillas, E., and Guadalupe, R. Martin, F. W. The tropical yams, their growth Selected varieties of Dioscorea alata L., the requirements and their potentials. Asian greater yam. J. Agric. Univ. Puerto Rico, Martin, F. W., and Rhodes, A M. The relation­ 49,1975,165-181. ship of Dioscorea cayenensis and D. rotundata.