The Dioscoreaceae are usually considered to comprise about 650 species of herbaceous plants within ten genera. They possess rhizomes and tubers which are edible (e.g. Dioscorea alata L., greater yam). Do not confuse the term yam with other species, e.g. sweet potato (see Chapter 31) and the aroids (see Chapter 24). The seeds are winged, show orthodox seed storage characteristics and can exhibit considerable dormancy. Detailed information is provided in this chapter for the genus Dioscorea only.
DIOSCOREA
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D. bulbifera L. |
potato yam, aerial yam, air potato |
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D. cayenensis Lam. |
yellow yam, yellow Guinea yam, attoto yam |
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D. composita |
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D. deltoidea |
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D. floribunda |
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D. hirtifloria Benth. |
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D. japonica Thunb. |
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D. nipponica Makino |
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D. odoratissima Pax. |
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D. opposita Thunb. [D. batatas Decne.] |
Chinese yam, cinnamon yam |
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D. praehensilis Benth. |
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D. preussii Pax. |
wild yam |
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D. quinqueloba Thunb. |
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D. rotundata Poir. |
white yam, white Guinea yam, Guinea yam, 8-months
yam |
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D. septemloba Thunb. |
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D. tenuipes Franch. & Sav. |
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D. tokoro Makino |
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D. villosa L. |
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I. Evidence of dormancy
Seed dormancy is a major problem in Dioscorea spp., sufficient in the past to have prevented improvement by breeding (7,8). Between 3 and 5 months after-ripening removes dormancy from most seeds, at least in West African collections (7,8,13,14). However, in Japanese collections 6 months after-ripening was only partly effective in removing dormancy (6). Within the species D. japonica, D. nipponica, D. quinqueloba, D. septemloba, D. tenuipes and D. tokoro, seeds of D. septemloba are the most dormant and seeds of D. nipponica the least dormant (5), although of course there is considerable variation in the degree of dormancy between lots within each species (5). A further problem is the absence of embryos in some seeds due to poor pollination and/or fertilization (7).
II. Germination regimes for non-dormant seeds
D. bulbifera
Constant temperatures: 30°C, 21d (3)
D. cayenensis
Calcium hypochlorite: pre-applied, 20 min, 10% (2)
D. composita
Constant temperatures: 30°C, diffuse light, 20d (9)
D. deltoidea
Constant temperatures: 25°C, diffuse light, 20d (9)
D. floribunda
Constant temperatures: 30°C, diffuse light, 20d (9)
D. hirtifloria
Constant temperatures: 30°C, 21d (3)
D. nipponica
Constant temperatures: 20°-29°C, 70d (5)
D. odoratissima
Constant temperatures: 30°C, 21d (3)
D. opposita
Constant temperatures: 25°C, light, 4000 lux, 16h/d, 30d (10)
D. praehensilis, D. preussii
Constant temperatures: 30°C, 21d (3)
D. rotundata
Constant temperatures: 25°-30°C (8, 13, 14)
Calcium hypochlorite: pre-applied, 20 min, 10% (2)
D. tenuipes
Constant temperatures: 20°C (4)
D. tokoro
Constant temperatures: 20°C, dark, 30d (6); 25°C (4)
III. Unsuccessful dormancy-breaking treatments
D. japonica
Constant temperatures: 11°C, 17°C, 23°C, 26°C, 29°C, 70d (5)
Pre-chill: 0°C, 65d, germinate at 26°C, 29°C, 85d (5)
D. preussii
Pre-chill: 5°C, 1,3w (1)
Pre-wash: 2d (1)
GA3: co-applied, 500 ppm (1)
D. quinqueloba
Constant temperatures: 17°-29°C, 70d (5)
Pre-chill: 5°C, 65d, germinate at 29°C, 85d (5); 5°C, 20,35d, germinate at 20°C, 65-80d (5)
D. septemloba
Constant temperatures: 11°-29°C, 70d (5)
Pre-chill: 0°C, 65d, germinate at 29°C, 85d (5)
D. tenuipes
Constant temperatures: 23°-29°C (5)
Pre-chill: 5°C, 50d, germinate at 29°C (5)
Light: fluorescent, 1500 lux (4); green, 1500 lux (4); far red, 1500 lux (4); blue, 1500 lux (4)
D. tokoro
Constant temperatures: 26°-29°C, 70d (5)
Pre-chill: 5°C, 50d, germinate at 29°C, 50d (5); 5°C, 30,50d, germinate at 25°C in light (4)
Light: fluorescent, 1500 lux (4); green, 1500 lux (4); far red, 1500 lux (4); blue, 1500 lux (4)
GA3: co-applied, 3x10-7 -3x10-4 M, at 25°C (4)
D. villosa
Constant temperatures: 20°-25°C, light or dark, 30d (12)
IV. Partly-successful dormancy-breaking treatments
D. japonica
Constant temperatures: 14°C, 20°C, 70d (5)
Pre-chill: -2° to 2°C, 100d, germinate at 20°C, dark, 40d (5); 0°C, 65d, germinate at 11°-23°C, dark, 85d (5); 5°C, 30,80d, germinate at 20°C, dark, 20-70d (5)
D. opposita
Constant temperatures: 25°C, light, 4000 lux, 16h/d, 30d (10)
Pre-chill: 0°C, 4,8w, germinate at 25°C in light, 4000 lux, 16h/d, 30d (10)
GA3: co-applied, 1, 30 ppm, at 25°C in light, 4000 lux, 16h/d, 30d (10)
D. preussi
Pre-soak: 45°C, 2d (1)
Potassium nitrate: co-applied, 10-2 M (1)
D. quinqueloba
Constant temperatures: 11°-14°C, 70d (5).
Pre-chill: 2°-11°C, 75d, germinate at 20°C, dark, 15d, (5); 5°C, 65d, germinate at 11°C, 17°C, 20°C, 23°C, dark, 85d (5); 5°C, 50,80d, germinate at 20°C, dark, 20-50d (5)
D. septemloba
Pre-chill: 0°-2°C, 65d, germinate at 20°C, dark, 60d (5); 0°C, 65d, germinate at 11°-20°C, dark, 85d (5); 0°C, 30,60d, germinate at 20°C, dark, 40-70d (5)
D. tenuipes
Constant temperatures: 11°-20°C, 70d (5)
Pre-chill: 5°C, 20d, germinate at 20°C, in dark or light (4); -2° to 17°C, 30d, germinate at 20°C, dark, 30d (5); 5°C, 35d, germinate at 11°C, 14°C, 17°C, 20°C, 26°C, dark, 50d (5); 5°C, 20,35d, germinate at 20°C, dark, 65-80d (5)
Light: red, 1500 lux (4)
GA3: co-applied, 3x10-7 -3x10-4 M, at 20°C in dark (4)
D. tokoro
Constant temperatures: 11°-23°C, 70d (5)
Pre-chill: 2°-17°C, 20d, germinate at 20°C, dark, 15d (5); 5°C, 35d, germinate at 26°C, dark, 50d (5); 5°C, 20-50d, germinate at 20°C, dark, 50-80d (5); 5°C, 30,50d, germinate at 25°C, dark (4); 5°C, 80d, germinate at 25°C, light (4)
Light: red, 1500 lux (4)
D. villosa
Removal of seed covering structures: cut through to endosperm, germinate at 20°-25°C, dark, 30d (12)
V. Successful dormancy-breaking treatments
D. opposita
Removal of seed covering structures: excise embryo, culture on Murashige and Skoog's medium with sucrose, 20 g/l, and agar, 7 g/l, at 25°C in light, 4000 lux, 16h/d (10)
D. quinqueloba
Pre-chill: 5°C, 65d, germinate at 14°C, dark, 85d (5)
D. rotundata
Removal of seed covering structures: clip wings (11)
D. tenuipes
Pre-chill: 5°C, 35d, germinate at 23°C, dark, 50d (5); 5°C, 50,80d, germinate at 20°C, dark, 20-50d (5); 5°C, 50,80d, germinate at 20°C in dark or light (4)
D. tokoro
Pre-chill: 5°C, 30d, germinate at 20°C, dark, 11d (6); 5°C, 35d, germinate at 11°-23°C, dark, 50d (5); 5°C, 80d, germinate at 20°C, dark, 20d (5); 5°C, 80d, germinate at 25°C, dark (4)
VI. Comment
Seed germination in Dioscorea spp. is inhibited - or at best not promoted - by white light (4,5) and/or high germination test temperatures, 23°-40°C (5,6). The response of germination to gibberellins appears complicated; germination is both promoted and inhibited by treatment with gibberellins (4,10). It appears that gibberellins may mitigate against the inhibition of germination by inhibitory light sources (and thus in these circumstances appear to be promotory), but that in the dark or in red light their action is generally inhibitory (4). Consequently it is suggested that treatment with gibberellins is not worthwhile.
Pre-chill treatments are particularly effective in removing dormancy (4,5,10). Optimum pre-chill treatment temperatures and subsequent germination test temperatures, however, vary between species and possibly between seed lots within a species (5). Nevertheless the following treatment is suggested for breaking dormancy and promoting germination in dormant seeds of Dioscorea spp.: pre-chill at 5°C for 30 days, then transfer to a germination test regime of 14°-17°C for 21-28 days; repeat this cycle (as many times as necessary) for those dormant (fresh) seeds which fail to germinate; apply red light throughout both the pre-chill and germination test treatments; if this is not possible, test the seeds in the dark. In addition it may be worthwhile to clip or prick the fresh seeds which fall to germinate after the first pre-chill germination cycle, since such treatments can be promotory over a wide range of species (11,12). Non-dormant seed lots can be tested for germination at 20°C, again either in red light or dark. Micro-organism contamination may be a problem in germination tests (2), particularly if these extend over considerable periods. Pre-treatment in a 10% calcium hypochlorite solution for 20 minutes has been reported to minimise such problems (2).
In ecological terms the suggested treatment of dormant accessions from tropical regions at low temperatures (pre-chill) may appear surprising. Certainly the evidence that such treatments are effective is limited to Japanese collections (4,5), and thus more work is needed. Nevertheless, within the Japanese collections accessions from the warmer climates had the lower optimum pre-chilling temperatures and benefitted most from longer pre-chill treatments (5,6). Moreover, evidence in other species (rice, for example) demonstrates that pre-chill treatments can be effective in breaking dormancy in accessions collected from regions where such low temperatures would never be experienced in the soil.
VII. References
1. Anonymous (1972). Yam seed germination. In IITA 1971 Annual Report, p. 106.
2. Anonymous (1976). Root and tuber improvement program. In IITA 1975 Annual Report, pp. 127-151.
3. Lawton, J.R.S. and Lawton, J.R. (1967). The morphology of the dormant embryo and young seedlings of five species of Dioscorea from Nigeria. Proceedings of the Linnean Society London, 178, 153-159.
4. Okagami, N. and Kawai, M. (1977). Dormancy in Dioscorea: gibberellin-induced inhibition or promotion in seed germination of D. tokoro and D. tenuipes in relation to light quality. Plant Physiology, 60, 360-362.
5. Okagami, N. and Kawai, M. (1982). Dormancy in Dioscorea: differences of temperature responses in seed germination among six Japanese species. Botanical Magazine Tokyo, 95, 155-166.
6. Okagami, N. and Kawai, M. (1983). Dormancy in Dioscorea: range, duration and timing of high-temperature treatment in germination inhibition of D. tokoro seeds. Plant and Cell Physiology, 24, 509-515.
7. Sadik, S. (1975). Root and tuber physiology with emphasis on yam improvement. In Proceedings of Physiology Program Formulation Workshop, pp. 35-40. IITA, Ibadan, Nigeria.
8. Sadik, S. (1976). Methods for seed germination and seedling establishment of yam, Dioscorea rotunda Poir. Technical Report No. 1, pp. 1-7, IITA, Ibadan, Nigeria.
9. Tyagi, M.C., Singh, M.P. and Bammi, R.K. (1973). The effect of temperature on seed germination in Dioscorea species. Planta Medica, 24, 294-296.
10. Yakuwa, T., Harada, T., Kasai, N. and Araki, H. (1981). Studies on the botanical characteristics of the genus Dioscorea. II. On the formation and germination of the seed in Chinese yam (c.v. Nagaimo). Journal of the Faculty of Agriculture, Hokkaido University, 60, 220-228.
11. Doko, E.V. (1973). Sexuality and reproductive biology in Ghanaian yam (Dioscorea species) cultivars. 1. Preliminary studies. Proceedings of the 3rd International Symposium of Tropical Root Crops, pp. 2-9, Ibadan, Nigeria.
12. Mitchell, E. (1926). Germination of seeds of plants native to Dutchess County, New York. Botanical Gazette, 81, 108-112.
13. Sadik, S. and Okereke, O.U. (1975). Flowering, pollen grain germination, fruiting, seed germination and seedling development of white yam, Dioscorea rotundata Poir. Annals of Botany, 39, 597-604.
14. Sadik, S. and Okereke, O. (1975). A new approach to improvement of yam Dioscorea rotundata. Nature, 254, 134-135.
