The Araceae comprise roughly 1500 species, mainly herbaceous, within more than 100 genera. The aroids provide edible roots (e.g. Amorphophallus campanulatus. (Roxb.) Blume, elephant yam), edible fruits (e.g. Monstera deliciosa Liebm., ceriman) and oils (e.g. Acorus calamus L., sweet flag). The fruits are usually berries. Seed storage behaviour is orthodox, at least in Colocasia and Xanthosoma spp.
SEED DORMANCY AND GERMINATION
The seeds are usually endospermic and can exhibit dormancy, although the problem may not be pronounced. Detailed information is provided for the genera Colocasia and Xanthosoma in this chapter.
caracu Koch & Bouché
cocoyam
X. sagittifolium (L.) Schott
cocoyam, tannia, tanier, yautia
I. Evidence of dormancy
Xanthosoma spp. show orthodox seed storage behaviour and can be stored at low moisture contents and temperature, but longevity is reported to be comparatively short (4). We have found no evidence of dormancy in the literature.
II. Germination regimes for non-dormant seeds
X. caracu
Agar: Hoagland's nutrient agar medium, room temperature, 18d (3)
X. sagittifolium
Constant temperatures: 28°-30°C, 4w, in sand or on top of paper (1,2)
Xanthosoma spp.
Constant temperatures: 25°-28°C in light, 12h/d (4)
III. Unsuccessful dormancy-breaking treatments
IV. Partly-successful dormancy-breaking treatments
V. Successful dormancy-breaking treatments
VI. Comment
Flowering in Xanthosoma spp. is sporadic and consequently seeds are only produced infrequently (1,4). Flowering can be promoted by foliar application of gibberellic acid at 250 ppm (1) or between 500 and 1500 ppm (4); pollination by hand is then required (1,4).
It is suggested that the seeds be tested for germination on top of filter papers at 25°C in light: a 28 day test should be adequate. For seedling production the seeds can be germinated on top of sterile soil (4), but do not let the temperature fall below 22°C (4). The seedlings can be transplanted between 14 and 21 days after sowing (4).
The apparent absence of dormancy in freshly harvested seeds of cocoyam may have been the result of the earlier foliar applications of gibberellic acid which were used to promote flowering in references (1) and (2). If this were to be the case then it would suggest that seed treatment with gibberellic acid is likely to be an effective dormancy-breaking treatment where seed dormancy is encountered in accessions of X. sagittifolium.
VII. References
1. Alamu, S., McDavid, C.R. and Duncan, E.J. (1982). Production of viable seed in gibberellic acid-treated tannia (Xanthosoma sagittifolium (L.) Schott) plants. Tropical Agriculture (Trinidad), 59, 333-334.
2. McDavid, CR. (1984). University of the West Indies, Trinidad (Personal communication).
3. Volin, R.B. and Zettler, F.W. (1976). Seed propogation of cocoyam, Xanthosoma caracu Koch & Bouché HortScience, 11, 459-460.
4. Wilson, J.E. (1980). Cocoyam breeding by flower induction, pollination and seed germination. Manual Series No. 4, 15pp., International Institute of Tropical Agriculture Ibadan, Nigeria.
COLOCASIA
C. esculenta (L.) Schott. [C. antiquorum var esculenta Schott.; Arum esculentum L.; Caladium esculentum Vent.] taro, eddo, dasheen, coco yam
C. gigantea (B1.) Hook. f.
I. Evidence of dormancy
Orthodox seed storage characteristics are shown by C. esculenta (4) and C. gigantea (3), although in the former seed longevity under ambient conditions is short (5). Seed dormancy can be exhibited in C. esculenta, for example after-ripening the seeds for 60 weeks at room temperature has been reported to result in the germination of a greater proportion of seeds (8), but tends to be less of a problem than is the case for freshly harvested seeds of C. gigantea which show considerable dormancy (3).
II. Germination regimes for non-dormant seeds
C. esculenta
Constant temperatures: 22°C in light, 18h/d (4,8); 25°C (5); 25°-28°C in light, 12h/d (9)
C. gigantea
Constant temperatures: 28°C (3)
III. Unsuccessful dormancy-breaking treatments
C. gigantea
Constant temperatures: 15°C, 28°C (3)
Warm stratification: 15°C, 6d, germinate at 28°C (3)
IV. Partly-successful dormancy-breaking treatments
C. esculenta
Constant temperatures: 22°C in light, 18h/d (8)
C. gigantea
Warm stratification: 40°C, 1,2d, germinate at 28°C (3); 15°/28°C, 15°/40°C, 28°/40°C (8h/16h), 6d, germinate at 28°C (3)
V. Successful dormancy-breaking treatments
C. gigantea
Warm stratification: 40°C, 3-20d, germinate at 28°C (3)
VI. Comment
Seeds of C. esculenta are reported to be easy to germinate (2,7) and can be tested in soil (5), between moist rolled paper towels (5), on top of paper (7,8), in agar (4,5,8,9) or in embryo culture (1); testing on top of paper is a more suitable germination test medium than either agar or soil (8). Immature seeds, however, show low germination (7). Seeds of C. gigantea can be more difficult to germinate. It is suggested that non-dormant seeds of Colocasia spp. be tested for germination on top of filter paper at 25°C. Dormant seeds can be tested in this way after the imbibed seeds have first received a 6 day warm stratification treatment at 40°C.
VII. References
1. Abraham, A. and Ramachandran, K. (1960). Growing Colocasia embryos in culture. Current Science, 29, 342-343.
2. Barrau, J. (1959). Fruits et graines du taro, Colocasia esculenta (L.) Schott. Journal d'Agriculture Tropicale et de Botanique Appliquee, 6, 436-438.
3. Hanson, J. and Imamuddin, H. (1983). Germination and storage behaviour of seeds of Colocasia gigantea Hook.f. Proceedings of the 6th Symposium of the International Society for Tropical Root Crops, Peru, February 1983.
4. Jackson, G.V.H., Ball, E.A. and Arditti, J. (1977). Seed germination and seedling proliferation of taro, Colocasia esculenta (L.) Schott. in vitro. Journal of Horticultural Science, 52, 169-171.
5. Kikuta, K., Whitney, L.D. and Parris, G.K. (1938). Seeds and seedlings of the taro, Colocasia esculenta. American Journal of Botany, 25, 186-188.
6. Pardales, J.R. Jr. (1981). Floral morphology and biology, fruit and seed set, seed germination and seedling development of taro. Annals of Tropical Research, 3, 169-176.
7. Shaw, D.E. (1975). Illustrated notes on flowering, flowers, seed and germination in taro (Colocasia esculenta). Research Bulletin, Department of Agriculture, Stock and Fisheries, Papua New Guinea, 13, 39-59.
8. Strauss, M.S., Michaud, J.D. and Arditti, J. (1979). Seed storage and germination and seedling proliferation in taro, Colocasia esculenta (L.) Schott. Annals of Botany, 43, 603-612.
9. Wilson, J.E. (1980). Cocoyam breeding by flower induction, pollination and seed germination. International Institute of Tropical Agriculture, Manual Series No. 4, 15 pp.
