The Sterculiaceae comprise about 750 species of trees, shrubs, and - rarely - herbaceous plants within about 50 genera which provide beverages (e.g. Theobroma cacao L., cocoa), masticatories (e.g. Cola spp.) and gums (e.g. Sterculia urens Roxb.). The fruits are pods. (woody drupes), capsules, and follicles. Seed storage behaviour is variable. Rulingia pannosa is orthodox and maintained in the long-term seed store at the Wakehurst Place Gene Bank, but Theobroma cacao L. is widely reported as recalcitrant. Seed storage behaviour in Cola spp. is not entirely clear: the seeds are treated as recalcitrant (that is they are not dried) but, if they are recalcitrant, then they are some of the longest-lived recalcitrant species so far reported.
SEED DORMANCY AND GERMINATION
The main food storage organ of the seeds is the cotyledons (two or more), but a residual endosperm may also be present as a thin membrane. The testa is thin, but can be quite tough and delay or prevent germination. Detailed information on seed dormancy and germination is provided in this chapter for the genus Cola. A brief summary of recommended germination test procedures and dormancy-breaking treatments for Theobroma cacao L. is provided in Table 68.1.
TABLE 68.1 Summary of germination test recommendations for species within the Sterculiaceae
|
Species and Authority |
Substrate |
Temperature |
Duration |
Additional directions |
Source |
|
Theobroma cacao L.
|
S |
25°-30°C |
21d |
light, continuous |
CHML |
|
|
27°C |
14d |
pre-soak, 24h |
Riley |
COLA
|
C. acuminata (P. Beauv.) Schott & Endl. |
abata kola, cola nut, goora nut |
|
C. nitida (Vent.) Schott & Endl. |
gbanja kola |
I. Evidence of dormancy
Seeds of Cola spp. can show considerable dormancy (1,5-7,12-14). Freshly harvested seeds of C. nitida take between 3 and 9 months to germinate whereas completely after-ripened seeds are reported to germinate immediately (6,9,13,14). After-ripening periods of between 3 months and 1 year are reported to be required to remove dormancy (1,6,7,13,14). The after-ripening treatments were carried out in moist environments (e.g. undried nuts in a polyethylene bag). Therefore the success of after-ripening treatments does not provide evidence of orthodox seed storage behaviour.
II. Germination regimes for non-dormant seeds
-
III. Unsuccessful dormancy-breaking treatments
C. nitida
Constant temperatures: 37°C, 45°C, 55°C (7)
Thiourea: pre-applied, 24,48h, 5000 ppm (1)
Removal of seed covering structures: part of hull (6)
IV. Partly-successful dormancy-breaking treatments
C. acuminata
Removal of seed covering structures: testa and tip of cotyledon (11); testa, then tease apart cotyledons (11)
C. nitida
Constant temperatures: 20°C, 30°C (7)
Alternating temperatures: 24°/30°C (7)
Warm stratification: 37°C, 2,3w, germinate at 25°C, 30°C (7)
Pre-soak: 24h (12)
Light: shade (7,8,9); fluorescent, continuous (7); 1000 fc (1)
Kinetin: pre-applied, 24,48h, 25-100 ppm, germinate at 37°-39°C in light, 1000 fc (1)
Thiourea: pre-applied, 24h, 2000 ppm (12); pre-applied, 24,48h, 1000, 2000 ppm, germinate at 37°-39°C in light, 1000 fc (1)
Removal of seed covering structures: testa and upper half of cotyledons (3,4,10); testa and tip of cotyledons (7,8,12); split nut (7,9); testa, then tease apart cotyledons, germinate at 20°C, 25°C, 35°C (12)
V. Successful dormancy-breaking treatments
C. nitida
Removal of seed covering structures: testa (5); testa, then tease apart cotyledons, germinate at 30°C (12); testa, then tease apart cotyledons, then pre-soak; 24h (12)
VI. Comment
In freshly harvested seeds of Cola spp. the embryo is 1-1.5 mm long and lies at the deepest point of the basal furrows in the cotyledons which are firmly held together (2,7,12). When placed in contact with a moist medium the nut imbibes moisture but germination may not occur for some weeks or months: only after considerable delays do the embryos begin to develop (7). If the cotyledons are gently teased apart without damaging their contact with the embryo or the seeds are pre-soaked or the cotyledon tip removed then the embryo imbibes more rapidly and the delay to germination is reduced (7,9,12). Consequently for dormant seeds it is suggested that testa removal and cotyledon parting be practised.
Quite high temperatures are required for germination. A constant temperature of 30°C has been reported as optimal (7,12). Although continuous exposure to 37°C will eventually kill the seeds, warm stratification at 37°C for 2 to 3 weeks with subsequent removal to a lower temperature can reduce the time taken by the seeds to germinate (7) - possibly by increasing the embryo's imbibition rate? Diffuse sunlight or fluorescent light are promotory (1,5,7), but direct sunlight (in nursery sowings) can reduce germination (5,8). It is suggested that the seeds be tested for germination in moist sand or between moist paper towels at 30°C, and recommended that attention be paid to keeping the germination test medium moist throughout the test.
VII. References
1. Ashiru, G.A. (1969). Effect of kinetin, thiourea, thiourea dioxide, light and heat on seed germination and seedling growth of kola (Cola nitida (Ventenant) Schott and Endlicher). Journal of the American Society for Horticultural Science, 94, 429-432.
2. Brown, D.A.L. (1970). A review of germination of kola seed (Cola nitida (Vent.) Schott & Endl.). Ghana Journal of Agricultural Science, 3, 179-186.
3. Brown, D.A.L. and Afrifa, M.K. (1971). Effect of cutting cola nut on the germination rate and subsequent seedling characters. Ghana Journal of Agricultural Science, 4, 117-120.
4. Brown, D.A.L. and Afrifa, M.K. (1972). Cola, germination investigations. Ghana, Cocoa Research Institute Tafo, Annual Report for 1970/1971, 38-39.
5. Clay, D.W.T. (1964). Germination of the kola nut (Cola nitida (Vent.) Schott and Endl.). Tropical Agriculture, Trinidad, 41 41, 55-60.
6. Dublin, P. (1965). Le colatier (C. nitida) en République Centrafricaine. Café-Cacao-Thé, 9, 97-115.
7. Eijnatten, C.L.M. van (1968). The germination of kola nuts, Cola nitida (Ventenant) Schott and Endlicher. Nigerian Agricultural Journal, 5, 72-82.
8. Eijnatten, C.L.M. van and Odegbaro, O.A. (1966). Kola, germination trials. Annual Report, Cocoa Research Institute of Nigeria 1964/1965, 4, 93-94.
9. Eijnatten, C.L.M. van and Quarcoo, T. (1968). Studies on kola, germination studies. Annual Report, Cocoa Research Institute of Nigeria 1966/1967, 46-50.
10. Godfrey-Sam-Aggrey, W. (1969). Cola production in Ghana. World Crops, 21, 196-199.
11. Ibikunle, B.O. (1975). The germination of Cola acuminata (P. Beauv.) Schott and Endl. Acta Horticulturae, 49, 75-83.
12. Ibikunle, B.O. and MacKenzie, J.A. (1974). Germination of kola (Cola nitida (Vent.) Schott and Endl.). Turrialba, 24, 187-192.
13. Karikari, S.K. (1973). The effect of maturity and storage on the germination of cola nut (Cola nitida (Ventenant) Schott and Endlicher). Ghana Journal of Agricultural Science, 6, 87-91.
14. Odegbaro, O.A. and Ogutuga, D.B.A. (1967). The influence of the time of storage on the germination of kola nuts. Annual Report, Cocoa Research Institute of Nigeria 1965/1966, 112-114.
