The Theaceae are often known as the Ternstroemiaceae (an older name) and are sometimes described also as the Camelliaceae. The Theaceae comprise about 200 species of trees and shrubs within 18 to 20 genera, but the only important species is tea, Camellia sinensis Kuntze (Thea sinensis L.). The fruits are capsules (e.g. Camellia) or berries (e.g. Cleyera). Seed storage behaviour in Camellia spp. was thought to be recalcitrant, but we now question this assumption - see the Comment on Camellia in this chapter.
SEED DORMANCY AND GERMINATION
The seeds are non-endospermic with a linear embryo, thick cotyledons and a thin testa. Tea seed germination is epigeal, but the testa can delay germination. Detailed information on seed dormancy and germination is provided in this chapter for the genus Camellia (including synonyms within Thea). In addition the algorithm below may be helpful in developing suitable germination test procedures.
RBG Kew Wakehurst Place algorithm
The first step in the algorithm is to pre-chill two samples of seeds at 2° to 6°C for 8w and then test for germination in alternating-temperature regimes of 26°/16°C (12h/12h) and 23°/9°C (12h/12h) with light applied for 12h/d during the period spent at the upper temperature. If the supply of seeds is limited then pre-chill one sample only and then test at 26°/16°C.
If the first step in the algorithm does not result in full germination then the second step is to pre-treat imbibing seeds at 26°C for 4w (i.e. a warm stratification treatment), then pre-chill at 2° to 6°C for 8w and, finally, test for germination in the alternating-temperature regime described in step one which gave the greater germination. If no difference in germination was observed during step one between the two alternating temperature regimes then test at 26°/16°C.
If the second step of the algorithm does not result in full germination then the third step is to experiment with further dormancy-breaking agents (see the information on Camellia for examples of likely suitable treatments) in addition to the warm stratification/pre-chill/alternating temperature regime described in step two.
CAMELLIA
|
C. sinensis (L.) Kuntze [C. thea Link; C.
theifera Griff.; Thea bohea L.; Thea sinensis L.; Thea
viridis L.; Thea viridis L.] |
tea |
I. Evidence of dormancy
The germination of untreated seeds of C. sinensis can be slow (4,7,8,11,12), particularly if the seeds are light or not ripe, but full and rapid germination can be achieved by various pre-treatments (4,6-8, 10-15). Seed dormancy is manifested by a delay to germination rather than by preventing germination: in 9-week germination tests no difference is seen in the cumulative germination of treated and untreated seeds, but the germination progress curves differ considerably (11). See the Comment for a discussion of seed storage characteristics.
II. Germination regimes for non-dormant seeds
Constant temperatures: 16°-18°C (12); 20°-25°C (4,8); 25°C (1,13,16)
III. Unsuccessful dormancy-breaking treatments
Constant temperatures: 5°C, 35°C (8); 27°C+ (4,12)
Pre-soak: 7d (11); 9d (15)
IV. Partly-successful dormancy-breaking treatments
Constant temperatures: 14°-17°C (4)
Pre-soak: (10); 1-8d, then crack shells (15)
Removal of seed covering structures: shells, then pre-soak, 1d (8); crack shells (8,10); crack shells, then pre-soak (8)
V. Successful dormancy-breaking treatments
Constant temperatures: 16°-18°C (12); 20°-25°C (4,8); 25°C, (1,13)
Pre-chill: 5°-10°C, 3w (7)
Pre-soak: (6); 1-4d (11); 5d (4); 1-7d, then remove shells (15); 1d, then pre-dry, sunlight (4)
Removal of seed covering structures: shells (4,8,10,12-14); shells, then pre-soak, 1-5d (15); crack shells at micropylar end (11); crack shells, then pre-soak, 2-5d, then remove shells (15)
VI.. Comment
The delay to seed germination in C. sinensis is caused by the seed covering structures (14). They act as a mechanical barrier to germination (11), and their removal promotes rapid, full germination (4,8,10-14). Pre-soaking, though widely applied, is not necessary if the shells are removed prior to testing for germination (15).
Germination test temperatures above 27°C are reported to be deleterious (4,8,12), resulting in the production of abnormal seedlings (12,13). However, this problem is reduced if the seed covering structures are first removed before testing for germination (12,13) - presumably because the period of exposure to the high temperature before germination occurs is reduced.
It is suggested that the seeds be tested for germination in moist sand at 20°C or 25°C (1,4,8,12,13). If the shells are removed prior to the test then a 50-day test period should be sufficient for most accessions (7), but intact seeds may require 4 to 5 months in test (4). It is reported that an easy method of cracking the shells is to place wet seeds in hot sunshine for 15 minutes (15). This technique might be useful for field sowings of large numbers of seeds, but is not suggested for use in laboratory germination tests.
C. sinensis has been reported to exhibit recalcitrant seed storage behaviour (5), since desiccation is reported to result in loss in viability (1,3,11). The viability period of seeds stored moist (but not fully imbibed) is short (2 to 4 weeks) at room temperature, but viability remains high at 5°-7°C after 9 months (11) or 2 years storage (7), or after six years' storage at 1°C (16). The critical moisture content - below which loss in seed viability occurs rapidly - is reported to be at about 23% fresh weight (11), although even in that report some seeds germinated after drying to 17% fresh weight (11).
However, we suggest that the evidence is not yet conclusive that C. sinensis shows recalcitrant seed storage behaviour. Indeed there are some grounds to suspect that the seeds may be orthodox. First the dried seeds' failure to germinate may result from delayed imbition. For example, in one investigation the dried seeds failed to re-imbibe to the moisture content reached by moist seeds when they were set to germinate (11). Second, and more striking, is the evidence that the majority of seeds of C. sinensis dried to equilibrium at either 0, 10, 25, 40, or 55% relative humidity remained viable for 20 weeks at 0°C (15). Moreover in this investigation the seeds were soaked for 2 days prior to the germination test and the shells cracked, a treatment which the authors acknowledged may have resulted in the seed rotting which they observed (15). As discussed elsewhere (Volume I), such treatments are known to damage very dry seeds of orthodox species - the damage to orthodox seeds being described as imbibition injury. Consequently we recommend that further investigations are required to clarify the seed storage behaviour of C. sinensis.
VII. References
1. Amma, S. (1978). [Rapid germinability test with sugars exuded from tea seed.] Study of Tea, 55, 1-6.
2. Andrews, D.N. (1966). The cold storage of tea seed. Revue Agricole et Sucrière de l'Ile Maurice, 45, 303-304.
3. Anonymous (1955). Effect of desiccation and seed dressings on germination of tea seeds and the resulting plants. Annual Report, Indian Tea Association Scientific Department, Tocklai, 1954, 1955, pp. 116-120.
4. Bonheure, D. (1962). La semence de théier d'Assam et le jardin semencier. Bulletin Information de L'institut National pour l'étude Agronomique du Congo Belge, 11, 119-140.
5. Chin, H.F. and Roberts, E.H. (1980). Recalcitrant Crop Seeds. Tropical Press, Kuala Lumpur, Malaysia.
6. Hume, P.F. (1955). Storage of tea seed. Tea Quarterly, 26, 93-95.
7. Katsuo, K., Toyao, T. and Kayumi, S. (1970). [The germination of tea seed. Part 1. Relation of the picking period and conditions for storage to the seed germination.] Study of Tea, 39, 14-19.
8. Katsuo, K., Toyao, T. and Kayumi, S. (1970). [The germination of tea seed. Part 2. Accelerating effect of pre-treatment on the seed germination.] Study of Tea, 39, 20-25.
9. Nakayama, A. and Harada, S. (1962). [Studies on the effect of temperature on the growth of the tea plant. Part 1. The effect of temperature on seed germination.] Bulletin Tea Research Station, Japan, 1, 1. (From Horticultural Abstracts, 1963, 33, 6380.)
10. Sanikidze, Z.G. (1975). [Measures for hastening and stimulating tea seed germination in Adygeya conditions.] Subtropicheskie Kul'tury, 5, 24-26. (From Horticultural Abstracts, 1977, 47, 2102.)
11. Sebastiampillai, A.R. and Anandappa, T.I. (1979). The influence of moisture and temperature of the germinability and longevity of tea (Camellia sinensis L.) seeds. Tea Quarterly, 48, 8-20.
12. Shaw, D.E. and Burnett, W.M. (1968). Investigation into the cause of leaf tumours of tea seedlings. Papua and New Guinea Agricultural Journal, 19, 167-192.
13. Toyao, T. and Kayumi, S. (1970). [Germination of tea seeds. Part 3. Abnormal growth of plumule caused by high temperature during germination.] Study of Tea, 40, 7-12.
14. Tubbs, F.R. (1932). The germination of tea seed. Tea Quarterly, 5, 66-69.
15. Visser, T. and de Waas Tillekeratne, L.M. (1958). Observations on the germination and storage of tea pollen and seed. Tea Quarterly, 29, 30-35.
16. Amma, S. and Watanabe, A. (1983). [Long-term storage of germ plasm in tea (Camellia sinensis (L.) O. Kuntze).] Bulletin of National Research Institute of Tea, 19, 29-57.
