The Euphorbiaceae comprise over 8000 species of herbaceous plants, shrubs and trees within about 280 genera which provide edible roots (e.g. Manihot esculenta Crantz, cassava), oils (e.g. Ricinus communis L., castor), edible fruits (e.g. Antidesma bunius (L.) Spreng., bignay), medicinal products (e.g. Croton tiglium L.) and other products (e.g. Hevea brasiliensis Muell.-Arg., para rubber). The fruits are usually dehiscent capsules, but sometimes dehiscent and berry- or drupe-like.
Seed storage behaviour in Euphorbiaceae is generally orthodox. For example, Euphorbia spp. are maintained in the long-term seed store at the Wakehurst Place Gene Bank. There are, however, important exceptions. For example, seeds of Hevea brasiliensis are recalcitrant.
SEED DORMANCY AND GERMINATION
Seeds can be either endospermic or non-endospermic; germination can be either epigeal or hypogeal. Dormancy can be a severe problem; many, but certainly not all the problems in germinating the seeds can be overcome by removing part of the seed coat and the surrounding structures. B.R. Atwater classifies seed morphology in some species as non-endospermic seeds with axile foliar embryos within woody seed coats and an inner semi-permeable layer (see Table 17.2, Chapter 17). Detailed information on seed dormancy and germination is provided in this chapter for the genera Aleurites, Hevea, Manihot and Ricinus. Table 37.1 provides a limited number of recommended germination test procedures and dormancy-breaking treatments for several other species. In addition the algorithm below may be helpful in developing suitable germination test procedures for species where no advice is provided in this chapter.
RBG Kew Wakehurst Place algorithm
The first step in the algorithm is to test seeds at constant temperatures of 16°C and 26°C with light applied for 12h/d in each case.
If this is not successful in promoting full germination then the second step in the algorithm is to chip the seeds and test at whichever temperature gave the greater germination in the first step.
If this is not successful in promoting full germination then the third step of the algorithm is to co-apply 7 x 10-4 M GA3 to the germination test substrate and test in the most promotory regime determined in steps one and two. (This may include a requirement to chip the seeds.)
TABLE 37.1 Summary of germination test recommendations for species within the Euphorbiaceae
|
Species and Authority |
Substrate |
Temperature |
Duration |
Additional directions |
Source |
|
Baccaurea motleyana Muell. Arg. |
S |
25°-30°C |
35d |
light, continuous |
CHML |
|
Euphorbia heterophylla L. |
TP |
20°/30°C |
16d |
light |
AOSA |
|
Euphorbia marginata Pursh |
TP |
20°C |
14d |
excise embryos or pre-chill, 5°C, 2m |
AOSA |
ALEURITES
|
A. Fordii Hemsl. |
China wood-oil-tree, tung-oil-tree |
|
A. moluccana Willd. [A. triloba
Forst.] |
candlenut, candle-berry-tree, varnish-tree |
|
A. montana (Lour.) Wils. |
tung, mu-tree |
I. Evidence of dormancy
Seeds of Aleurites spp. tolerate desiccation to at least 3% moisture content (5,9), and can be stored dry (7) at low temperatures (3). Thus despite reports that they are short-lived (3) and the preference of many growers to store the seeds moist in cool conditions, seeds of Aleurites spp. show orthodox, not recalcitrant, seed storage characteristics. Seed germination is slow, erratic and low and thus problematic for growers (1,4,6,7,8). The seed coat is reported to be the cause of delayed germination (1,8,9).
II. Germination regimes for non-dormant seeds
A. Fordii
Constant temperatures: 24°-32°C (7)
III. Unsuccessful dormancy-breaking treatments
A. Fordii
Pre-soak: fruit, 48h (5)
Removal of seed covering structures: crack seed coat (5); crack seed coat, then store in oxygen, 100%, 18d (5)
Scarification: emery wheel (5)
Tergetol: pre-applied, 12,24,48h, 0.2, 0.4% (8); pre-applied, 48h, 0.1% (8)
Morpholine: pre-applied, 48h, 1% (5)
A. moluccana
Removal of seed covering structures: crack exocarp (1); crack exocarp, then pre-soak, 10 min (1)
Scarification: sulphuric acid, 5, 10, 15, 20%, 10-30 min (1); hydrochloric acid, 5, 10, 15, 20%, 10-30 min (1); nitric acid, 5, 10, 15, 20%, 10-30 min (1)
Pre-soak: 100°C, 10,50 min (1)
A. montana
Removal of seed covering structures: crack seed coat by exposure to sunlight, 1-10h (9)
Pre-soak: cold or 60°-80°C, 6,24,48h (9)
IV. Partly-successful dormancy-breaking treatments
A. Fordii
Pre-chill: 0°C, 3°C, 4°-15°C, 4m (4); seed or fruit, 40-140d (7)
Warm stratification: 25°C, 41d (5); 9°-24°C, 41d (5); 9°-24°C, 30d, then 25°C, 42d (5)
Pre-soak: 12-48h (8)
Tergetol: pre-applied, 12,24h, 0.1% (8)
Morpholine: pre-applied, 12-48h, 0.25, 0.5, 1% (8)
Removal of seed covering structures: crack seed coat (9); seed coat, then pre-chill, 40-140d (7)
A. montana
Warm stratification: 100-150d (9)
Removal of seed covering structures: crack seed coat (9)
V. Successful dormancy-breaking treatments
A. Fordii
Warm stratification: 9°-24°C, 75-100d (5)
A. montana
Scarification: by hand, near hilum (9)
VI. Comment
No reports on laboratory germination test procedures for seeds of Aleurites spp. have been found; those summarised above concern treatments prior to field sowings. For nursery sowings sphagnum moss, sand or soil are suitable (8,9); seeds should be sown 2.5 cm deep (8), with the hilum at the side or top - since germination is greatly reduced when seeds are sown with the hilum at the bottom (9). The period required for germination can be considerable. For example, there may be a 60 day delay before the first seeds to germinate begin to emerge through the soil surface for A. Fordii (6), and a 75 day delay for A. moluccana (1).
Although substantial pre-chill - 3°-5°C, 4 months (4,7) - or warm stratification - 9°-24°C, 3 months (5) - treatments result in substantial increases in subsequent germination, their major effect appears to be in enabling seed moisture content to rise (5,7), suggesting that much of the delay to germination may be caused by the seed coat delaying imbibition. Certainly cracking the seeds coats of freshly harvested seeds is promotory and reduces the time taken to germinate, but for stored seeds the treatment may be damaging (9). Rather than crack the seed coat, it is better to scarify it by hand near the hilum (9). It is suggested that this be done and the seeds then tested for germination in moist (sterile) sand at 25°C. However, investigations into the response of germination to a range of constant and alternating temperature regimes might be tried since an alternating temperature regime could possibly be more suitable than a constant 25°C.
VII. References
1. Eakle, T.W. and Garcia, A.S. (1977). Hastening the germination of lumbang (Aleurites moluccana (L.) Willd.) seeds. Sylvatrop, 2, 291-295.
2. François, M.T. (1936). Sur l'analyse de graines de tung (Aleurites Fordii) cultiveés au Maroc (Récolte 1933). Agronomie Coloniale 25, 89-91.
3. Large, J.R., Fernholz, D.L., Merrill, S. Jr. and Potter, G.F. (1947). Longevity of tung seed as affected by storage temperatures. Proceedings of the American Society for Horticultural Science, 49, 147-150.
4. Li, L.Y. (1943). The influence of stratification of tung seeds upon emergence and establishment of seedlings in the nursery. New Zealand Journal of Science and Technology, Section A, 25, 43-48.
5. Merrill, S. Jr. (1947). Germination of early-planted and late-planted tung seeds as affected by stratification and various seed treatments. Proceedings of the American Society for Horticultural Science, 49, 151-157.
6. Merrill, S. Jr., Slick, W.A., Painter, J.H. and Brown, R.T. (1941). Effect of planting date on germination of tung nuts in the nursery. Proceedings of the American Society for Horticultural Science, 39, 153-156.
7. Sharpe, R.H. and Merrill, S. Jr. (1942). Effect of stratification and time of planting on germination of tung seed. Proceedings of the American Society for Horticultural Science 40, 286-291.
8. Shear, C.B. and Crane, H.L. (1943). Germination of the nuts of the tung tree as affected by penetrants, substrata, depth of planting, and storage conditions. Botanical Gazette, 105, 251-256.
9. Webster, C.C. (1948). The effect of seed treatments, nursery technique and storage methods on the germination of tung seed. East African Agricultural Journal, 14, 38-48.
HEVEA
|
H. brasiliensis (Willd. ex Adr. de Juss.)
Muell.-Arg. |
para rubber |
I. Evidence of dormancy
H. brasiliensis shows recalcitrant seed storage characteristics: that is the seeds are killed by desiccation (4,8,9,12). Consequently, since the seeds cannot be safely dried, the seeds are killed by exposure to sub-zero temperatures (4). Seed longevity in moist storage conditions is short (2,3,6,8,11,14).
Although freshly harvested rubber seeds are reported to show no dormancy (2), seeds from certain clones show the slow, erratic and low germination typical of dormant seed populations (5,6,7). Moreover, after-ripening of rubber seeds in moist storage has been shown to reduce this problem (12). A high partial pressure of carbon dioxide can induce dormancy in the seed (8).
II. Germination regimes for non-dorment seeds
Constant temperatures: 25°C (13); 27°C (8); 28°C in diffuse light, 12-13 h/d (4,10,11)
Alternating temperatures: 25°/30°C, light, 24h/d, 21d (15)
III. Unsuccessful dormancy-breaking treatments
Pre-soak: 12h (7); c. 100°C, 5-10 min (7)
Scarification: concentrated sulphuric acid, 2-5 min (7); file seed coat (7)
Light: direct sunlight through clear plastic, 0.5-2h (7)
IV. Partly-successful dormancy-breaking treatments
-
V. Successful dormancy-breaking treatments
Pre-soak: 16h (5)
Scarification: file seed coat near hylum (5)
Removal of seed covering structures: seed coat (5); micropylar cap (6)
VI. Comment
Moist sand is generally reported to be the best germination medium for both laboratory tests and nursery sowings (1,4,6,10,11,13). The seeds should be sown deep enough just to bury the micropyle under the surface with the grooved side of the seed underneath (6). 25°C is a suitable germination test temperature, but direct sunlight should be avoided (7). Although the majority of seeds germinate 3 to 25 days after sowing (6,10), where dormancy is present 56 days, or more, may be required before germination is complete (7). After 14 days in test seeds which have not begun to germinate can be scarified by hand and returned to the germination test. There is one report that testing the seeds between wet hessian bags rather than moist sand is preferable (7). It would be worthwhile testing whether a rolled paper towel test is more suitable for laboratory germination tests; an investigation of the response of seed germination to alternating temperatures would also be worthwhile. Finally a tetrazolium test can be used as an alternative test of viability (10) if this is required.
VII. References
1. Amma, C.K.S. and Nair, V.K.B. (1977). Relationship of seed weight and seedling vigour in Hevea. Rubber Board Bulletin, 13, 28-29.
2. Ang, B.B. (1977). Problems of rubber seed storage. In Seed Technology in the Tropics (eds. H.F. Chin, I.C. Enoch, and R.M. Raja Harun), pp. 117-122, Universiti Pertanian Malaysia, Malaysia.
3. Cardoso, M., Zink, E. and Bacchi, O. (1966). [A study on the storage of Hevea seeds.] Bragantia, 25, 35-40. (From Horticultural Abstracts, 1968, 38, 4488.)
4. Chin, H.F., Ariz, M., Ang, B.B. and Hamzah, S. (1981). The effect of moisture and temperature on the ultrastructure and viability of seeds of Hevea Brasiliensis. Seed Science and Technology, 9, 411-422.
5. Dahle, H.E. (1938). [Germination experiments with Hevea seed.] Bergcultures, 12, 1270. (From Horticultural Abstracts, 1939, 9, 264.)
6. Dijkman, M.J. (1951). Hevea. Thirty years of research in the Far East, 43pp. University of Miami Press, Coral Gables, Florida.
7. Keleny, G.P. and Van Haaren, A.J.H. (1967). Progress report on rubber seed investigation. Papua New Guinea Agriculture Journal, 19 19, 72-87.
8. Kidd, F. (1914). The controlling influence of carbon dioxide in the maturation, dormancy, and germination of seeds. Part II. Proceedings of the Royal Society, Series B, 87, 609-625.
9. Koopman, M.J.F. (1963). Result of a number of storage experiments conducted under controlled conditions. b. Other than agricultural seeds. Proceedings of the International Seed Testing Association, 28, 853-860.
10. Mohamad Husin, S., Chin, H.F. and Hor, Y.L. (1981). Viability test on Hevea seeds by the tetrazolium method. Journal of the Rubber Research Institute of Malaysia, 29, 44-51.
11. Pa, O.T. and Koen, L.I. (1963). Results on storage test with seeds of Hevea Brasiliensis. Menara Perkebunan, 32, 183-192.
12. Pereira, J.D.P. (1977). [Conservation of viability of Hevea seeds.] Pesquisas Agropecuaries Brasileira Serie Agronomicas.
13. Van der Hoop, D.J.N. (1930). [Germination experiment with Hevea seed.] Archives Rubber Cultivation Nederl. Indie, 14, 81-84.
14. Wycherley, P.R. (1971). Hevea seed. Part 3. Planter, 47, 405-410.
15. Chin, H.F., Hor, Y.L. and Mohd Lassim, M.B. (1984). Identification of recalcitrant seeds. Seed Science and Technology, 12, 429-436.
MANIHOT
|
M. anomala |
|
|
M. esculenta Crantz |
cassava, tapioca, manioc, mandioca, yuca |
|
M. gracilis |
|
|
M. longepetiolata |
|
|
M. oligantha |
|
|
M. pentaphylla |
|
|
M. salicifolia |
|
I. Evidence of dormancy
Cassava (M. esculenta) seeds are generally dormant for several months after harvest (1). However, estimating the proportion of dormant seeds in populations is quite difficult unless the range of environments over which non-dormant seeds will germinate is taken into account (3). Wild Manihot spp. can exhibit extreme seed dormancy (9).
II. Germination regimes for non-dormant seeds
M. esculenta
Constant temperatures: 34°-40°C (3)
III. Unsuccessful dormancy-breaking treatments
M. anomala
Constant temperatures: 18°-22°C (9)
M. esculenta
Constant temperatures: 4°-25°C, dark (7); 19°-31°C (3); 20°C, 25°C, 40°C (4)
Alternating temperatures: 19°-25°/28°C, 19°/22°C, 19°/25°C, 22°/25°C (4-20h/4-20h) (3); 19°-28°/31°C, 19°/34°-40°C (20h/4h) (3); 20°/25°C (16h/8h) (4)
Pre-chill: (8); 24h (5)
Pre-soak: 24h (5); 60°C, 0.5, 1h (5)
Scarification: acid (8); sulphuric acid, 0.25, 0.5, 1h (5)
Removal of seed covering structures: (5); file (5)
M. gracilis, M. longepetiolata, M. oligantha, M. pentaphylla, M. salicifolia
Constant temperatures: 18°-22°C (9)
IV. Partly-successful dormancy-breaking treatments
M. anomala
Alternating temperatures: 26°/38°C (16h/8h), 35d (9)
M. esculenta
Constant temperatures: 30°C, 21d (8); 30°C, 35°C, (4); 30°C, 37°C, dark, 35d (7)
Alternating temperatures: 25°/35°C (16h/8h), 42d (2); 19°-28°/34°-40°C, 34°-40°/34°-40°C (16h/8h or 8h/16h), 19°-37°/34°-40°C (4h/20h), 28°-34°/37°-40°C (20h/4h) (3); 25°/30°C, 30°/35°C, 35°/40°C, 20°/30°C, 25°/35°C, 30°/40°C, 20°/35°C, 25°/40°C (16h/8h) (4)
GA3: pre-applied, in red light, germinate at 30°-35°C (6)
Pre-dry: 60°C, 14d (1)
Scarification: by hand, near micropyle (8)
M. gracilis, M. longepetiolata, M. oligantha, M. pentaphylla, M. salicifolia
Alternating temperatures: 26°/38°C (16h/8h), 35d (9)
V. Successful dormancy-breaking treatments
M. esculenta
Light: red, 24h, germinate at 28°C in dark (8)
VI. Comment
The following laboratory procedure has been reported for germinating cassava seeds (8): remove the cork-like caruncles and scarify in the region of the micropyle until the radicle is slightly visible; then sterilize the seeds in either 10% hydrogen peroxide or 0.15 N sodium hypochlorite for 30 minutes; rinse three times; pre-soak for 16-24 hours in aerated distilled water; sterilize again in 10% hydrogen peroxide for 1 minute; irradiate with red light for one day; and finally test for germination at 28°C in the dark. Unfortunately no evidence is provided of the success or otherwise of this lengthy procedure (8). Moreover, it is clear from other results that higher constant temperatures are required for germination (3,4), that the optimum constant germination test temperature may be altered by storage (4), and that often alternating temperature germination test regimes provide a further stimulus, promoting germination to higher levels than at constant temperatures (3).
From results of germination tests under a wide range of constant and alternating temperature regimes it is clear that certain minimum conditions must be provided before cassava seeds will germinate; the maximum temperature during part of the day must exceed 30°C and the mean temperature must be 24°C or more (3). However, the conditions required for full germination are rather more stringent; the maximum temperature during part of the day must be between 36° to 40°C, the amplitude of the alternation must be between 3°-18°C, and the mean temperature must be 33°C or more (3). Alternating temperature regimes have also been shown to promote the germination of some, but not all, dormant seeds of wild Manihot spp. (9).
In view of these requirements an alternating temperature regime of 38°/30°C (16h/8h) has been recommended with a minimum test duration of 21 days for M. esculenta (3); for dormant seeds of other Manihot spp. a minimum test duration of 42 days is suggested here. In addition it is suggested that red light be applied intermittently - see Chapter 6.
VII. References
1. Anonymous (1981). Germplasm development. Seed germination studies. In Cassava Program, Annual Report 1980, p.32, CIAT, Cali, Colombia. (From Field Crop Abstracts, 1983, 36, 878.)
2. Ellis, R.H., Hong, T.D. and Roberts, E.H. (1981). The influence of desiccation on cassava seed germination and longevity. Annals of Botany, 47, 173-175.
3. Ellis, R.H., Hong, T.D. and Roberts, E.H. (1982). An investigation of the influence of constant and alternating temperature on the germination of cassava seed using a two-dimensional temperature gradient plate. Annals of Botany, 49, 241-246.
4. Ellis, R.H. and Roberts, E.H. (1979). Germination of stored cassava seed at constant and alternating temperatures. Annals of Botany, 44, 677-684.
5. Martin, F.W. and Ruberté, R. (1976). Germination and longevity of cassava seeds. Tropical Root and Tuber Crops Newsletter, 9, 54-56.
6. Mendes, R.A. (1981). [Improvement of seed germination of cassava (Manihot esculenta Crantz).] In Anais i Congresso Brasileira de Mandioca, 1, Pesquisas agronômicas, pp. 523-533. (From Seed Abstracts, 1982, 5, 2748.)
7. Mumford, P.M. and Grout, B.W.W. (1978). Germination and liquid nitrogen storage of cassava seed. Annals of Botany, 42, 255-257.
8. Nartey, F. (1978). Manihot esculenta (Cassava, Tapioca, Manioc, Mandioca, Yuca): Cyanogenesis, Ultrastructure and Seed Germination, 262 pp., Munksgaard, Copenhagen.
9. Nassar, N.M.A. and Teixeira, R.P. (1983). [Seed germination of wild cassava species (Manihot spp.).] Ciencia e Cultura, 35, 630-632.
RICINUS
|
R. communis L. |
castor bean, castor oil plant, palma christi |
I. Evidence of dormancy
Freshly harvested seeds of R. communis show slow, erratic, low germination (1-4,6,8,9,12). After-ripening at room temperature for four (2), nine (6) or several (3) months is reported to avoid this problem in subsequent germination tests. The seed covering structures (testa and caruncle) are reported to be the major cause of poor germination (1,3,4,8,9). The seeds show orthodox storage behaviour.
II. Germination regimes for non-dormant seeds
BP; S: 20°/30°C (16h/8h): 14d (AOSA, ISTA)
In addition if the growth of moulds interfere with a germination test AOSA rules recommend that the caruncles be removed.
Constant temperatures: 25°C (13); 27°C (10,11,12); 30°C (13)
Alternating temperatures: 20°/30°C (16h/8h) (4,10,11,13); 20°/35°C (16h/8h) (13)
III. Unsuccessful dormancy-breaking treatments
Urea: pre-applied, 24h, 24% (5)
Scarification: sulphuric acid, 10%, 1,2d (7); concentrated nitric acid, 1 min (9); emery paper (7)
IV. Partly-successful dormancy-breaking treatments
Pre-soak: 24h (5); 2-3d (7)
V. Successful dormancy-breaking treatments
Pre-soak: pour on boiling water, then leave to cool, 24h (8)
Removal of seed covering structures: caruncle (9,12); caruncle, then scarify testa at caruncle end with emery paper (3,4,8); seed coat, germinate in moist sand (9)
VI. Comment
In addition to poor germination, difficulties in seed germination tests of R. communis may also be caused by the growth of fungi and bacteria. Removal of the caruncles from seeds reduces the delay to germination and can increase the proportion of seeds germinating (3,4,8,9); it can also reduce fungal and bacterial growth during germination tests (4,12). The latter problem can be further reduced by testing for germination in moist sand rather than paper towels (10,11). Consequently it is suggested that the seeds be tested for germination as follows: remove caruncle and hand scarify testa at the caruncle end (but do not damage the endosperm), and then test in moist (sterile) sand at 20°/30°C (16h/8h).
VII. References
1. Atsmon, D. (1958). Germination inhibition in castor-beans. Bulletin of the Research Council of Israel, 6D, 260.
2. Brighan, R.D. (1965). Delayed germination and seedling emergence of castorbean (Ricinus communis L.). Crop Science, 5, 79 -83.
3. Engelhardt, M. and Vincente, M. (1963). [How to make recently harvested castorbean (Ricinus communis L.) seeds germinate.] Biológico Brasil, 29, 191. (From Horticultural Abstracts, 1964, 34, 1391.)
4. Heit, C.E. (1949). Germinating castor-bean seed in the laboratory. Proceedings of the Association of Official Seed Analysts, 39, 114-117.
5. Kurdikeri, C.B. (1974). Soaking of castor in water promotes germination. Current Research, 3, 102.
6. Lago, A.A., Zink, E., Razera, L.F., Banzatto, N.V. and Savy-Filho, A. (1978). [Seed dormancy of three castorbean cultivars.] Bragantia, 38, 41-44.
7. Patel, G.J. and Jaisani, B.G. (1962). Studies on castor germination. Indian Oilseeds Journal, 6, 106-111.
8. Purseglove, J.W. (1968). Ricinus communis L. In Tropical Crops. Dicotyledons. pp. 180-186, Longmans, London.
9. Reilhes, R. (1942). Influence du tégument sur la germination des graines de ricin (Ricinus communis). Comptes Rendus de la Société de Biologie, 136, 70-73.
10. Stafford, R.E. and Metzer, R.B. (1970). Germination of four castor seed lots in paper towels, kimpak, and sterile sand. 14 pp., Texas Agricultural Experiment Station, Progress Report 2773.
11. Stafford, R.E. and Metzer, R.B. (1971). Relationships among laboratory germination, greenhouse, and field emergence of four castor seed lots. Agronomy Journal, 63, 805-808.
12. Williams, J.H. and Kittock, D.L. (1969). Management factors influencing viability of castor bean (Ricinus communis) seed. Agronomy Journal, 61, 954-958.
13. Carneiro, J.W.P. and Pires, J.C. (1983). [Influence of temperature and substrata in the germination of castorbean seeds.] Revista Brasileira de Sementes, 5, 127-131.
