The Portulacaceae comprise about 200 species of fleshy herbaceous plants and small shrubs within about 20 genera, a few of which are cultivated as pot-herbs (e.g. Talinum triangulare (Jacq.) Willd., waterleaf). The fruits are usually dehiscent capsules and the seeds show orthodox storage behaviour. For example, Portulaca and Calandrinia spp. are maintained in the long-term seed store at the Wakehurst Place Gene Bank.
SEED DORMANCY AND GERMINATION
The seeds have a curved or ring-like embryo and can exhibit considerable dormancy. B.R. Atwater classifies seed morphology as endospermic seeds with peripheral linear embryos (see Table 17.1, Chapter 17). Treatments to the seed coat, light and alternating temperatures tend to promote seed germination. Detailed information on seed dormancy and germination is provided in this chapter for the genus Talinum only, but recommendations for germination test procedures and dormancy-breaking treatments for other species are summarised in Table 59.1.
TABLE 59.1 Summary of germination test recommendations for species within the Portulacaceae
|
Species and Authority |
Substrate |
Temperature |
Duration |
Additional directions |
Source |
|
Claytonia perfoliata Donn ex Willd. |
BP |
10°C |
21d |
|
ISTA |
|
Portulaca grandiflora Hook. |
TP; BP |
20°/30°C; 20°C |
14d |
light, pre-chill, potassium nitrate |
ISTA |
|
TP |
20°/30°C |
7d |
light, pre-chill, 5°C, 14-21d |
AOSA |
|
|
|
20°/30°C |
14d |
light, test at 10°C, potassium nitrate |
Atwater |
|
|
Portulaca oleracea L.
|
TP; BP |
20°/30°C |
14d |
pre-chill |
ISTA |
|
S |
20°/30°C |
10d |
light |
Everson |
TALINUM
|
T. triangulare (Jacq.) Willd. |
waterleaf |
I. Evidence of dormancy
At harvest seeds of T. triangulare exhibit considerable dormancy (1-4).
II. Germination regimes for non-dormant seeds
Constant temperatures: 20°C in light (3)
Alternating temperatures: 10°/30°C (12h/12h) in light (1)
III. Unsuccessful dormancy-breaking treatments
Warm stratification: 25°C, dark, 5,10,20d, germinate at 21°C, dark/light (24h/24h) (3)
Pre-soak: (2); 45°C, 75°C, 90°C (4)
Light: dark, at 20°-22°C (1,3); dark, at 25°-30°C, 28°-35°C (1); dark, at 6°-10°C, intact or scarified seeds (1); light, continuous, at 25°-30°C, 28°-35°C (1); light, 1-5d, 700 lux, 21°C, then dark, 21°C (3)
Scarification: concentrated sulphuric acid (4); sulphuric acid, 0.1, 2 N, 10 min (2)
Hydrogen peroxide: pre-applied, 10 min, 6% (2)
Acetone: (4)
Sodium hypochlorite: (4)
Thiourea: (4)
Potassium nitrate: (4)
IV. Partly-successful dormancy-breaking treatments
Alternating temperatures: 6°-10°/28°-35°C (12h/12h), dark (1); 28°-35°/6°-10°C, light/dark (12h/12h) (1)
Pre-chill: 4°C, dark, 5,10,20,30d, germinate at 21°C in light, continuous, 700 lux (3); 4°-5°C, 30d, in activated carbon-paste (2); 4°C, dark, 5d, then warm stratification, 25°C, dark, 15d, germinate at 21°C in light, continuous (3); 4°C, dark, 10d, then warm stratification, 25°C, dark, 10d, germinate at 21°C in light, continuous (3)
Warm stratification: 25°C, dark, 5,10,20d, germinate at 21°C in light, 5-25d, 700 lux, then 21°C in dark (3); 25°C, dark, 5,10,20d, germinate at 21°C in light, continuous, 700 lux (3); 25°C, dark, 5d, then pre-chill, 4°C, dark, 15d, germinate at 21°C in light, continuous (3)
Pre-soak: 60°C (4); 60°C, 2 min (2)
Light: continuous, at 20°-22°C (1,3); white, 0.25-8h/d, 700 lux, at 21°C (3); 10-25d, 700 lux, at 21°C, then dark, at 21°C (3) Scarification: sand paper, then pre-soak, 3h (2)
Removal of seed covering structures: chip, then pre-soak, 20 min (2); chip, then thiourea, pre-applied, 20 min, 5% (2)
Thiourea: pre-applied, 20 min, 5% (2)
V. Successful dormancy-breaking treatments
Warm stratification: 25°C, dark, 30d, germinate at 21°C in light, continuous (3); 25°C, dark, 10d, then pre-chill, 4°C, dark, 10d, germinate at 21°C in light, continuous (3)
Removal of seed covering structures: prick, germinate at 20°-22°C in light, continuous (1)
VI. Comment
Light appears to be an essential requirement for the germination of seeds of T. triangulare (1,3); 8 hours light per day is sufficient to promote the maximum response (3). Alternating temperatures (1), pre-chill (2,3), warm stratification (3), and pricking the seed coat (1) can all be promotory and it is suggested that these be incorporated in dormancy-breaking and germination test routines as follows: prick the seed coat with a needle then imbibe the seeds at 25°C in the dark for 10 days, transfer to 4°C in the dark for a further 10 days and then test for germination at 10°/30°C (12h/12h) with light applied for 8 hours per day during part of the period spent at the higher temperature. For some (less dormant) accessions the germination test regime alone may be sufficiently promotory. Incidentally, pre-chilling after a warm stratification treatment is more promotory than pre-chilling first (3).
VII. References
1. Agble, F. (1970). Germination of seeds of Talinum triangulare, Ghana Journal of Science, 10, 29-32.
2. Fawusi, M.O.A. (1979). Germination of Talinum triangulare L. seeds as affected by various chemical and physical treatments. Annals of Botany, 44, 617-622.
3. Nwoke, F.I.O. (1982). Effects of photoperiod on germination of seeds of Talinum triangulare (Jacq.) Willd. Annals of Botany, 49, 23-29.
4. Stephens, C.E. (1967). The genetical basis of flower colour variation in Talinum triangulare (Jacq.) Willd. and cytological studies of Hydrocleis nyphoides (Willd.) Buchenau. M.Sc. Thesis, University of Ghana. [Cited by Agble, F. (1970).]
