Physiological dormancy; the world's most common expression of seed dormancy
Physiological dormancy (PD) is an endogenous dormancy, associated with the seed embryo and thought to be caused by a physiological inhibiting mechanism.
Seeds have physiological dormancy if:
- Application of gibberellic acid (GA3) increases germination
- Dry after-ripening or dry storage increases germination
- Excised embryos produce normal, healthy seedlings
- Scarification increases germination (in some PD cases)
- Up to 3 months of cold (0-10°C) or warm (=15°C) stratification increases germination
- Dry after-ripening shortens the length of cold stratification required
Seeds have deep physiological dormancy if:
- Application of GA3 does not increase germination
- Excised embryos produce abnormal seedlings
- Seeds require more than 3 months of cold stratification to germinate1.
Examples of physiological dormancy at the TSCC:
Up to 3 months warm or cold stratification is often a germination requirement of seeds with physiological dormancy. Juncus antarcticus (Juncaceae) seeds were collected at approx. 1310m on Mt Rufus in Lake St Clair National Park, Tasmania. Seeds required 8 weeks cold stratification (5°C) before they would germinate at 27/15°C (fig. 1).
Figure 1. Germination response of Juncus antarcticus. Click image for more details.
These results suggest that in the field, physiological dormancy of J. antarcticus is alleviated during wet, winter months, prior to seeds germinating in late spring/early summer.
Epacris marginata (Ericaceae) is a heath species endemic to Tasmania. Seeds were collected in October (spring) from the Tasman Peninsula. Tests carried out so far indicate that seeds require warm stratification (27/15°C) before germination can occur at 15°C (fig. 2).
Figure 2. Germination of Epacris marginata. This species appears to require warm stratification to germinate at intermediate temperatures. For more information click image.
These results suggest that E. marginata bears physiologically dormant seeds which lose dormancy over the summer before germination can occur in the autumn.
Other examples of physiological dormancy:
Goodenia fascicularis (Goodeniaceae) seeds collected in south-west Queensland, Australia did not respond to application of GA3. However seeds did responded to mechanical scarification (chipping) and de-coating prior to germination testing. For example chipping above the radicle stimulated 71% germination at 20°C.
However, de-coating seeds caused seedlings to grow abnormally long radicles (roots), before seedlings became stunted and died. Similarly, chipping above the cotyledons resulted in stunted radical growth and chipping close to the radicle meant that cotyledons were not able to fully emerge from the seed coat (fig. 3). Stunted growth can suggest that, despite a germination response, some seed dormancy still remains. These G. fascicularis seeds were consequently diagnosed with deep physiological dormancy2.
Figure 3. Abnormal germination of Goodenia fascicularis (Goodeniaceae) seeds mechanically scarified in the vicinity of the cotyledons (a) and the radicle (b). Image courtesy of Gemma Hoyle.
- Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
- Hoyle GL, Steadman KJ, Daws MI and Adkins SW. 2008. Physiological dormancy in forbs native to south-west Queensland: Diagnosis and classification. South African Journal of Botany. 74: 208-213.