Seed Dormancy

Seed dormancy can be defined as the failure of mature, intact seeds to germinate under favourable conditions. Environmental conditions necessary for germination include sufficient water, oxygen, light, and an appropriate temperature. Dormancy occurs because some property of the seed prevents germination. When mature, healthy seeds are tested over a range of conditions and fail to germinate; they are described as dormant.

 

Why has seed dormancy evolved?

Seed dormancy has evolved in plants to ensure survival from the time of seed dispersal to the time of germination and seedling establishment. It is the vital mechanism that connects one generation with the next. There are many types of seed dormancy (see below). Plant life form, regeneration strategy and, particularly, external environmental and climatic factors, either in the natural habitat or in the seed storage or germination environment, may all influence the type of dormancy demonstrated.

 

Why is dormancy a problem to seed conservationists?

There are two main reasons. First, when seeds do not germinate in routine germination monitoring at the MSB, this may lead to an underestimate of the true viability of the seed collection. In extreme cases this scenario could result in the collection being discarded as dead. In fact, the seed collection may simply be dormant, and require an appropriate dormancy breaking treatment to allow germination.

Secondly, there is little point in conserving seeds unless the seeds can be converted back into plants for use (e.g. in habitat restoration, re-introduction, or research). It is important to understand the appropriate conditions for germinating a particular collection of seeds.

 

Seed Dormancy Types

The MSB uses Baskin and Baskin’s (2004) classification of 5 main dormancy types:

• Physiological - caused by a mechanism within the seed embryo, or an embryo covering structure, which prevents germination. Physiologically dormant seeds in the Apiaceae, Iridaceae, Liliaceae, Papaveraceae and Ranunculaceae families usually respond positively to a period of moist storage under cold or warm conditions before sowing at the optimum germination temperature.
• Morphological - refers to a seed embryo which is underdeveloped or not fully formed at seed dispersal. The embryo continues to grow after dispersal, and germination is prevented until the embryo reaches a species-specific critical length. Examples can be found in the Annonaceae, Apiaceae, Orchidaceae, Orobanchaceae and Ranunculaceae families.
• Morpho-physiological - occurs in seeds with underdeveloped embryos that also have physiological dormancy (see above). The embryo must grow to a species-specific critical size and the physiological dormancy of the embryo must be broken before germination can occur. Examples can be found in the Apiaceae, Araceae, Fumariaceae, Liliaceae and the Magnoliaceae families.
• Physical - due to seed or fruit coat being resistant to water uptake, thereby preventing germination. It can also be due to the presence of inhibitors within the seed covering structure or the presence of a hard, woody fruit wall. Seeds with physical dormancy are often chipped or covering structures are removed at the MSB to allow water uptake or remove chemical inhibitors or mechanical constraint and hence allow germination. Examples can be found in the Anacardiaceae, Burseraceae, Cistaceae, Fabaceae, Geraniaceae, Malvaceae, and Rhamnaceae families.
• Combinational - occurs in seeds that have a combination of impermeable seed or fruit coats and physiologically dormant embryos. It is sometimes referred to as combined dormancy. For germination to occur, both types of dormancy must be broken. The order in which each type of dormancy must be broken depends on the species. Examples are Ceanothus (Rhamnaceae), Tilia (Tiliaceae) and Rhus (Anacardiaceae).

Further reading:

Baskin, C. C. and Baskin, J. M. (2004) A classification system for seed dormancy. Seed Science Research 14, 1-16.
Baskin, C. C. and Baskin, J. M. (1998) Seeds: Ecology, Biogeography and Evolution of Dormancy and Germination. Academic Press.