If a viable (living) seed doesn't germinate at appropriate temperatures, with water and oxygen (and perhaps light too), it probably posesses dormancy.
The subject of seed dormancy has been active in a research sense for at least 100 years1, and has been known to exist for far longer. However describing exactly what seed dormancy is can be troublesome and numerous definitions have been accepted and rejected by seed scientists over the years.
A dormant seed is not a 'failure'
Figure 1. Planocarpa nitida (Ericaceae) at the TSCC; So far no germination has taken place. Stone fruited Ericaceae seeds are frequently difficult to germinate.
Because dormant seeds don't germinate, dormancy was, for many years, considered to be some sort of seed defect or inactivity that meant that even a viable seed simply could not germinate.
However investigation has proven that viable seeds, including dormant seeds, continually sense and respond to their environment2, so dormant seeds are not inactive.
Not only that but dormant seeds can germinate after dormancy has been removed or alleviated, and seedlings then develop normally. For more information, see topics covered under 'Dealing with dormancy'.
Germination and dormancy are two very different things
Dormancy and germination work on different time scales and are affected by different environmental factors, (or are affected differently by the same environmental factors). We know this because seed scientists can make predictions about dormancy and germination separately3. For example, moisture, nitrate and light were found to influence germination of Polygonum persicaria (Polygonaceae) seeds, where as temperature influenced not germination, but dormancy status of the seeds4.
With regard to dormancy alleviation of Australian species, seed scientists are beginning to make a distinction between factors that affect dormancy and those that affect germination5.
Dormancy is not a matter of time
A seed isn't dormant simply because it hasn't germinated within a few weeks of germination testing. The time it takes for a seed to germinate can not be used to diagnose dormancy, unless germination tests last up to 90 days; the equivalent of a full season in the field (e.g. spring, autumn or 'the rainy season', etc.). If a seed is still viable after more than 90 days in germination conditions, and hasn't germinated, it may be dormant (fig. 2).
Dormancy is not caused by environmental conditions
Figure 2. Gahnia trifida (Cyperaceae) has shown slow and sporadic germination at the TSCC. This seedling emerged following 260 days in germination test conditions!
The environmental conditions surrounding a seed do not cause seed dormancy. If a seed has no water, oxygen or temperatures adequate for metabolism and growth, it will not germinate, regardless of whether or not it possesses dormancy.
For example, seeds in a packet on the shelf in your garden shed will not germinate, but this doesn't necessarily mean they're dormant!
Instead this is called quiescence and is sometimes referred to as 'pseudodormancy'6, or 'enforced dormancy'7, but it is not really 'dormancy' at all.
Dormancy is a physiological block within the seed
Vleeshouwers et al. (1995)2 accurately described dormancy as
…a block or blocks within the seed that prevent germination, as distinguished from the absence of factors required to evoke germination.
Dormancy is a seed characteristic
Vleeshouwers et al. went on to carefully described dormancy as '…a seed characteristic, the degree of which defines what conditions should be met to make the seed germinate.'
Other useful definitions of seed dormancy
A useful definition of seed dormancy will separate dormancy mechanisms from germination requirements, the process of germination and the environmental conditions surrounding the seed.
Simpson (1990)8 defined dormancy as:
…temporary failure of a viable seed to germinate…in a particular set of environmental conditions that later evoke germination when the restrictive state has been terminated by either natural or artificial means.
Bell (1999)9 stated that seeds ‘must not be in a state of dormancy and the environmental requirements for germination of that seed must be met', before germination can occur.'
Eira and Caldas (2000)10 defined dormancy as
…a state in which the development or germination of a viable seed is blocked by one of many possible limitations located within the seed itself, and went on to discuss dormancy and germination as concurrent processes rather than in sequence.
Dormancy can range between all and nothing
Dormancy is today considered to be a 'plastic' seed characteristic, the status of which can range from any value between all (maximum dormancy) and nothing (non-dormancy). A non-dormant seed has the capacity to germinate over the widest range of normal physical environmental conditions possible for the genotype11.
For more on seed dormancy please view the following pages.