When trying to answer the question ‘are these seeds dormant?’, a simple ‘yes’ or ‘no’ answer is not always adequate and ‘dormancy status’ needs consideration.
Dormancy status refers to the ‘amount’ of physiological dormancy (PD) the seeds possess. Dormancy is a ‘plastic’ seed characteristic, the status of which can range from any value between all (maximum dormancy) and nothing (non-dormant).
Unfortunately, seed dormancy status cannot yet be measured directly. However a satisfactory impression of the dormancy status of a large seed population can be obtained by carrying out a significant range of different germination tests1.
Consider the seed’s lifetime
The dormancy status of a seed at a given moment is not only influenced by its current physiological state, but also by the conditions it has experienced during its entire existence, including development on the parent plant, maturation, and post-harvest handling and storage. It is also good to note the importance of seed collecting and storing in relation to dormancy status.
Degree of dormancy
The ‘degree’ of PD can be thought of in terms of the range of conditions in which the seed will germinate.
It is generally accepted that when dormancy is induced, the temperature range over which the seeds will germinate begins to narrow, until germination will not occur at any temperature and the seed is considered dormant. Conversely, when dormancy is alleviated there is a widening of this range so that a non-dormant seed has the capacity to germinate over the widest range of normal physical environmental conditions possible for the genotype2.
For example, dormancy of Aesculus hippocastanum (Sapindaceae) seeds was alleviated by cold stratification which resulted in a widening of the temperature range in which seeds would germinate3.
Therefore it is the degree of dormancy that defines the germination requirements of a seed1, and germination requirements are likely to be more specific, the greater the degree of dormancy.
Seed scientists commonly use a range of germination test temperature regimes in order to determine the degree of dormancy exhibited by a seed lot.
Depth of dormancy
Physiological dormancy is said to be caused by a physiological inhibiting mechanism or PIM4. With respect to the PIM, physiological dormancy was recently divided into three types or ‘depths’2 (table 1);
Table 1. Characteristics of dormancy in seeds with non-deep, intermediate and deep physiological dormancy (according to Baskin and Baskin, 2004).
Level of dormancy
The depth of physiological dormancy is defined by the response of seeds to dormancy alleviating treatments and takes into account resulting seedling health and normality. However, very few seed dormancy studies attempt to diagnose dormancy depth and it has been suggested that no fixed boundaries exist between the levels discussed in table 1 5.
Dormancy duration and timing
The terms ‘primary’ and ‘secondary’ dormancy refer to the timing of occurrence of dormancy, which is another facet of a seed lot’s dormancy status. Primary dormancy describes seeds that posses dormancy at the point of dispersal. Secondary dormancy describes non-dormant seeds entering or re-entering dormancy. This latter phenomenon is known as ‘dormancy cycling’6 (fig. 1).
Dormancy cycling can occur many times in nature before a seed eventually germinates. In the field, physiological dormancy is alleviated during the period before conditions are expected to be just right for germination and seedling growth. Secondary dormancy will be induced if conditions turn out to be unsuitable for plant survival7.
Dormancy cycling is evidence that seeds continually sense their environment and adjust their dormancy status accordingly, as they wait for the best time and/or place to germinate.
Dormancy cycling in weeds
Studies upon arable weeds have provided the most evidence of dormancy cycling. For example, Veronica hederifolia (Scrophulariaceae) seeds buried in the field and retrieved at monthly intervals were dormant at the beginning of the experiment, and exhibited dormancy/non-dormancy/conditional dormancy cycling throughout a 12 month investigation8.
- Vleeshouwers LM, Bouwmeester HJ and Karssen CM. 1995. Redefining seed dormancy: An attempt to integrate physiology and ecology. The Journal of Ecology 83: 1031-1037.
- Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
- Pritchard HW, Steadman KJ, Nash JV and Jones C. 1999. Kinetics of dormancy release and the high temperature germination response in Aesculus hippocastanum seeds. Journal of Experimental Botany 50: 1507-1514.
- Nikolaeva MG. 1977. Factors controlling the seed dormancy pattern. In: The physiology and biochemistry of seed dormancy and germination. Khan AA, ed. New York, North-Holland Publishing Company: 51-74.
- Nikolaeva MG. 2001. An update of Nikolaeva’s seed dormancy classification and its relevance to the ecology, physiology, biogeography and phylogenetic relationships of seed dormancy and germination. Botanicheskii Zhurnal 86: 1-14.
- Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.
- Benech-Arnold RL, Sanchez RA, Forcella F, Kruk BC and Ghersa CM. 2000. Environmental control of dormancy in weed seed banks in soil. Field Crops Research 67: 105-122.
- Mennan H and Zandstra BH. 2006. The Effects of depth and duration of seed burial on viability, dormancy, germination and emergence of Ivyleaf Speedwell (Veronica hederifolia). Weed Technology 20: 438-444.