CSAAC (Jan 2011)
In seed testing, dormant seeds are defined as viable seeds, rather than hard seeds, which fail to germinate when provided the prescribed germination conditions. Hard seeds are defined as “seeds which remain hard at the end of the test period because their impermeable seed coats prevent the absorption of water.”
Botanically, dormancy means the “inactive phase of viable seeds.” This dormancy provides the dual purpose of enabling the seed to survive adverse environmental conditions and to give the seed time to mature fully prior to germination.
One of the reasons for dormancy is a growth inhibiting substance on the seed coat like coumarin. Only when it has been removed or destroyed will the seed germinate.
The germination process of seed testing has to address seed dormancy in order to determine the maximum potential of germination of a seed lot.
One method to break dormancy is chilling prior to the test or during the test after it has been going on for a few days. During these chilling periods, the seed is kept at a constant temperature of not less than 5°C or more than 10°C for a period of anywhere from three days to three months depending upon the seed variety.
The use of alternating temperatures such as 20-30°C or 10-30°C assists in breaking dormancy. In some cases, preheating at high temperatures prior to germination testing is effective.
Chemical methods of breaking dormancy are the application of potassium nitrate (KNO3) and gibberellic acid (GA3) on the substrate instead of water.
Soaking the seeds in water for a period of 4–24 hours is recommended to assist in hard seed dormancy. Pre-washing seeds with running water at 25°C prior to planting is another method.
Light is very effective in the breaking of dormancy. It will destroy the coumarin found on some seeds. Cool white fluorescent of 750–1250 lux for eight hours during a 24-hour period is recommended.
Scarification can be done by puncturing the seed with a needle away from the embryo, clipping the seed, or using an abrasive, like sand paper, thus enabling the hard seed coat to take on water.
For many seeds, you need to use these dormancy breaking techniques during every germination test. They simulate outside environmental conditions. For various reasons, like physiological dormancy, hard seed coats, after-ripening (maturing) requirements or inhibitory substances present, even with dormancy breaking techniques you may still have hard or fresh seeds at the end of a test. A tetrazolium (TZ) test can be done at the end of the testing period to determine if any dormant or hard seeds are truly viable. In Canada, a TZ test is performed only for informational purposes with the exception of western wheatgrass.
Seed dormancy is always a challenge in the seed testing laboratory. If you have concerns about dormancy in your seed lot, please talk to your knowledgeable CSAAC Accredited Seed Analyst.