2. At Incotec lettuce seed is incubated for 48 hours at 20° C in total darkness (this is called the 20 DD test). If not photodormant, all the viable seed should all sprout within that time. After that 48 hours, the seed that sprouted in the dark is counted. The test then is returned to the germinator for 5 more days, but this time with a light-dark cycle of 12 and 12 hours. It is quite normal for the seeds that did not sprout in the dark (those that are photodormant) to then completely germinate after being exposed to light. The test results might then appear as follows:
20DD = 50% (after 2 days in the dark)
20 LD = 100% (same seed after 5 more days with light)

The above test information indicates that 50% of the seed did not sprout after 48 hours in darkness. In this case, 50% of the seed population is considered to be photodormant.

3. We recommend priming for a seed lot when the 20DD test result shows more than 10% of the seed population is photodormant. In the above case 100% of the seed sprouted after light was provided, indicating that the absence of light was the only reason the photodormant seed had not sprouted.

4. Priming is routinely used to alleviate photodormancy. For example - after priming, the same seed could give test results as follows:
20DD = 100%
20LD = 100%