Germination testing is used to determine the correct method for bringing seeds out of dormancy. A seed typically consists of the embryonic plant with its food reserve wrapped in a seed coat, which acts as a protective layer between the embryo and outside elements. Some scientists define germination as the emergence of the radical (the first root), but at the Native Seed Bank here at the San Diego Zoo Safari Park, we define it as the emergence of the radical and the emergence of at least one cotyledon (first leaf). This is because some seeds may have enough energy to produce the tip of a root but then lack the ability to develop further.
Most seeds enter a dormant stage in nature, and we can extend this state for decades by lowering their moisture content and storing them at cold temperatures for our Native Seed Bank. However, when seeds enter this phase in their life cycle, it can be very difficult to get them to “wake up” again. In nature, seed germination is triggered by factors such as fire, water, and temperature changes. To test the germination ability of seeds placed in storage, we try different dormancy-breaking mechanisms to determine which protocol is effective. Scientists look at things like the species’ natural habitat and what triggers have worked with closely related species. The process is further complicated by the fact that species require different combinations depending on how long they have been in storage. For example, a trigger that works for seeds stored for 10 years may not work for seeds stored for 50 years. Another complicating variable is that even within the same species, different populations may have been reproductively isolated enough so that their seeds have different requirements.
At the Safari Park, all seed lots placed into the Seed Bank are testable once they have been stored for at least one month. Twenty or so seed lots that have been placed in storage around the same time period are then removed and soaked in water overnight before different triggers are tested on them. For some seed lots, this initial soaking is the only stimulus they need, but others have additional requirements.
Some of the other tests we perform include:
Here we expose seeds to changes in temperature by placing them in moist, cold conditions that alter them in physiological ways so that they are able to germinate when warmer conditions follow. This simulates winter followed by spring. The same is true for warm stratification, which allows seeds to germinate in response to cooling temperatures similar to what they would naturally encounter in the fall.
In nature, one environmental condition that triggers germination is fire. Either heat, the chemicals released by burned plant matter, or a combination of the two causes seeds to come out of dormancy. To mimic the chemicals released by burned plants, we collect samples of different chaparral species and turn them into charcoal, which is then mixed in water along with the actual smoke, to give us “smoke water.” In some cases, when this water is absorbed through the seed coat, it triggers the seed to grow.
Hot Water Soak and Boiling Water Dip:
There are some species that need to have their seed coat damaged before they can absorb enough water to germinate. One way this occurs in nature is by the high acidity found in animal stomachs (to which seeds are exposed when they are ingested). Instead of soaking seeds in acid, (which can damage the seeds in the wrong concentrations), we use a hot water bath or a boiling water dip. This allows the seed coat to become permeable without damaging the embryo within.
Discovering the correct combinations is like working a time-consuming, yet fascinating, puzzle. Despite all the details that have to be taken into account, germination testing is an extremely important area of study. After all, what’s the point of seed banks if we are unable to germinate the seeds?
Lauren Anderson is an intern at the San Diego Zoo Institute for Conservation Research through the Bureau of Land Management’s Seeds of Success Program. Read her previous post, Native Seed Bank.