The Challenge

Allocasuarina duncanii twins

Figure 1. Allocasuarina duncanii (Casuarinaceae) seedlings. Click image for more details.

Over time seed banks develop a wealth of knowledge and experience in regarding to germination. Even so it should be appreciated that much of the germination testing carried out, follows a process of trial and error. Currently the germination of only a tiny fraction of seed bearing plants has fallen under scientific scrutiny. To compound matters, some of the very early published work failed to record factors that we now know are crucial to interpreting results; where seeds came from, when they were collected, how they were stored and for how long.

Chorizandra australis (ii)

Figure 2. Woody achenes of the Australian sedge Chorizandra australis (Cyperaceae). Achenes are small, single seeded fruits and are commonly dispersed by many plant species.

It is well documented that the germination behaviour exhibited by many plants is quite plastic. Optimal germination conditions vary not only from species to species, but also within a species from location to location. Even collections made from the same species in the same location can have differing germination requirements from one year to the next.

Consequently, seed workers can not assume that conditions that evoke germination for one collection will evoke germination from another of the same species.

We do know however that some seed traits are strongly linked to taxonomy, morphology or location, therefore we can at least apply some framework to our investigations.

So how do we do that?

Investigating seed germination requirements

Persoonia gunnii seeds

Figure 3. Cross section of Persoonia gunnii (Proteaceae) stones revealing a pair of seeds with well developed embryos. Thick woody coats like this seldom pose a barrier to water or seedling growth.

The seed itself and the environment from which it came can reveal many clues regarding germination requirements. Here we list a few of the important questions you may want to ask yourself before attempting to germinate viable seeds that you know very little about.

Is it a seed?

The very first thing to check is that you are actually dealing with seeds. This may sound obvious but many plant species will shed seeds that appear to be perfectly healthy, but in fact have little or no content (see an example here). Seed bank workers and seed scientists always conduct seed viability tests to assess seed quality prior to germination testing.

Seed morphology

Seed morphology plays an important role in germination requirements. You may need to ask yourself:

  • Are seeds dispersed within a fleshy fruit or other covering structure that needs to be removed prior to sowing seeds?
  • Are seeds able to absorb water? If not, they probably have physical dormancy.
  • How small are the seeds? Germination of tiny seed is more likely to be light dependent.
  • How small is the embryo? Diminutive and/or undifferentiated embryos may require time to develop post dispersal i.e. seeds have morphological dormancy.
Seed Embryo Variation

Figure 4. Sample of internal seed morphology.

Figure 4 is an illustration showing a small sample of the variation observed in internal seed morphology (Seed embryos are yellow. Endosperm is shaded white):

  1. Tiny Undifferentiated embryo (e.g. Ranunculaceae)
  2. Small differentiated embryo (e.g. Apiaceae)
  3. Linear embryo (e.g. Oleaceae)
  4. Bent embryo (e.g. Brassicaceae)
  5. Large investing embryo (e.g. Mimosaceae)

Consider the plant life cycle and natural environment

Equally as important to consider is the plant life cycle and the environment it has evolved in:

  • What is the life cycle of the plant?
  • What time of year are mature seeds dispersed in the wild?
  • When do they germinate in the wild? Level of documentation for this varies – it’s very good in the UK, but elsewhere information can be very scarce. If this can’t be determined then…
  • When and/or what is the most likely window for seedling recruitment? i.e. Does the habitat offer clear disturbance events (such as fire), that offer the opportunity for seedlings to establish with little competition?
  • What are the environmental conditions in the plant’s natural habitat before, during and after seed germination?

Whenever possible, germination testing should begin within 7 to 10 days of harvesting seeds1. This is because post-harvest handling may cause seeds to undergo changes, and then there is no way of knowing what the germination response of fresh seeds is, or how germination requirements may have changed over time. This is particularly important if seeds exhibit physiological dormancy.

N.B. Almost all tests conducted by the TSCC are performed on stored seeds, not fresh seeds, and usually commence about 6 months after collection. This is standard operation for seed banking as germination of stored seed is the focus of the program.

For some species dormancy can be avoided by collecting very young / immature seeds. For example, Gahnia grandis (Cyperaceae) seeds will rapidly germinate when collected immature and sown immediately, but when fully mature (and possessing greater storage longevity), these seeds are deeply dormant. Since immature seeds have lower longevity potential, they are useless for conservation seed banking as they will rapidly degrade in storage to become nonviable.

Time between seed dispersal and germination

A useful strategy is to think about the journey the seed takes between dispersal and germination. It is particularly important when dealing with physiologically dormant seeds, to have some information about conditions during the time between natural seed dispersal and when they might germinate:

  • Are seeds exposed to wet or dry conditions?
  • Are seeds exposed to warm or cool conditions?
  • If so, for how long?
  • Are these conditions necessary for germination or not?
  • Are seeds buried or on the soil surface when they germinate?

Much information can be gained from collecting seeds at the time of natural dispersal, and sowing them on soil where they receive natural temperature and soil moisture conditions, either outside or in a non-heated greenhouse. How long is it before seeds germinate? What conditions do they experience in the meantime?

Air and/or soil temperature records can reveal the temperatures seeds are exposed to before and during germination. You might consider purchasing a temperature data logger or obtaining temperature, humidity and rain event data for a general area from www.bom.gov.au.

If seeds do not germinate whilst under the soil, they may require light for germination. It may be worth sowing seeds both on and under the soil surface and noting germination patterns. Seeds less than 3 or 4 mm in size are more likely to require light for germination and should be sown on or close to the soil surface.

The TSCC Germination Database

The TSCC Germination Database provides you with a listing of the relative success of various seed germination protocols. This information is generated through germination testing of seeds currently held in the Tasmanian Seed Conservation Centre.

Please note, the TSCC seed bank focuses on the preservation of Tasmania’s wild flora, therefore the database holds little information on the germination behaviour of cultivated garden plants. However we do have some data on a few introduced weed species.

You can search for germination data via plant family or genus. Simply select the family or genus that you are interested in and press the report button next to it. This will generate a PDF file for downloading.

References:

  1. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.