Calystegia marginata cut-test (ii)

Figure 1. Cross section of Calystegia marginata (Convolvulaceae) seed. The thickened seed coat forms an extremely effective barrier to water.

To recap…

Dormancy is a seed characteristic, manifesting as a block or series of blocks that prevent germination under otherwise favourable moisture, temperature and gaseous conditions1.

Physical dormancy is an exogenous dormancy, considered to be on the outside of the seed; associated with the seed’s external covering structures such as the seed coat or pericarp.

High viability (i)

Figure 2. Vigorous germination of physically dormant Calystegia marginata seeds at the TSCC following a chipping treatment.

Seeds exhibiting physical dormancy possess covering structures that prevent the seed from absorbing (or imbibing) water. Physical dormancy is present in at least 15 plant families, including Fabaceae, Malvaceae, Convolvulaceae and Rhamnaceae2.
Figure 2. Vigorous germination of physically dormant Calystegia marginata seeds at the TSCC following a chipping treatment.

Overcoming physical dormancy

In the field, depending on the species and habitat, various environmental factors cause PY seeds to become permeable to water and able to germinate. For example; seed coat erosion, herbivory, insect damage, animal digestion, high temperatures and fire. For many plant families, natural openings that allow water to enter seeds with physical dormancy have been discovered, and ‘unplugging’ these openings allows subsequent germination. In the wild, this is environmentally controlled. For many PY species, germination is rapid once water is able to enter the seed (fig. 2). An Australian example of PY familiar to many Tasmanians is the flush of Silver Wattle (Acacia dealbata, Mimosaceae) following a bush fire. N.B. The hard seededness of Acacia (Mimosaceae/Fabaceae) species is well known, however Acacia harpophylla bear non-physically dormant seeds.

Calystegia marginata - chipped seed

Figure 3. Chipped seed of Calystegia marginata. A section of the seed coat has been carefully removed to permit entry of water and enable germination.

In the laboratory

We can mimic natural factors in order to germinate physically dormant seeds ex situ.

  • Removal of fruit or other seed covering structures.
  • Chipping/surgical treatment /- mimics herbivory, insects, digestion.
  • Cracking – achieved at high temperatures – mimics fire/high summer temps.
  • Soaking – softening the seed coats – mimics rains.
  • Embryo excision

The TSCC routinely deals with PY by chipping seed coats, which is usually performed with a scalpel under a dissection microscope. However ‘heat shock’ treatments have been applied to some species with success.

References:

  1. Veeshouwers LM, Bouwmeester HJ and Karssen CM. 1995. Redefining seed dormancy: An attempt to integrate physiology and ecology. The Journal of Ecology 83: 1031-1037.
  2. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.