Seeds exhibiting physical dormancy (termed PY by scientists) possess covering structures that prevent the seed from absorbing (or imbibing) water.

Seed covering structures are either permeable or impermeable to water. Differences in permeability to water can be exhibited across a population of seeds, but not for individual seeds1.

Examples of physical dormancy

Cuscuta seedlings

Figure 1. Cuscuta tasmanica germinating at 15°C. Like many members of the Convolvulaceae (Bindweed family) C. tasmanica exhibits physical dormancy.

As a seed trait physical dormancy has perhaps the strongest taxonomic association. It is predominantly displayed by members of the following plant families;

  • Fabaceae (predominate in all three subfamilies- Caesalpinioideae, Mimosoideae, Faboideae)
  • Malvaceae (including Sterculiaceae, Tiliaceae and Bombacaceae)
  • Convolvulaceae (fig. 1)
  • Cistaceae
  • Rhamnaceae (typical even in small seeded genera like Pomaderris, Spyridium and Cryptandra),

but can also occur in other families such as Geraniaceae, Sapindaceae (fig. 3), Lauraceae and Cochlospermaceae.

Figure 2 shows the germination response of the bindweed Calystegia marginata. Physical dormancy is commonly exhibited by members of the bindweed family, Convolvulaceae. At the TSCC, C. marginata underwent three germination tests; two in which the seed coat was chipped with a scalpel and a third in which the seeds were sown intact. As you can see, seeds of this species are clearly unable to absorb water and germinate until the seed coat is scarified in some way.

Germination of Calystegia marginata

Figure 2 shows the germination response of the bindweed Calystegia marginata.

Graph plotting percentage germination over time (days) under two different temperature regimes after scarification. Seed coats were chipped using a scalpel. Click image for more details.

Germination of Dodonaea filiformis

Figure 3. Dodonaea filiformis germinating at 20°C.

Figure 3. Dodonaea filiformis germinating at 20°C. Physical dormancy is often encountered amongst members of the Sapindaceae (Soapberry family). Click image for more details.

Seed size and thickness of covering structure are no indication of PY. Seeds of the clover genus Trifolium (Fabaceae) possess PY despite being only 2 to 4mm in diameter. Conversely stoned seeds typical of cherries and plums and displayed in a wide range of families are fully permeable to water and therefore not physically dormant.

Quiescence

Most PY seeds germinate readily once water is able to penetrate the seed coat, therefore these seeds may be considered quiescent, not dormant, since they simply lack the water necessary for germination.

Case study

A TSCC case study of the PY genus Pomaderris has revealed that some Pomaderris species combine physical dormancy with a physiological dormancy.

References:

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