Morphological dormancy; the seed embryo is not fully grown

Morphological dormancy (MD) is an endogenous dormancy, associated with the seed embryo. Inside a morphologically dormant seed, the embryo is undeveloped or not fully grown, therefore, although the seed has been dispersed, the embryo needs to grow inside the seed before the seed can germinate.

Seeds have morphological dormancy if:

  • The seed embryo is rudimentary or linear (underdeveloped).
  • Most of the seed interior is occupied with endosperm and the embryo is only 0.1% of the seed volume or less.

Morpho-physiological dormancy (MPD) or combined dormancy

Morphological dormancy (MD) is often found in combination with physiological dormancy (PD), otherwise known as morpho-physiological dormancy or combined dormancy; MD + PD or MPD1.

Seeds have morpho-physiological dormancy if:

  • The seed embryo is rudimentary or linear (underdeveloped).
  • Seeds require physiological dormancy to be alleviated, even after the seed embryo has fully developed2.
Fraxinus excelsior longitudinal section

Figure 1. Longitudinal section of Fraxinus excelsior seed.

Examples of morphological and morpho-physiological dormancy:

Germination of Fraxinus excelsior (Oleaceae) seeds (fig. 1) requires a warm temperature treatment during which the immature embryo grows in size, before a period of chilling alleviates the embryo’s physiological dormancy and enables seeds to germinate3.

Erythronium albidum (Liliaceae) embryos are underdeveloped at dispersal and grow over the subsequent summer. Once fully developed they become sensitive to cold stratification which alleviates dormancy during winter to enable seeds to germinate the following spring4.

Is a seed dormant just because we can’t see the embryo growing?

Some studies have proven that embryos considered to be morphologically dormant can, unbeknown to the naked eye, begin to develop very soon after seeds are dispersed or sown i.e. without any dormancy alleviating treatment.

Therefore some consider MD and MPD to be merely a function of the need for the embryo to grow and develop before visible germination is recognised, and since the embryo is growing (even though we can’t see it), it is arguably not dormant.

For example, Anemone nemorosa (Ranunculaceae) embryos developed considerably inside the seed before the radicle could be seen, and did not exhibit developmental arrest before aquiring desiccation tolerance, as is normal for seeds with MD or MPD5.

References:

  1. Nikolaeva MG. 1977. Factors controlling the seed dormancy pattern. In: The physiology and biochemistry of seed dormancy and germination. Khan AA, ed. New York, North-Holland Publishing Company: 51-74.
  2. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.
  3. Villiers TA and Wareing PF. 1965. The growth-substance content of dormant fruits of Fraxinus excelsior L. Journal of Experimental Botany 16: 533-544.
  4. Baskin JM and Baskin CC. 1985. Seed germination ecophysiology of the woodland spring geophyte Erythronium albidum. Botanical Gazette 146: 130-136.
  5. Ali N, Probert R, Hay F, Davies H and Stuppy W. 2007. Post-disperal embryo growth and acquisition of dessication tolerance in Anemone nemorosa L. seeds. Seed Science Research 17: 155-163.