DORMANCY CLASSIFICATION

Different Types of Seed Dormancy

Ideally we would like to classify dormancy

We humans like to categorise things, and seed dormancy is no exception. Ever since seed scientists discovered that a number of different types of seed dormancy exist, they have been interested in trying to assign the different types to groups within a system of categories.

Ideally, classifying different types of dormancy by attributing them to different factors, locations or durations can not only help us to categorise plant species, but also improve our ability to germinate dormant seeds.

In reality it’s not that simple

Leptecophylla juniperina cut-test

Figure 1. Typical multi-seeded stone of Leptecophylla juniperina (Ericaceae). Stone fruited Ericaceae usually exhibit dormancy and are problematical to germinate.

However nature rarely co-operates with our idealism, and despite numerous attempts to classify different dormancy types, the subject remains controversial and no universally accepted dormancy classification scheme exists. Classifying different dormancy types is proving difficult, largely because many dormancy mechanisms are not well understood yet. In addition, dormancy is not an ‘all or nothing’ seed characteristic. The amount of dormancy a seed exhibits, otherwise known as the seed’s dormancy status, can change over time and vary within and between seed lots.

In 1990 Simpson1 declared that seed classification terminologies were ‘…unable to cover the reality of the great range of interactions between seed morphology, physiology and biochemical functions, and the diverse environments which are always in flux, that together determine the nature of dormancy in any one particular seed.’

Today many people still believe that on the whole, the type of seed dormancy expressed by a plant species cannot be described using a single term.

Dormancy: inside or outside?

Despite the controversy surrounding dormancy classification, most seed scientists do agree that the location of different dormancy mechanisms can be either inside the dormant seed and/or around the outside of the dormant seed.

In 1969 M. G. Nikolaeva published what is still regarded by many today as the most comprehensive attempt to classify the different types of seed dormancy2. Nikolaeva was the first person to allocate names and numerical symbols to the different types of dormancy, and the first to distinguish between:

These terms are still considered useful and relevant today (table 1).

Endogenous dormancy may be due to physiological or morphological characteristics of the seed embryo, or a combination of both (table 1). The different types of endogenous dormancy are physiological dormancy (PD), morphological dormancy (MD) and morpho-physiological dormancy (MPD).

Exogenous dormancy may be due to a physical, chemical or mechanical characteristic of the seed coat or fruit (table 1). The different types of exogenous dormancy are physical dormancy (PY), chemical dormancy and mechanical dormancy.

N.B. The following pages describe how we classify different types of exogenous and endogenous dormancies, based on our current understanding. However this information is designed to introduce you to the idea of dormancy classification and is not meant to be a definitive approach to dormancy classification since the subject is continuously under scientific scrutiny.

Table 1. A proposed classification system for seed dormancy, attempting to locate different types of dormancy mechanisms (modified from Nikolaeva, 19773).

Dormancy Type Description Further Classification
Endogenous: Related to the seed embryo and/or endosperm
Physiological (PD) Physiological inhibiting mechanisms (PIM) of the embryo that differs in depth. – Deep (strong PIM)- Intermediate

– Non-deep (weak PIM)

Morphological (MD) Embryo underdeveloped or undifferentiated. n/a
Combined (MD+PD) PD united with under development of embryo. – Morpho-physiological- Intermediate simple

– Deep simple

– Deep simple epicotyl

– Deep simple double

– Intermediate complex

– Deep complex

Exogenous: Related to properties of the external seed covering structures (pericarp/seed coat)
Physical (PY) Tissues impermeable to water. n/a
Chemical Tissues contain chemical germination inhibitors. n/a
Mechanical Tissues restricting embryo expansion and development. n/a

Dormancy classification can be subjective

It is important to exercise caution when trying to diagnose dormancy type. The dormancy classification that we assign to a species can be influenced by our choice of experiments and seed treatments.

For example, seeds that germinate following removal of a small proportion of the seed coat could be diagnosed with either physical, mechanical or physiological dormancy (to understand why, see further explanations of these dormancy types).

Similarly, morphological dormancy may be overlooked if, during your investigations, you fail to check whether or not the seed embryo is growing inside the seed (see Morphological dormancy).

References:

  1. Simpson GM. 1990. Terminology and definitions of dormancy. In: Seed Dormancy in Grasses. Cambridge, England, Cambridge University Press: 43-59.
  2. Nikolaeva MG. 1969. Physiology of Deep Dormancy in Seeds. Izdatel’stvo “Nauka”, Leningrad. Translation from Rusian by Z. Shapiro. National Science Foundation, Washington D.C.
  3. 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.

Endogenous Dormancy

Dormancy on the inside

Endogenous dormancy is considered to be inside the seed; associated with the seed embryo and/or endosperm (table 1).

Table 1. A proposed classification system for endogenous seed dormancy types (modified from Nikolaeva, 1977).

Dormancy Type Description Further Classification
Endogenous:  Related to the seed embryo and/or endosperm
Physiological (PD) Physiological inhibiting mechanisms (PIM) of the embryo that can differ in depth.
  • Deep (strong PIM)
  • Intermediate
  • Non-deep (weak PIM)
Morphological (MD) Mature seed contains under-developed or undifferentiated embryo.
Combined (MD + PD) PD coupled with an under-developed embryo.
  • Morpho-physiological
  • Intermediate simple
  • Deep simple
  • Deep simple epicotyl
  • Deep simple double
  • Intermediate complex
  • Deep complex
Stones of Cenarrhenes nitida germinating.

Figure 1. Cenarrhenes nitida (Proteaceae) seedlings at the TSCC. This species shows a clear requirement for cold stratification to alleviate physiological dormancy. For more details click on image.

Physiological dormancy

Physiological dormancy (PD) is the most common expression of seed dormancy1, and is thought to be caused by a physiological inhibiting mechanism (PIM) of the seed embryo. Not only is PD classified into three levels:

  1. Non-deep,
  2. Intermediate, and
  3. Deep (table 1);

but it was recently proposed that non-deep PD be further classified into five types based on patterns of change in physiological response to temperature during dormancy relief (table 2)2.

There are also examples of Tasmanian / Australian seeds that exhibit physiological dormancy.

Table 2. A three tier hierarchical classification system (modified from Baskin and Baskin, 2004).

Class Level Type
Physiological dormancy (PD)
  • Deep
  • Intermediate
  • Non-deep
Of non-deep PD:1 2 3 4 5
Morphological dormancy (MD) Does not include seeds with undifferentiated embryos
Morpho-physiological dormancy (MPD)
  • Non-deep simple
  • Intermediate simple
  • Deep simple
  • Deep simple epicotyl
  • Deep simple double
  • Non-deep complex
  • Intermediate complex
  • Deep complex
Does not include seeds with undifferentiated embryos
Physical dormancy (PY) Probably needs to be subdivided
Physical and physiological dormancy (PY + PD) Non-deep PD Probably both Type 1 and Type 2 are represented

Too complex?

Seed scientists are sometimes criticised for failing to classify the type of dormancy their seeds possess when publishing experimental results3, but this may be due to the fact that classification systems are so complex, especially for physiological dormancy.

Morphological dormancy (MD)

Stilbocarpa polaris: Fruit cross section

Figure 2. Cross section of a Stilbocarpa polaris (Apiaceae) fruit revealing three seeds. This species is proving to be very difficult to germinate at the TSCC, possibly due to MPD which has been documented for other Apiaceae species. For examples see refs 4,5 & 6.

Some plant species disperse seeds in which the embryo is undeveloped or not fully grown. In these cases, the embryo needs to grow inside the seed before the seed can begin to germinate. These seeds, although mature, are prevented from germinating by morphological characteristics of the embryo, and are therefore said to have morphological dormancy (MD, table 1).

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 MPD (table 1 and fig. 2).

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. &  3. Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
  3. Baskin JM and Baskin CC. 1991. Nondeep Complex Morphophysiological Dormancy in Seeds of Osmorhiza claytonii (Apiaceae). American Journal of Botany, Vol. 78, No. 4: 588-593.
  4. Vandelook F, Bolle N and Van Assche JA. 2007. Seed Dormancy and Germination of the European Chaerophyllum temulum (Apiaceae), a Member of a Trans-Atlantic Genus. Annals of Botany. 100(2): 233-239.
  5. Vandelook F, Bolle N and Van Assche JA. 2008. Seasonal dormancy cycles in the biennial Torilis japonica ( Apiaceae), a species with morphophysiological dormancy. Seed Science Research. 18: 161-171.

Physiological Dormany

Physiological dormancy; the world’s most common expression of seed dormancy

Physiological dormancy (PD) is an endogenous dormancy, associated with the seed embryo and thought to be caused by a physiological inhibiting mechanism.

Seeds have physiological dormancy if:

  • Application of gibberellic acid (GA3) increases germination
  • Dry after-ripening or dry storage increases germination
  • Excised embryos produce normal, healthy seedlings
  • Scarification increases germination (in some PD cases)
  • Up to 3 months of cold (0-10°C) or warm (=15°C) stratification increases germination
  • Dry after-ripening shortens the length of cold stratification required

Seeds have deep physiological dormancy if:

  • Application of GA3 does not increase germination
  • Excised embryos produce abnormal seedlings
  • Seeds require more than 3 months of cold stratification to germinate1.

Examples of physiological dormancy at the TSCC:

Up to 3 months warm or cold stratification is often a germination requirement of seeds with physiological dormancy. Juncus antarcticus (Juncaceae) seeds were collected at approx. 1310m on Mt Rufus in Lake St Clair National Park, Tasmania. Seeds required 8 weeks cold stratification (5°C) before they would germinate at 27/15°C (fig. 1).

Germination of Juncus antarcticus

Figure 1. Germination response of Juncus antarcticus. Click image for more details.

These results suggest that in the field, physiological dormancy of J. antarcticus is alleviated during wet, winter months, prior to seeds germinating in late spring/early summer.

Epacris marginata (Ericaceae) is a heath species endemic to Tasmania. Seeds were collected in October (spring) from the Tasman Peninsula. Tests carried out so far indicate that seeds require warm stratification (27/15°C) before germination can occur at 15°C (fig. 2).

Germination of Epacris marginata

Figure 2. Germination of Epacris marginata. This species appears to require warm stratification to germinate at intermediate temperatures. For more information click image.

These results suggest that E. marginata bears physiologically dormant seeds which lose dormancy over the summer before germination can occur in the autumn.

Other examples of physiological dormancy:

Goodenia fascicularis (Goodeniaceae) seeds collected in south-west Queensland, Australia did not respond to application of GA3. However seeds did responded to mechanical scarification (chipping) and de-coating prior to germination testing. For example chipping above the radicle stimulated 71% germination at 20°C.

However, de-coating seeds caused seedlings to grow abnormally long radicles (roots), before seedlings became stunted and died. Similarly, chipping above the cotyledons resulted in stunted radical growth and chipping close to the radicle meant that cotyledons were not able to fully emerge from the seed coat (fig. 3). Stunted growth can suggest that, despite a germination response, some seed dormancy still remains. These G. fascicularis seeds were consequently diagnosed with deep physiological dormancy2.

Goodenia fascicularis germination
Figure 3. Abnormal germination of Goodenia fascicularis (Goodeniaceae) seeds mechanically scarified in the vicinity of the cotyledons (a) and the radicle (b). Image courtesy of Gemma Hoyle.

References:

  1. Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
  2. Hoyle GL, Steadman KJ, Daws MI and Adkins SW. 2008. Physiological dormancy in forbs native to south-west Queensland: Diagnosis and classification. South African Journal of Botany. 74: 208-213.

Morphological & Combinational Dormancies

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.

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.

Exogenous Dormancy

Dormancy on the outside

Exogenous dormancy is considered to be on the outside of the seed; associated with the seed’s external covering structures such as the seed coat or pericarp (table 1).

Table 1. A proposed classification of exogenous dormancy1.

Dormancy Type Description
Exogenous: Related to properties of the external seed covering structures (pericarp/seed coat)
Physical  (PY) Tissues impermeable to water (preventing seed imbibition).
Chemical Tissues contain chemical germination inhibitors.
Mechanical Tissues restricting embryo expansion and development.
Calystegia marginata cut-test (ii)

Figure 1. Cross section of Calystegia marginata showing convoluted folded embryo sitting within endosperm. Also note crystalline layers within the seed coat. A PY seed

Physical dormancy

Seeds exhibiting physical dormancy (PY) possess covering structures that physically prevent the seed from absorbing or imbibing water (fig. 1). Since water is necessary for germination, PY seeds must be made permeable to water before germination can begin, which is usually achieved by chipping or cracking the seed coat.

There are examples of Tasmanian / Australian seeds that possess physical dormancy, however physical dormancy can be overcome.

Chemical dormancy

Chemical dormancy refers to chemical germination inhibitors in seed covering tissues preventing germination (table 1). However chemical dormancy is seldom observed alone, but is found in combination with other dormancy types. For example most seeds from which germination inhibitors have been isolated also exhibit physiological dormancy (PD), and in many studies the effects of inhibitors have been tested on seeds after PD was alleviated, making it unclear as to whether inhibitors would have prevented germination of non-dormant seeds or not2.

In conclusion, dyagnosing chemical dormancy is difficult. However if the factor preventing germination is thought to be chemical, and not physical or physiological, it may be leached (washed or soaked) from the seed, or deactivated3.

Mechanical dormancy

Mechanical dormancy is said to be caused by seed covering tissues mechanically restricting embryo expansion and development (table 1). However the ‘mechanical dormancy’ of many species has been found to be the result of endogenous factors such as low embryo growth potential and therefore actually physiological dormancy.

For example, when the embryo growth potential increases to the point of radicle emergence, germination occurs without the need to alter the resistance of restraining seed covering structures4. A small study conducted at Kew’s Millennium Seed Bank looked at the benefit of acid scarification on germination of a range of stone fruited and woody seeded Rosaceae species. Most species displayed a physiological dormancy that, once addressed, led to germination; the tough covering structures apparently easily overcome. No benefit of acid scarification was observed and in fact at high levels, scarification was detrimental (unpublished work).

It has been suggested that both exogenous and endogenous factors determine the dormancy of some Iridaceae (Iris) seeds. For example it was concluded that the hardness of the seed coat was the main cause of dormancy in Iris lorteti (Iridaceae), in which radicle protrusion was found to require a pressure of up to 135 atm (1368 newtons of force/cm2). This is one of the highest seed coat resistances ever recorded! Only when the testa was cut at the micropylar end of the seed, did germination occur. However, extracts prepared from the endosperm of I. lorteti acted to inhibit germination, and interactions between this inhibitor and the hardness of the testa were not ruled out5.

Seeds that are prevented from germinating by their fleshy fruit and not their seed coat are considered quiescent, not dormant6. To prevent confusion, it is suggested that dormancy be evaluated after seeds are released from fruits.

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. Nikolaeva MG. 2001. An update of Nikolaeva’s seed dormancy classification and its relevance to the ecology, physiology, biogeography and phylogenetic relationships of seed dormancy and germination. Botanicheskii Zhurnal 86: 1-14.
  3. Baskin C and Baskin J. 2001. Seeds. Ecology, Biogeography and Evolution of Dormancy and Germination. London: Academic Press.
  4. &  6. Baskin JM and Baskin CC. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
  5. Blumenthal A, Lerner HR, Werker E and Pojakoff-Mayber A. 1986. Germination preventing mechanisms in Iris seeds. Annals of Botany 58: 551-561.