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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.
Figure 1 - Calystegia marginata cut-test Figure 1. Cross section of Calystegia marginata (Convolvulaceae); 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.