Seed Dormancy Mechanisms
Seed Dormancy: After-Ripening
In a broad sense, after-ripening describes the loss of the dormant state in a seed over some period of time. In the strictest sense, after-riening refers to the loss of dormancy mechanisms imposed by the Mother Plant.
Seeds maintained in dry storage or imbibed in the soil seedbank tend to lose this maternal control over dormancy and germination without any applied dormancy breaking methods over a period of days to years depending upon the species. After-ripening is a period of :quiescence that the seed must go through to finalize the separation from the Mother Plant and become autonomous and on its own.
Another type of after-ripening involves seeds with rudimentary embryos. With this after-ripening, the rudimentary embryo must develop into a full embryonic axis before germination can occur. This is a physiological process that may occur over a period of time or may have to be stimulated by certain environmental factors to proceed.
In some species (such as members of the Apiaceae, carrot family), there is a dependence upon where the seed is positioned with the inflorescence as to how rudimentary or developed the embryo is at physiological maturity. Those seeds with a well developed embryo germinate readily while those with rudimentary embryos must develop fully over time to germinate.
Seedcoat Dormancy: Hardseededness
Hardseededness is where the seedcoat or pericarp surrounding the seed present a physical barrier to the uptake of water. This type of dormancy is common in the Fabaceae, Convolvulaceae, Geraniaceae, Malvaceae, lamiaceae, and the Poaceae plant family members. Physical abrasion (scarification), or freezing and thawing may be needed to allow water uptake and germination to proceed.
Hardseededness (in its broadest sense), may also prevent germination in many trees and shrubs where the seed unit is a nut or a drupe. With this type of hardseededness, the seedcoat or pericarp presents a physical barrier to the expansion of the germinating seed.
Seedcoat Dormancy: Impermeable Membranes
With this type of dormancy, membranes within the pericarp, seedcoat, or sometimes in the endosperm of the seed form a barrier that is permeable to the imbibition of water but impermeable to the uptake of oxygen. Generally, cool temperature between 10C and 15C allow oxygen to make its way into the seed while warm temperatures prevent oxygen uptake.
This type of seed dormancy is often the basis of why certain species need alternating temperatures in order to germinate. Since germination is fueled by the respiration of stored food within the seed, without oxygen, germination cannot occur.
Another variation of impermeable membranes inhibiting germination is found in some species where a membrane layer around the radicle inhibits the maximum uptake of water imbibition into the radicle and presents a physical barrier to the expansion and protrusion of the radicle during germination. These layers often have light dormancy associated with them that needs to be overcome to allow the radicle to emerge.
To continue the discussion on seed dormancy go to:
Physiological Dormancy Mechanisms.
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