Factors affecting seed dormancy

1. Embryo dormancy

a. Rudimentary embryo:

• In few cases, the embryo is rudimentary and poorly developed at seed maturity.
• It is necessary for such embryo to continue their development during the dormant period before they can germinate successfully.
• Example; the seeds of many orchids that are apparently ripened contain a rudimentary embryo and the germination of such seeds naturally gets delayed until the embryo develops properly.

b. Immature embryo:

• The embryo appears to be fully developed but unable to resume growth promptly when provided with a suitable environment.
• This phenomenon is noticed especially in the seeds of Rosaceous plants (apple, pear, cherry, blackberry) and conifers.
• The seeds can be induced to germinate after a cold treatment and this process of low temperature treatment is known as stratification.

2. Dormancy due to seed coat

• From a chemical standpoint seed coat consists of complex mixture of polysaccharides, hemicelluloses, fats, waxes and proteins.
• During the seed ripening, the chemical components of seed coat become dehydrated and form a hard, tough protective layer around the embryo.
• The seeds of certain plants (Papilionaceae, Malvaceae, Liliaceae, Chenopodiaceae, Solanaceae) have very hard seed coats that are impermeable to water (impermeable layer of palisade cells).
• The seed remain dormant in the soil until the impermeable layer of testa decay by the action of soil microorganisms.

3. Presence of germination inhibitors

• This dormancy is due to presence of specific chemical substances that suppresses the process of germination in the seed.
• These chemicals are called germination inhibitors (Examples; abscisic acid, ferulic acid, dormin etc).
• Germination in some seeds can be induced by Gibberellins and Cytokinins while germination of non-dormant seeds can be inhibited by exogenous

4. Light:

I. Positively photoblastic seed (germination induced by single irradiation): ex. Tobacco, lettuce etc.

ii. Negatively photoblastic seed (germination inhibited by exposure to light): ex. Allium, Helleborus niger etc.

iii. non-photoblastic seed (no effect of exposure to light): ex. Tomato, cucumber etc.

5. Genetic dormancy or cultivar variations:

• In this case, dormancy is primarily determined by the genetic makeup of the seeds and varies widely among species and even within species.
• The intensity of dormancy in rice varieties is controlled by a varying number of partially dominant genes that have cumulative unequal effect.