Dormancy is the condition of a seed when it fails to germinate even when the environmental conditions usually considered necessary for germination are available. Reasons of seed dormancy :
(i) Impermeable seed coats : Seed coats can be impermeable to water (e.g., Chenopodium), gases (apple).
(ii) Inhibitors : ABA is a plant hormone which is responsible for seed dormancy in many plants.
(iii) Immature embryo.
Ethylene is a gaseous plant hormone which is responsible for ripening of fruits.
Gibberellins are plant hormone which promotes growth of stem and leaves e.g., GA-3.
Cytokinines are plant hormone which promotes cell division, e.g., Dihydrozeatin