1. Impermeability of seed coat to water: due to seed coat structure, which is hard enough to
restrict the entry of moisture into the seeds, thereby preventing seed
germination. For Example: Malvaceae, Leguminoseae, Lilaceae
2. Impermeability of seed coat to gases; is related to the insufficient intake of oxygen by seeds
due to impermeability of seed structure enclosing embryo. Ex: Graminaceae,
fruit crops & forest trees
3. Mechanical resistances of seed coat: growth of embryo is checked due to extremely hard
seed/fruit structure such as seed coat, endosperm per carp etc., Ex: Acacia
species.
4. Inhibitors
Present: Many plant species invest chemicals
in the developing seeds, and these chemicals inhibit the development of the
embryos. They keep the embryos dormant.
Obviously the seed must have some way to eliminate these chemicals before they
can sprout.
5. Insufficient Development:
If a seed's embryo is not completely developed, some additional maturation
may be needed before the seed can sprout. This happens in seeds with
little-to-no storage
material invested in the seed.
Examples include orchid seeds. Such a seed needs a
symbiosis
with fungi
to feed the developing embryo until the embryo is mature enough to penetrate
the seed coat.
6. Abscisic Acid (ABA)
Many temperate zone species that use inhibitors use abscisic acid. This chemical induces dormancy in the embryo. The germination of this seed need a pre-treatment (see: stratification or vernalization – to degrade the abscisic acid