Factors That Influence Seed Dormancy:
Seed germination is the process by
which the uptake of water leads to the rupture of the seed coat
by the radicle or the shoot. The blockage of any one of the steps
leading to the germination can, and likely will, cause a state of
dormancy (a failure to germinate) in the seed. There are many factors
that play a role in the germination of seeds.
Seed germination may be blocked
by the absence of some external factors. External factors include
water, proper temperature, proper mixture of gases, and proper light
requirements. However, some seeds may fail to germinate even if
external conditions are favorable due to some internal factor.
A hard seed coat is one internal
factor. A hard seed coat can cause dormancy by depriving the seed
of water and gases, and it can even mechanically restrict the growth
of the embryo. The Leguminosae family has the largest number of
species of plants which produce seeds with hard seed coats. Some
seeds have a one-way valve that lets moisture out but it does not
take moisture in. The dormancy of seeds with these characteristics
can be broken artificially by scarification.
is the process by which a seed coat is made permeable to water and
/or oxygen, and the seed coat is weakened so the expansion of the
embryo is not restricted. Scarification can be done mechanically
by cracking or scratching the seed coat with a knife or other abrasive
materials. In nature this can be done by the mechanical abrasion
of rock particles, by alternate freezing and thawing, or by bacterial
action. Chemical scarification can be accomplished by dipping seeds
in strong acid, such as sulfuric acid, or strong solvents, such
as acetone or alcohol for seconds up to several minutes. In nature
this can happen when animals eat and expel seeds. Acids form the
animals' stomachs can weaken the seed coats.
Other factors influence the dormancy
of seeds. Growth inhibiting substances can be present in the seed
coat, seed interior, or in tissues surrounding the seed that prevent
a seed from germinating. For example, pears, apples, citrus fruits
and other fleshy fruits contain inhibitors to keep the seed from
germinating while still in the fruit. They will germinate only after
being removed and washed. Also, many desert plants have inhibitors
in their seed coats that prevent a seed from germinating unless
there has been sufficient rainfall for the seedling to become established.
Many plants produce seeds that must
go through a period of after ripening before they are able to germinate.
This is where the seed remains dormant from the time the seed falls
to the ground in autumn until its germination in the spring. Meanwhile,
the seeds are covered by leaves and winter snows. During this time,
the seed's embryo fully develops with aid from the food materials
stored in the endosperm.
The amount of time that a seed will
remain viable (capable of germinating) can range from days to many
years depending on the type of seed and the conditions of storage.
Some seeds such as certain willows, orchids, and cottonwoods remain
viable for only a few days or weeks. Seeds of the Pumpkin Family
can remain viable to several years. Wheat seeds that have been properly
stored have been reported to have been better than 30% viable for
more than thirty years. On the contrary, a few species of both monocots
and dicots produce seeds with no dormancy period. The red mangrove,
a tropical tree, is one example.
There are many factors that affect
the dormancy of seeds. External factors, as well as internal factors,
play very important roles in the germination of seeds. Favorable
environmental conditions alone are not enough for some seeds to
sprout into beautiful plants. Dormancy is nature's way of giving
plants a better chance to survive.