Seed
Germination,
Dormancy and Priming
Terminology
pollination - deposition of pollen on the stigma of the
pistil.
fertilization - the union of male and female gamete
(nuclei, 1N) to produce zygote (2N).
double
fertilization - in higher plants only (angiosperms)
-
union of 1 1N male gamete with 1 1N female gamete (the egg) to
produce
a 2N zygote; and union of 1 1N male gamete with 2 1N
polar
nuclei to produce a 3N endosperm.
apomixis - development
of an embryo without fertilization; hence, it is not true sexual
propagation even
though it produces a seed.
parthenocarpy - development of fruit without seeds.
vivipary - germination of seeds inside the fruit
while still attached to the parent plant.
Seed
Dormancy Terminology
|
Primary Dormancy |
||
|
Old Term |
New Term |
Definition |
|
Quiescence |
Ecodormancy |
Dormancy imposed by an external unfavorable environmental factor or external structure. Example: too dry, external hard seed coat |
|
Correlative Inhibition |
Paradormancy |
Dormancy imposed by physiological factor external to the embryo. Example: inhibitors in testa or pericarp |
|
Rest or Physiological Dormancy |
Endodormancy |
Dormancy imposed by a physiological factor internal to the embryo. Example: embryo rest |
|
Secondary Dormancy |
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|
Photodormancy |
|
Dormancy due to lack of light (red) in light requiring seeds. |
STAGES OF SEED GERMINATION
1st Stage
a) imbibition - initial absorption of
water to hydrate seed
b) activation of metabolism - increased
respiration and protein synthesis
2nd
Stage
a) digestion of stored food - for example,
starch to sugars in cotyledon or endosperm
b) translocation to embryo
3rd
Stage
a) cell division and continued growth and
development of seedling
SEED DORMANCY
CAUSED
BY TYPE DORMANCY HOW OVERCOME?
1) Dry
Seeds:
dehydration of seed quiescence sow in moist environment
2) Seed Coat Dormancy or Hardseededness:
hard seed coat impermeable quiescence scarification - physical or
to water and gases chemical
abrasion of seed coat.
3) Embryo Rest:
low growth promoters rest stratification - cold (35-40 oF
and/or high growth (or physiological moist storage for 4-12 weeks.
inhibitors in embryo dormancy)
4) Double Dormancy:
hard seed coat plus quiescence scarification then stratification
embryo rest and rest (or
physiological
dormancy)
5) Chemical Inhibitors:
inhibitors in pericarp (fruit correlative I) remove fleshy pericarp (fruit
wall) or testa (seed coat) inhibition wall) or testa (seed
coat).
2) leach in running water if
pericarp
or testa is dry.
6) Immature Embryo:
underdeveloped or developmental I) after ripening - store for 4-6 rudimentary embryo dormancy weeks
under ambient
conditions
2) warm stratification - warm moist storage.
3) embryo culture
7) Light
Requirement
phytochrome in Pr form secondary I) expose to any white light
dormancy 2) expose to red light
3) sow shallow or on surface
Seed Treatments to Enhance
Germination
Seed Priming
Seed
priming is a seed treatment that
imbibition and activation of the initial metabolic events associated with seed
germination, but prevents radicle emergence and growth. Obviously, seeds are tolerant of desiccation,
and even though during seed priming imbibition allows water uptake, the
tolerance to desiccation is not lost.
Thus, the seed can be dried again and stored. If the seeds are primed too long, desiccation
tolerance will be lost, and the seeds may loose viability upon re-drying. The secret to successful seed priming is to
stop the priming treatment at just the right time to allow re-drying.
The advantage of primed seed is that when the
primed seeds are planted their germination is faster and more uniform.
Types of
seed priming
1) Osmopriming (osmoconditioning):
This
is the most common technique used. The seeds are soaked in an osmotic solution
to allow imbibition and metabolic activation, but the osmotic conditions do not
allow expansion and growth of cells.
Osmotica used are: mannitol, polyethyleneglycol (PEG) or salts
such as KCl.
2) Hydropriming:
Imbibition is obtained by partially hydrated the seeds using a limited amount of water by exposing them to a limited amount of water, using very humid air or exposing them for a short time in warm water.
3) Matrix priming:
A solid, insoluble matrix is used to obtain a water solution with low water potential. The matrix potential keeps the water potential low. Vermiculite, diatomaceous earth or cross-linked highly water-absorbent polymers are used.
Hormone Treatments to Enhance Germination
Seeds of some species are very difficult and slow to germinate due to primary and secondary dormancy, the need for after ripening periods, immature embryos, etc. Many of the seeds respond to hormones to increase the speed of germination, uniformity of germination and/or percent germination. For example, the seeds of many tropical foliage plants are difficult to geminate, but respond to hormonal treatments. Hormones can also be added to seed priming treatments.
1) cytokinin – 100 to 200 mg/liter for a 12-24 hour soak.
2) gibberellic acid – 200 to 1,000 mg/liter fro 12-24 hour soak