Spring 2022
David Wm.
Reed
Seeds
Germination
Priming
Dormancy
.
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.
Types of Seed Germination
|
|
Epigeous Germination – Dicot ·
hypocotyl
elongates & bends ·
cotyledons
emerge from soil ·
apical meristem
protected in cotyledons during emergence |
|
|
Hypogeous Germination – Dicot ·
epicotyl
elongates & bends ·
cotyledons stay
below soil ·
apical meristem
protected by leaves of plumule during emergence |
|
|
Hypogeous Germination – Monocot (not always called hypogeous) ·
epicotyl
elongates ·
reduced
cotyledon & endosperm (stored food) stay below soil ·
apical meristem
protected inside coleoptile |
Images from Pearson Education, Inc., Benjamin Cummings
STAGES
OF SEED GERMINATION
Stage 1
· imbibition - initial absorption of water to hydrate
seed
· leak
solutes – may occur due to membrane
leakiness
· activation
of metabolism - increased
respiration
· release - stored protein (a-amylase)
and mRNA may be released
Stage
2
·
transcription - to produce new mRNA
·
translation - to produce new
proteins
· digestion
of stored food - for
example, starch to sugars in cotyledon or endosperm
· mobilization
– food reserves towards
embryo
· DNA
synthesis – begins
· cell
division – begins
· radicle
emergence – signifies the
end of Stage 2 and beginning of Stage 3
·
Stage
3
· continued growth and development: mobilization
of food reserves, cell division, seedling emergence, and seedling growth and
development
Seed Treatments to Enhance Germination
Seed Priming
Seed priming is a seed treatment that allows imbibition and activation of the some of the metabolic events associated with Stage 1 and Stage 2 of seed germination, but prevents radicle emergence and growth, thus prevents the start of Stage 3 of seed germination.
The changes that occur during priming can be:
· RNA synthesis increases, or the seed resume RNA synthesis quicker
· Protein synthesis increased significantly
· Enzymes involve in mobilization of reserves increases, such as a-amylase, malate dehydrogenase or isocitrate lyase.
· Gene expression may occur or change
But, DNA synthesis does not increase because the seeds are kept in the Stage 2 lag phase, and cell division usually starts at the end of Stage 2 and throughout Stage 3.
Many seeds are tolerant of desiccation. Even though during seed priming imbibition allows water uptake, the tolerance to desiccation upon dehydration is not lost. Thus, after priming the seed can be dried again and stored. If the seeds are primed too long, desiccation tolerance may 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 without the loss of seed viability.
From: Seed priming, Lutts et al, New Challenges in Seed Biology
Advantage of Seed Priming
1) Germination time is greatly reduced;
especially reduced lag phase
2) Germination is
much faster and more uniform.
- See standard
deviation bars in the top graph.
- See
germination example below
Disadvantages:
1) Typically,
primed seeds cannot be stored as long as unprimed seeds.
2) Some primed
seeds may require cool storage temperatures.
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 hydrating seeds using a limited amount of water
· using very humid air
· 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 or slow to germinate due to primary and secondary dormancy, the need for after ripening periods, immature embryos, etc. Many of these 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 for 12-24 hour soak
Seed Longevity and Storage
Seed Longevity
Recalcitrant Seeds:
- Do not tolerate significant dehydration
after maturity.
- Most begin to loose viability when
moisture content drops below 25%
- May not have an after ripening period.
- Will not have pronounced dormancy.
- Sow the seeds soon after maturity.
- Examples: oak, maple, pecan, walnut, elm,
many tropical plants
Orthodox Seeds:
- Tolerate drying after maturity.
- Can be stored for significant periods at
4-10% moisture
- May remain viable for years, some for
decades, maybe centuries.
Example, viable seeds found in ancient excavations, lotus in dry lake bed. - Examples: majority of horticultural and
agronomic crop plants.
Dormancy: many orthodox seeds have various types of dormancies that must be
satisfied before germination can occur.
– see next section.
Seed Storage
Orthodox seeds can be stored under conditions that maintain vigor,
germination and viability.
- Maintain at least 4-6% seed moisture
content
- Many tolerate room temperature, but it
may beneficial to reduce the temperature to 40-50o F.
- Low to moderate humidity
Harrington Rule of Thumb (James Harrington, Professor of Vegetable Crops, UC Davis)
·
For every decrease of 1% seed moisture content, the life of the
seed doubles.
This rule is applicable when
moisture content between 5 and 14%.
·
For every decrease of 5°C (10°F) in storage temperature the life
of the seed doubles.
This rule applies between 0°C to
50°C.
·
50/50 rule
o 50 oF / 50 %RH
o oF + %RH <=100%
o
Good seed storage is achieved when the % of relative humidity in
storage environment and the storage temperature in degrees Fahrenheit add up to
hundred but the contribution from temperature should not exceed 50°F.
Take Home Lesson, in general best is cool temperature and moderate
humidity.