(75594248) seed germination
TRANSCRIPT
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Seed germinationrequires:
SeedGermination
1. Viable seed: Seed must be alive(embryo) 2. Correct environmental conditionsincluding:
a. Waterb. Temperature
c. Oxygend. Light
3. Lack of dormancy or dormancyreleased
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Viviparyseedsmaynot dryorbecomedormant
resulting in
precociousgermination.Maturationisnotcomplete;often
germinatewhilestillattached toplant.
Postharv
estspro
uting.
Reduced/little
dormancyseeddriesandmaturesbutseed(emb
ryo)dorm
ancy is either absent orreduced and will readilygeminate under properconditions.
Dormant seedrequiressome type of after-ripeningprocess before it cangerminate. Dormancyimposes a restriction onability to readilygerminate.
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Germination begins with the imbibition or uptake of water.
Can be viewed, as the change is fresh weight due mostly to theuptake of water. Occurs in three phases:
Phase Iwater uptake by imbibitionPhase IILag phase little if any increase in FWPhase IIIradicle emergence
Phase
I
Water
upta
k
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e is related to the low water potential of dry seeds (-100mPa).
cell
= m
+
+ p
cell = cell water potential/ chemical potential of water and is relatedto the free
energy, G, required for water to move from a high to low potential.
m
= matric potential and is related to the interaction of water
withmacromolecules of the seed and with surfaces (cell walls,
macromolecules). Important because initial water uptake byseed is driven by this component.
=
osmotic
potential
and
is
related
to
the
dissolved
solutes
in
the
cytoplasm
(organic acids, amino acids, sugars, ions), expressed as a negativevalue and water
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moves to a more negative. Thus as solute concentration increases
becomesmore negativegreater chemical potential with increased soluteconcentration.
p = turgor pressure, force or pressure due to cell wall and membrane
How does this relate to seed germination? Phase I (imbibition) ismostly related to matric potential and is associated with theinteraction of water will surfaces and macromolecules. Phase 1 occurs
in two stages: 1st involves rapid uptake of water associated with
surface interactions (cellulose hydration) and 2nd involve linearincrease in water content associated with hydration of membranesand establishment of membrane function.
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During this stage since membranes are not completely functional,
metabolites can leak from the seedsenvironmental stress thatextend this time interval (low temperature) can reduce seedgermination and viability.
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Reduced ambient water potential during phase 1 results in:--reduced seed water content--extends length of phase II--delays and/or blocks entry into phase III
Phase IILag Phase
Char
acterized
by
lit
tle
o
r
no
wate
r
upt
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ake.therefore increase in FW lags. Highly active metabolically
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Important Phase II events:--mitochondria become active and respiration and ATP synthesisincreases--protein synthesis increases and new proteins are synthesized from
newmRNAs
--metabolism of storage reserves begins--specific enzymes are produced such as ATPase, hydrolases for storagereserves.
Phase IIIradicle emergence
1st visible evidence of germination is emergence of the radicleembryonic root.This results from cell enlargement with little cell division.
Once radicleemerges,
cell
division
begins
in
the radicle tip/meristem
Two types of germination/radicle emergence
One step emergence
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Usually occurs in seed without endosperm (Brassica, pea areexamples) the testa (seed coat) ruptures and initial radicle elongationresult in the completion of germination. Radicle does not rupture endosperm since it does not exist. ABA does not inhibit testa rupture,but inhibits subsequent radicle growth.
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Two Step EmergenceRupture of testa (seed coat) and endosperm are two separateevents. This is wide-spread in the plant kingdom. ABA appears toinhibit endosperm rupture but not testa rupture and appears to becounteracted by GA.
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What is known about the regulation of seed germination and role ofplanthormones?
GA promotes the induction of cell wall hydrolases necessary for endosperm weakening and rupture
ABA inhibits the induction of cell wallhydrolases which inhibits endospermweakening and endosperm rupture
GA promotes and ABA inhibits embryo growth
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Model for the regulation of seed germination
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Key points:
Some factor releases seed dormancy (in above model, light and GAact to release dormancy)
GA promotes -1-3 glucanase accumulation
ABA inhibits endosperm rupture but not testa rupture
Ethylene
stimulated
-
1-
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3 glucanase accumulation but does not affect dormancy
Class I -1-3 glucanase accumulate prior to endosperm rupture andpromote radicle
emergence by weakening of endosperm.
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Environmental Factors that Influence Seed Germination
Main factors include:WaterTemperature
OxygenLight
Waterfundamental to seed germination
Most species have a threshold water potential (wateravailable in a givensubstrate) that results in optimum emergence.
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Temperature
Next to water is most important factor
In dry state seeds are very resistant to temperatures, both high
andlow
Temperature affects both germination rate and percentage
3important
temp
e
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rature points can be used to characterize seed germination
1. minimumlowest temp for germination to occur2. maximumhighest temp above which injury of dormancy occurs
3. optimumhighest % or seed germinate and the highest rate
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Classification of seeds based on temperature requirements
1. Cool-temperature tolerant; usually plants from temperate zonesand can
germinate over a wide temperature range. Optimum usually not
cool(24
-
30C) but can tolerate cool temperature (broccoli, carrot, cabbage)
2. Cool-temperature requiring; require low temperatures forgermination and fail to germinate at warmer temperatures usually above 25C (celery, lettuce, onion).
3. Warm temperature requiring; fail to germinate below about 10-15C and tend to be tropical or subtropical in origin; can beinjured by low temperature and affect plant vigor (tomato, beans,pepper)
4. Alternating temperature; fluctuations in day/nigh temp
canimprove
germination and is usually at 10C difference (many wild species).
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Oxygen or aerationO
2is required for respiration, excess water limits diffusion of O
2.Trade offbetween sufficient water and to much water.
LightBoth Quality (wavelength) and duration (photoperiod) can
influence some specieslettuce and red/far-red light is classicexample. Usually small seeded species will often germinate betterwith light exposure.