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Physiology of germination and dormancy
Alison A Powell
University of Aberdeen
UK
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Dry seed
Physiological germination (radicle protrusion)
Normal seedling
(ISTA germination)
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Dry seeds
•Low moisture content
•High matric potential
(-350 to -50MPa)
Low water potential
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Radicle protrusion
Water potential gradient
(seed vs environment)
results in rapid water uptake
(physical process)
Water potential of seed
and environment at equilibrium
Priming
Mobilisation
of food reserves,
decreased
water potential
Water uptake
Imbibition
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Membrane changes during imbibition?
Phosphatidyl head group
Fatty acid
Hydrated, liquid crystalline Dry gel Hydrated, liquid crystalline
Dry seed Imbibed seed
Rapid water uptake Imbibition
damage
Reduced
germination
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Embryo Storage tissue
I: Activation of enzymes
Active mitochondria
II: Synthesis of enzymes / mitochondria
Temporary anaerobiosis (CO2/ O2)
Ethanol / lactate produced
Taken from Bewley and Black, 1994
Seeds: Physiology of development and germination
Respiration during imbibition and germination
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Protein and RNA synthesis
• Resume minutes after hydration
• rRNA, tRNA, some mRNAs: retained in dry seed
• Initial synthesis:
Translation of mRNAs (mRNA turnover)
Polysomes
Proteins synthesised
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DNA synthesis
• DNA repair
• DNA replication
• DNA repair
– DNA damage
• drying/ rehydration; storage
– Single and double strand breaks
• endonuclease activity, free radicals, base loss
– Repair by DNA polymerases and ligases
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0
20
40
60
80
100
0 24 48 72 96 120 144
Time from set to germinate (h)
Germ
ination (
%)
F
H
I
Germination
at 20oC
Lag period
Repair
Longer lag period
More repair needed
Repair also during priming
Deterioration
A
B
Time course of germination
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Lag period, repair and normal vs abnormal seedlings (oil seed rape)
B
Slow germination
Low germination at 5d
Long lag period
A
Faster germination
High germination at 5d
Short lag period
B: deteriorated lot
More repair needed
Repair incomplete
Few normals
0
20
40
60
80
100
0 20 40 60 80 100
Germination (%) after 2 days at 20°C
No
rmal
germ
inati
on
(%
) (2
0°C
)..
A: less deteriorated
Less repair needed
Fewer abnormals, more normals
R2 = 0.62***
From Khajeh Hosseini, Nasehzadeh and Matthews,2010
Seed Science and Technology,38, 602-611
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• DNA synthesis
2C DNA
4C DNA
Doubling of DNA
Increased β tubulin
Cell cycle
G1
Normal cell
growth
S Phase
MitosisG2 growth phase
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• DNA synthesis
Dry seed
2C DNA
4C DNA
Cell cycle
G1
Normal cell
growth
Imbibition
S Phase
Mitosis
After radicle protrusion
G2 growth phase
Doubling of DNA
Increased β tubulin
Priming: 4C DNA produced when germination is advanced following repair
Implications for seed storage( Powell et al, 2000 Journal of Experimental Botany, 51,2031-2043)
High germination, high vigour (little deterioration).
Advancement, 4C DNA synthesis, reduced longevity
High germination, low vigour (deteriorated seed)
Repair, little advancement, improved longevity
2C DNA
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• Radicle extension and protrusion:
– cells expand
– increased turgor; cell walls yield
– causes unknown
– possible role for expansins
• proteins involved in cell wall relaxation in vegetative growth; loosen H bonds?
• Production of normal seedling
– Mobilisation of storage reserves
– Seedling growth
Final stages of ‘germination’
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Dormancy
• Prevents germination even in conditions adequate for germination
– Evolutionary adaptation
– Bet-hedging
• Two types of dormancy
– Primary dormancy
• part of genetic programme of seed development and maturation
– Secondary dormancy
• Mature imbibed seed
• Induced by environment
• Occurs in non-dormant seeds + initially with primary dormancy
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Primary dormant seeds
Non-dormant seeds Secondary dormant seeds
Breaking of
primary
dormancy
Breaking of
secondary dormancy
Induction of
secondary dormancy
Germination
Non germination
Germination
Courtesy of Françoise Corbineau
Dormancy : inability to germinate in apparently favorable conditions
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Factors that maintain dormancy
• Maternal– Testa / pericarp/ endosperm/ megagametophyte
(gymnosperms)• Mechanical
• Natural chemical inhibitors
• Permeability (water, gases)
• Embryo– Endosperm:
• Restraint of radicle growth
– Hormones:• ABA/GA3 antagonism;
• embryo sensitivity to ABA and GA3
– Genetics• Interaction of dormancy promoting + germination repressing loci vs
germination promoting loci
Dormancy classification: Baskin and Baskin
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Dormancy breaking
• 2 processes– Dormancy breaking
• Dormancy breaking agent
• Threshold value; single event or incremental events
• >1 factor may be effective
• Increased range of conditions in which germination will occur
– Germination• Requires right conditions even after dormancy broken
e.g. Summer annuals: Dormancy broken by low temperature;Germination requires a higher temperature
Dormancy cycling• Primary dormancy decays
• Increased range of conditions in which germination will occur, until non-dormant
• But if germination not triggered, dormancy re-established
.
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Factors breaking dormancy
• Environmental
– Temperature
• Dry after-ripening; alternating temperature; stratification
– Light
• Light / dark; single doses
• Chemical
– Inorganic
• CO2, nitrate, nitrite
– Organic
• Varied; butenolides (KAR1)
• Hormonal
– ABA / GA balance
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Role of hormones in breaking dormancy
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0
20
40
60
80
100
0 24 48 72 96 120 144
Time from set to germinate (h)
Germ
ination (
%)
F
H
I
Seed physiology Seed testing
Thank you!