Physiology of Seed Germination

1
Physiology of Seed
Germination
Miller B. McDonald
Seed Biology Program
Department of Horticulture and Crop Science
Ohio State University
Columbus, OH 43210-1086
[email protected]
2
1
Seed Germination
„
„
Fundamental to crop/food production
Fulfills role of seed as a plant
multiplication unit
3
Seed Germination
„
Definitions
„
„
„
Seed physiologist: Protrusion of the primary root
Seed technologist: Emergence and development
from the seed embryo of those essential
structures which are indicative of the ability to
produce a normal plant under favorable conditions
Others: Resumption of active growth of the
embryo
4
2
Seed Germination
„
Two germination
types based on
location of storage
reserves
„
Epigeal germination
– above soil surface
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
5
Seed Germination
„
Two germination
types based on
location of storage
reserves
„
„
Epigeal germination
– above soil surface
Hypogeal
germination – below
soil surface
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
6
3
Seed Germination
„
Requirements for Germination
„
Water
„
„
„
„
Essential element for enzyme activation
Permits translocation of storage reserves
Field capacity moisture optimum for
germination of most seeds
Critical moisture content concept
„
„
„
Corn – 30%
Wheat – 40%
Soybeans – 50%
7
Seed Germination
„
Requirements for Germination
„
Gases
„
„
„
„
Air (20% O2, 0.03% CO2, 80% N2)
Oxygen most critical, required for respiration
High levels of CO2 inhibitory
Nitrogen no effect
8
4
Seed Germination
„
Requirements for Germination
„
Temperature
„
„
„
Different seeds have different temperature
optima
Viability of dry seeds generally not affected at
liquid N2 (-196oC) or at 90oC
General rule: Temperate seeds require lower
temperature than tropical seeds; wild species
require lower temperature than domesticated
species
9
Seed Germination
„
Pattern of Germination
„
Maintenance phase – reduced metabolic
activity (dormancy – ABA, metabolic
blocks)
10
5
Seed Germination
„
Pattern of Germination
„
“Trigger agent” – factor that elicits
germination but whose presence is not
required throughout (e.g., light,
temperature); shifts balance from inhibitor
to promotor
11
Seed Germination
„
Pattern of Germination
„
“Germination agent” – factor that must be
present throughout germination (GA3)
12
6
Seed Germination
„
Sequence of events for germination
„
Water imbibition
„
Dependent on the composition of the seed
„
„
A physical process
Q10 = 1.5 to 1.8
13
Seed Germination
„
Sequence of events
for germination
„
Water imbibition
„
Dependent on the
chemical composition
of the seed
„
From Mayer, A. M. and A. Poljakoff-Mayber.
1975. The Germination of Seeds.
Occurs in both dead
and live seeds
14
7
Seed Germination
„
Sequence of events
for germination
„
Water imbibition
„
Dependent on the
chemical composition
of the seed
„
„
Proteins (“zwitterions”) cause
swelling
Proteins>starch>
lipid
15
Seed Germination
„
Sequence of events
for germination
„
Water imbibition
„
Seed coat
permeabilty
„
„
„
Heat killed seeds
imbibe more rapidly
than viable seeds
Greatest near
micropyle/hilum
Muciliages enhance
imbibition
16
8
Seed Germination
„
Sequence of events
for germination
„
Ψseed = Ψπ + Ψm + Ψp
Water imbibition
„
„
Availability of water
Determined by
„
„
Soil matrix
Water relations of
the seed
17
Seed Germination
„
Sequence of events for
germination
„
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Water imbibition
Ψ = water potential
Ψ of water = 0
Clays different than sand
Water moves to more
negative water potentials
(Seed at -16 MPa, Soil at
– 8 MPa; water moves to
seed)
18
9
Seed Germination
„
Sequence of events
for germination
„
„
Water imbibition
Seed-soil contact
important
„
Dependent on
„
„
„
Presence of
mucilage
Seed shape
Seed size
After Harper and Benton, 1966.
19
Seed Germination
„
Sequence of events
for germination
„
„
„
After Dasberg, 1971.
Water imbibition
Distance over which
water flows rarely
exceeds 10 mm (1/2
inch)
Seed water uptake
necessary for
germination is
species specific
20
10
Seed Germination
„
Sequence of events
for germination
„
„
Water imbibition
Imbibition is not
uniform
„
Soybean
„
Usually starts on
back side of seed
where the coat is
thinnest
After McDonald, et al. 1988.
21
Seed Germination
„
Sequence of events
for germination
„
„
Water imbibition
Imbibition is not
uniform
„
Soybean
„
„
After McDonald, et al. 1988.
Usually starts on
back side of seed
Most water
absorbed in radicle
pocket
22
11
Seed Germination
„
Sequence of events
for germination
„
„
Water imbibition
Imbibition is not
uniform
„
Soybean
„
„
After McDonald, et al. 1988.
Presence of hour
glass cells creates a
reservoir of water
Water starts
germination
process in radicle
23
Seed Germination
„
Sequence of events
for germination
„
„
Water imbibition
Imbibition is not
uniform
„
Corn
„
OOhh
After McDonald, et al. 1994.
66hh
12
12hh
Water moves into
radicle rapidly due
to black layer
openings
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12
Seed Germination
„
Sequence of events
for germination
„
„
00hh
66hh
Water imbibition
Imbibition is not
uniform
„
Corn
„
24
24hh
Water moves
through seed coat
slowly due to
starches
48
48hh
After McDonald, et al. 1994.
25
Seed Germination
„
Sequence of events for
germination
„
Enzyme activation
„
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Triphasic pattern of
water uptake
Phase I occurs in dead
and living seeds
„
„
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
„
Immediate release of
gases
Not dependent on
metabolism
Is a consequence of
matric forces
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13
Seed Germination
„
Sequence of events
for germination
„
Enzyme activation
„
„
Triphasic pattern of
water uptake
Phase II is lag period
„
„
„
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
Period of active
metabolism
Enzyme activation
No visible growth
27
Seed Germination
„
Sequence of events
for germination
„
Enzyme activation
„
„
Triphasic pattern of
water uptake
Phase III is
associated only with
germination
„
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
Period when
primary root
protrusion occurs
and active growth
resumes
28
14
Seed Germination
„
Sequence of events
for germination
„
Enzyme activation
„
Pattern of respiration
follows water uptake
pattern
„
„
Triphasic
Absence of seed
coat (open circles)
enhances water
uptake
After Kollöffel, 1967.
29
Seed Germination
„
Sequence of events
for germination
„
Enzyme activation
„
„
Increased respiration
leads to increased
ATP synthesis
ATP synthesis permits
subsequent growth
„
„
A) water uptake, O2
and germination
B) Nulceotide levels
(ADP, ATP, total
nucleotides)
After Pradet, et al., 1968.
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15
Seed Germination
„
Monocots
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Endosperm becomes rich
in soluble carbohydrates
and peptides
Translocated to scutellum
Lipids and glucose
transformed to sucrose
Sucrose and amides
translocated to
embryonic axis for
energy and new proteins
causing growth
After McDonald, 1994.
31
Seed Germination
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Dicots
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After McDonald, 1994.
Starch and lipids
degraded to form
sucrose in cotyledons
Protein degraded to
form amides in
cotyledons
Compounds
translocated to
embryonic axis for
growth and enzyme
formation
32
16
Seed Germination
„
Sequence of events for germination
„
Radicle protrusion is the final sequence of
germination
„
„
Seedling becomes autotrophic
Germination is ended by visible primary root
growth
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Seed Germination
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Metabolism
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Only substances taken up are H2O and O2
Initially, have a net loss in dry weight
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„
CO2
Leakage of substances from seed
As growth continues, increase in dry
weight due to photosynthesis (fixing of
CO2)
Consider changing “sink” strength
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17
Seed Germination
„
Metabolism
„
Soybean
„
„
„
Dry weight changes
Cotyledon (C) loses
energy
Hypocotyl (H) first
part to grow followed
by radicle (R),
plumules (P), and
epicotyl (E)
After Oota, et al., 1953.
35
Seed Germination
„
Metabolism
„
„
Corn
Water uptake
changes
„
„
Axis takes up more
water than
endosperm
Scutellum takes up
the least amount of
water
After Ingle, et al., 1964.
36
18
Seed Germination
„
Metabolism
„
Corn
„
Dry weight changes
„
„
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„
Whole seedling dry
weight decreases
Endosperm loses
dry weight
Axis increases in
dry weight
Slight decrease in
dry weight of
scutellum
After Ingle, et al., 1964.
37
Seed Germination
„
Metabolism
„
Corn
„
Soluble protein
changes
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„
Whole seedling
increases
Endosperm
increases, then
decreases
Axis increases
Scutellum little
change
After Ingle, et al., 1964.
38
19
Seed Germination
„
Metabolism
„
Corn
„
Nucleic acid changes
„
„
„
Whole seedling and
axis increase
Endosperm no
change
Scutellum no
change
After Ingle, et al., 1964.
39
Seed Germination
„
Metabolism
„
Corn
„
Lipid changes
„
„
„
Decrease in whole
seedling,
endosperm
Major change
occurs in the
scutellum
No change in the
axis
After Ingle, et al., 1964.
40
20
Seed Germination
„
Metabolism
„
Castor bean (Ricinus
communis)
„
Oil storing seed
„
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„
Primarily in
endosperm
Cotyledons fleshy
and leaf-like
Concerned with
degradation of lipids
to fatty acids
From Bewley, J. D. and M. Black. 1978. Physiology
and Biochemistry of Seeds.
41
Seed Germination
„
Metabolism
„
Castor bean (Ricinus
communis)
„
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„
After Yamada, 1955.
Glyoxylate cycle
Reducing sugar
(glucose, fructose)
Non-reducing sugar
(sucrose)
Sucrose appears first
and then declines as
it is hydroyzed into
glucose and fructose
for energy
42
21
Seed Germination
„
Metabolism
„
Castor bean (Ricinus
communis)
„
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„
Fatty acids are used
in Glyoxylate cycle
Energy primarily lost
from endosperm
where greatest oil
concentration exists
Lipase is enzyme
responsible for
breaking lipids into FA
After Yamada, 1955.
43
Seed Germination
„
Metabolism
„
Protein hydrolysis
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„
„
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Proteases
Probably under
hormonal control
(GA3)
Occurs more rapidly
in cotyledons and
then declines
Little activity in axis
After Yamada, 1957.
44
22
Seed Germination
„
Metabolism
„
Phosphorous
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„
„
Phytin primary
phosphorous store
Hydrolyzed by
phytase
Declines rapidly with
germination
After Mayer, 1958.
45
Seed Germination
„
Metabolism
„
„
„
„
DNA synthesis detected late, not necessary
for cell elongation
Long-lived (stored) mRNA responsible for
early enzymatic activity
mRNA stored during development
DNA —> mRNA —> enzymes
46
23
Seed Germination
„
Conclusions
47
48
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