Download Seed Germination

Text Pages: 76 (Box), 466 (coconut), 502-509,
631-632, 637-638
Ben Lockwood and Charlie
Hartman
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Microbodies
Imbition
Dry seed germination
Germination: external factors
Phytochrome
After-ripening
Special requirements
Roles of hormones
 Single
membrane-bound
organelles
 0.5 – 1.5 mm diameter
 +/- crystalline body of protein
 Two
types
• Peroxisomes
• Glyoxysomes
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Photorespiration (glycolic
acid metabolism)
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Common in green leaves
Text, Fig. 3-16, p. 45
 Site
of b-oxidation and the
glyoxylate cycle
 Stored lipids converted to carbs
during seed/spore germination
 Adsorption
of water by a dry
material resulting in its swelling
(or onto internal surface)
 Dry material (cellulose) has a
very low (negative) water
potential.
Why?
 Cellulose:
ionic/polar (net - charge)
 Water: polar (partial + and - charge)
◦ Thus, water adheres to cellulose, causing
swelling.
Why or why not?
 Mature
seeds are dry
◦ only 5-20% water
 Imbibition
is necessary
◦ for metabolic activity, i.e., to activate
enzyme activity
 Water
 Oxygen
 Temperature
 Light
requirement
◦ phytochrome (red/far red response)
 Activates
metabolism (imbibition)
 Enzymes
needed
◦ Activates enzymes present
◦ Synthesis of other enzymes
◦ Digest and use stored nutrients
◦ Respiration for ATP production
◦ DNA/RNA/protein synthesis
 Early
stages of germination are
entirely anaerobic
 Becomes aerobic when seed coat
ruptures
 Why does respiration switch from
anaerobic to aerobic when O2
becomes available?
 If
soil is waterlogged
◦ Maybe insufficient oxygen available
for aerobic respiration
◦ Seed will either:
fail to germinate, or
fail to grow into a seedling
 Wide
range of germination
◦ Min temp: 0-5oC
◦ Max temp: 45-48oC
◦ Opt range: 25-30oC
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Phytochrome response
Some need exposure to red light at 660 nm to
germinate
Require light
◦ Small seeds of weeds, lettuce, grasses, etc.
◦ Very little nutrient storage
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Require absence of light
◦ poppies and geraniums
Text, Fig. 28-19, p. 633
Text, Fig. 28-18, p. 633
1
2
3
4
5
Red -- Far Red
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Far Red -- Red .
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R -- FR -- R .
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FR -- R -- FR -- R -- FR .
FR -- FR -- FR -- FR -- R.
No
Yes
Yes
No
Yes
Why do small weed seeds not germinate on the
floor of a dense temperate forest, but do
germinate if several trees are removed?
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These plants require Pr to germinate. Red
light (660 nanometers) does not reach the
bottom of the forest, inhibiting the
conversion of Pfr into Pr.
Why?
 The
embryo is physiologically
immature
 Seed coat impermeable to H2O
and O2
 Must
undergo a process called
after-ripening before they will
germinate.
 Function
(Adaptive Significance):
◦ Ensures seed will germinate under
favorable conditions:
 Not during winter
 Controlled hormones
 Abscisic Acid -- inhibits
 Gibberellins -- stimulates
 In
our area
◦ Triggered by low temperature of winter
 As
temperatures increase in spring
◦ ABA breaks down
◦ GA is produced initiating after-ripening
◦ Tambalacoque or Dodo tree (Sideroxylon
grandiflorum, and the Dodo bird)
◦ Mulberry and coralberry
◦ Promotes wider dispersal of species
Today Sideroxylon grandiflorum seeds are
encouraged to germinate by being fed to turkeys or
by turning them in a gemstone polisher.
Symphoricarpos orbiculatus -- Coralberry
 Scarification
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Many seeds are sealed within a tightly sealed
endocarp or the seed coat is extremely hard and
covered with a thick waxy coat
Such seed germinate only after the seed coat is
scratched or cracked
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Tumbling along in the rushing water of a gravelly
stream
Bacterial or fungal action
Freeze-thaw cycles
Abrasive handling by squirrels or other animals
Passing through digestive system of some animals
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Especially of desert species
Ensures the seed will germinate
only during those rare intervals
when desert rainfall provides
sufficient water
Ex: Artemisia spp., Aloe sp.
http://www.csdl.tamu.edu/FLORA/schoepke/alo_fe_1.jpg
 Plants
of the California
chaparral community
 Manzanita (Arctostaphylos
viscida)
 Many
pines
◦ Jack pine
Text Fig. 22-9, p. 505
Manzanita (Arctostaphylos viscida)
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Serotinous cones (cones remaining closed
until the occurrence of high temperatures
such as created by a fire)
http://web2.uqat.uquebec.ca/ferld/recherches/Jackpine.htm
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Maturing seed cones at the end of their
second season of development (September)
http://www.unbf.ca/forestry/species/pinusbnk.htm
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Seed cones open on a burnt branch after passage
of a fire - a normal way in which seed-shedding
becomes possible and the species regenerates
http://www.unbf.ca/forestry/species/pinusbnk.htm
Selective Advantage:
 Favors
survival of the seedling
because competitors have been
killed by the fire
 Moisture
content of most seeds
drops to 5-20%
Selective Advantage?
Prevents the seeds from germinating
while they are inside the moist fruit
of the parent plant
Examples:
 Fleshy fruits such as apples, pears,
etc.
 Low
temperature stimulates afterripening
 Also necessary for the breakdown
of ABA present in the seed
 Horticultural
practice that mimics
exposure to cold
 Expose seeds to 5oC for 100 days
 Used by horticulturists to germinate
seeds of many ornamental plants and
fruit trees
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Seeds and Seed Germination
Eastern Connecticut State University
http://koning.ecsu.ctstateu.edu/plants_human/seedg
erm.html
 -Dormancy
is extremely valuable,
because it allows for plants to
survive by going dormant (non
active) over periods of coldness.
 Gibberellic
acid (GA)
 Abscisic acid (ABA)
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ABA = Abscisic Acid (plant hormone)
Text Fig. 27-12, p. 613
 Initiates
dormancy and prevents
germination
 Does this (presumably) by
preventing synthesis of hydrolytic
enzymes
 ABA is antagonistic to GA
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GA = Gibberellic Acid (plant hormone)
Text Fig. 27-14, p. 614
 To
overcome seed dormancy
 Imbibition stimulates the embryo to
release GA
 GA causes:
◦ Cellular elongation of embryo
◦ Synthesis of hydrolytic enzymes (such as
amylase, etc.)
Barley Seed (Hordeum vulgare)
Text Fig. 27-16, p. 615
 Compare
and contrast the floral
characteristics of wind-pollinated and
animal-pollinated flowers.
 Speculate why it is that animal
pollination is more common in the
desert.
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Eichhorn, Evert, Raven. Biology of Plants.
Seventh ed. 2005. W.H. Freeman and Co.
Publishers.