Seed Germination

download report

Transcript Seed Germination

Text Pages: 76 (Box), 466 (coconut), 502-509,
631-632, 637-638
Ben Lockwood and Charlie
Dry seed germination
Germination: external factors
Special requirements
Roles of hormones
 Single
 0.5 – 1.5 mm diameter
 +/- crystalline body of protein
 Two
• Peroxisomes
• Glyoxysomes
Photorespiration (glycolic
acid metabolism)
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
 Cellulose:
ionic/polar (net - charge)
 Water: polar (partial + and - charge)
◦ Thus, water adheres to cellulose, causing
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
◦ phytochrome (red/far red response)
 Activates
metabolism (imbibition)
 Enzymes
◦ 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
 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
Phytochrome response
Some need exposure to red light at 660 nm to
Require light
◦ Small seeds of weeds, lettuce, grasses, etc.
◦ Very little nutrient storage
Require absence of light
◦ poppies and geraniums
Text, Fig. 28-19, p. 633
Text, Fig. 28-18, p. 633
Red -- Far Red
Far Red -- Red .
R -- FR -- R .
FR -- R -- FR -- R -- FR .
FR -- FR -- FR -- FR -- R.
Why do small weed seeds not germinate on the
floor of a dense temperate forest, but do
germinate if several trees are removed?
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.
 The
embryo is physiologically
 Seed coat impermeable to H2O
and O2
 Must
undergo a process called
after-ripening before they will
 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
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
Tumbling along in the rushing water of a gravelly
Bacterial or fungal action
Freeze-thaw cycles
Abrasive handling by squirrels or other animals
Passing through digestive system of some animals
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.
 Plants
of the California
chaparral community
 Manzanita (Arctostaphylos
 Many
◦ Jack pine
Text Fig. 22-9, p. 505
Manzanita (Arctostaphylos viscida)
Serotinous cones (cones remaining closed
until the occurrence of high temperatures
such as created by a fire)
Maturing seed cones at the end of their
second season of development (September)
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
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
 Fleshy fruits such as apples, pears,
 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
Seeds and Seed Germination
Eastern Connecticut State University
 -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)
ABA = Abscisic Acid (plant hormone)
Text Fig. 27-12, p. 613
 Initiates
dormancy and prevents
 Does this (presumably) by
preventing synthesis of hydrolytic
 ABA is antagonistic to GA
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
Eichhorn, Evert, Raven. Biology of Plants.
Seventh ed. 2005. W.H. Freeman and Co.