Seed GerminationText Pages: 76 (Box), 466 (coconut), 502‑509,
631‑632, 637‑638
Ben Lockwood and Charlie Hartman
Microbodies Imbition Dry seed germination Germination: external factors Phytochrome After-ripening Special requirements Roles of hormones
Objectives
Single membrane-bound organelles0.5 – 1.5 mm diameter+/- crystalline body of protein
Microbodies
Two types•Peroxisomes•Glyoxysomes
Microbodies
Peroxisomes
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
Glyoxysomes
Question for thought: What is imbition and why does it occur?
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.
Imbibition
Cellulose: ionic/polar (net - charge)Water: polar (partial + and - charge)
◦Thus, water adheres to cellulose, causing swelling.
Why does imbition occur?
Why?
Question for thought: Is germination of dry seeds possible if the seed does not imbibe water?
Why or why not?
Mature seeds are dry◦ only 5-20% water
Imbibition is necessary◦for metabolic activity, i.e., to activate enzyme activity
No, germination cannot occur in most seeds. Why?
WaterOxygenTemperatureLight requirement◦phytochrome (red/far red response)
External Factors Aiding Seed Germination
Activates metabolism (imbibition)◦Activates enzymes present◦Synthesis of other enzymes
Enzymes needed◦Digest and use stored nutrients◦Respiration for ATP production◦DNA/RNA/protein synthesis
Water
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?
Oxygen
If soil is waterlogged◦Maybe insufficient oxygen available for aerobic respiration
◦Seed will either:fail to germinate, orfail to grow into a seedling
Oxygen con’t
Wide range of germination◦ Min temp: 0-5oC◦ Max temp: 45-48oC◦ Opt range: 25-30oC
Temperature
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
Require absence of light◦ poppies and geraniums
Light Requirement
Phytochrome
Phytochrome (Pr) [Blue
Pigment]
Text, Fig. 28-19, p. 633
Phytochrome
Text, Fig. 28-18, p. 633
1 Red -- Far Red . . .2 Far Red -- Red . . .3 R -- FR -- R . . . .4 FR -- R -- FR -- R -- FR .5 FR -- FR -- FR -- FR -- R.
NoYesYesNoYes
Lettuce Seed Germination
Data interpretation:
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.
Answer
Question for thought: even when external conditions
are favorable, some seeds will fail to germinate.
Why?
The embryo is physiologically immature
Seed coat impermeable to H2O and O2
Two Most Common Reasons
Must undergo a process called after-ripening before they will germinate.
Physiologically ImmatureSeeds (Embryo)
Function (Adaptive Significance):◦Ensures seed will germinate under
favorable conditions: Not during winter Controlled hormones
Abscisic Acid -- inhibits Gibberellins -- stimulates
After-Ripening
In our area◦Triggered by low temperature of winter
As temperatures increase in spring◦ABA breaks down◦GA is produced initiating after-ripening
After-Ripening con’t
Special Requirements for Germination in
Some Seeds
◦ Tambalacoque or Dodo tree (Sideroxylon grandiflorum, and the Dodo bird)
◦ Mulberry and coralberry
◦ Promotes wider dispersal of species
1) Digestion
Turkeys Take Over
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
2) Mechanical Cracking of Seed Coat
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
How does the seed coat becomes scratched?
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.
3) Excessive Rain to Leach Growth Inhibitors out of the Seed Coat
Aloe ferox
http://www.csdl.tamu.edu/FLORA/schoepke/alo_fe_1.jpg
4) Fire (Intense Heat)
Plants of the California chaparral community
Manzanita (Arctostaphylos viscida)
Many pines◦Jack pine
Text Fig. 22-9, p. 505
Manzanita (Arctostaphylos viscida)
Pinus banksiana -- Jack Pine 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
Jack Pine con’t
Maturing seed cones at the end of their second season of development (September)
http://www.unbf.ca/forestry/species/pinusbnk.htm
Jack Pine con’t
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
Fire (Intense Heat) con’t
Moisture content of most seeds drops to 5-20%
Selective Advantage?
5) Seeds Must Dry Before They Will Germinate
Prevents the seeds from germinating while they are inside the moist fruit of the parent plant
Examples:Fleshy fruits such as apples, pears,
etc.
Seeds Must Dry con’t
Low temperature stimulates after-ripening
Also necessary for the breakdown of ABA present in the seed
6) Exposure to Cold
Horticultural practice that mimics exposure to cold
Expose seeds to 5oC for 100 daysUsed by horticulturists to germinate
seeds of many ornamental plants and fruit trees
Stratification
Seeds and Seed Germination Eastern Connecticut State University http://koning.ecsu.ctstateu.edu/plants_human/seedg
erm.html
Seed Germination Web Site
-Dormancy is extremely valuable, because it allows for plants to survive by going dormant (non active) over periods of coldness.
Dormancy
Gibberellic acid (GA)Abscisic acid (ABA)
Plant Hormones in Germination
Role of ABA 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
Role of ABA con’t
Role of GA GA = Gibberellic Acid (plant hormone)
Text Fig. 27-14, p. 614
To overcome seed dormancyImbibition stimulates the embryo to
release GAGA causes:
◦Cellular elongation of embryo◦Synthesis of hydrolytic enzymes (such as
amylase, etc.)
Role of GA con’t
Role of GA con’t
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.
Thought Questions
Eichhorn, Evert, Raven. Biology of Plants. Seventh ed. 2005. W.H. Freeman and
Co. Publishers.
Citations