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