Seeds
Global Seed
Market
• $42 Billion
Importance of Seeds
• Perpetuation &
Propagation of the
Species
• Corn
Diversity in Form
– 1 Seed ---> 1,000 to 2,000
Seeds
• Wheat
– 1 Seed ---> 20 to 30 Seeds
• Soybean
– 1 Seed ---> 50 to 150
Seeds
Diversity in Size & Shape
1
Apomixis
How Seeds Form
• “Away From Mixing”
• Normally Sexual
Reproduction
• Pollination
• Asexual Seed Production
• Embryos Form
Spontaneously From
Cells That Bypass Meiosis
– Wind
– Pollinators
• Fertilization
• Same Genotype as
Mother
– Self vs. Cross
• ‘Parthenogenesis’
•
A.D. Daniels photo
• Advantages
– Assured Reproduction in Absence of
Pollinators
– Maternal Energy Not Wasted in Unfit
Offspring
– Avoiding Male Energy Cost of Producing
Pollen
• Disadvantages
– Can’t Control Genetic Mutations
– Narrow Ecological Niches
– Inability to Adapt to Changing Environments
Steps in Pollination
• Transfer of Pollen to
Receptive Stigma
• Self Pollination
– Anther & Stigma on
Same Flower
A.D. Daniels photo
• Cross-Pollination
– Anther & Stigma on
Separate Flowers
– Pollen Germinates
– Pollen Tube Grows Down
Style Toward Embryo Sac
– Tube Enters Micropile & Releases 3 Nuclei Into
Embryo Sac
• Double Fertilization
How Seeds Spread
• Birds &
Other
Animals
• Water
• Wind
• Hooks
• Drop & Roll
– Zygote
– Endosperm
2
• Bryophyllum crenatodaigremontianum
Vivipary
• Seeds Germinate Prematurely on Plant
• Wet Weather a Factor
• Genetic Mutation
• ‘Normal’ Seeds Do Not Exhibit Vivipary
– High ABA
– High Osmotic Potential
Parthenocarpy
• Ovary
Develops
Into Fruit
Without
Fertilization
• Fruits
Typically
Seedless
• Auxin
Hormones & Seed Development
• Higher Level of Hormones in Seeds
• Role in Seed Development
– Growth & Differentiation
– Accumulation of Food
– Development of Fruit Tissue
• Cytokinins
– Free Auxin High During Development
– High in Developing Seeds
– Conjugated in Mature Seeds
– Highest During Cell Division Stages of Embryo
– Auxin Signals Fruit to Continue to Develop
•
Gibberellins
• Abscisic Acid
– High in Developing Seeds
–
Minor Role in Development
– Induces Storage Proteins
–
Conjugated Forms at Seed Maturity
– Potent Germination Inhibitor
–
Parthenocarpy in Some Crops
–
Germination & Control of Seed Dormancy
– Minor Role During Seed Development
–
Can Overcome Need for Stratification
– May Play Role in Browning
• Ethylene
3
Ripening
Seed Components
• Moisture Content Drops to
≤30% While Seed Attached
to Plant
• Storage Tissue
• Further Drying During
Harvest
• Protective Outer Covering
– Cotyledons & Some Endosperm in Dicots
– Starchy Endosperm in Monocots
– Seed Coat
– Usually to 4 to 6%
– Pericarp—Part of Fruit Covering
• Some Seeds Do Not Dry
Below 30 to 50%
– Recalcitrant Seeds
http://www.absoluteastronomy.com/topics/Embryo
Structure & Function
• Fruit
– Contains Mature Ovary
& 1 or More Ovules
Surrounded By an
Ovary Wall
• Seed
– A Mature Ovule Containing an Embryo
http://www.beeculture.com/content/pollination_handbook/tomato.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Angiosperm.html
Seeds & Seedlings
• Every Seed Is Tiny Embryo
• Contains Leaves, Stems & Root Parts
Waiting for Right Conditions to Allow It to
Germinate & Grow
• Seeds Protected By Coat
– Thin Germinates More Quickly
– Thick Protects Embryo From Environment
Photos By Dr. Rob Bowker, Glendale Community College, Glendale, Arizona: http://web.gccaz.edu/~lsola/Flower/flwr3key.htm
Parts of a Dicot Seed
• Embryo
–
–
–
–
–
Radicle
Plumule
Hypocotyl
Epicotyl
Cotyledons
• Endosperm
– Short-Term Food Supply Formed at
Fertilization
– Used By Embryo to Fuel Its Growth
• Seed Coat
4
3. Raphe
Function of Dicot Seed Parts
–
1. Hilum
–
Scar Where Seed
Attached to
Ovary Wall
4. Testa
–
Seed Coat
5. Cotyledons
2. Micropyle
–
Slight Ridge
Along
Edge of Seed
–
Minute Scar
Where Pollen
Tube Entered the
Ovule Before
Fertilization of
Egg
Food Reserves
For Seed
Dormancy &
Germination
Chapter 6 - Structure of Higher Plants
6. Plumule
–
Germination of a Bean Seed (a Dicot)
Contains Embryonic Foliar Leaves &
Epicotyl
After a bean seed has been sown in moist soil,
it imbibes water and swells, the seed coat bursts and the
radicle emerges
7. Hypocotyl
–
The radicle grows
Down, and the
hook of the bean,
the hypocotyl,
emerges above
the soil, carrying
the cotyledons
With it.
The stem region
just above the
cotyledons and
the first trifoliate
leaves is called
the epicotyl.
Elongates to Pull the Cotyledons &
Plumule Out of the Ground in Epigeal
Emergence
8. Radicle
–
–
Develops Into
Primary Root
at Germination
Becomes Tap
Root
tab
Monocot Seed
• Embryo
Practical Horticulture 5th edition
By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky
© 2011, 2007, 2002, 1988 Pearson Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
• Caryopsis
– Seed Coat Is Fused
to Ovary Wall
• Corn, Sorghum,
Wheat, Oats, Rye,
Barley, Millet, Rice
– Radicle
– Plumule
• Cotyledon
– Scutellum
– Coleoptile
• Endosperm
• Seed Coat
http://www.geochembio.com/biology/organisms/maize/#kernel
http://www.life.uiuc.edu/plantbio/digitalflowers/Fruits/39.htm
5
4. Coleoptile
Function of Monocot Seed Parts
–
1. Pericarp
–
Protection
2. Endosperm
–
Protective
Sheath For
Leaves During
Emergence
5. Plumule
Carbohydrates &
Other Energy
Compounds For
Embryo During
Germination
–
–
Embryonic
Foliar Leaves
Becomes Shoot
3. Cotyledon
–
Seed Leaf (Scutellum
& Coleoptile)
Chapter 6 - Structure of Higher Plants
Germination of a Corn Plant (a Monocot)
6. Radicle
–
A corn seed planted in moist soil
imbibes (absorbs) water From the soil.
Develops
Into the
Primary
Root
Germination begins With emergence
of the radicle (the primary root) and
the plumule (the primary shoot).
7. Coleorhiza
–
These two enlarging axes form
the primary body of the plant.
Protective
Sheath
Around the
Radicle
The radicle grows Down through
the coleorhiza, From which the
primary root develops and the
secondary roots branch.
A mature corn plant can
develop roots 2 m (6.1 ft) long.
tab
Practical Horticulture 5th edition
By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky
© 2011, 2007, 2002, 1988 Pearson Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
Germination
Seed Shape, Size & Color
1. Stage I: Activation/Imbibition
• Shape
–
– Determines Kind
of Planting
Equipment
Rapid Absorption of Water
• Protein Synthesis Activated
• Enzyme Driven
• Need to Plant
Seeds Uniformly
1. Stage II: Lag Phase/Digestion &
Translocation
• Size
– Depth of Planting
–
• Color
Metabolic Processes Kick in
2. Stage III: Cell Division & Elongation
– Affects
Marketability
Milkweed seed With spider: A.D. Daniels photo
–
–
Radicle Emerges 1st
Then Plumule Emerges
6
Epigeal vs. Hypogeal Emergence
• Epigeal Germination
– Cotyledons Emerge
Above Soil Surface
– Results From
Hypocotyl Extension
• Epigeal Advantages
– Cotyledons May Protect
Apical Meristem if
Unfavorable Weather
Conditions Occur
– Herbivores May Damage
the Cotyledons & Not
Destroy the Entire
Plant
• Hypogeal Advantages
• Hypogeal
• Germination
– Takes Less Energy For
Emergence
– Can Plant Deeper
– Frost Protection
– Seed Cotyledons
Remain Underground
– Results From Epicotyl
Elongation
http://sangmeshwar.com/epigeal-germination-bean-484.html
Toby Back photo
http://theseedsite.co.uk/germinating.html
Seed Propagation
• Major Natural
Method
• Efficient &
Widely-Used
Seed Production Industry
• Most Seed Crops Produced by
Companies That Specialize in
Breeding & Seed Production
• Expected to Introduce
Improved Cultivars
• Expected to Produce HighQuality Seeds
Seed
Production
Industry
Viability Determination
• Standard Germination Test
– At Least 400 Seeds Picked at Random
– Divided Into Lots of 100
– If Any Lots Differ More Than 10%, Retest
– Average of the 4 Tests Is Official Percentage
• Rolled Towel Test
• Petri Dish Test
7
• Excised-Embryo Test
Purity Determination
– Woodies
– Excise Embryo
• Percentage by Weight of Pure Seed in a
Sample
– Germinated in Petri Dish
• Requires Trained Seed Analyst
– Soak Seeds 1 to 4 Days Until Swollen
• X-Ray Analysis
– Mechanical Disturbance
– Absence of Vital Tissues
– State or Private Seed Lab
– Looks for Foreign Material Including Seeds of
Other Species
– Insect Infestation
– Cracked or Broken Seed Coats
– Age
Treatments to Improve
Germination
Seed Protectants
• Pelleted Seeds
• Seed Protectants
– Round Uniform Shape & Size
• Germination Enhancement
– Aids Precision Mechanical
Sowing
• Inoculation With N-Fixing Bacteria
• Coatings to Help Mechanical Sowing
– Seed Treated With a Pesticide Must Be Colored
Germination Enhancement
• Seed Sizing
– Higher Potential for Viability & Vigor
• Seed Priming
– Seeds Tumbled in a Pan
• Polymer Films
– Coat Seeds to Improve
Flowability
Factors Affecting Germination
• Seeds Lose Vigor & Viability
• Key Factors
– Moisture Content
– Controlled Hydration to Reduce Emergence Time
•
4% to 6%
– Seeds Soaked Then Dried to Near Original Weight
•
Higher Moisture Allowed if Temp Is Reduced
– Seeds Held in an Imbibed Condition But Without
Radicle Emergence
• Pregermination
– Fast & Uniform,  $, Short Shelf Life
– Temperature
•
Reduced Temp Lengthens Storage Life
•
Can Offset Adverse Effect of High Moisture
•
Around 0F Increases Storage Life of Most Seeds
8