Seeds and Seed Germination
Seeds and seed germination

Seeds are normally the
product of sexual
reproduction
Pollination
Fertilization
Embryo development
Mature seed
Seeds are for propagation

The biological function of seeds is for
propagation of the species

How does seeds help in propagation of
the species?
Diversity among seeds

Seeds are very diverse in term of size:
Thousands of Begonia seeds
– Begonia seed weigh 10-20
micrograms
– Coconuts weigh more than a
kilogram
Diversity among seeds
In terms of
adaptation to
survive various
environments
until conditions
are favorable for
germination
In terms of method
of dispersal
Seeds are for propagation
 Protect
the zygote against physically injury
 Store
foods for seedling-- before it becomes
independent
 Remain
dormant to survive harsh environment
 Disperse
the seeds to capitalize on their genetic
variability
 Respond
to environment cues and germinate at
the right time and place
Seeds are alive!

To function in propagation, seeds must be
alive

Seeds respire, albeit slowly
– consume O2, produce CO2 and H2O

Seeds have a finite lifespan

they cannot be stored indefinitely

Which kind of seeds can remain viable longer?
Seeds of tropical plant or temperate plant?
Seeds are alive!

Seeds of many tropical plants remain viable
for only a short time, a few days
– Tropical plants grow in environments that do not
have a winter season through which seeds must
survive before the favorable growing conditions of
spring arrive
General features of seeds
Embryo
– Root and shoot, in a miniature form
Food reserves
– Allow seedling to grow before it is capable of
performing photosynthesis
Seed coat
– Provides protection from the environment
Monocots and Dicots

Flowering plants (angiosperms):

Dicotyledonous plants with two seed leaves
Monocotyledonous plants with one seed leaf

Dicots
Angiosperms
flowering plants
200,000 species
Monocots
50,000 species
includes beans, roses, cacti,
melons, citrus
includes grasses, lilies, orchids,
palms
Seed structure
Cotyledon
Plumule
Seed coat or
testa
Radicle
Micropyle
Endospermous and nonEndospermous seeds



Takes place in the fruit on the parent plant
Endospermous seeds: Retain the endosperm tissue,
and is surrounded by a layer of living cells, the
aleurone layer.
Non-endospermous seeds: The endosperm tissue is
absorbed by the cotyledons. The cotyledons then
become the food reserve for the seed.
Endospermous or nonEndospermous seeds?
Endospermous and nonEndospermous seeds
A LS of maize grain / fruit
SEED DORMANCY
•
It is a phenomenon in certain seeds in which they
would not germinate if given an optimal condition
( water, oxygen, optimum temperature ).
•
Dormancy can be seen in seeds ( eg: legumes ),
buds, spore & food storage organs ( tubers ).
•
i.
ii.
iii.
Due to many factors include:
Lack of oxygen
Dryness
Presence of substances that inhibit germination
GROWTH UNDER EXTREME CONDITION
DORMANCY
• A period in the life cycle of many animals & plants when their
metabolic activities become minimum & growth stop.
• Is a resting stage
• It can occur in the adult, egg, pupa, spore or seed stage.
• A way of protecting an organism against unfavourable conditions
such as insufficient food, cold ( winter ) & dry ( drought ).
• It is controlled by hormones that
~ response to physiological in plants & animals
~ affecting the behaviour in animals
CHAPTER : GROWTH
What physiological changes
lead to dormancy

Metabolism falls
 Number of organelles per cell _____
 Dehydration – water content _____
 Vacuoles in cells _____
 Food reserves become _____ _____ _____
What physiological changes
lead to dormancy
 Metabolism
falls
 Number of organelles per cell falls
 Dehydration – water content falls
 Vacuoles in cells deflate
 Food reserves become dense crystalline
bodies
SEED Germination
Dormancy of these seeds may be broken by one or
more of the following:
(1)light, sunlight being the most effective;
(2)low temperatures (1 to 5 degrees Celsius [33.8 to 41
degrees Fahrenheit]) for several weeks;
(3)day/night fluctuating temperatures of 1 to 10 degrees
Celsius (41 to 50 degrees Fahrenheit);
(4)chemicals, such as nitrate in the soil, or applied
hormones (gibberellins) in the laboratory; and
(5)fire.
CHAPTER : GROWTH
Dormancy mechanism is related to the
seeds’ natural environment
Seeds that require light
involves a receptor protein,
phytochrome
Seeds that need a period
of low temperature
The requirement for
alternating
temperatures
The requirement for fire
is particularly important for small, winddispersed weed seeds.
the cold winter may cause the
parent plant to die, and thus remove
competition for space in the spring.
will prevent germination of seeds
beneath dense vegetation because the
latter dampens the day/night
temperature fluctuations; these seeds
will germinate only when there is little
vegetation cover, again reducing
competition with established plants.
CHAPTER : GROWTH
Dormancy mechanism is related to the
seeds’ natural environment
Seeds that require light
involves a receptor protein,
phytochrome
Seeds that need a period
of low temperature
The requirement for
alternating
temperatures
The requirement for fire
CHAPTER : GROWTH
Maintaining dormancy

Physical barriers
The seed coat (testa) is waxy = waterproof
and impermeable to oxygen
 Physical state – dehydrated
 Chemical inhibitors present e.g. salts,
mustard oils, organic acids, alkaloids
 Growth promoters absent
The breaking of dormancy
Break down of barriers
Abrasion of seed coat (soil
particles)
Decomposition of seed coat
(soil microbes, gut enzymes)
Cracking of seed coat (fire)
Destruction and dilution of
inhibitors
Light, temperature, water
Change in physical state rehydration
Production of growth
promoters
Seed Germination: Emergence of Radicle through Seed Coat
To break dormancy seeds need: Water Warm Temperature
So if you want to store seeds what are the conditions?
Dry Cold
Dormant seeds need more than moisture and warmth:
Dormancy is caused by:
Thick Seed Coat
Thin Seed Coat
Insufficient Development
Inhibitor: Abscisic Acid
Is overcome by:
Scarification
Light
or
nick digest
scrub fire
freeze-thaw cycles
Dark
Kentucky Coffee Tree
Lettuce
or
Pea
Soil Fungus Association
Orchids
Stratification > Vernalization
Most CT feral plants
6 weeks at 4° C
Inhibitor: Phenolics
Example:
20° C
Leaching by Repeated Rain
Cacti
Germination
STAGE
EVENTS
PREGERMINATI
ON
(a) Rehydration – imbibition of water.
(b) RNA activated & protein synthesis
starts.
(c) Increased metabolism – increased
respiration.
(d) Hydrolysis (digestion) of food reserves
by enzymes.
(e) Induction of cell division & cell growth.
GERMINATION
(a) Rupture of seed coat.
(b) Emergence of seedling, usually radicle
first.
POST
(a) Growth of root and shoot axis. Transport
GERMINATIO
of materials from food stores to growing
Stages leading to cell division
Mitchondria
reconstituted
Respiration
Initially
anaerobic
ATP
Later aerobic
Soluble sugars
RNA activated
Protein synthesis (0.5h)
Enzymes (proteins)
DNA synthesis (45h)
http://www.rbgsyd.nsw.gov.au/
Mitosis (70h)
Mobilization of food
reserves



Control by growth promotors such as
gibberellin and growth inhibitors such as
abscisic acid
These directly affect the genes for enzyme
synthesis or the activity of the enzymes
themselves
The growth substances are affected by
environmental factors (e.g. light,
temperature, humidity)
The control of food reserve
hydrolysis

Negative feedback control of enzymes
Negative
feedback
Starch + H20

 - amylase
Maltose
The action of the enzyme also limited by
substrate
 Once all the starch in an amyloplast is
hydrolysed the enzyme stops work
Therefore the release of the stored food is
adjusted to suite the demand
The mobilisation of food
reserves
Carbohydra
tes
Proteins
Lipids

Starches
(amylopectin
& amylose)
e.g. Zein
Oils
Amylases
Maltose and
glucose
Proteases
Amino acids
Lipases
Fatty acids &
glycerol
The food reserves are stored as large insoluble
macromolecules
 They are hydrolysed using enzymes to smaller
soluble molecules for transport
Triggering factors for germination
Light, chilling
or water (rain)
trigger the
inactivation of
ABA, which
makes dormant
seeds able to
germinate.
ABA
ABA
ABA
GA
After seeds take up water, GA is released from
the embryo to signal aleurone.
GA
The aleurone responds by synthesizing and secreting
digestive enzymes (-amylase) to hydrolyze stored
nutrients in the endosperm.
-amylase
Nutrients (ex. sugars) absorbed from the endosperm by
the scutellum (cotyledon) are consumed by the seedling
during germination.
The growth of seedling starting with the roots first, then
shoot growth follows.
Barley Seed Germination
Fruit+Seed Coat
Endosperm
starch
maltose
sugar
cotyledon
exocytosis
-amylase
monocot
Aleurone Layer
Storage Protein
RNA
shoot apex
Embryo
GA
radicle apex
water
imbibition
DNA
Amino Acids
Capsella Seed:
Seed Coat
Endosperm
Embryo
Shoot Apex
Cotyledons - dicot
Hypocotyl
Radicle
Root Apex
Micropyle
Lettuce Seed Germination
shoot apex
starch
Seed Coat
sugar
cotyledons
-amylase
dicot
RNA
Embryo
DNA
phytochrome
radicle apex
photoactivation
water
imbibition
photoreversibility
660 nm
Pfr
Pr
730 nm
dark
red and
white light
stimulate
germination
Germination of seeds
1. Utilization of stored reserves
– In cotyledons or endosperm tissue
– During germination, enzymes are made
that convert stored reserves (large
molecules) into compounds that can be
used by the seedling (smaller molecules)

starches  sugars

lipids, fats  sugars

proteins  amino acids
Germination of seeds
2. Transport of compounds into growing
seedling through vascular system
– These compounds have two functions
Support respiration in the embryo
 Provide a source of building blocks (carbon,
nitrogen, etc.) for the seedling

3. Expansion and growth of seedling
– Root radicle elongates down, hypocotyl expands
up
– Establishment of root system and emergence of
shoot
Seedling establishment

Shoot emerges and is exposed to light
 Chlorophyll is produced and seedling starts
to perform photosynthesis
 Seedling is no longer dependent on reserves
from the seed
 If stored reserves are consumed before
photosynthesis is established, the seedling
will die
Seedling establishment

Growth of the seedling can be measured
in many ways
– Length
Increases after seed imbibes
– Fresh weight
 Increases as seedling grows
– Dry weight
 Declines initially as stored reserves are
consumed by respiration, increases once
photosynthesis is established

Conclusions

Seeds are alive but dormant
 Comprise an embryonic plant and stored
reserves
 Germination requires
– Water - for imbibition
– Oxygen - for respiration
– Suitable temperature

Outcome of successful germination is a
seedling capable of independent growth
TYPES GERMINATION
• Radicle will emerge first from the seed
• Next, the shoot tip breaks through the soil surface
• Types of germination:
i.
epigeal
- cotyledons appear above the ground
{ hypocotyl forms a hook and pushes aboveground, raising
cotyledons }
ii.
hypogeal
- cotyledons remain underground
{ epicotyl forms a hook and shoot tip is lifted out of the soil }
the
…TYPES GERMINATION
…TYPES GERMINATION
Aggregate fruits
Receptacle is the fruit