Seed Germination: Structure of a Seed

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Seed germination is defined as the sum of events that begin with the hydration of the seed and culminate in the emergence of the embryonic axis (usually the radicle) from the seed coat.
By C. Kohn, Waterford WI
2. Outside, In
 When you look at a seed, what you are
actually seeing is the seed coat.
 The seed coat performs much of the same
work that your own coat performs …
 It provides protection against entry of parasites,
 It protects against mechanical injury
 In some seeds, it buffers against unfavorably
high or low temperatures.
 It stops germination until the right time
 What would happen if the seed coat failed in fall?
3. Parts of a seed
4.  Inside of the seed coat, an embryo is protected.
 The embryo is an immature plant with all of the
parts of an adult plant.
 A close looks shows leaves and roots, albeit very small
leaves and roots
 The leaves of the embryo are called plumules
 The leaves are sheathed by a cotyledon
 The embryonic roots are called radicles
 The embryonic stem is called the hypocotyl
 The seed is filled with nourishment for the new
plant; this source of nutrition is called the
5. Parts of a seed
6.  The first step of germination is the
absorption of water – a lot of water
 Absorption of water activates enzymes in
the seed that stimulate growth.
 These enzymes break down starches in the
endosperm into sugars that can be used for
 The deciding factor in whether or not a
seed germinates is whether or not energy
is available for growth and cell division.
7. Stages leading to cell
Mitchondria Initially Soluble
reconstitute anaerobic sugars
d Later aerobic
Protein synthesis (0.5h)
Enzymes (proteins)
DNA synthesis (45h)
© 2008 Paul Billiet ODWS
8. Seed Dormancy
 If moisture is the key to starting seed
germination, it should be clear what is necessary
for seed dormancy
 A waterproof seed coat
 No oxygen
 Chemical inhibitors
 For germination to occur, these must be negated
 Coat broken down; oxygen available
 Water absorbed
 Growth promoters activated, inhibitors are inhibited.
PREGERMINATION (a) Rehydration – imbibition of water.
(b) RNA & protein synthesis stimulated.
(c) Increased metabolism – increased respiration.
(d) Hydrolysis (digestion) of food reserves by
(e) Changes in cell ultrastructure.
(f) Induction of cell division & cell growth.
GERMINATION (a) Rupture of seed coat.
(b) Emergence of seedling, usually radicle first.
POST GERMINATION (a) Controlled growth of root and shoot axis.
(b) Controlled transport of materials from food
stores to growing axis.
(c) Senescence (aging) of food storage tissues.
© 2008 Paul Billiet ODWS
10. Germination Factors
 Water is clearly the most important
factor in germination; an adequate
continuous supply of water is necessary
for complete emergence.
 Water functions a triggering enzyme for
starch conversion into sugar, turgor
pressure for moving the radicle root down
and the cotelydons up, and for
transporting nutrients and enzymes within
the seed
11. Germination Factors
 Light is another key germination factor; light can either
stimulate or inhibit seed germination
 Some crops have a requirement for light to assist seed
germination (e.g. begonias, impatiens, lettuce)
 Others germinate best in the dark
 This is determined by how the seed would naturally be
 Small seeds must sprout on the surface of soil because they
lack a suitable endosperm to supply the needed nutrients;
these are typically aided by light exposure
 Large seeds contain enough nutrition to grow underground
when photosynthesis is not possible. These seeds are more
likely to germinate in dark conditions.
12. Germination Factors
 Oxygen is a third factor
 Cellular respiration is necessary for plants
to grow; oxygen is necessary to complete
 Oxygen removes metabolic waste from the cell
 Without oxygen, waste is not removed and the
cellular metabolism is slowed.
 If oxygen supply is limited during
germination, emergence may not occur due
to inhibited growth.
13. Germination Factors
 A favorable temperature is necessary to allow
for plant growth
 Temperature not only affects the germination
percentage but also the rate of germination
 For every species of seed, there is an optimal
soil temperature for germination
 At that temperature, the maximum number of
seeds will germinate and in less time than at any
other temperature.
 Many seeds germinate best around room temp.
14. The Process of Germination
 Germination is a complex process
 A pre-formed plant (embryo) inside of
the seed coat must turn the endosperm
(starch) into sugar
 This sugar powers cell division (mitosis);
the addition of cells will cause the
embryonic roots, leaves, and stems to
grow, expand, and develop.
15.  During germination, the radicle (embryonic root)
emerges due to mitosis fueled by the breakdown
of starch into sugar
 Under warm conditions, this process will take 4-5 days
 Under cool conditions, this process takes longer
 Initially the radicle grows in what direction the kernel tip
is pointing.
 Later, smaller roots will emerge from the radicle at
varying angles
 These roots will absorb the nutrients necessary for
growth and development when the endosperm is
completely consumed.
17.  The coleoptile, a protective sheath covering the
shoot, pushes through the soil until it reaches light.
 Upon reaching light, the plumule emerges
 If the kernel is planted too deep, emergence will not
 The time between planting and emergence is
determined mostly by temperature; warmer temps =
reduced emergence time.
 This is only true to an extent; excessively high temps
can also increase emergence time.
 Soil compaction can increase the time to emergence
 Lack of moisture can also increase emergence time
18. Sprouting Time
 Many temperate-zoned species use
chemical inhibitors to induce dormancy in
 In fall the seeds are exposed to abscisic
acid (ABA) by the plant to prevent the
seeds from sprouting before winter
 Over winter, enzymes in the seed degrade
the abscisic acid and by spring it is gone
19.  Seeds of some species can be caused to sprout
earlier by exposure to cool or cold temperatures
 Seeds of temperate species that were exposed to
the cold will sprout earlier in a greenhouse than
those planted directly.
 This process is called vernalization
 Vernalization is the intentional exposing of seeds to
cool conditions to increase the breakdown of chemical
inhibitors and stimulate the production of growth or
flowering enzymes.
 Many annuals are facultative – vernalization is not
necessary for development but does speed it up
20. Hot climate vernalization
 If vernalization, or exposure to cold, is
necessary or helpful for seed growth and
development, how does vernalization
work in warmer climates?
21. Phenolic Compounds
 Desert plants typically have long dry
seasons and short wet seasons
 When the rains come, phenolic
compounds are leached from seeds
 Phenolic compounds inhibit growth and
development in the seed
 Because they are water soluble, phenolic
compounds are washed out of the seed
when growth is most likely – after a rain.