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PART III:
SEED DORMANCY
Seed dormancy is the failure of seed to germinate even though optimal
environmental conditions exist.
It is due to physical, physiological or
chemical barriers within the seed.
In most wild and even cultivated plants
this is of value. For example, it is very common for seeds to be produced at
the end of the summer or early fall.
Conditions at this time are often
perfect for germination, but if germination took place there would not be
time for much seedling growth before the first killing frost.
It is,
therefore, a survival advantage to the species if its seeds remain dormant at
this time.
Seed dormancy is caused by many factors, but three very common ones
are: (1) impermeable seed coat, (2) dormant embryo, and (3) chemical
inhibitors within the seed. The easiest of these to demonstrate is dormancy
due to an impermeable seed coat.
Exercise 5.
Dormancy due to an impermeable seed coat.
Impermeable seed coats prevent water absorption.
Without water, the
seed’s reserve foods are not hydrolyzed (made soluble) and the digestive
enzymes are not activated, so the embryo cannot grow.
a. Select two Honey Locust (Gleditsia triacanthos) pods and extract 20
seeds.
b. As a group of four students: (1) scarify 10 seeds by using a razor
blade to chip the seed coat until an opening is visible, wrap them in a
rag-doll tester. (2) Leave an equal number of seeds intact and wrap in
a separate tester. (3) Label the testers and store them in a plastic
bag in a warm (21 degrees C.) place for five to seven days.
Make
certain the testers do not dry out during the germination period.
c. After five to seven days, unfold the testers and count the number of
seeds that have germinated. Compute the germination percentage. Record
in Table 9-3 below.
Number of Seeds
Tested
Number of Seeds
Germinated
Germination
Percentage
Scarified Seeds
Intact Seeds
Table 9-3. Honey Locust germination results.
PART IV:
ENVIRONMENTAL CONDITIONS INFLUENCING SEED GERMINATION
Exercise 6.
Oxygen requirements for germination.
a. Select 20 dry bean seeds.
b. As a group of four students: (1) Prepare two rag-doll testers as per
class procedure, each with 10 seeds. (2) Thoroughly soak both testers.
(3) Place one tester in a Zip-lock bag and seal after removing as much
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