Agriculture Reference
In-Depth Information
Paired Tests
may also be useful in determining the dormancy of tree and shrub seeds requiring a pre-
chill treatment. Since dormancy may vary according to geographic location or year of collection, it may
be helpful to test seeds from the same seed lot with and without prechilling, a procedure known as “paired
tests.” This enables the analyst to determine the depth of dormancy existing among seed lots exposed to
differing environmental conditions. The AOSA rules specify that four 100-seed replicates be used for each
of the paired tests.
Research results suggest that the physiological changes during prechill are associated with modiica-
tions of endogenous inhibitor-promoter hormonal levels which favor the promoter following the treatment.
Imbibition must occur for these changes to take place; prechilling dry seeds does not affect dormancy.
While the ultimate objective of prechilling is increased germination, it does subject the seed to a
certain amount of biological stress. The seed is physiologically active, even at such low temperatures, for
long periods and may also be subjected to fungi that subsist on exudates from the seed. Consequently, such
stressful treatments which promote germination may actually reduce seed vigor. Furthermore, as an adap-
tive mechanism, seed dormancy is strongest following seed maturity, declining as seeds age. Prechilling is
therefore most effective for freshly harvested seed lots, but may reduce seed quality if applied to carryover
or stored seeds.
High-Temperature Treatment (Predry)
Some seeds require exposure to high temperatures to break dormancy. This process is called “predrying”
and is accomplished by placing “dry” seeds in a shallow layer at temperatures of 35 to 40°C for 5 to 7 days
with a provision for air circulation. This method of breaking dormancy is useful in many cereal crops that
also respond to prechilling or gibberellic acid applications to break dormancy. It is thought that high tem-
perature treatments are required to accelerate shifts in inhibitor-promoter hormonal balance which favor the
promoter that normally occurs in dry seeds during long-term storage.
Light
The AOSA and ISTA rules recommend light exposure for seeds which often fail to germinate properly
without light. The rules do not treat light-dormant and nondormant seeds differently. All seeds are routinely
exposed to light during germination and the results presented without regard to dormancy.
Light intensity and quality as well as the duration of exposure affect the germination of many species,
particularly lower seeds. Light sensitivity is governed by the cell wall protein pigment,
phytochrome
, that
exists in one of two physiologically important forms dependent on the last light exposure. One form (P
FR
)
results from light exposure and permits germination, while the other (P
R
) results from prolonged darkness
or high temperature and prevents germination. The response of a dormant seed in light changes phyto-
chrome to the physiologically active form (P
FR
) and permits germination. Gibberellic acid can overcome
light dormancy in many species and is thought to be a inal product of the light reaction. Another com-
pound, potassium nitrate, also appears to be linked to relieving light dormancy in a number of species and
is often prescribed in the rules for this purpose.
use of Chemicals to Help break dormancy
Potassium Nitrate (KNO
3
).
Potassium nitrate has enjoyed popularity as the compound-of-choice for helping
to break the dormancy of many species (e.g., many grasses) for a number of years. Its effectiveness was
discovered when it was noted that seeds germinated better in a complete nutrient solution than in distilled
water. By successive deletion of each of the compounds comprising the nutrient solution, KNO
3
was found
to be the germination promoter. KNO
3
has been popular with seed analysts because it is effective and is con-
