Abstract |
- This study was undertaken to determine the
effects of different chilling treatments during the dormant
season upon growth of Douglas-fir seedlings0 In
addition it was planned to determine whether seedlings
native to various geographic areas differ in their chilling
requirements0
The hypotheses were advanced, based on previous
studies, that chilling at near L&00F. was more effective in
satisfying the chilling requirement of plants than chilling
at temperatures near 32°F., or 50°F., and that plants
native to areas of low elevation, characterized by mild
winters, require less chilling than plants native to
areas of high elevation characterized by long and severe
winters.
Verification or rejection of these hypotheses was
undertaken by subjecting dormant seedlings of Douglas-fir,
native to several geographic areas, to chilling under
different temperature treatments for different lengths of
time.
Experiment I was designed to compare the effects
of chilling for three, six, and twelve weeks at various
temperatures (32, L&0, 50, and 60°F., and out-of-doors)
on two-year-old seedlings from eight geographic areas.
After chilling, they were moved to a warm greenhouse and
exposed to short days (nine hours).
The effectiveness of the treatments in breaking
dormancy was expressed in number of days from end of treatment
to bud burst, percentage of plants which broke
dormancy, and amount of shoot elongation.
In general 40°F. was the most effective treatment
followed by 32°F., out-of-doors, 50, and 60°F., in
that order. The latter temperature had little or no
effect in breaking dormancy compared with the other
treatments.
Twelve weeks of chilling were more effective than
chilling for shorter periods at all temperatures except
60°F. At six weeks 40°F. was the most effective treatment,
32°F. was less effective, while 50, 60°F., and out-of-
doors treatments were ineffective in breaking dormancy.
Four weeks of chilling were ineffective regardless of the
chilling treatment.
Genetic differences were observed among the
plants from the eight areas chilled at 40°F. for twelve
weeks. Plants native to the highest areas represented in
the experiment resumed growth earlier than those from the
lower elevations.
Experiments II and III differed from Experiment I
since the dormant seedlings had germinated and grown only
about three months prior to chilling. In these experiments
plants were chilled for four, eight, and twelve
weeks. After the chilling period half of the plants were
grown under long days (normal days plus two hours of
artificial light) while the other half were maintained on
short days (nine hours).
The results of Experiment II were similar to
those of Experiment I with respect to the greater effectiveness
of 40°F. compared with the other temperature8.
Long days seemed to compensate for lack of adequate chilling
except in plants chilled for twelve weeks at 40°F.,
indicating that the chilling requirements of these plants
had been more adequately satisfied than by any of the
other treatments.
Experiment III was designed to compare the effectiveness
of chilling under fluctuating day and night
temperatures with that of chilling at a constant temperature.
Fluctuating day and night temperatures of
40/32°F. (40°F. day, 32'F. night) were about as effective
as continuous chilling at 32°F. or 40°F. Fluctuating
temperatures of 50/32°F., and 50/40°F. appeared to be
about as effective as continuous chilling at 50°F. Chilling
at 50°F. during the day, irrespective of the temperature
during night, delayed bud burst in comparison with chilling at 32, 40, or 40/32°F.
Genetic differences were observed among the
plants from the different areas in Experiments II and 1110
Plants native to high elevations (6000' elev.) resumed
growth in lower percentages, resumed growth later, and
grew less than plants from tower elevations (3000' and
3600' elev.).
The principal contribution of this study has
been the demonstration of differences in effectiveness
of several chilling treatments in breaking the dormancy
of seedlings. Variations in the chilling requirements of
seedlings native to different areas has also been shown.
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