An important cultural tool in the greenhouse production of tree
seedlings in the Pacific Northwest is the control of photoperiod. By
artificially lengthening the period of daily exposure to light, it is
possible to increase both the magnitude and duration of seedling
growth. By shortening the photoperiod, one can induce dormancy and
hasten the development of frost hardiness,
A variety of supplemental lighting and light blocking techniques
are currently used in the production of Douglas-fir and ponderosa
pine seedlings. However, there is little information on the relative
advantages of alternative lighting and light blocking schemes in terms
of biological benefits or production costs. Three experiments were
therefore conducted to investigate the effects of a variety of photoperiodic
regimes on controlling the growth, dormancy and frost
hardiness of seedlings of these two species. The objectives of these experiments were to determine the
following: 1) the minimum light intensity, of an eight-hour period of
supplemental light, capable of delaying dormancy and increasing the
vegetative growth of newly germinated seedlings in the fall; 2) the
effects of a variety of nighttime lighting treatments on maintaining
seedlings in an actively growing condition in the early spring; and
3) the effects of varying daylengths and varying intensities of light
leakage on the development of frost hardiness of seedlings in the late
summer and early fall.
In all three experiments there were pronounced differences
between the responses of Douglas-fir seedlings and those of ponderosa
pine seedlings.
For Douglas-fir seedlings, an eight-hour period of supplemental
lighting delayed dormancy and increased both stem elongation and dry
weight when the intensity of the light provided was ten or more foot
candles. Intensities of one foot candle or less did not cause any
increase in the magnitude or duration of seedling growth. A variety
of nighttime lighting regimes, including several intermittent lighting
treatments, a two-hour night break, and 16- and 22-hour photoperiods,
successfully extended the period of active growth of newly germinated
winter-sown seedlings. Finally, shortening the photoperiod in the late
summer and early fall substantially increased the frost hardiness of
seedlings, but relatively low intensities of light leakage reduced
seedling hardiness. For ponderosa pine seedlings, there was no obvious threshold
supplemental light intensity above which dormancy was delayed and
below which it was induced. Ten foot candles or more resulted in an
increase in height growth, but there was no consistent relationship
between light intensity and dry weight, bud set or basal diameter. For
pine seedlings in the second experiment, growth was greatest for
those exposed to only a natural photoperiod. Lastly, shortening the
photoperiod in the late summer and early fall resulted in a general
decrease in the level of frost hardiness developed in ponderosa pine
seedlings.
These results suggest that the control of photoperiod can be a
highly effective tool for regulating the annual growth cycle of Douglas-fir
seedlings, but is relatively ineffective for ponderosa pine seedlings
