Photoperiodic responses of Douglas-fir (Pseudotsuga Menziesii (Mirb.) Franco) and Ponderosa pine (Pinus ponderosa) seedlings Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9p290d39t

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  • 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.
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