Graduate Thesis Or Dissertation
 

The physiology of dormancy of western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings

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  • The physiology of dormancy of western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings was examined. Many areas proved to exhibit similarities to those elucidated for Douglas-fir, although significant differences were also found and necessitate the use of caution in extrapolating data from one species to the other.. Dormancy of western hemlock seedlings was initiated most rapidly under a regime consisting of an 8 hour photoperiod, a warm temperature regime (25°/20°C), and moderate moisture stress (12-15 bars, PMS). A regime of warm temperature and moderate moisture stress was able to induce dormancy under a 16 hour photoperiod, although not as rapidly as with the short photoperjod. The photoperiod experienced by the seedling in the fall was found to have a slight but significant effect on the date of spring bud break after natural overwintering, with a long photoperjod during the fall delaying the date of bud break. The chilling requirement o western hemlock was found to be considerably less than that of Douglas fir. If seedlings were preconditioned with six weeks of mild, short days the requirement was met by four weeks of a constant 5°c temperature. Seedlings not receiving this beneficial pretreatment generally had their chilling requirement fulfilled by six to eight weeks of constant chilling. Cold storage of western hemlock seedlings for four weeks during October resulted in delayed bud break if they were then placed in a growth room or very high mortality if they were planted into a cold frame. A daily photoperiod during cold storage reduced mortality although it was still high. Cold storage during December or February was found to have no adverse effect on the seedlings.. A 16 hour daily photoperiod during February storage provided seedlings which resumed growth most rapidly in the growth room and earliest in the cold frame. Western hemlock was found to have four distinct phases of dormancy similar to those which occur in Douglas-fir. The third phase during which the chilling requirement is being fulfilled is shorter for western hemlock and results in the transition into the fourth phase earlier than Douglas fir. The phase of dormancy which the seedling is in appears to have a controlling influence on the response of the seedling to a variety of treatments and cultural manipulations. Xndole3-acetjc acid and abscisic acid were identified as endogenous hormones in western hemlock and the seasonal variation of these hormones was determined, The ratio of IAA to BA correlated well with the phase5 oedoancy, although no causal relationship was established. bscisic acid level was found to increase with increasing moisture ste experienced by the seedling, while the photoperiod experienced by the seedling had little effect on the abscisic acid level. A threshold moisture stress of about 12 bars was found to be the point where IBA levels began to increase rapidly in the needles of western hemlock seedlings with the most rapid increase occurring between 16 and 22 bars.
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