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Plant nursery
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===Hardening off, frost hardiness=== Seedlings vary in their susceptibility to injury from frost. Damage can be catastrophic if "unhardened" seedlings are exposed to frost. Frost hardiness may be defined as the minimum temperature at which a certain percentage of a random seedling population will survive or will sustain a given level of damage (Siminovitch 1963, Timmis and Worrall 1975).<ref name="simi">Siminovitch, D. 1963. Evidence from increase in ribonucleic acid and protein synthesis in autumn for increase in proto plasm during frost hardening of black locust bark cells. Can. J. Bot. 41:1301β1308.</ref><ref name="timm">Timmis, R.; Worrall, J. 1975. Environmental control of cold acclimation in douglas-fir during germination, active growth, and rest. Can. J. For. Res. 5:464β477.</ref> The term LT<sub>50</sub> (lethal temperature for 50% of a population) is commonly used. Determination of frost hardiness in Ontario is based on electrolyte leakage from mainstem terminal tips 2 cm to 3 cm long in weekly samplings (Colombo and Hickie 1987).<ref name="colom2">Colombo, S.J.; Hickie, D.F. 1987. A one-day test for determining frost hardiness using the electrical conductivity technique. Ont. Min. Nat. Resour., For. Res. Note 45. 4 p.</ref> The tips are frozen then thawed, immersed in distilled water, the electrical conductivity of which depends on the degree to which cell membranes have been ruptured by freezing releasing electrolyte. A β15 Β°C frost hardiness level has been used to determine the readiness of container stock to be moved outside from the greenhouse, and β40 Β°C has been the level determining readiness for frozen storage (Colombo 1997).<ref name="colom1">Colombo, S.J. 1997. The role of operational frost hardiness testing in the development of container stock hardening regimes in Ontario. New. For. 13:449β467.</ref> In an earlier technique, potted seedlings were placed in a freezer chest and cooled to some level for some specific duration; a few days after removal, seedlings were assessed for damage using various criteria, including odour, general visual appearance, and examination of cambial tissue (Ritchie 1982).<ref name="rit">Ritchie, G.A. 1982. Carbohydrate reserves and root growth potential in Douglas-fir seedlings before and after cold storage. Can. J. For. Res. 12:905β912.</ref> Stock for fall planting must be properly hardened-off. [[Pinophyta|Conifer]] seedlings are considered to be hardened off when the terminal buds have formed and the stem and root tissues have ceased growth. Other characteristics that in some species indicate [[dormancy]] are color and stiffness of the needles, but these are not apparent in white spruce.
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