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Rates of dehardening were measured in seedlings of Juglans nigra L. grown from seed collected in Tippecanoe Co., Indiana, transferred indoors during January, and exposed to warm days plus either warm or cool nights. Five seedlings from both cool and warm night treatments were sampled at 4-day intervals until foliation occurred. Stem sections from each seedling were subjected to test temperatures of +3, -4, -11, -18, -25, or -32°C. Freezing injury was detected visually and by a modified triphenyl tetrazolium chloride test of living bark viability. Eight days elapsed before seedlings exposed to warm nights dehardened to the point that they suffered injury at -18°C. Cool nights delayed similar dehardening by about 16 days but did not prevent it. Absence of injury to living bark at -4°C, even when seedlings were in full leaf, is attributed to the abilitiy of the cell contents to undercool to between -6 and -9°C before ice formed. Seedlings were not highly susceptible to freezing injury due to loss of hardiness by living bark during unusually mild weather in late winter. Forest Sci. 21:313-317.
Professor, Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, U.S.A.
Publication date: September 1, 1975
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Forest Science is a peer-reviewed journal publishing fundamental and applied research that explores all aspects of natural and social sciences as they apply to the function and management of the forested ecosystems of the world. Topics include silviculture, forest management, biometrics, economics, entomology & pathology, fire & fuels management, forest ecology, genetics & tree improvement, geospatial technologies, harvesting & utilization, landscape ecology, operations research, forest policy, physiology, recreation, social sciences, soils & hydrology, and wildlife management.