-4- 



dispersed pure water. Apple trees do not survive where winter 

 temperatures frequently drop below about -40° because below that 

 temperature ice forms in the living wood cells, causing death. 



Apple bark, cambium, and bud tissues, do not depend on deep 

 supercooling. Investigations have shown that these tissues survive 

 our winters by moving the freezable protoplasmic water outside the 

 cells to sites where ice formation does no apparent damage. As the 

 temperature drops below freezing, ice begins to develop in cracks 

 in the bark, in intercellular spaces, and between bud scales. This 

 creates a vapor pressure gradient favoring movement of protoplasmic 

 water toward the ice. The protoplasm becomes dehydrated rather than 

 freezing, but considerable dehydration does not harm acclimated 

 tissue. Experimentally, apple bark, cambium, and bud tissues have 

 been subjected to temperatures more than 100°F below zero without 

 injury if the temperature drop is not too rapid. The ability of 

 these tissues to survive is thought to be limited by the rate at 

 which water can move out of the protoplasm when temperature drops. 



If the temperature drops at a rate of degrees per minute these 

 tissues can be injured by ice formation within the cells even when 

 fully acclimated. Air temperature drop in nature is normally at 

 rates of degrees per hour. The temperature fall on the morning of 

 December 17, 1975 was about ICF in 5 hours (2°F per hour;) between 

 26° and 16°F. 



If the bark and cambium had been fully acclimated on December 

 17, 1975, we would not expect that intracellular ice could have 

 formed, in response to the 2° per hour temperature drop. On the 

 other hand, the 60 °F temperature of December 15 may have deaccli- 

 mated the tissue. Then the tree would have had only 5 or 6 hours 

 of exposure to temperatures below 28°F before the 16° temperature 

 occurred. Recall that a period of days or weeks at sub-freezing 

 temperature is needed to induce much acclimation. 



Temperatures of bark and cambium tissues apparently do some- 

 times drop at rates faster than water can move out to safe freezing 

 sites. It can happen in winter v>?hen air is calm and well below 

 freezing, and the south, or southwest, side of the tree trunk is 

 exposed to direct low angle sunshine. Tissue temperature can go 

 to 70 or 80°F under these conditions. When the sunlight is suddenly 

 cut off by shading, or sunset, the bark temperature returns to 

 ambient air temperature very quickly. This sometimes results in 

 bark or cambium kill on the south or southwest side of tree trunks. 

 It can be prevented by applying a reflective white latex paint to 

 the trunks. 



Knowledge of the relative hardiness of different tissues at 

 different times of the year can be helpful when trying to determine 

 when injury might have occurred, if it is discovered much later. 

 It has been shown that sapwood is the hardiest tissue in the early 

 fall. But by midwinter, cambium is the hardiest tissue, bark tissue 

 is slightly less hardy, and sapwood is least hardy. 



