Rewarming Rates on Survival

Summilt1tary. The survival rates of the cortical parenchymal cells of mulberry tree were determined as a fuinction of cooling and rewarming rates. When cooling was carried out slowly at 10 to 15° per mintute, all of the cells still remained viable even when rewarmed either rapidly or slowly. Survival rates gradually decreased to zero as the cooling rate increased from about 150 to 20000 per minute. In the intermediate cooling rates, when the cells were cooled at the rates lower than 140 per mintute, from -2.2° to about -100, these cells could sturvive subsequent raipid cooling and rewarming. However, at cooling rates above 10000 per minute and with rapid rewarming, the effect of cooling rate reversed and survival increased, reaching a maximum at about 200,0000 per minute. As the cooling rate increased above 150 per minute, sturvival rates became increasingly dependent on ithe rewarming rate, with rapid rewarming becoming less deleterious than slow rewarming. The temperature range at which damage occurred duiring rewarming following removal from liquid nitrogen and in which growth rate of ice crystallization was grea,test, was -300 to -400. The survival rates even in the prefrozen cells at -30° decreased considerably by keeping them at -300 for 10 minutes after removal from liquid nitrogen. This fact indicates that intracellular freezable water remains to some degree even in the prefrozen cells at -300. After removal from liquid nitrogen, all cells retained their viability, when they were passed rapidly through a temperature range between -50° and -2.50 within about 2 seconds, namely at the rates greater than 10000 per minute. These observations are explained in terms of the size of the crvstals formed within the cortical cells.