Survival of Plant Tissue at Super-Low Temperature VI. Effects of Cooling and Rewarming Rates on Survival.

The survival rates of the cortical parenchymal cells of mulberry tree were determined as a function of cooling and rewarming rates. When cooling was carried out slowly at 1 degrees to 15 degrees per minute, 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 15 degrees to 2000 degrees per minute. In the intermediate cooling rates, when the cells were cooled at the rates lower than 14 degrees per minute, from -2.2 degrees to about -10 degrees , these cells could survive subsequent rapid cooling and rewarming.However, at cooling rates above 1000 degrees per minute and with rapid rewarming, the effect of cooling rate reversed and survival increased, reaching a maximum at about 200,000 degrees per minute. As the cooling rate increased above 15 degrees per minute, survival rates became increasingly dependent on the rewarming rate, with rapid rewarming becoming less deleterious than slow rewarming.The temperature range at which damage occurred during rewarming following removal from liquid nitrogen and in which growth rate of ice crystallization was greatest, was -30 degrees to -40 degrees . The survival rates even in the prefrozen cells at -30 degrees decreased considerably by keeping them at -30 degrees 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 -30 degrees . After removal from liquid nitrogen, all cells retained their viability, when they were passed rapidly through a temperature range between -50 degrees and -2.5 degrees within about 2 seconds, namely at the rates greater than 1000 degrees per minute.These observations are explained in terms of the size of the crystals formed within the cortical cells.