X-Ray Inactivation of Yeast at Freezing Temperatures

Several recent investigations have dealt with the dependence of X-ray sensitivity of unicellular organisms suspended in aqueous media on the temperature during irradiation (1-3). It has been found that X-ray sensitivities at freezing temperatures are materially less than those typical for cells at nonfreezing temperatures. Stapleton and Edington (2) have found that for E. coli the phase change from liquid (0? C) to solid (-22? C) decreases the radiosensitivity by a factor of approximately 3. A decrease in the irradiation temperature to -72? C further lowers the sensitivity by 17 %, and to -196? C by another 50 %. This general picture was also found by Houtermans for the same organism (3). Wood has found that in haploid yeast the sensitivity decreases twofold in the region from 0? C (liquid phase) to -10? C (solid phase) and decreases an additional 25 % in going from -10? to -30? C (1). The explanation that has been given for these results is that a substantial part of the radiation inactivation due to indirect effects mediated by and through the water of the biological system is lost in frozen suspensions. It has been assumed that at freezing temperatures diffusion processes would be materially decreased because of the loss of water through freezing and because of a diminution in diffusion constants in the ice phase. In this paper the radiosensitivity of haploid yeast in the temperature range 10? to -72? C is examined. As previously reported, the above general pattern is followed down to approximately -30? C. In the range from -30? to -72? C, however, there is a drastic increase in radiosensitivity (2.5-fold) to a value typical of the liquid phase. To explain these apparently anomalous results it is necessary to consider not only the degree of cellular freezing, but also the position of the frozen water, whether within or without the cell wall.