386 MUTATION AND PLANT BREEDING 



storage time increased, sensitivity to oxygen decreased. This particu- 

 lar relation appears to hold under all but one set of conditions. When 

 seeds are subjected to a temperature of 75° C for 24 hours before they 

 are irradiated, they subsequently show no sensitivity to storage. How- 

 ever, their sensitivity to oxygen immediately after irirradiation is as 

 great as in non-heat treated seeds and, furthermore, it persists for at 

 least 48 hours (Figure 10). These last data suggest that there is no 

 relation between the effects of oxygen and storage on seeds. This 

 problem will be discussed later. 



There appear to be two reasonable explanations as to why a pre- 

 irradiation treatment with heat prevents the dissipation of sensitivity 

 to oxygen. First, that the cell membranes are modfied by the tempera- 

 ture treatment so that they become impermeable to oxygen. Second, 

 that radiosensitive molecules within the nucleus are so re-oriented by 

 high temperature that the reactive sites are enfolded within them 

 and are not exposed to oxygen until hydration occurs. Both of these 

 explanations suggest that heat-treated seeds would retain a radiation- 

 induced labile state more or less indefinitely under appropriate condi- 

 tions. The data presently available support this suggestion. 



In their recent report Powers, et al. (21) have demonstrated that, 

 in bacteria, maximum protection from X-irradiation by high temper- 

 ature (80° C) only occurs when the temperature treatment is admin- 

 istered during or immediately after irradiation. Furthermore, maxi- 

 mal thermorestoration or thermal annealment, as they have chosen 

 to call this phenomenon, takes place only in the absence of oxygen. 

 In contrast with the spore work, for the first few hours following 

 irradiation there is a more rapid increase in the injury to X-rayed 

 seeds when they are maintained at 85° C (Figures 11 and 12). Inter- 

 estingly enough, however, seeds maintained at the higher temper- 

 atures for periods of 48 hours or more showed some thermorestoration 

 of injury. One other significant fact is that sensitivity to oxygen was 

 completely eliminated after a 15-minute post-irradiation heat treat- 

 ment at either 75° C or 85° C, whereas it persisted for considerably 

 longer periods of time at 20° C. 



Speculation seems warranted concerning the mode of action of 

 high temperatures on critical molecules that results in the elimination 

 of injury from storage, but not from oxygen, when the temperature 

 treatment is oiven before irradiation and the reverse when it is given 



