388 MUTATION AND PLANT BREEDING 



2. The interchange frequency increases exponentially with dose 

 when seeds are hydrated aerobically immediately after X-irradi- 

 ation but linearly with dose when they are hydrated anaerobically. 

 For both conditions of hydration the mutation frequency 

 increases linearly with dose. However, aerobic hydration results 

 in the production of more mutations per roentgen than does 

 anaerobic hydration. 



3. Pre-irradiation temperature treatments for 24 hours at 75° C, 

 showed a protective effect over a wide range of doses of X-rays. 

 The protective effect of the treatment was manifest when the seeds 

 were hydrated either aerobically or anaerobically and was not asso- 

 ciated with a detectable change in water content. Furthermore, 

 the percentage protection obtained, per unit of injury, under 

 both conditions of hydration was similar. It was also demonstrated 

 that the protective effect of the heat treatment against a subse- 

 quent dose of X-rays persisted for at least 2 months between heat- 

 ing and irradiation provided the humidity of the storage atmos- 

 phere was not changed. In addition, a pre-irradiation temperature 

 of 75° C eliminated the increase in injury that usually accom- 

 panies post-irradiation storage. However, such a pre-treatment 

 did not modify the sensitivity of the seeds to post-irradiation 

 hydration in the presence of oxygen. 



4. In contrast with the effects of a pre-irradiation heat treatment, 

 a short post-irradiation storage temperature of either 75° C or 

 85° C generally enhanced the rate of increase in injury that was 

 observed during post-irradiation storage at room temperature. 

 However, 48 hours storage at both of these temperatures resulted 

 in a distinct thermorestoration of injury. Of particular significance 

 was the demonstration that treatment of the seeds at either 75° C 

 or 85° C for as little as 15 minutes after they were irradiated 

 eliminated their sensitivity to post-irradiation hydration in the 

 presence of oxygen. 



Part II. Theoretical Considerations Relating to Acute 

 Irradiation of Successive Generations 



Actually since 1930, but particularly since the conclusion of the 

 Second World War, ionizing radiations have been used by plant 

 geneticists in efforts to induce heritable changes that will be of eco- 



