CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 383 



might depend upon the distribution of energy within critical sites. 

 As stated earlier, with X-rays there would be some sensitive volumes 

 that were sparsely ionized and some that were densely ionized. If 

 post-irradiation oxygen treatment influenced only sites that received 

 one or a few ionizations, effecting a single chromosome break, the 

 frequency of such sites, and their corresponding breaks, would 

 increase linearly with the dose while the frequencies of interchanges 

 between such breaks would increase at a power greater than unity. 

 To test this consideration seeds were treated with one of a wide 

 range of doses of X-rays and then immediately hydrated either aerobi- 

 cally or anaerobically. The seeds were then permitted to develop into 

 mature plants and the frequency of interchanges was determined at 

 microsporogenesis. 



The data obtained (Figure 5) are in agreement with the hypothe- 

 sis presented, for the frequency of interchanges in this material 

 increased exponentially when the seeds were hydrated in the presence 

 of oxygen and linearly when they were hydrated anaerobically. 



These data have greater significance than merely in apparently 

 resolving the confusion that has heretofore existed concerning the 

 interchange-dose relationship. They suggest that many radiation- 

 induced breaks are in fact "potential" breaks or "lesions", that either 

 become manifest as actual breaks or are eliminated, depending upon 

 the presence of oxygen in the system. 



In regard to these lesions, it is worth considering whether or not 

 it would be correct to use the term "restitution" in referring to the 

 elimination of breaks under conditions of anaerobic hydration. Wolff 

 and I have discussed this matter in personal communications and are 

 agreed that, if the relationship between dose and interchange fre- 

 quency is exponential for seeds hydrated in the presence of oxygen, 

 which it is, then it is reasonable to conclude that "restitution" (as the 

 term is defined in radiobiological literature) occurred in the material 

 hydrated in the presence of nitrogen. 



The effect of temperature treatments before, during, and after 

 irradiation on X-ray-induced injury in seeds has been studied over a 

 period of 30 years (25). Unfortunately, in the early work, the pro- 

 found effects of slight changes in the water content of seeds on their 

 senstitvity to pre- and post-irradiation treatments was not appreciated. 

 Consequently, there was often little correspondence between differ- 



