CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 387 



after irradiation. Let us assume that the injury occurring during 

 storage results from the transfer of energy from the site of initial 

 absorption (primary event) to a second more sensitive site (secondary 

 event), whereas injury from oxygen results directly from oxidation 

 of the primary event. Let us further assume that it is a matter of 

 chance whether or not oxidation occurs at the primary site before 

 the energy associated with it is transmitted to a secondary site. If such 

 was the case, it is conceivable that pre-treatment with heat could 

 modify the structure of the critical molecule in such a way that the 

 energy associated with the primary event could not be transferred or 

 otherwise dissipated. Under such a circumstance the oxygen-sensitive 

 primary site should persist indefinitely, which is precisely what the 

 data indicate (Figure 10). 



Using the same model, the effect of a post-irradiation heat treat- 

 ment, which rapidly eliminates sensitivity to oxygen, could be 

 explained by assuming that the thermal energies involved were 

 sufficient to enhance the rate at which energy was transferred away 

 from the site of primary absorption. 



Summary 



Dormant seeds of barley with an embryo water content of 4 per 

 cent were used for biophysical studies concerned with the influence 

 of pre- and post-irradiation treatment conditions on the manifestation 

 of seedling and genetic injury. In the course of the investigations, 

 designed to examine some of the environmental parameters that were 

 known to affect radiosensitivity, the following observations were 



made: 



1 . Seeds germinated immediately after X-irradiation and then grown 

 for 8 days showed a near normal distribution of seedling heights 

 about the mean. However, when seeds were stored at room tem- 

 perature before germination, the distribution of seedling heights 

 about the mean became progressively skewed, as a function of 

 time, for a period of at least 8 days. Determination of the extent 

 of genetic injury in different seedling height classes in material 

 that was stored for 8 days showed that there was an inverse relation 

 between seedling height and genetic injury. On the average, seeds 

 that fell into the shortest height class evidenced between two and 

 four times as many interchanges and mutations as seeds that fell 

 into the tallest height class. 



