CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 385 



nitrogen. They show that the linear components in both curves are 

 indistinguishable from one another. This is rather good evidence that 

 oxygen, at least, has no discernible effect on breaks resulting from 

 dense clusters of ionization. 



From these considerations, it seems reasonable to suggest that 

 most, if not all, environmental factors that effect radiosensitivity 

 either mitigate or enhance the probability of sparsely ionized sites 

 becoming biologically detectable. On the assumption that the detect- 

 able events are a manifestation of genetic injury, one could conceive 

 of both metastable (reversible) states within the chromosome and in 

 the ambient layer as initial stages. Distinguishing between these two 

 possibilities would be difficult, if not impossible, in studies with cells. 

 Possibly studies with virus particles should be expanded in an 

 attempt to resolve the problem. 



At this point it should be emphasized that, because of the sig- 

 moidal nature of the curve expressing seedling injury as a function 

 of dose (Figure 7), it is often necessary to choose specific doses to 

 demonstrate the significance of particular kinds of pre- and post- 

 irradiation treatments. This accounts for the fact that 15,000 r was 

 used to show the protective effect of a pre-irradiation temperature 

 of 75° C (Figures 8, 9, 10), whereas 7,500 r was used to show the effect 

 of post-irradiation temperature (Figure 11) on the progression of 

 injury from storage. 



It is now appropriate to look at the interrelations between 

 hydration, storage, and oxygen availability in connection with the 

 manner in which they influence the manifestation of radiation-incited 

 changes as seedling injury. It can be stated with certainty that the 

 addition of water to the seed after irradiation, irrespective of other 

 pre- and post-irradiation treatment conditions, has a quenching influ- 

 ence on those radiation-induced changes which might otherwise 

 progress for a considerable period of time. Furthermore, absorption 

 of water is equally effective in eliminating injury progression under 

 both anaerobic and aerobic conditions, although more total injury 

 may occur in the material subjected to aerobic hydration under 

 specific treatment regimes. 



Most previous investigators have suggested that the increase in 

 injury which occurs during storage includes the oxygen-sensitive 

 component (5, 12). It was demonstrated (5) that as post-irradiation 



