374 MUTATION AND PLANT BREEDING 



Relation of pre- and post-irradiation temperature to manifestation 

 of injury 



Pre-irradiation temperature and the oxygen effect. — In previous 

 investigations, prior to the demonstration that, following irradiation, 

 seeds were injured by aerobic hydration and protected by anaerobic 

 hydration, it was shown that protection from X-rays was also afforded 

 to barley seeds when they were treated with a barely sub-lethal tem- 

 perature immediately before irradiation (6, 24, 26). Present studies 

 have proved that a pre-irradiation heat treatment of 75° C or 85° C 

 gives effective protection when the treatment is for a duration of 24 

 hours. Furthermore, the protective effect is not necessarily associated 

 with a detectable water loss and cannot be attributed to this factor. 

 It has also been demonstrated that two months can elapse between 

 the temperature treatment and irradiation without loss of protection, 

 provided the seeds are not hydrated in the interim. 



Because protection from X-irradiation was obtained by both pre- 

 irradiation temperature treatment and by anaerobic post-irradiation 

 hydration, it was deemed essential to determine whether or not the 

 two types of protection were complementary. To determine this point 

 seeds that had been heated for 24 hours at a temperature of 75° C and 

 then X-rayed with a wide range of doses were immediately hydrated, 

 either aerobically or anaerobically, and compared with seeds that 

 were not subjected to the temperature treatment but otherwise 

 handled in an identical manner (Figure 7). 



These data show that the protective effect of the heat treatment 

 was obtained under both conditions of hydration. Because other data 

 (Figure 3) suggest a casual relation between genetic damage and 

 seedling injury, and because it has been demonstrated that a pre- 

 irradiation treatment with heat reduces the frequency of interchanges 

 in X-irradiated seed (26), it is apparent that the protective effect 

 afforded by post-irradiation hydration in the presence of nitrogen and 

 pre-irradiation treatment with heat must be complementary; in the 

 sense, that is, that they are both influencing the manifestation of the 

 same kinds of genetic injury. 



Pre-irradiation temperature and storage effect.- — As previously 

 indicated (5, 12), earlier work has shown that the injury to barley 

 seeds from a given dose of X-rays can be enhanced by post-irradiation 

 storage. In addition, data have been presented which show that a pre- 



