366 MUTATION AND PLANT BREEDING 



ment was shown to be dependent on temperature. Thus, the post- 

 irradiation increase in injury to seeds was negligible over a period of 

 96 hours at the temperature of solid carbon dioxide. 



Studies complementary to those on the relation of time and tem- 

 perature to the manifestation of post-irradiation injury have shown 

 that the availability of oxygen to the seed following X-irradiation 

 also has a profound effect on the expression of injury. Thus, seeds that 

 are stored or hydrated in the presence of oxygen immediately after 

 X-irradiation are more severely injured than are seeds that are stored 

 or hydrated anaerobically (5, 10). Significantly, sensitivity to oxygen 

 following irradiation diminishes as a function of time and can be 

 completely eliminated by a brief post-irradiation temperature treat- 

 ment administered at 75° C. 



In a recent series of comprehensive and refined experiments, 

 using bacterial spores which will tolerate desiccation to levels com- 

 parable to those commonly used in the seed work, Powers, et al. (21) 

 have corroborated and expanded upon much of the work that has 

 been done with seeds. This parallel response, at least in the more basic 

 considerations, of a multicellular and a unicellular organism is a 

 fortunate circumstance in that it oives credence to the wealth of data 

 accumulated in biophysical studies with seeds that has often been 

 considered exceptional and has, accordingly, too often been ignored. 

 Furthermore, analyses of these two organisms supplement one 

 another. The bacterial spore techniques are amenable to rapid 

 analysis of large populations to determine the relative lethality of 

 specific treatments; whereas the seed material is ideal for the most 

 refined cytogenetic and genetic studies, which have resulted in pin- 

 pointing the lethal consequences of the treatments as originating in 

 the chromosome. 



The work with seeds was given further significance in an origi- 

 nal experiment by Zimmer, et al. (31) in which it was shown that the 

 paremagnetic resonance spectra obtained in X-rayed barley seeds 

 was similar to that obtained from free radicals. Equally relevant was 

 the demonstration that irradiation in air gave rise to more magnetic 

 centers than irradiation in the presence of nitrogen. Later studies 

 have confirmed and extended this work (11, 13). 



It is of interest that much higher dosages were necessary to obtain 

 clear electron spin resonance signals (ESR) than were necessary to 



