380 



MUTATION AND PLANT BREEDING 



100 



o 



z 

 o 





80 



60- 



o 



UJ 



40 



8 12 



24 



48 

 TIME STORED (HRS.) 



72 



96 



Figure 12. — Initial thermal enhancement of and ultimate thermal re- 

 covery from X-ray -induced storage injury. 



injury. Apparently the radiation-induced damage is maintained in 

 relatively labile state by the low temperature because, on removal 

 from this temperature to temperatures of either 20° C or 85° C, the 

 progression of injury, as a function of time, is similar to that obtained 

 when seeds are stored at these temperatures immediately after 

 irradiation (Figure 11A and B). 



Discussion 



Seedling injury (growth retardation during a finite period) fol- 

 lowing germination of irradiated seeds has been used commonly as 

 an indication of the genetic injury that has been induced in the cells 

 of the meristem. The validity of using this estimate is reasonable on 

 a priori grounds. In addition, it has been repeatedly demonstrated 

 that there is a good correlation between seedling injury and genetic 

 injury over a wide range of doses with X-rays and other irradiations. 

 Despite this, the problem of the relation between seedling height 

 and genetic injury has long been of concern because the distribution 

 of seedling heights about the mean was often more skewed in X-rayed 

 material than in neutron-irradiated seed (9). The problem took on 

 added significance when studies were undertaken on the effects of 

 post-irradiation storage on seedling injury because the variability in 

 seedling heights in the stored populations became extreme; actually, 

 it often approached the bimodal. 



Clearly, there was a need to determine if the suspected relation- 



