400 MUTATION AND PLANT BREEDING 



Mericle, R. P.: One factor which has not been given much consideration 

 to date at this meeting is that of dose-rate. By this, I do not mean chronic 

 versus acute, but rather, within the framework of acute or semiacute. 

 In our proembryo irradiations of bailey, for example, we have achieved 

 more than a 6-fold increase in mutation rate to albinism by a 4-fold 

 decrease in dose-rate, when total dose and irradiation conditions were 

 identical. These mutations were in all cases ones in which the mutant- 

 carrying sector encompassed two or more heads in the X-l plants, and 

 are probably of a similar nature since segregation ratios in the X-2 

 were the same. With the continued interest in increasing mutation 

 rates, while at the same time decreasing the frequency of gross chromo- 

 somal aberrations, we suggest that, especially in instances when it is 

 desirable to use large radiation doses, the usual dose-rates be reduced 

 by x / 4 , : /j , or so. An optimum balance between total dose and rate 

 of delivery should be sought, since lowering the dose-rate too much may 

 result in loss of effective mutation frequencies. 



