444 MUTATION AND PLANT BREEDING 



some aberrations in irradiated cells must be decreased. Such a decrease 

 will lead to a more favorable ratio of mutations to chromosome 

 aberrations. 



Some progress has already been achieved in decreasing frequen- 

 cies of chromosome aberration in irradiated cells. Several reports 

 (7, 12, 13, 17, 28) have indicated that manipulation of various factors 

 of seed environment may decrease X-ray-induced chromosome aber- 

 ration frequencies and maintain or increase the Mi plant survival 

 and Mo mutation frequency. 



A convincing demonstration of a possible control over the ratio 

 of radiation-induced mutations to aberrations has been heat-shock 

 post-treatments in our laboratory (23, 24). 3 Seeds were frozen in dry 

 ice at about —78° C, exposed to 80 Kr and 100 Kr, then immediately 

 plunged into water at 60° C for 1 minute, and hydrated in distilled 

 oxygen-free water at 32° C for \i/ 2 hours. Compared with the non- 

 shocked treatments, there was an appreciable increase in survival of 

 the Mi plants from the heat-shocked seeds. Mutation yield from these 

 treatments was higher than any previously recorded for irradiated 

 barley seeds. It was determined that the improved survival rate after 

 heat-shock was in part due to a reduction of chromosome aberrations 

 (Table 1). We have more recently found that certain growth- 

 regulator treatments after X-radiation also reduce chromosome 

 aberration frequencies without decreasing mutation frequencies. 



The mechanism conditioning this more favorable ratio of muta- 

 tions to chromosome aberrations is yet unknown. Apparently it must 

 affect processes that either cause mutations and gross chromosome 

 aberrations at different frequencies or that result in restitution or 

 repair of chromosome damage. 



Considerable support for the possibility of obtaining more favor- 

 able ratios of mutations to chromosome aberrations in irradiated 

 seeds has come from experiments involving certain chemical muta- 

 gens. It must be recalled here that most chemical mutagens are con- 

 sidered to be radiomimetic since their effects and even possibly their 

 basic mechanisms of action are similar to those of ionizing radiations. 

 Diethyl sulfate and ethyl methane sulfonate have produced excep- 

 tionally high frequencies of mutations in barley (11, 15, 18, 19, 20, 



3 More recent studies have established that the heat shock response, at least in part, 

 depends on oxygen and moisture in the pre-radiation storage environment of the seeds. 



