278 CHROMOSOME ABERRATION PRODUCTION 



quency when the cells are irradiated with alpha particles, as compared 

 with a very marked decrease when x-rays are used. These observations 

 have been confirmed in preliminary results obtained by Conger (un- 

 published) in experiments on the irradiation of mature pollen grains of 

 Tradescantia with alpha particles. 



Although these comparisons suggest very strongly that H2O2 may be 

 the product involved in the oxygen effect, they do not establish this 

 point unequivocally. The possibility exists that other products of the 

 radiodecomposition of water may be concerned. The observations of 

 Krenz and Dewhurst (18) on the effect of dissolved oxygen on the oxida- 

 tion of ferrous sulfate in aqueous solution by gamma rays can apparently 

 best be explained by a mechanism involving the HO2 radical. The 

 marked similarity between the magnitude of the decreased oxidation of 

 ferrous sulfate in the absence of oxygen (to about one-fourth) and the 

 decrease in aberration frequency obtained in the early experiments with 

 Tradescantia is noteworthy. However, the absolute magnitude of the 

 decrease in aberration frequency occurring in the absence of oxygen 

 apparently depends on the dose and the intensity in a particular experi- 

 ment. Further, the experimental results with alpha particles in the 

 presence and absence of oxygen appear to make it unlikely that HO2 

 is the intermediate involved. These results are in agreement with the 

 view, on radiochemical grounds, that H2O2 is formed directly in large 

 amounts by alpha particles because of the very close proximity of the 

 OH radicals produced in the center of the particle track [Gray (13)]. 

 The failure of O2 to increase the yield of H2O2 apparently indicates that 

 the usual reaction for peroxide formation by way of the intermediate 

 HO2 radical is relatively unimportant [Bonet-Maury and LeFort (5)]. 

 It thus appears that H2O2, rather than HO2, may be responsible for 

 chromosome breakage. There are, however, additional mechanisms by 

 which HO2 may be produced from peroxide, and the possibility cannot 

 be entirely eliminated that this radical may also be to some extent an 

 effective agent in chromosome breakage. 



Additional evidence on the presumptive role of H2O2 should be ob- 

 tained from experiments with enzyme inhibitors such as cyanide which 

 should block the action of catalase and permit H2O2 accumulation. A 

 suggestive slight mutagenic effect of cyanide alone, which has been in- 

 terpreted on this basis, has already been reported by Wagner et al. (30) 

 in Neurospora. The experiments of Mottram (23), which show that 

 cyanide increases the sensitivity of roots of Vicia faha to x-rays, as 

 judged by inhibition of growth, also lend support to this possibility. In 

 the Neurospora experiments a direct mutagenic effect of H2O2 was also 

 noted. In experiments utihzing enzyme poisons to elucidate the mecha- 



