280 CHROMOSOME ABERRATION PRODUCTION 



[Lea and Catcheside (19)]. It now appears most likely that an indirect 

 mechanism is involved, in which irradiation of the oxygen-containing 

 aqueous medium in the cell leads to the production of some substance 

 which in turn produces chromosome breaks. 



It should be recalled, however, that a substantial aberration frequency 

 is still produced by irradiation in the absence of oxygen (at least in so 

 far as oxygen can be removed from these cells). The question thus 

 arises whether there are two mechanisms for chromosome-break produc- 

 tion, one involving direct ionization of the chromosome molecules and 

 the other an indirect effect from the irradiated aqueous medium, and 

 further whether the relative importance of the two mechanisms may be 

 judged by the degree of the oxygen effect. That such is the situation is 

 by no means clear. It seems possible, in fact, that at least some of the 

 aberrations induced in the absence of oxygen may also be the result of 

 an indirect effect, being produced by substances other than H2O2 or 

 HO2, such as OH radicals, resulting from the radiodecomposition of es- 

 sentially oxygen-free water. 



Attempts have been made to test this point by experiments (Giles 

 and Beatty, unpublished) designed to minimize the effectiveness of the 

 OH radical by promoting, during irradiation, the back reaction to form 

 H2O [Allen (1)]. To do this, inflorescences were exposed to 400 r of 

 x-rays in atmospheres of hydrogen, both at normal pressure and at 3 

 atm above normal. Interchange frequencies were essentially the same 

 for the tw^o exposures, and although both values were somewhat lower 

 than those obtained in comparable exposures in helium or nitrogen, the 

 difference is not significant. If it is valid to assume that hydrogen would 

 in fact react to remove OH radicals formed during irradiation, the failure 

 to detect a reduced aberration yield in these experiments supports the 

 view that chromosome breakage produced by x-rays in the absence of 

 oxygen may all result from direct ionization of the chromosome mole- 

 cules. It should be pointed out, however, that this conclusion is based 

 on the assumption that reactions leading to H2O2 production or suppres- 

 sion in cells from which oxygen has been removed as completely as pos- 

 sible are similar to those occurring in oxygen-free pure water. There 

 is as yet little experimental evidence on this point, and it is quite pos- 

 sible that the complexity of the cellular environment may cause very 

 different reactions to occur. 



Unfortunately it does not appear to be experimentally feasible to as- 

 sess the relative importance of the indirect and direct effects on chromo- 

 somes by the method normally employed for enzymes and viruses— that 

 of determining the effect of a constant radiation dose w^hen the solute 

 concentration is varied over a considerable range. The nearest approach 



