742 



RADIATION BIOLOGY 



peak frequencies of various chromatid aberrations although the magni- 

 tudes of the peaks was greater at low than at high temperatures. These 

 results indicate that there had been no major modification by tempera- 

 ture of the timing of the nuclear cycle. 



The discovery of the oxygen effect (to be discussed later) served to 

 reopen the problem of the temperature effect, especially the interpreta- 

 tion of this effect as operating solely on the recovery mechanism. Since 

 X-ray-induced aberration frequencies are positively correlated with 

 oxygen concentration and oxygen is more soluble in water at low than at 

 high temperatures, it seemed possible that all the apparent temperature 



1.0- 



0.10 0.20 0.30 



VOLUME PER CENT DISSOLVED OXYGEN 



040 



O DISSOLVED 2 FROM PARTIAL PRESSURE OF 2 - 27°C 



• DISSOLVED 2 FROM 5% ? AT VARIOUS TEMPERATURES 

 Fig. 10-7. Comparative effects of temperature and dissolved oxygen on chromosome 

 interchange yield induced by 400 r of X rays. (Giles, Beatty, and Riley, unpublished.) 



effects might be due to an indirect effect on oxygen availability. How- 

 ever, experiments of Giles, Beatty, and Riley (1951; and unpublished) 

 indicate that the relation between chromosomal aberration yield and 

 temperature can be attributed only in part to an effect of temperature on 

 oxygen solubility. In addition, there appears to be a rather large effect 

 of low temperature alone in increasing aberration frequency, such that 

 chromosome interchanges and interstitial deletions are much more fre- 

 quent than expected on the basis of an oxygen solubility effect alone at 

 low as compared to high temperatures as long as oxygen is present during 

 irradiation (Fig. 10-7). Whether this additional effect of temperature — 

 beyond that portion probably associated with oxygen solubility and thus 

 very likely with the initial breakage frequency — is to be attributed to an 

 influence on the recovery mechanism, in accordance with the earlier 

 interpretations, is not clear. It is also possible that the additional effect 

 may result from an influence of temperature on the formation or effective- 



