W. D. JACKSON 



ageing in seed^'', and it seems as if this ageing effect can, as far as chromo- 

 some breakage is concerned, be attributed to oxygen. Similar effects of 

 oxygen in breaking chromosomes are known from the experiments of Conger 

 and Fairchild^^ using pollen. Again chromosome breakage due to ageing 

 pollen has been reported. The type of compound involved is not known, but 

 it is likely that peroxy compounds similar to HgOg and HO, are involved. 

 Fetner^*^ and Brinkman and Lamberts'^'' have reported damage by ozone 

 and the remarkable efficiency of this gas in inactivating oxidase enzymes has 

 been shown by Todd^^, The fact that oxygen is itself effective in causing 

 breakage would indicate that its effect in irradiation produced breakage is 

 on breakage rather than on reunion. The remarkable similarity in the 

 pattern of breakage shown by Jackson and Barber^*^ is additional evidence 

 for this idea. 



A knowledge of the breakage produced by oxygen alone, and the breakage 

 induced by a given X-ray dose in the absence of oxygen, allows the total 

 effect of irradiation in various synthetic atmospheres to be partitioned into 

 its component parts {Figure 4). When this is done it is seen that in conditions 

 of metabolic rest the component due to the interaction of oxygen with 

 irradiation accounts for much of the damage. This component is greater in 

 dormant cells than in non-resting cells. An analysis of the distribution of 

 breaks caused by these separate components provides additional information 

 on the type of process operating in each component as well as evidence of 

 the effect of oxygen on breakage and reunion. The proportion of various 

 aberration types remains remarkably constant. In this material the absence 

 of chromatid breaks and the very low rate of non-sister reunion is a feature 

 of all components. Since these types show no change in frequency and the 

 proportion of sister unions to open breaks apparently remains constant, these 

 results would support the thesis of Giles and Conger that oxygen exerts its 

 effect by altering breakage rather than reunion. In addition, an analysis of 

 the distribution of breaks between cells shows that while the damage pro- 

 duced in the absence of oxygen has a Poissonian distribution, the distribution 

 produced by irradiation with oxygen, and by treatments with oxygen alone, 

 are significantly over-dispersed. This would indicate that the mechanism 

 of radiation damage approximates direct action in the absence of oxygen 

 but is distinctly cumulative in action in the presence of oxygen. Since the 

 breakage due to oxygen alone is produced by a cumulative action the 

 inference is again, that the effect of oxygen in irradiation experiments is 

 directed through the breakage mechanism. 



The post-storage experiments confirm the previous findings^^- ^^' '^^' ^^■^'^ 

 that storage of irradiated seed in atmospheres containing oxygen increases 

 radiation damage. The present experiments show, in addition, the depen- 

 dence of this process on oxygen concentration and length of storage. A par- 

 tition of the damage into its component elements {Figure 6) shows the 

 magnitude of this post-storage effect and its importance at low levels of 

 oxygen concentration. An analysis of the breakage distribution shows that 

 the amount of over-dispersion increases with post-storage and with oxygen 

 concentration. Even when the contribution to the dispersion produced by 

 oxygen alone is removed, it can be shown that the post-storage effect 

 introduces additional over-dispersion. 



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