Till, I.ll r.-SP.W OF MUTAGEN'S I'RODUCIT) l.\ CKLLS BV IRRADIAIION 



period produced by subsequent doses given in the presence of BAL. It was 

 proposed that irradiation ])roduces a delay in reunion of" breaks by the 

 destruction of some metaboHc activity. This activity could be protected by 

 BAL so that the recovery time was reduced. 



In contrast to these experiments using reducing agents, Wolff and Luip- 

 pold*'*'^^ and Cohn^®'^"-*^ have tried respiratory inhil^itors in fractionation 

 experiments. These experiments extend the original findings of Zirkle^'-*' **", 

 King and Schneidcrman''^ King et al.^'^, Schncidcrman and King®^ and 

 Haas el al.^* in which carbon monoxide and carbon dioxide were both found 

 to increase the breakage induced by irradiation if oxygen was also present, 

 although these gases had no efTect in the absence of oxygen. Haas et al.^* 

 also showed that both carbon monoxide and carbon dioxide increase the 

 effect of irradiation at slow dose rates to equal the effect at high dose rates. 

 In the fractionation experiments of Wolff and Luippold, and Cohn, it was 

 shown that respiratory inhibitors like carbon monoxide, cyanide, dinitro- 

 phcnol, and low temperatures have the opposite effect to that produced by 

 BAL or other anoxic conditions. Respiratory inhibitors extend the 'open 

 period ' of the breaks beyond normal, \vhereas BAL and anoxic conditions 

 reduce the 'open period'. They conclude from this that an active oxidative 

 metabolism, probably that providing high-energy phosphate bonds, is 

 required to get reunion of the broken ends. This oxidative metabolism is in 

 some way inhibited by irradiation, a similar effect being produced by 

 compounds which interfere with the cytochrome system. BAL and other 

 reducing agents enable a faster recovery after inhibition, or in some way 

 protect the oxidative metabolism from irradiation damage. 



It was shown by Wolff and Atwood^^ th^t the breaks which rejoin quickly 

 after irradiation are capable of uniting with breaks which remain 'open' for 

 long periods. It was also shown that there was a period of greater or less 

 duration within which there was no rejoining. On the basis of these facts 

 it was considered unlikely that qualitative differences between the breaks 

 themselves could account for the observed difference in reunion times. 

 Wolff and Luippold^^ Abrahamson"^ and Cohn^^ claim that qualitative 

 differences between the breaks do exist. The general argument put forward 

 was that certain breaks closed quickly because they were closed by ionic 

 bonds. The breaks which remained 'open' for long periods were thought to 

 be closed by covalent bonds with high-energy i-equirements. This difference 

 was not observed in fractionation experiments by Cohn^^ in which carbon 

 monoxide produced equal delay in both types of breaks. It would thus seem 

 that there is little qualitative difference between breaks which unite quickly 

 and those which unite slowly. 



The investigation of the oxygen effect under conditions of metabolic rest 

 provides another useful avenue to test the breakage-reunion hypothesis. 

 The present set of experiments was designed to exploit this method using 

 dormant seed. The results of these, and experiments using similar states of 

 rest, enable interesting comparisons to be made with the other methods of 

 modifying the normal metabolic state. 



Storage in oxygen causes chromosome breakage in dry seed {Figures 1 and 2) 

 and this breakage appears to be identical in form and distribution with that 

 induced by irradiation. These observations agree with the general results of 



204 



