W. D. JACKSON 



(2) storage in various oxygen-nitrogen mixtures sliows that oxygen causes 

 breakage in dry seed as it does in pollen. Breakage increases exponentially 

 with oxygen concentration. A dose of 1 -5 kr of soft X irradiation given in 

 the absence of oxygen produces approximately the same damage as 14 days' 

 storage in pure oxygen, 



(3) the effect of 1 -2 kr of X irradiation on seed pre-stored for 14 days in 

 various oxygen-nitrogen mixtures, and also that stored for an additional 

 14 days' post-storage in these mixtures, has been partitioned into a direct 

 irradiation component, a direct oxygen component, an oxygen-irradiation 

 component, and an oxygen irradiation post-storage component. The inter- 

 action-components account for more than t\vo-thirds of the damage, 



(4) breakage continues during the post-storage period if oxygen is present. 

 Other experiments showed that the total number of deletions rises rapidly 

 in the early post-storage period and approaches a constant value after about 

 10 days' post-storage in air. In oxygen the breakage continues at an increased 

 rate. Substances which can be leached from both old seed and from fresh 

 seed previously irradiated in air, can cause a significant increase in chromo- 

 some breakage in seed which has previously been given a small sensitizing 

 dose of irradiation. The effect of oxygen in increasing radiation damage is 

 thought to arise from extended breakage or opening of 'latent' breaks by 

 the combined presence of oxidative mutagens with long mean free paths. 



Aly original experiments on post-storage effects were extended by Air. R. Crowden 

 as an honours project under my supervision. His results are incorporated in his honours 

 thesis [University of Tasmania, 1954). I have availed myself of this work and have 

 incorporated his results with those from my later experiments. I wish to acknowledge 

 the use of this data and to thank Professor Barber for his helpful suggestions with the 

 rn^nuscript. references 



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207 



