THE LIFE-SPAN OF MUTAGENS PRODUCED IN C:ELLS 15^ IRRADIATION 



\\as about thai picjtlucctl 1)\ Icachatcs from irradiated fresh seed. When 

 irradiated aged seed was used to obtain the leachate, breakage in the test 

 seed increased approximately three-fold over the unirradiated aged seed 



p*c 



ice 



"W" 



t 



11 



?0 © O CteOOOoOp»^ 



s-c 



Donor 

 material leached 



No 

 irradiation 



Irradiated 

 4 kR X; rays 

 in air 



None 



Fresh seed Ayr. old seed 



Figure 9. The effect of leachates from aged and fresh seed, irradiated and non- 

 irradiated, on the chromosome breakage induced in fresh test seed by a small 



sensitizing dose of irradiation 



trials. Because of the large variance the probability of obtaining this 

 difference by chance was 0-01. 



The spontaneous breakage in aged seed used in the leaching experiment 

 measured in a pooled sample from 10 root tips was 0-57 acentric chromo- 

 some fragments per cell. The agent producing chromosome breakage 

 appears to be quite independent of the germination inhibitor. There is no 

 significant difference in germination of seed soaked in leachates from irra- 

 diated and unirradiated donor seeds, although there is a significant increase 

 in the germination of seed soaked in leachate from old seed, as compared 

 with leachates from fresh seed. Thus although old seed contributes com- 

 pounds causing increased breakage, it apparently contributes less germin- 

 ation inhibitor. The effect of irradiation would seem to be merely additive 

 to the effects of ageing. There was no significant change in the type of 

 aberrations produced or in the 'stickiness' of the chromosomes. A slightly 

 increased frequency of metaphases with undercondensed nucleic acid was 

 noticed in treatments with leachates from aged seed. 



DISCUSSION OF RESULTS 



The early conceptions of radiation damage to chromosomes were based 

 largely on the physical principles^ Final damage was considered to result 

 from the separate events of breakage and reunion of the chromosome threads. 



200 



