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MUTATION AND PLANT BREEDING 



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LINEAR ION DENSITY, IONS/ M 



Figure 16. — The influence of ion density on chemical change and chro- 



mosome breakage. After Gray (55). 



peak of 8 to 11 between approximately 50 to 70 kev/[.i. It then falls 

 considerably and flattens out in the LET range between roughly 120 

 and 220 kev/|i. Less extensive but somewhat similar observations 

 were made by Larter and Elliott (89) who found thermal neutrons 

 more effective in producing translocations than X-rays, or beta rays 

 from S 35 and P 32 . It also has been found that the dense ionization 

 associated with thermal neutron treatment caused more breakage per 

 unit inhibition of seedling growth than did X-rays (18). 



In addition to the LET effect on RBE's for chromosome break- 

 age, there is evidence that breaks caused by high-ion density tracks, 

 such as alphas, are less likely to rejoin than those caused by less dense 

 tracks of X- or gamma rays (87). The RBE's of densely ionizing radi- 

 ation with respect to various types of mutations also have been 

 studied by several investigators. Theoretically, for point mutations 

 low-ion density should be more efficient; however, the effect depends 

 upon the kind of mutation produced (1, 13, 108, 109, 199). For 

 example, Ehrenberg and Nybom (35) found a higher percentage of 

 erectoides mutations in barley with thermal neutrons than with X 

 rays. Other studies show that radiations of different ionization den- 

 sities sometimes produce characteristically different spectrums of 



