CYTOLOGICAL EFFECT OF CHRONIC 



GAMMA IRRADIATION AND THE 



PROTECTIVE PROPERTY OF CERTAIN 



CHEMICALS AGAINST THE RADIATION 



INDUCED CHROMOSOME ABERRATIONS 



Knut Mikaelsen 



Institute of Genetics and Plant Breeding, Agricultural College of Norway, Vollebekk, 



Norway 



Most radiation effects are studied after acute radiation treatments over a 

 short period of time. Effects of continuous or chronic radiation treatments 

 over longer periods of time have been httle investigated until recent years. 

 Some results on effects of chronic gamma irradiation have been reported from 

 gamma field radiation or other use of ^"Co^--. At the Agricultural College 

 of Norway we had a ^^-Ir isotope at work in 1953 and from 1954 a ^''Co 

 source was operating in a gamma field. Some cytological results of chronic 

 radiations and the protective property of certain chemicals against chromo- 

 some aberrations induced by irradiation will be presented. 



Cuttings of Tradescantia paludosa were found most suitable for this study and 

 were used as experimental material. Root development was initiated by 

 placing the cuttings in tap water with continual aeration at ordinary green- 

 house temperature. 



When the cuttings showed good development of primary roots, they were 

 transferred to specially designed Incite vessels which contained Hoagland 

 and Snyder's^ nutrient solution. After 24 hours a fresh nutrient solution was 

 substituted for the old and the plants were exposed to chronic gamma radia- 

 tion from ^^'Co. The roots were fixed immediately following the exposure 

 period. 



Two different experiments were performed. In the first experiment the 

 plants were irradiated at dose rate of 12-5, 25, 50 and lOOr/day. In 

 another experiment the same dose rates were applied during a 48-hr expos- 

 ure ; thus the roots received twice the total dose as in the previous experi- 

 ment. Both acentric fragments and bridges were scored at anaphase, and 

 the results are summarized in Table I. 



Both fragment and bridge frequencies seem to be almost linearly pro- 

 portional to dose. It is noteworthy, that the bridge frequencies are so low 

 after chronic irradiation. The ratios between the frequencies of fragments 

 and bridges is approximately 6 in all series, while after acute exposures to 

 X-rays and neutrons the formation of bridges is much more frequent. Thus, 

 restitutions resulting in chromatid exchanges seem to be difficult during the 

 chronic radiation exposures. 



By comparing the 24-hr and 48-hr exposure experiments, it seems obvious 

 that the dose rate has been a more important factor in the yield of aberrations 



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