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Two‐hour treatments with 5 times 10‐3 M caffeine strongly enhanced the frequencies of chromosomal aberrations obtained in root tips of Vicia faba 5 to 6 hours after irradiation with X‐rays. The caffeine treatments were effective only when given after X‐irradiation and when the time between X‐rays and caffeine was less than 60 minutes. In contrast to the frequency of aberrations produced by caffeine alone, which increased when the temperature was lowered, that produced by the combined X‐ray‐caffeine treatments was higher at 27°C than at 15°C. The aberration frequency produced by X‐rays in combination with caffeine was not markedly influenced by sodium azide, an inhibitor of oxidative phosphorylation. A comparison was made between the effects of caffeine, theophylline and 1,3,7,9‐tetramethyluric acid alone and in combination with X‐rays In unirradiated root‐tip cells, 1,3,7,9‐tetramethyluric acid produced the highest frequency of aberrations, whereas theophylline was practically inactive. When given as post‐treatments to X‐rays, caffeine produced the largest and 1,3,7,9‐tetramethyluric acid the smallest increase in the frequency of chromosomal aberrations.

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