Effects of X-ray dose fractionations with various intervals in inducing somatic mutations in the stamen hairs of Tradescantia clone KU 9

The effects of X-ray dose fractionations in inducing somatic mutations were studied in the stamen hairs of Tradescantia clone KU 9 heterozygous for flower color (blue/pink). Young inflorescence-bearing shoots with roots were prepared, and were exposed to X rays acutely. A dose-response regression line with a slope of 1.454 on a log-log graph was obtained for single acute X-ray doses of 0.255 to 1.03 Gy, and it showed that somatic mutation frequency increased curvilinearly with increasing X-ray dose. When 1.00 to 1.15 Gy of X rays were fractionated into two acute doses of about halves given with intervals of 5 to 120 min, decreases in induced mutation frequency were observed. The mutation frequencies induced by the fractionated doses with intervals of 5 and 10 min were not significantly different from those expected for the total single doses. However, the mutation frequency decreased significantly at 5% level with 20and 30-min intervals, and decreases were highly significant at 0.1% level when the interval was prolonged to 40 to 120 min. The results obtained indicate that the interaction between the first and second doses began to reduce between 10 and 20 min later, and disappeared by 60 min later. That is, the DNA and/or chromosomal breaks induced by the first dose began to be rejoined (repaired) or healed between 10 and 20 min later, and all of them were rejoined or healed by 60 min later, losing their abilities to interact with the DNA and/or chromosomal breaks induced by the second dose.