STUDIES ON THE PRODUCTION OF BUD SPORTS IN TULIPS BY IONIZING RADIATION

V. EFFECTS OF TOTAL DOSE, FRACTIONATION AND TEMPERATURE ON SOMATIC MUTATION

Abstract

The tulip flower develops inside the bulb during storage, and the color spots appearing on the perianth were induced by gamma-irradiation. If irradiation was carried out in the critical period shortly before the completion of perianth formation, the color spots appeared at random in single or a few epithelial cells. As previously reported, the extent of mutation was reduced at low intensity as compared with high intensity exposure. Therofore, the spots are considered to be due to somatic mutation of epithelial cell being caused by chromosomal deletion through irradiation.
In the present paper, in order to elucidate the nature of the genetic change, i, e., whether genic or chromosomal, quantitative studies of total dose, fractionation and temperature effects on the somatic mutation have been studied. The results are summarized as follows:
1) The data refer to the number of mutant cells per unit area. Therefore, growth inhibition of the perianth was undesirable. But measurements of the size of perianth showed very rarely inhibition after irradiation by 200r in total dose in October.
2) Cell division of the epithelium at the time of irradiation, in October and November, was estimated from the average number of mutant cells per spot, which was 1.13 and 1.02 at 200r, respectively. These figures indicate that the epithelial cells of the perianth may not undergo division after November.
3) Frequency of color spot occurrence after single acute exposure was examined at doses of 25 to 200r at 600r/h. The dose response curve is rather exponential.
4) A total dose of 100 or 200r was fractionated into 2 or 4 doses of 50r and 100r, respectively, and with intervals of 30 to 1440 minutes and the effects were compared with that of single dose. No reduction of mutation was observed when the interval lasted 30 minutes, while in the case of fractionation into 4 doses at the same interval, the number of mutations was reduced as compared with a single dose. When the interval lasted 1440 minutes at the total dose of 100r and 200r, the number of mutation was markedly reduced to that obtained at non-fractionated dose.
5) The number of mutations increased at low temperature (-2° to 2°C) during irradiation as compared with that obtained at 12.5°C. These results show that chromosome reunion after deletion may be involved in the color spot formation induced by gamma-rays.