THE IMMEDIATE EFFECTS OF LOW DOSES OF X-RADIATION ON THE FREQUENCY OF SEVERAL MITOTIC STAGES IN THE ALLIUM ROOT TIP

¹ The Department of Zoology and Entomology, The University of Tennessee, Knoxville, Tennessee

1. Root tip cells of germinated Allium seeds were given doses of 128 or 512 r of x-rays. The mitotic effects of this treatment were determined by making counts, in fixed preparations, of cells in several mitotic stages at regular intervals following x-raying.

2. After treatment with 128 r of x-rays virtual disappearance of cells in late prophase, prometaphase, metaphase, anaphase and telophase was observed between three and four hours. No observable change occurred in the interphase frequencies, but an increase was observed in early prophase one and one-half hours subsequent to treatment.

3. After receiving 512 r of x-radiation, late prophase, prometaphase, metaphase, anaphase and telophase cells fell to minimal frequency between one and one-half hours and three hours. The fall in frequencies after this dose was noticeably more rapid than following the 128 r dose. An increase was noted in interphase frequency two hours following treatment and in early prophase three hours following treatment. Again, late prophase frequency did not reach zero, but was at its lowest point five hours subsequent to treatment.

4. Minimal mitotic activity in the root tip cells of Allium cepa is reached between three and four hours subsequent to treatment with 128 r of x-rays, and between two and one-half and three hours following treatment with 512 r of x-rays. Between treatment and fixation all root tips were maintained at 32° C.

5. Reversion of cells from late prophase to early prophase is indicated when the cells are treated with 128 or 512 r of x-radiation and from early prophase to interphase when the cells are treated with 512 r of x-rays. It is probably due to this latter reversion that the minimal mitotic rate was reached in less time by the cells treated with the larger dose of x-rays.

6. Late prophase, which includes the latter portion of middle prophase of certain other investigators, seems to be the most sensitive and early prophase the second most sensitive stage to the mitosis-inhibiting effect of x-radiation.