Nuclear and Cell Division in Allium cepa as Influenced by Slow Neutrons and X-Rays

Abstract

1. Bulbs of Allium cepa, variety Ebenezer, were exposed to X-ray dosages of 100 r, 1000 r, or 10,000 r, or to show neutrons produced in the Argonne heavywater pile. The neutron-treated material was exposed to an average neutron flux of 5.8 x 1010 slow neutrons/sq cm/sec for periods of ~2, 4, 8, or 16 minutes. 2. Plants from treated bulbs in both series were markedly retarded in growth with respect to control plants. All plants from bulbs treated with slow neutrons died as did also those treated with the highest dosage of X-rays. 3. Bulbs from control and treated series were sprouted in soil. Cytological analysis of root tips collected at 48-, 52-, and 96-hour intervals showed that the effects of both types of treatment followed a pattern that was chronologically determined: a general, but temporary, inhibition of mitosis; abnormal nuclear division necessitated by structural chromosome aberrations; abnormal cell division directly following aberrant nuclear division; and, lastly, degeneration of nuclei and cells which had been severely damaged either by the original treatment or by disorientation in normal call processes during the preceding stages. 4. Chromosomal aberrations of all types-dicentrics, centric and acentric rings, acentric fragments, and dot deletions-were observed in root-tip cells from both treatments. Dosage had no apparent effect on type of aberrancy induced. Pycnosis of many nuclei at the higher dosages was of common occurrence. 5. Aberration frequency induced by 2-, 4-, or 8-minute exposures to slow neutrons was higher in all cases than that produced by either 100 r or 1000 r. Frequencies in the 10,000-r series approximated those found for the 16-minute dosage with slow neutrons. 6. The degree of fragmentation produced at the higher treatment levels by both X-rays and slow neutrons gave a more accurate picture of the shattering effect on chromosomes than did the extent of formation of dicentrics and rings. 7. The observation of pseudo-amitotic nuclei and daughter nuclei accompanied by micronuclei does not constitute evidence of induced amitosis as a result of treatment. These are stages in a time continuum of the general effects of radiation on chromosome breakage, abnormal nuclear division, and subsequent abnormal cell division. 8. The marked inhibition of mitosis found in the present study and the low dosage-curve exponents obtained for aberration frequency indicate that with regard to quantitative determination of the relationship between excessive treatment with slow neutrons and response of the organism, aberration frequency alone does not yield a wholly representative or valid index of the true picture of dosage effect.