The DNA Content of Nuclei in the Embryonic Root Apices of Dry Seeds ofAllium Cepaand their Radiation Response

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Abstract
DNA cytophotometry showed the contemporary occurrence of nuclei with 2C and nuclei with 4C DNA content in the radicle meristems of dry seeds of two varieties ot Allium cepa; in some meristems all nuclei measured were 2C. When seeds were germinated in 3H-thymidine, very few cells (0,18%) were found to enter mitosis unlabelled, thus demonstrating that most of 4C nuclei in radicle meristems were not in the G2 phase, but belonged to cells of the differentiated line. When seeds were irradiated in the dry state, the first post-irradiation mitoses to appear showed almost exclusively chromosome type aberration. Only at the following collection times a certain amount of anaphases with chromatid type aberration (B') (up to 50% in the first mitotic cycle) was seen. As the cell population responding to X-rays with B' did not appear in the first germination period, it is presumed that the chromatid response does not represent the response of the late G1 period. It seems, therefore, very probable that the chromatid type damage is induced by an indirect effect of X-rays. This secondary effect is clearly overlapping with the chromosome response of the G1 period.