FURTHER STUDIES OF GAMMA‐IRRADIATED WHEAT AND THEIR RELEVANCE TO USE OF MITOTIC INHIBITION FOR DEVELOPMENTAL STUDIES

Abstract

After gamma irradiation of wheat grain, embryos can germinate and grow without detectable mitotic figures and without increases in tissue‐cell numbers. Roots growing without DNA synthesis after irradiation show the 2 striking examples of normal nuclear behavior on the anatomic level that occur in unirradiated roots: (1) trichoblast nuclei migrate into the root hairs and change shape; and (2) nuclei in sieve‐tube elements and vessel elements disappear. Nuclei that do not synthesize DNA do synthesize RNA, some of which moves into the cytoplasm. Therefore, the irradiation that prevents DNA synthesis and mitosis has not completely inactivated all nuclear functions. On a purely descriptive level, the irradiation that prevents cell division seems to prevent the initiation at the apical “meristem” of immature vascular elements but does not prevent maturation in those vascular elements detectable before irradiation. Although the ultimate extent of growth is reduced in those structures which suffer radiation‐induced mitotic inhibition, it is not reduced in those structures (epiblast, coleorhiza, root hairs) which develop without cell division in unirradiated controls and, therefore, have no mitosis for the radiation to inhibit. This established generalization leads to the inference that the smaller sizes attained by gamma‐plantlet roots and leaves may be attributed largely or almost entirely to the radiation‐induced mitotic inhibition rather than to other actions of radiation. Irrespective of the validity of this last inference, the extent of growth theoretically possible without cell division is at least as great as the observed growth of gamma‐plantlets under given experimental conditions. Also reported are variations in radiation sensitivity among different batches of grain, among different organs within the same embryo, and among different cell types within the same organ. These variations are discussed with reference to suggestions and precautions concerning the use of radiation‐induced mitotic inhibition for developmental studies.