Alan H. Haber

Dormancy Resulting from Gamma-irradiation of Lettuce Seed

SummaryLettuce seeds given 300–1000 kr of gamma-rays show inhibition of germination when subsequently put into water. That this failure to germinate was caused by seed dormancy was suggested by the fact that such irradiated seeds did germinate if treated with any of the following germination-stimulating agents: gibberellic acid, kinetin, thiourea, pricking, or light. Germination was accompanied by cellular expansions and cell divisions, irrespective of radiation and chemical treatments. The first four agents mentioned were fully effective under conditions where continuous white light was not. The chemical treatments, which reversed the gamma-radiation effects on germination, gave no reversal of any of the following gamma-radiation effects: inhibition of respiratory rates, production of chromosomal aberrations, and reduction in seedling height. Consequently, the chemical treatments that enabled gamma-irradiated seeds to germinate had radioprotective activity specific for the germination process. These resu...

Radiosensitivity and Rate of Cell Division: "Law of Bergonié and Tribondeau"

The rate of cell division decreases as tobacco leaves grow, and older leaves grow without any cell divisions. Disks were cut from leaves having differing degrees of cell division at various developmental stages. In the chlorenchy, induced susceptibility to photodestruction of chloroplasts was used to measure sensitivity to gamma radiation as a function of the rate of cell division. The same biological effect can thereby be studied both in dividing and in nondividing tissues of the same morphological and physiological cell types. The radiosensitivities were approximately the same, irrespective of the extent of cell divisions.

GAMMA-RAY-INDUCED ESR SIGNALS IN LETTUCE: EVIDENCE FOR SEED-HYDRATION- RESISTANT AND -SENSITIVE FREE RADICALS.

Abstract The effects of wetting and redrying lettuce seeds on γ-radiation-induced electron-spin-resonance (ESR) signals have been studied. Seeds were wetted and then dried at room temperature over a desiccant, followed by equilibration in an atmosphere until the water content for both the wetted-redried seeds and their unwetted controls was approximately 6–7 per cent. When γ-irradiated seeds were completely wetted and redried, most, but not all, of the radiation-induced signal was eliminated. The radiation-induced signal that persisted after wetting and redrying was approximately 8 per cent of the total radiation-induced signal in unwetted seeds. The derivative curve of the signal from un-irradiated seeds, both before and after wetting and redrying, has a peak-to-peak line width of 6·5 G. The corresponding line width of the radiation-induced signal for doses less than 1 Mrad was not detectably different from the endogenous signal width. At very high doses (5–20 Mrad), the radiation-induced, water-resistant signal was from 4 to 12 per cent wider and the radiation-induced signal in unwetted seeds was from 9 to 20 per cent wider. No difference in g values (2·006-2·007) were detected resulting either from irradiation or from wetting and redrying. Studies of power saturation, however, did indicate a difference between the radiation-induced resonances that persisted after wetting and redrying vs. the radiation-induced resonances that were present in unwetted controls. These results indicate a difference in the molecular environment of such radiation-induced unpaired electrons that persist after hydration vs. the molecular environment of radiation-induced unpaired electrons in unwetted seeds. Reirradiation of seeds that had been wetted and redried after receiving 8 Mrad was as effective in inducing resonances as was irradiation of control seeds that had not been irradiated before having been wetted and redried. This indicates that the radiation-induced resonances that persist after hydration and redrying do not influence the subsequent induction of resonances by radiation. This last finding also suggests that the radiation-induced signal that disappears on seed hydration can be reversibly eliminated by hydration and reinduced by reirradiation.

ULTRASTRUCTURE OF AUXIN-INDUCED TUMORS OF THE COLEORHIZA-EPIBLAST OF WHEAT

When wheat is germinated in high concentrations of certain auxins, the coleorhiza-epiblast grows in an excessive and disorganized manner and resembles a callus. Wheat was germinated in 10-3 M indoleacetic acid or in water. There was greater net synthesis of DNA, RNA, and protein in the tumor tissue than in control tissue. Control and tumor tissue was fixed for electron microscopy 1, 2, 3, and 4 days after sowing. In contrast to the controls, many more lipoidal bodies appeared in 1-day-old, auxin-treated tissue and thereafter diminished in number. In 2- and 3-day-old tumor tissue, small membraneous fragments were prominent but disappeared in older tissue. With time, the following changes became progressively more pronounced: cytoplasmic vacuolation, appearance of myelin figures, polyribosome configurations and extensive profiles of rough endoplasmic reticulum, and sloughing of cytoplasmic contents into and accumulation of electron-dense material in vacuoles. Possible factors in susceptibility of certain tissues to tumor formation are discussed.

Mitoses in thermodormant lettuce seeds with reference to histological location, localized expansion, and seed storage.

SummarySeeds from one lot of “New York” lettuce were sown in each of 5 successive years to compare the effect of duration of storage on the capacity for mitosis in thermodormant seeds with the effect of storage on the capacity for germination under conditions favorable for germination. Whereas the capacity for mitosis in absence of germination increased as a function of duration of seed storage, the capacity for germination itself decreased steadily over the period studied. Thus, the apparent “deterioration” of stored seed, as measured by decreased germination, does not necessarily indicate a general deterioration of all cytologic processes. Histological study of thermodormant lettuce seeds demonstrated that in some seeds mitosis and cytokinesis can occur without either overall or localized expansion of the embryo-axis. Although localized expansion occurred in some seeds, there was no expansion of the embryo-axis as a whole in any of the seeds, the localized expansion being accompanied always by localized compression elsewhere in the axis. Restraint of the embryo by the endosperm, which remained intact, could account for the prevention of overall expansion of those embryos in which localized expansion and compression occurred. Mitoses in such embryos occurred both within and outside the regions of localized expansion. Thus, even in those embryos with localized expansion, mitotic activity is not necessarily correlated with expansion. Mitoses occurred in each of the primary tissue meristems of the hypocotyl-radicle, mainly within the apical 0.5 mm of the radicle. In this regard, thermodormant embryos resemble growing roots of lettuce seedlings. These findings demonstrate that mitotic activity and localized expansion, either separately or together, can occur in the embryo without germination.

Growth of embryos on synthetic media after excision from gamma-irradiated (525 kr) and unirradiated wheat grains*

Abstract Embryos exclusive of scutella were excised from γ-irradiated (525 kr) and unirradiated wheat grains and cultured in darkness on defined media. Growth of the embryos from irradiated grain occurred without detectable cell divisions. Absence of a nitrogen source reduced growth of irradiated and unirradiated embryos to similar extents. Absence of carbohydrate completely prevented detectable growth in both irradiated and unirradiated embryos. The growth responses of the irradiated embryos to single and mixed carbohydrates were similar to the growth responses of unirradiated embryos in the following respects: (1) dependency of growth on d -glucose concentration, (2) growth on either sucrose or fructose comparable with growth on d -glucose, (3) lesser but significant growth on either d -ribose or d -xylose, (4) incapacity to grow on d -galactose, d -mannose, or l -arabinose, and (5) reduced growth after addition of either l -arabinose or d -mannose to media containing a near optimal concentration of d -glucose. Similarly, addition of d -galactose to media containing a near optimal concentration of d -glucose reduced coleoptile and root growth of both irradiated and unirradiated embryos. By contrast, however, addition of d -galactose to media containing a near optimal concentration of d -glucose greatly reduced first leaf growth of the unirradiated embryos but had little or no effect on reducing the first leaf growth of the irradiated embryos. This differential effect of d -galactose on growth of the first leaf was the only major difference in gross growth characteristics found between the irradiated embryos growing without detectable cell division and the unirradiated embryos.

Seedling irradiation of ‘gamma-plantlets’: Demonstration and explanation of a leaf growth stimulation

Abstract A dose of 500 krads of gamma rays to dry seeds (‘seed irradiation’) prevents mitosis during germination of wheat seedlings; such seedlings are called ‘gamma-plantlets’. Their growth is relatively little affected by increasing the dose of the seed irradiation beyond 500 krads. As growing seedlings, however, gamma-plantlets are sensitive to additional irradiation (‘seedling irradiation’). Leaf growth is reduced by seedling irradiation doses of 50 krads in intact plants and 25 krads in seedlings excised from the endosperm. The effectiveness of a given dose is decreased by dose fractionation. Consequently some repair mechanisms survive the heavy seed irradiation and can operate independently of processes directly or indirectly involving cell division. Growth stimulation was observed in the first leaf, but not the coleoptile, after a seedling irradiation dose of 25 krads was given to intact gamma-plantlets. Similarly there was a stimulation of second leaf growth but not of stem growth. These growth stimulations were absent when exogenous nutriments were substituted for the endosperm immediately following the seedling irradiation. Associated with the leaf growth stimulation in the intact gamma-plantlets was an accelerated breakdown of endosperm starch to soluble carbohydrate without any greater amylase activity. Comparison of gamma-plantlets from two different varieties of wheat disclosed an apparent correlation between the presence of the leaf growth stimulation and faster breakdown of endosperm starch resulting from the seedling irradiation. We suggest that the leaf growth stimulation, which in this system is not masked by certain inhibitory radiation effects (i.e. decreased mitotic activity and nuclear imbalance resulting from division of cells having chromosomal aberrations), results from the more favorable carbohydrate nutrition caused by an apparently nonspecific promotion of endosperm breakdown.

5-Fluorodeoxyuridine-thymidine interactions unrelated to DNA synthesis

Summary Thymidine reduces the uptake of 5-fluorodeoxyuridine into germinating wheat seedlings. Thymidine also reverses fluorodeoxyuridine-induced growth inhibition that is unrelated to any DNA synthesis. Thus, thymidine reversal of fluorodeoxyuridine-induced growth inhibition may not prove conclusively that the growth inhibition resulted entirely from inhibition of DNA synthesis.

Cell Division, Development, and Radiation Injury

This paper deals with two interrelated ideas: (a) the relation of cell division to radiation injury and death, and (b) the consequent usefulness of certain irradiated systems for studying the role of cell division in development.

Cytokinins and mitotic inhibition in “gamma-plantlets”

After irradiation of dry grains with 500 krads of gamma rays, the cytokinin content of wheat seedlings germinating without cell division rose from a barely detectable quantity in the dry grains to an amount similar to that in seedlings from unirradiated controls. The chromatographic mobility of the cytokinins from the two sources suggests that they are identical. Treatment of the “gamma-plantlets” with cytokinins and several combinations of other growth substances failed to induce cell division. It is concluded that accumulation of extractable cytokinins can be uncoupled from cell division and that failure of cell division in gamma-plantlets is not due to a gross deficiency of extractable cytokinins.