Lloyd A. Schairer

Some factors affecting the responses of plants to acute and chronic radiation exposures

Abstract The radiobiological responses of a large number of species of higher plants have been studied after acute or chronic irradiation of growing plants mostly with γ radiation from Co60. Some acute treatments were made with X-rays. The radiation responses studied were of four main types: chromosome breakage, somatic mutation, growth inhibition and lethality. Somatic mutations in petals of several species of plants heterozygous for flower colour show responses most readily explicable on the basis that they result from deletion of the dominant locus, i.e., in most respects they behave as chromosome breakage events and not as point mutations. The yield of somatic mutations is reduced by dose fractionation of acute exposures or, within limits, by reduced dose rates with chronic exposures. The frequency per r of somatic mutations and of chromosome deletions produced under chronic irradiation is higher during periods of slow growth than during periods of fast growth. Definite evidence of long-term cumulative dosage effects was described for Pinus and Taxus. Over a long period (several years) at dose rates as low as 5 r/day, cumulative effects are expressed as severe morphological deformity, severe growth inhibition, or even death. A correlation was shown between the acute and daily chronic doses necessary to produce severe growth inhibition in young plants of several different species. To produce equivalent effects the acute dose required averages about thirteen times the daily chronic dose. In diploid species a clear relationship was shown between the average nuclear volume of apical meristem cells and tolerance to chronic gamma radiation. The larger the nuclear volume the greater the sensitivity of the nucleus and ultimately of the whole plant. The frequency of somatic mutation per r and nuclear size were shown to be directly related. Preliminary data also indicate a similar relationship between nuclear volume and frequency of chromosome aberration and for three species between average amount of DNA per nucleus and radiosensitivity (the more DNA the greater the sensitivity). Two polyploid series (Chrysanthemum and Sedum) showed increasing resistance as the degree of polyploidy increased. Preliminary data suggest that increasing chromosome number without known polyploidy and without a changing nuclear volume has a protective effect. A theoretical model was presented describing a possible explanation for the protective effect. The protection offered by doubling the chromosome number without polyploidy may be greater than that afforded by polyploidy at a doubling level. Some theoretical aspects relating to the interrelationship of certain major factors determining radiosensitivity are presented. Analysis of the relationship between sensitivity of different species and nuclear volume indicates that a constant (or nearly constant) number of ionizations per nucleus is required to produce growth inhibition or death. This relationship supports the concept that a cluster effect such as chromosome breakage (deletion) may be the critical event or events. Differential sensitivity results in part from the different doses required to produce a unit number of breaks in nuclei of different average volumes. The protective effect associated with increasing chromosome number or polyploidy is assumed to result from the reduced seriousness of an average break or deletion as the chromosome number increases. Clarification of the manner in which radiosensitivity is determined in different species should make predictions of radiosensitivities possible in the very near future. This should hold for animal species as well as for plants. Extrapolation of certain kinds of radiobiological data from one species to another can now be made on a reasonably sound basis.

Relationship between Nuclear Volumes, Chromosome Numbers, and Relative Radiosensitivities.

An inverse relationship between a volume estimated to be associated with interphase chromosomes and acute lethal exposure to x-or gamma radiation has been found in 16 plant species. The apparent differences in radiosensitivities found would seem spurious, since the estimated average energy absorbed in the nucleus per chromosome (3.6 x 106 ev) approaches a constant (variation less than fourfold) in spite of wide ranges of lethal exposures (0.6 to 75 kr), of nuclear volumes (43 to 1758 �3), and of somatic chromosome numbers (6 to 136). The regression line obtained can be used to predict the radiosensitivities of other plant species if their nuclear volumes and chromosome numbers are known.

Comparative effects of ionizing radiation and two gaseous chemical mutagens on somatic mutation induction in one mutable and two non-mutable clones of tradescantia

Abstract The X-ray dose responses of mutable clone 0106 of Tradescantia (mutable for blue to pink), and its parent clone 02 have been determined for pink and colorless mutations in stamen hair cells, and are compared to the previously determined X-ray response for pink mutations of a third unrelated clone, clone 4430 (hybrid of T. subacaulis and T. hirsutiflora). X-ray response curves are compared to the response curves of the same three clones after exposure to the gaseous phase of the alkylating agent ethyl methanesulfonate (EMS) and the fumigant and gasoline additive 1,2-dibromoethane (DBE). X-irradiation induces a pink mutation rate in mutable clone 0106 that is significantly higher than that of the nearly identical pink mutation rates in clones 02 and 4430. However, the colorless mutation rates of clones 02 and 0106 are not significantly different from one another. In clones 02 and 0106, pink mutations occur more frequently than colorless mutations at lower doses, but colorless dose—response curves saturate at higher doses than do those for pink mutations. Exposure—response curves for EMS and DBE have characteristics similar to those of X-ray response curves: exponential rise followed by an area of saturation. However, it was found that the relative sensitivities of the three clones to the gaseous mutagens and to ionizing radiation do not parallel one another. Where clones 02 and 4430 are equally sensitive to X-rays, at equal mutagen concentration clone 4430 is 6–7 times more sensitive to EMS and 7–9 times more sensitive to DBE than is clone 02. Mutable clone 0106 shows intermediate sensitivities to both EMS and DBE.

The radiosensitivity of gymnosperms. I. The effect of dormancy on the response of Pinus strobus seedlings to acute gamma irradiation

Abstract Seedlings of Pinus strobus (white pine) were exposed for 16.5 hr to Co60 gamma radiation at dosages from 50 to 1000 r during a period of dormancy or a period of active growth. The effect on growth was determined by measurement of needle lengths. The data show the dormant plants to be considerably more resistant than the actively growing plants. Measurements of interphase nuclear volumes in apical shoot meristems made at various times during the year show a large volume during stages of active growth and much smaller volumes during dormancy. The data strongly suggest that the differences in sensitivity of dormant and actively growing plants may be largely the result of differences in nuclear volumes of these stages.

THE RELATIVE BIOLOGICAL EFFICIENCY OF GAMMA RAYS AND FISSION NEUTRONS IN PLANT SPECIES WITH DIFFERENT NUCLEAR AND CHROMOSOME VOLUMES.

Young seedlings of five plant species (Vicia faba, Pisum sativum, Helianthus annuus, Raphanus sativus, Arabidopsis thaliana) with widely different nuclear and interphase chromosome volumes (ICV) were acutely irradiated with various doses of60 Co gamma rays and fission neutrons. Meristematic shoot apices of control plants were fixed at the time of irradiation, and interphase nuclear and chromosome volumes were calculated from measurements of nuclear diameters. The relation between nuclear or chromosome volume and radiosensitivity as well as the possible influence of differences in these volumes on RBE has been studied. Radiation injury was evaluated on the basis of plant survival, plant dry weight, plant height, and pollen abortion. Dose-effect curves were obtained from the two kinds of radiations; the doses necessary to produce a 50% effect were calculated for each species and each end point used. The doses required for a 50% effect were highest for survival, lowest (and about the same) for dry weight and...

Influence of temperature on spontaneous and radiation-induced somatic mutations in tradescantia stamen hairs

Abstract Three diploid clones of Tradescantia which are heterozygous or hemizygous for flower color, and which exhibit a 35-fold range in spontaneous mutation frequency, have been compared in their response to increased temperatures. The endpoint studied is the expression of pink and colorless somatic mutations, which arise in stamen hair cells spontaneously and following exposure to ionizing radiation and chemical mutagens. Increased temperature (26.5 or 28° vs. 16 or 19°) significantly increased the spontaneous pink mutation frequency in stamen hair cells of all three clones. Colorless mutations in clone 0106 were not increased significantly at the higher temperature. The influence of increased temperature on radiation-induced pink mutation frequency was examined in clone 02. Temperatures in the range of 16–28° did not influence the magnitude of mutation response after exposure to 100 rad of X-rays; however, the timing of the mutation response was altered both in time of onset and in duration.

Monitoring Ambient Air for Mutagenicity Using the Higher Plant Tradescantia

Final assessment of human health effects resulting from exposure to harmful environmental agents may rest with mammalian test system results. In vitro systems are short-term assays used most frequently for extrapolation to humans. However, no single assay system is adequate and the more expensive long-term tests must be augmented by multiple assays designed for redundancy or to fill gaps in present state of the art of environmental monitoring. The Tradescantia stamen hair test system is one such assay offering redundancy as well as filling the gap of monitoring ambient air for mutagenic agents. The flower color locus in heterozygous clones of Tradescantia mutates when exposed to such agents as fumigants, solvents, chemical additives or catalysts, and compounds requiring activation such as benzo (a) pyrene. The stamen hair system responds to low levels of airborne compounds. The Tradescantia stamen hair system was used as an in situ monitor for mutagens in ambient air in polluted industrial sites. Preliminary results from many sites showed a significant increase in mutation rate. The environment most consistently mutagenic was that downwind from petroleum refineries. No specific compounds or groups of compounds have as yet been correlated with the positive sites. (ERB)

A Search for the Identity of Genotoxic Agents in the Ambient Air using the Tradescantia Bioassay

The theme of the conference on “Application of Short-Term Bioassays in the Analysis of Complex Environmental Mixtures” concerned the state of the art of bioassay systems from cultured microbes to laboratory animals and their application to the assessment of human health effects of airborne environmental contaminants. The major emphasis for short-term bioassays has been placed on bacterial and mammalian cell lines. However, for increased perspective on the state of the art of specific in vitro assays, it is important to consider the environmental impact on whole organisms by reviewing the contributions made by in vivo assays. The more classical nonmammallian in vivo systems such as Drosophila, Zea mays, and Tradescantia are characterized by well-defined genetic bases, versatility in mode of treatment, relatively low cost, short term, and/or high sensitivity to both physical (radiation) and chemical mutagens (Underbrink et al., 1973; Vogel and Sobels, 1976; Nix and Brewen, 1978; Plewa, 1978; Schairer et al., 1978a).

The effects of space flight factors and gamma radiation on flower production and microspore division and development in tradescantia

Abstract The interaction between 85 Sr gamma radiation (average exposure approximately 220 R) and space flight factors (vibration, acceleration, weightlessness, etc.) on the number of flowers produced and on microspore division and development was studied in Biosatellite II. In the actual flight one package of 32 young Tradescantia clone 02 plants was irradiated during the two-day flight, and another package was used as an unirradiated flight control. Two similar packages, one of which was irradiated, were maintained as non-flight controls in a similar spacecraft at the launch site. Various postflight experiments were conducted in an attempt to attribute the effects observed in the orbited plants to specific flight factors. In these postflight ground tests plants were grown simultaneously in both the flight and non-flight spacecraft under conditions similar to those recorded during the actual flight. In the clinostat and additional vibration experiments individual packages rather than the whole spacecraft were used. After each treatment, open flowers were collected for at least 26 days. Some of the morphological and cytological effects observed in the flight material can be attributed to low gravity (weightlessness) and others to the internal environment of the spacecraft. Those effects attributed to weightlessness include (1) an increase in number of flowers produced in both orbited samples compared with their non-orbited controls and (2) various kinds of mitotic abnormalities associated with altered spindle behavior in microspores. Effects attributed to spacecraft environmental factors such as the increased concentration of ethylene measured in the flight vehicle include (1) an increase in the mortality rate of microspores, (2) an increase in the frequency of developmental abnormalities indicated by a change in the number and shape of the nuclei in young pollen, and (3) an increase in flower bud blasting which led to a change in the pattern of flower production.

THE EFFECT OF CHRONIC GAMMA IRRADIATION ON APICAL DOMINANCE OF TREES

Abstract Cherry and apple trees subjected to low doses of gamma irradiation (average of 16 and 34 r/day) for ten years produced atypical branch development and reversal of dominance which could be attributed to the decrease or inactivation of auxin resulting from the irradiation.