Dependence of the mutation spectrum in a shuttle plasmid replicated in human lymphoblasts on dose of gamma radiation.

The frequencies and types of mutations induced in the target gene, supF-tRNA, of the shuttle vector pZ189 were analysed following the replication of the gamma-irradiated plasmid in the human lymphoblastoid cell line, GM606. The mutation frequency measured in progeny of unirradiated pZ189 was 1.02 x 10(-4), increasing to 17.5 x 10(-4) at 1000 cGy, and to 63.4 x 10(-4) at 5000 cGy, approximately 17- and 62-fold over background levels, respectively. Simultaneously, the number of plasmids capable of replicating in Escherichia coli decreased with increasing radiation dose to 4% of the control value at 5000 cGy. Electrophoresis of the irradiated DNA showed a correlation between increases in mutation frequency and decreases in plasmid survival, and the formation of open-circular and linear DNA. The majority of the spontaneous (69.8%) and induced mutations (85.7%) at 1000 and 79.4% at 5000 cGy) were base substitutions and were generally of similar types among all groups. However, changes at 2500 (12.7%) and 5000 cGy (13.2%) involving A:T base pairs were greater than those in unirradiated controls (3.4%) or those at 1000 cGy (2.0%). This increase in A:T base pair mutations could be a result of reduced repair fidelity when the DNA is extensively damaged by high doses of ionizing radiation.     

Reactions of OH Radicals with Poly(U) in Deoxygenated Solutions: Sites of OH Radical Attack and the Kinetics of Base Release

Summary

Pulse radiolysis of N₂O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2·9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t_½ < 4 min at 0°C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50°C the half lives are 83 min and 26 h, respectively (E_a (fast) = 68 kJ mol^?1, E_a (slow) = 89 kJ mol^?1, A (fast) = 1·5 × 10⁷ s^?1, A (slow) = 1·9 × 10⁹ s^?1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.     

Lack of Dose Rate Modification (0·0049 vs. 0·12 Gy/min) of Fission-neutron-induced Neoplastic Transformation in C3H/10T½ Cells

Summary

Clonogenic survival and neoplastic transformation of asynchronous cultures of C3H/10T½ cells were used to assay the effect of dose protraction of reactor-produced fission neutrons. Cells were exposed to eight neutron doses ranging from 0·05 to 0·9 Gy delivered at 11·7 or at 0·49 cGy/min. For each dose level, high and low dose rate irradiations were performed on the same day. At each dose a similar effectiveness of fission neutron irradiation at high or low dose rates was measured for both cell survival and transformation. The combined high and low dose-rate data were analysed by two- or three-parameter models. Depending on the model used, values of the effectiveness per unit dose derived as parameters of linear terms of the respective dose-response curves were 0·9–1·2 Gy^?1 for clonogenic survival and 5–8 × 10^?4 Gy^?1 for neoplastic transformation. It is concluded that the modification of fission neutron dose-response curves by dose rate is negligible or absent in the range of doses and dose rates examined, in contrast to results with other sources of fission or fast neutrons.     

Lethal and Mutagenic Effects of 252Cf Radiation in Cultured Human Cells

Summary

HeLa MR cells were exposed to radiation emitted from a man-made spontaneously fissioning isotope, californium-252. The neutron to gamma-ray ratio in the radiation dose was measured to be 2·0. The extrapolation number of the dose-survival curve was 1·3 and the Do was 200 cGy. A dose-dependent increase in mutation to 6-TG^r (6-thioguanine resistant) was observed. The relative biological effectiveness (r.b.e.) for cell killing of the neutrons from ²⁵²Cf, calculated relative to high-dose-rate X-rays, was 2·6 at 50 per cent survival. The r.b.e. for mutation induction was 2·7 at a mutation frequency of 5 × 10^?5 per surviving cell.     

The Radiolysis of Dilute Solutions of Lysozyme

Summary

Pulse radiolysis was used for the study of transients formed from lysozyme solutions. Reactions of these transients with cysteine and oxygen were analysed kinetically. The rate constant for the reaction of lysozyme with e^?_aq was 5·2 × 10¹⁰ M^?1 sec^?1 and for lysozyme with OH was 3·1 × 10¹⁰ M^?1 sec^?1. For the reaction of the lysozyme transient (λ_max 400 mμ) with cysteine and oxygen, the rate constants were 4·1 × 10⁶ M^?1 sec^?1 and 3·38 × 10⁸ M^?1 sec^?1 respectively. The corresponding figures for the lysozyme transient (λ_max 310 mμ) were 4·6 × 10⁸ M^?1 sec^?1 and 1·6 × 10⁸ M^?1 sec^?1. These data are correlated with those obtained from steady-state radiolysis and elucidate an aspect of the known radiobiological oxygen effect.     

Mutation Induction by 125iodoacetylproflavine,a DNA-intercalating Agent,in Human Cells

Summary

Survival and the induction of mutations at the hprt and tk loci were measured in TK6 human lymphoblastoid cells following treatment with the DNA-intercalating agent ¹²⁵iodoacetylproflavine (¹²⁵IAP). ¹²⁵IAP was readily taken up into the cells, was localized to the nucleus, and was released rapidly following resuspension of the cells in fresh medium. Treatment with ¹²⁵IAP for 24 h yielded a D₀ of 110 decays/cell and an induced mutant fraction of 0·13 × 10^?6 per decay at the hprt locus and 0·4 × 10^?6 per decay at the tk locus. Molecular analyses of ¹²⁵IAP-induced hprt mutants by Southern blot revealed a high proportion of large-scale changes at this locus. When these results are compared with those observed with ¹²⁵IdUrd, ¹²⁵IAP shows a reduced effectiveness per decay, related perhaps to the non-covalent nature of intercalator binding, resulting in reduced energy deposition in the DNA.     

The influence of the b‐value combination on apparent diffusion coefficient based differentiation between malignant and benign tissue in cervical cancer

Purpose:

To analyze the influence of different b‐value combinations on apparent diffusion coefficient (ADC)‐based differentiation of known malignant and benign tissue in cervical cancer patients.

Materials and Methods:

A total of 35 patients with stage IB1, IB2, IIA cervical cancer underwent a 3.0T MRI scan prior to radical hysterectomy and pelvic lymph node dissection. Conventional T1‐ and T2‐weighted sequences and a diffusion‐weighted sequence (b = 0, 150, 500, 1000 seconds/mm²) were performed. Regions‐of‐interest (ROI) were drawn on ADC maps derived from five different b‐value combinations (0, 500; 0, 150, 500; 0, 1000; 0, 150, 500, 1000; 150, 500, 1000 seconds/mm²). The influence of the b‐value combination on ADC‐based differentiation of benign and malignant tissue was analyzed using receiver‐operating‐characteristics curves.

Results:

For all b‐value combinations, ADCs were significantly lower (P < 0.001) in cervical malignancies (1.15 ± 0.21·10^?3; 1.10 ± 0.21·10^?3; 0.97 ± 0.18·10^?3; 0.97 ± 0.23·10^?3 and 0.85 ± 0.18·10^?3 mm²/second respectively to the aforementioned b‐value combinations) than in benign cervix (2.08 ± 0.31·10^?3; 2.00 ± 0.29·10^?3; 1.62 ± 0.23·10^?3; 1.54 ± 0.21·10^?3 and 1.42 ± 0.22·10^?3 mm²/second respectively). The diagnostic accuracy was high for all b‐value combinations and without statistical differences between the combinations.

Conclusion:

ADC‐based differentiation of benign from malignant cervical tissue is independent of the tested b‐value combinations. The results support the inclusion and possible pooling of studies using different b‐value combinations in meta‐analyses on ADC‐based tissue differentiation in cervical cancer. J. Magn. Reson. Imaging 2010;32:376–382. © 2010 Wiley‐Liss, Inc.

     

Pulse Radiolytic Study of the Interaction of SO−·4 with Deoxynucleosides. Possible Implications for Direct Energy Deposition

Summary

Using the technique of pulse radiolysis it has been demonstrated that the interaction of SO^?·₄ with deoxynucleosides (k ～ < 2 × 10⁸–2·3 × 10⁹ dm³ mol^?1 s^?1) in aqueous solution at pH 7·0 results in the formation of the corresponding one-electron oxidized radicals which either deprotonate or hydrate to yield OH adducts. Based upon the ease of oxidation of the deoxynucleosides, dG, dA, dC, dT, by SO^?·₄, the apparent redox potentials are in the order dG ? dA ≈ dC > dT. With the exception of deoxyuridine, the deoxynucleoside radicals produced on interaction with SO^?·₄ have been shown to have oxidizing properties based upon the interactions with tetranitromethane and the nitroxyls, TMPN and NPPN. The deoxynucleoside radicals (dG, dA and dC) do not interact with oxygen (k < 10⁶ dm³ mol^?1 s^?1) in contrast to the interaction observed with the thymidine radical (k = 2·5 × 10⁷ dm³ mol^?1 s^?1). The implications of these findings are presented in terms of the properties of the discussed radicals as relating to those of potential DNA base radicals (positive centres) produced by direct energy deposition within DNA. The use of SO^?·₄ to mimic, to some extent, the effects of direct energy deposition in DNA may assist in our understanding of the resulting molecular processes relevant to radiobiological studies.     

Physiological Effects of Tritiated Water on Rhodotorula Gracilis as Influenced by Cystamine

Summary

The effects of tritium-emitted beta-radiations have been tested on the germination time, the period of lag, the cellular permeability and the respiratory quotient of Rh. gracilis treated with tritiated water in the presence of different concentrations of cystamine.

In our experimental conditions, cystamine at the metabolically-inactive concentration 2·5 × 10^?4 M showed no radioprotective activity on the cells, while at the metabolically-active concentration 2·5 × 10^?2 M cystamine was capable of counteracting the effects of beta-rays. The cellular permeability remained unchanged in every treatment.

The radioprotective action therefore seems to depend on the metabolic alterations brought about by the radioprotectant itself.     

Radiation Protection and Measurement for Low-Voltage Neutron Generators. NCRP Report No. 72

Inactivation of mouse C3H 10t½ cells in plateau-phase (7·8 × 10⁴ cells/cm²) was studied by using α-particles from the irradiation facility installed for radiobiological experiments at the 3 MV Tandem accelerator, University of Naples. Silicon detectors and CR39 plastic track detectors were employed for dosimetric purposes. The cells were exposed to high LET monoenergetic α-particles (energy of 1·8 MeV at the centre of the cell nucleus, track-averaged LET of 177 keV/µm and dose-rate of 1·1 Gy/min) and low-LET 80 kVp X-rays. The X-ray survival curve showed a significant shoulder (α/β = 9 Gy) while the survival curve for α-particles was close to exponential. The mean lethal dose of α-particles was 0·77 ± 0·02 Gy and the RBE was 5·2 at 80% survival and 3·0 at 5% survival. Survival of exponentially growing cells (2 × 10⁴ cells/cm²) following irradiation with the α-particle beam is also reported. The nuclear areas of 10T½ cells were measured as 299 ± 9 µm² and 250 ± 8 µm² for cells in log phase and plateau phase, respectively. The inactivation cross-section, obtained from the mean lethal dose, was 34 µm² and 37 µm² for cells in log phase and plateau phase, respectively. These values appear to be the maximum measured values for the inactivation cross-section of 10T½ cells as a function of the α-particle LET. This saturation cross-section is very similar to the saturation values reported in the literature for other mammalian cell lines.     

Model Studies for the Direct Effect of High-energy Irradiation on DNA. Mechanism of Strand Break Formation Induced by Laser Photoionization of Poly U in Aqueous Solution

Summary

Laser flash photolysis of polyuridylic acid (poly U) in anoxic aqueous solutions leads to biphotonic photoionization of the uracil moiety followed by the formation of single strand breaks (ssb). The rate constant for ssb formation (1·0 s^?1, obtained from the slow component of conductivity increase at 23°C and pH 6·8) increases with decreasing pH to 235 s^?1 at pH 3·5. The activation energy (pre-exponential factor) was measured to be 66 kJ mol^?1 (5 × 10¹¹ s^?1) at pH 6·8. Addition of dithiothreitol (DTT) or glutathione (GSH) prevents ssb formation by reacting with a poly U intermediate (rate constant = 1·2 × 10⁶ and 0·16 × 10⁶ dm³ mol^?1 s^?1, respectively). Since with OH radicals as initiators very similar data have been obtained for the kinetics of ssb formation and for the reaction with DTT, we conclude that photoionization of the uracil moiety in poly U leads eventually to the same chemical pathway for ssb formation as that induced by OH radicals. Furthermore, we propose that protection by DTT and GSH occurs via H donation to the C-4′ radicals of the sugar moiety of DNA and to the C-4′ and the C-2′ radicals of poly U.     

Rates for Repair of pBR 322 DNA Radicals by Thiols as Measured by the Gas Explosion Technique: Evidence that Counter-ion Condensation and Co-ion Depletion are Significant at Physiological Ionic Strength

Summary

Rates of repair of pBR 322 plasmid DNA radicals by thiols of varying net charge (Z) at pH 7 and physiological ionic strength were measured using the oxygen explosion technique. The extent of conversion of supercoiled to relaxed circular plasmid was measured by HPLC as a function of the time of oxygen exposure before or after irradiation, the time-courses being fitted by a pseudo-first-order kinetic expression with k₁ = k₂[RSH]. Values of k₂ (M^?1 s^?1) were: 2·1 × 10⁵ (GSH, Z = ?1), 1·4 × 10⁶ (2-mercaptoethanol, Z = 0), 1·2 × 10⁷ (cysteamine, Z = +1), 6·6 × 10⁷ (WR-1065 or N-(2-mercaptoethyl)-1,3-diaminopropane, Z = +2). The approximately 6-fold increase in rate with each unit increase in Z is attributed to concentration of cationic thiols near DNA as a consequence of counter-ion condensation and reduced levels of anionic thiols near DNA owing to co-ion depletion. The results are quantitatively consistent with chemical repair as a significant mechanism for radioprotection of cells by neutral and cationic thiols under aerobic conditions, but indicate that repair by GSH will compete effectively with oxygen only at low oxygen tension.     

The Absence of Late Effects of Radiation on the Cellularity of the Mouse Thymus

Summary

The in vitro cell survival of a human cervix carcinoma cell line (HX156c) has been assessed using ⁶⁰Co γ-rays administered at either high (150 cGy/min) or low (3·2 cGy/min) dose rate. Recovery during low dose-rate irradiation was observed; the dose reduction factor at 10^?2 cell kill for 150 versus 3·2 cGy/min was around 1·3. An insight into the possible underlying mechanisms of this recovery process has been investigated by addition of non-toxic concentrations of various agents thought to inhibit eukaryotic DNA repair. Agents were added 2 h prior to irradiation and removed after 24 h exposure. Differential effects among the inhibitors were observed; aphidicolin had no effect on cell survival, novobiocin, hydroxyurea and 3-aminobenzamide reduced survival by a similar extent at both dose rates, β-ara A and caffeine reduced survival to a greater extent during low dose-rate irradiation. β-ara A and caffeine seemed to exert their effects mainly by increasing the alpha component of the acute survival curve. Since survival curves obtained at dose rates of around 3 cGy/min help define a dominant component of the initial slope of the acute curve we have demonstrated that β-ara A and caffeine modify the initial slope, probably by inhibiting DNA repair processes involved in the sparing of tumour cells during protracted irradiation.     

The Effect of Chronic Radiation on the Humoral Immune Response of Rainbow Trout (Onchorhynchus Mykiss Walbaum)

Two separate experiments have examined the effect of exposing rainbow trout to chronic γ-radiation, commencing immediately after fertilization. In experiment 1 the period of exposure extended for 20 days with groups receiving mean dose rates of 1·87, 3·73 and 9·03 mGy h^?1, and mean total accumulated doses of 0·83, 1·66 and 4·01 Gy respectively. At 5 months of age fish were tested for specific antibody response to dinitrophenol coupled to keyhole limpet haemocyanin (DNP-KLH) and there was no significant difference in titre between irradiated groups and unirradiated controls. In experiment 2 the exposure period was extended to 246 days from fertilization. Mean dose-rates to the three groups used were the same as in the first experiment until hatching at 21 days and then lower with rates of 0·99, 1·9, and 4·66 mGy h^?1 to the free-swimming fish. The mean total accumulated doses over the whole irradiation period were 5·43, 10·53 and 25·43 Gy respectively. The antibody response to DNP-KLH was significantly lower in trout receiving the highest dose-rate when compared with those of unirradiated controls or the lowest dose-rate group. The significance of these results is discussed in relation to radiation levels in areas of radioactive waste disposal, and results from a similar study published previously.     

Stem Cells in Bone and Bone Marrow after Contamination with Bone-seeking Radionuclides

Summary

The effects of time, mass and oxidation state on plutonium gastrointestinal absorption and tooth adsorption were studied during and after chronic ingestion of plutonium-238 (IV) or (VI) (1·55–15·60 kBq/ml) in 6·5 mm bicarbonate medium by fed rats via drinking water for 8 days to 3 months. Animals were killed during the ingestion to follow the kinetics of whole-body storage and clearance of plutonium. At 1·55 kBq/ml the amount of plutonium retained in the skeleton increased continuously during the 85 days of ingestion and reached a plateau thereafter. This plutonium retention was therefore dependent on the total mass administered but not proportional to this mass, as the fraction of administered plutonium retained decreased during the first 22 days of ingestion and then stabilized. This is reflected by the gastrointestinal transfer (f₁), which had risen to (3·80 ± 0·82) × 10^?5 on Day 3 of ingestion and then decreased to a stabilized value of (1·07 ± 0·06) × 10^?5 from Day 30 to the end of the ingestion period. In the liver, the amount of plutonium retained reached a plateau, which lasted from Day 30 to the end of ingestion. The kidneys and spleen were also found to be retention sites. By Day 3 of ingestion, for a mass ingested of 5 × 10^?7 g/kg of body mass, the maximum mean value of f₁ we found was smaller than the 10^?4 recommended by ICRP Report 30. The oxidation state had no effect on f₁. Large plutonium deposition was observed on the teeth. For both oxidation states (IV) and (VI), about 0·10% of the administered dose was deposited on the teeth after 3 days of ingestion, whatever the plutonium concentration administered. However, whereas the amount of plutonium (IV) deposited did not change throughout the ingestion period, tooth deposition of plutonium (VI) decreased.     

The International System of Units (SI)

Serial blood samples were taken from four healthy individuals (three males, one female, aged between 26 and 51 years) in 3-monthly intervals during 1 year. Leucocyte suspensions were prepared and exposed to 3 Gy of ¹³⁷Cs γ-rays or left unirradiated as controls. In a cytokinesis-blocked (CB) micronucleus (MN) assay significant inter- and intra-donor variations of background and radiation-induced MN incidences became apparent. The two sources of variation lead to an extra variance σ²_I, in addition to the sample variance σ²_e of MN incidences. The contributions of the different components to the total variance were estimated by means of a variance component model. The deviation σ_I for the mean background MN level of 1·53 × 10^?2 MN/CB cell was +- 0·67 × 10^?2 and for the mean radiation-induced MN level of 0·53 MN/CB cell it was ± 0·10. The contribution of the intra-individual variance to σ²_I was about 50% for background MN levels and 75% for radiation-induced MN frequencies. With respect to the application of the CB-MN assay as a biological dosimetry system, the consequences of the present findings for calibration purposes and low-dose estimation are discussed. The calculation of the variance components is explained in an appendix, which serves also as an example for the adaptation of analysis of variance techniques to the evaluation of data derived from scoring of MN, as well as from scoring of metaphase chromosomal aberrations.     

Association for Radiation Research/Netherlands Radiobiological Society

Summary

The kinetics of pulse radiolytically induced intramolecular radical transformations: Trp/N^·→Tyr/O^·, Tyr/O^·→Trp/N^·, Met/S∴Br→Trp/N^· and Met/S∴Br→Tyr/O^· has been investigated at various pH levels and temperatures in model peptides: Trp-(Pro)_n-Tyr, Trp-(Gly)_n-Tyr, Trp-(Pro)_n-Met (n = 0–3), Tyr-Phe-Met-Arg-Phe-NH₂·2AcOH, Met⁵-enkephaline and [d-Ala]²Met⁵-enkephaline. The rate constants of the Trp/N^·→Tyr/O^· transformation at pH 8 were found to decrease exponentially with the distance between Trp and Tyr in the proline peptides, while in the glycine peptides the rate of the transformation is less dependent on the number of bridging glycine residues. The activation energies determined fall into the range 10–20 kJ mol^?1 irrespective of: (i) the ionization state of tryptophyl radical and tyrosine, (ii) type of bridging amino acids, and (iii) reversal of the direction of the electron transfer upon tyrosine OH group ionization. The activation entropies at 298 K for the peptides of the glycine and proline series are negative and rather high, and fall into the relatively narrow range of ?90 to ?140 J mol^?1 deg^?1. These activation parameters seem to indicate that a tunnelling mechanism is involved in the electron transfer between strictly oriented aromatic moieties of Trp and Tyr. The variation of the activation parameters with average separation distance in the case of Trp-(Pro)_n-Tyr shows a predominance of the electronic factor over the nuclear in determining the distance dependence of the electron transfer rate. The intramolecular Met/S∴Br→Tyr/O^· transfer proceeds with the rate constant of 1·1 × 10⁵ s^?1 in Met⁵-enkephaline and 5·7 × 10⁴ s^?1 in [d-Ala]²Met⁵-enkephaline. The activation parameters for this transformation E_a = 30 kJ mol^?1 and ΔS⁴₂₉₈ = ?62 J mol^?1 deg^?1 are close to those of the Trp/N^·→Tyr/O^· transformation, suggesting a similar mechanism for the electron transfer.     

Temporal and anatomical variations of brain water apparent diffusion coefficient in perinatal cerebral hypoxic-ischemic injury: Relationships to cerebral energy metabolism

Cerebral apparent diffusion coefficients {ADCs) were determined in nine newborn piglets before and for 48 h after transient hypoxia-ischemia. Phosphorus MRS revealed severely reduced cerebral energy metabolism during the insult and an apparently complete recovery 2 h after resuscitation commenced. At this time, mean ADC over the imaging slice (ADC_global) was 0.88 (0.04) × 10^{? 9} m² · s^{? 1} (mean (SD}), which was close to the baseline value of 0.92 (0.4) × 10^{? 9} m² · s^{? 1}. In seven of the animals, a “secondary” failure of energy metabolism then evolved, accompanied by a decline in ADC_global to 0.64 (0.17) × 10^{? 9} m² · s^{? 1} at 46 h postresuscitation (P < 0.001 versus baseline). For these seven animals, ADC_global correlated linearly with the concentration ratio [phosphocreatine (PCr)][inorganic phosphate (Pi)] (0.94 r < 0.99; P > 0.001). A nonlinear relationship was demonstrated between ADC_global, and the concentration ratio [nucleotide triphosphate (NTP)]/ [Pi + PCr + 3 NTP]. The ADC reduction commenced in the parasagittal cortex before spreading in a characteristic pattern throughout the brain. ADC seems to be closely related to cerebral energy status and shows considerable potential for the assessment of hypoxic-ischemic injury in the newborn brain.     

Differential expression of flowering genes in Arabidopsis thaliana under chronic and acute ionizing radiation

Abstract

Purpose: Chronic and acute irradiations have drastic effects on flowering stage that plays an important role in further seed development and can determine seed yield. The expression of the key flowering genes, AP1, CO, GI, FT, FLC, and LFY, sensitive to irradiation repair gene RAD51 and the proliferation gene PCNA2 were studied in the wild-type Arabidopsis thaliana (Columbia ecotype) under chronic and acute irradiations.

Materials and methods: Chronic irradiation was performed using the radioactive isotope ¹³⁷СsCl in two total doses of 3 cGy and 17 cGy, with the dose rate of 10^?7 cGy/s and 6.8 10^?6 cGy/s, respectively. The plants were grown under chronic irradiation during 6 weeks, from seeds till the 6.3 stage of flowering. For acute exposure, the plants were X-ray irradiated one time at the 5.0 development stage (20 days old) by a total dose of 15?Gy with the dose rate of 89 cGy/s.

Results: After chronic irradiation with the 3 cGy dose the irradiated plants demonstrated 8?±?2.8 days earlier flowering than in the control group. However, at the 17 cGy chronic and at the 15?Gy acute doses plants showed 14?±?3.7 and 2?±?1.4 days later flowering, respectively. The 3 cGy chronic exposure significantly increased the expression of the CO gene by a factor of 1.152 (1.087–1.217 95% C.I.) and decreased the expression of the FT gene by a factor of 0.128 (0.021–0.396 95% C.I.). The 17 cGy chronic exposure decreased expression of the AP1 gene by a factor of 0.872 (0.803–0.940 95% C.I.) and the LFY gene by a factor of 0.471 (0.306–0.687 95% C.I.). The 15?Gy acute exposure decreased the expression of the AP1 gene by a factor of 0.104 (0.074–0.144 95% C.I.) and the PCNA2 gene by a factor of 0.346 (0.238–0.488 95% C.I.).

Conclusions: The increased expression of the CO gene and decreased expression of the AP1 and FT genes under the lower dose of chronic exposure were associated with earlier flowering. The acute exposure increased the expression of the PCNA2 gene and decreased the expression of the flowering genes, except AP1. The flowering was delayed under both the higher dose of chronic exposure and under acute exposure, but it was less affected by the latter. Presumably, it was related to the activation of DNA repair under the 3 cGy chronic and 15?Gy acute irradiations.     

Subject Index

Summary

Using conductivity detection, pulse radiolysis experiments showed that solvent protonation of the electron adducts of cytosine, 5-methyl cytosine and 2′-deoxycytidine occurs with rate constants k ≥ 2 × 10⁴M^?1s^?1. The protonated electron adducts transfer an electron to p-nitroactetophenone (PNAP) with rate constants ranging from 3·5×10⁹ to 5·3·10⁹M^?1s^?1. The transfer is quantitative (G = 2·7), as shown by conductometric and spectroscopic measurements. In the presence of O₂ no electron transfer to O₂ takes place, implying that O₂ adds to the protonated electron adduct radicals.

No electron transfer from the H- and OH-adducts of the cytosine derivatives, either to PNAP or to O₂, takes place near neutral pH. It is suggested that the differences in the reaction behaviour of the H-adduct radicals and the protonated electron adduct radicals towards PNAP can be accounted for if different radicals are formed by H-addition and protonation of the electron adduct. The H atoms most probably add to the C-5-C-6 double bonds, whereas the electron adducts are protonated at N-3 and/or 0-2.