Investigation of biological-rhythm patterns: physiological and biochemical effects in herbaceous plants exposed to low-level chronic radiation - part 1: nonirradiated seeds

HYPOTHESIS

Because reactive oxygen species are involved in the regulation of biological rhythms, we hypothesized that intra-annual variability of seed progeny quality at low doses of ionizing radiation (LDIRs) would differ from that of background plants.

MATERIALS AND METHODS

We conducted 12 consecutive experiments using the roll culture method by germinating seeds (monthly for 3 weeks) of six herbaceous plant species (Bromus inermis, Geum aleppicum, Plantago major, Rumex confertus, Silene latifolia, and Taraxacum officinale) growing under conditions of chronic radiation in the East Ural Radioactive Trace (EURT). We assessed physiological (seed viability and abnormality frequency) and biochemical (low-molecular-weight antioxidants, LMWAs) parameters of seedlings.

RESULTS

Total absorbed dose rates of maternal plants (TADRplants) and seed embryos (TADRseeds) in the EURT exceeded background levels by 1-3 and 1-2 orders of magnitude, respectively. Nonlinear dependencies on TADR were mainly characteristic of physiological and biochemical parameters. For most populations of the studied species (B. inermis, G. aleppicum, R. confertus, and S. latifolia), seedling survival and root length decreased in the autumn-winter period, while the frequency of abnormal seedlings increased. The content of LMWAs could be ranked as R. confertus > B. inermis > G. aleppicum > S. latifolia, in good agreement with the presence of anthocyanin pigmentation in the plants. The lowest synthesis of antioxidants in seedlings was observed in winter. A high LMWA content promoted growth and reduced the frequency of abnormal seedlings.

CONCLUSIONS

These results underscore a multistage nature of the impact of LDIRs on intra-annual biological rhythms in plants. High heterogeneity in reference group 'wild grasses' and diversity of their radiobiological effects should help to develop methods of radiation protection for natural ecosystems and facilitate approaches used by the International Commission on Radiological Protection.HighlightsAbsorbed dose rates for six plant species in the East Ural Radioactive Trace (EURT) area range from 0.11 to 73.89 µGy h-s (plants) and 0.11 to 6.88 µGy h-s (seed embryos).Intra-annual rhythms of physiological and biochemical parameters in the EURT zone differ from those in background seedlings.Plants in the EURT area exhibit a wide range of trait variability, asynchrony of the manifestation of the effects, nonlinear dose-response relations, and hormesis.A high content of low-molecular-weight antioxidants (LMWAs) is associated with low frequency of developmental abnormalities and high viability of seed progeny.

Complexity of responses to ionizing radiation in plants, and the impact on interacting biotic factors

In nature, plants are simultaneously exposed to different abiotic (e.g., heat, drought, and salinity) and biotic (e.g., bacteria, fungi, and insects) stresses. Climate change and anthropogenic pressure are expected to intensify the frequency of stress factors. Although plants are well equipped with unique and common defense systems protecting against stressors, they may compromise their growth and development for survival in such challenging environments. Ionizing radiation is a peculiar stress factor capable of causing clustered damage. Radionuclides are both naturally present on the planet and produced by human activities. Natural and artificial radioactivity affects plants on molecular, biochemical, cellular, physiological, populational, and transgenerational levels. Moreover, the fitness of pests, pathogens, and symbionts is concomitantly challenged in radiologically contaminated areas. Plant responses to artificial acute ionizing radiation exposure and laboratory-simulated or field chronic exposure are often discordant. Acute or chronic ionizing radiation exposure may occasionally prime the defense system of plants to better tolerate the biotic stress or could often exhaust their metabolic reserves, making plants more susceptible to pests and pathogens. Currently, these alternatives are only marginally explored. Our review summarizes the available literature on the responses of host plants, biotic factors, and their interaction to ionizing radiation exposure. Such systematic analysis contributes to improved risk assessment in radiologically contaminated areas.

Plant adaptation to ionizing radiation: Mechanisms and patterns

Adaptation to environmental stressors is an essential property of plants that allows them, despite an immobile lifestyle, to survive in a changeable environment. The chain of successive events culminating in the final radiobiological reaction begins with the absorption of energy of ionizing radiation in the cell. Starting from stochastic acts of molecular injury formation, radiation damage gradually acquires deterministic features, which are expressed in a limited number of phenomena that complete plant radiation damage. As plants undergo specialization, the differences between plants and animals become more pronounced, leading to distinct responses to radiation. Chronic radiation exposure may activate biological mechanisms resulting in increased radioresistance of the population. The higher the level of radiation exposure and the sensitivity of plants to radiation, the more intensive the selection. Depending on the circumstances, enhanced radioresistance of a population can be achieved in different ways or has not evolved at all. High dose rates of chronic irradiation leаd to selection for the efficiency of repair systems, while low dose rates activate epigenetic mechanisms that lead to the maintenance of oxidative balance, additional synthesis of chaperones, and control of TEs transposition. Due to huge differences in the radiosensitivity of organisms that make up the ecosystem, irradiation can result in disruption of connections between components of ecosystems which may lead to consequences that can differ drastically from those expected at the organismal and population levels. Therefore, the use of ecological knowledge is essential for understanding the responses of populations and ecosystems to radiation exposure.

Multiannual Assessment of Quality of Plantago major L. Seed Progeny from Kyshtym Radiation Accident Area: Weather-Dependent Effects

The effects of low-dose radiation that are observed in plant populations in radioactively contaminated areas are variable. One of the reasons is the influence of fluctuating weather conditions and the interaction of radiation with weather factors. This article summarizes results of 12-year research on the viability and radioresistance of greater plantain (Plantago major L.) seed progeny growing in the East Ural Radioactive Trace (EURT) zone and in control (nonradioactive) areas, with consideration of weather conditions' variability. The EURT was formed by the Kyshtym accident, which occurred in 1957 at the Mayak Production Association. Absorbed dose rates of P. major parental plants in the pollution gradient were 14.5-165.9 μGy h^-1, which correspond to a low-dose range. Seed progeny quality was evaluated as seed weight, the survival rate, and root length of 21-day seedlings. Interannual variability in the studied parameters was high, and their ranges overlapped between EURT groups of seeds and control groups in most cases. The number of significant correlations between the parameters of seed quality and weather conditions was higher in EURT groups than in control populations. In the control groups of seeds, 88.9% of correlations were negative, whereas in the EURT groups, 78.5% were positive.

Stress memory in two generations of Plantago major from radioactive and chemical contaminated areas after the cessation of exposure

HYPOTHESIS

The differences in viability, root length, and pro/antioxidant features of Plantago major seedlings identified in seed progeny formed in areas of radioactive and chemical contamination can persist in subsequent generations after the elimination of the stress.

MATERIALS AND METHODS

The seed mixtures of F1 generation were collected from P. major natural populations (P plants) growing for a long time in the East Ural Radioactive Trace, the Karabash Copper Smelter zone, and background area. The seeds of F2 generation were obtained from F1 generation plants grown on experimental plots with 'clean' agricultural background; F3 generation was grown from F2 generation on the same plots. The viability of seed progeny was estimated by survival rate and root length. Pro/antioxidant features were determined spectrophotometrically by malondialdehyde content, superoxide dismutase and catalase activities, and total content of low molecular weight antioxidants in seedlings.

RESULTS AND CONCLUSIONS

The hypothesis about the persistence of effects from chronic exposure to ionizing radiation and chemical contamination in the generations' sequence of P. major after the removal of stress was confirmed only partially. The data obtained indicated that changes in the prooxidant and antioxidant features of plants in response to low doses of ionizing radiation can persist for at least in two generations after the stress removal. In the case of long-term exposure to chemical contaminants, we observed the persistence of the effect in a succession of generations only on the morphological indicator of root length.