Effects of Ionizing Radiation on Fukushima Flora: Comparison
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After the Fukushima Dai-ichi Nuclear Power Plant (FNPP)PP accident in March 2011 much attention was focused to the biological consequences of ionizing radiation and radionuclides released in the area surrounding the nuclear plant. This unexpected mishap led to the emission of radionuclides in aerosol and gaseous forms from the power plant, which contaminated a large area, including wild forest, cities, farmlands, mountains, and the sea, causing serious problems. Large quantities of 131I, 137Cs, and 134Cs were detected in the fallout. People were evacuated but the flora continued to be affected by the radiation exposure and by the radioactive dusts’ fallout. 

  • ionizing radiation
  • Fukushima Dai-ichi
  • Plant
  • flora

1. Background

Following the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident in March 2011, due to the Great Eastern Japan earthquake and tsunami, massive amounts of radioactive materials were released into the environment. Due to the direction of the wind, the great majority of these materials poured into the Pacific Ocean; however, some of them spilled over to coastal areas, causing soil contamination by radionuclides, mainly in Fukushima prefecture [1]. Among the radionuclides most deposited in the soil, 137Cs is the most dangerous as it has a relatively long half-life compared to other radioactive substances released by FNPP [2], this is way the authors have focused the attention on this radionuclide for this work. In addition, 137Cs contaminated soil binds strongly to clay and the migration rate of clay-bound 137Cs exhibits low mobility, less than 1 cm per year, suggesting that most of 137Cs is superficially distributed in the soil. 137Cs can emit γ-rays; hence, unusually high air dose rates continue over land areas [3]. In addition, although the number of radionuclides released in the coastal area decreased, they continued to diffuse from the FNPP through the aquifers. Consequently, all the flora and fauna present at the time of the accident received and continue to receive high doses of radiation from Fukushima. Therefore, the finding of adverse effects in wild organisms in the Fukushima area resulting from long-term, low-dose radiation exposure is of great concern [4]. Over the years, several investigations have tried to determine the levels of contamination with radioactive materials or to estimate the doses of radiation exposure in terrestrial organisms living around Fukushima. However, there are few studies on the impacts of environmental radiation on wild organisms. Furthermore, flora and wildlife are strongly influenced by human activities [4][5]. Following the incident, the Japanese government designated “Areas where residents are not allowed to live” and “Areas where residents are expected to have difficulty returning for a long time” near the FNPP which have higher annual radiation doses to 20 mSv. The result was a mass evacuation from these areas in the long term. While radiation levels in most of the evacuation zone are not considered extremely lethal to wildlife, land use change due to decontamination activities and the cessation of agricultural activities are believed to significantly affect flora, fauna, and ecosystems in these areas [6][7][8][9].

2. The Effects of Ionizing Radiation on Plants

Ionizing radiation can deposit energy in a system and it is ubiquitous in the environment. Its source can be natural such as radioactive materials and cosmic rays or artificial, such as nuclear power plants [10]. Recently, there has been a strong interest in the health of organisms in radioactively contaminated sites such as those of Chernobyl and Fukushima Dai-ichi. Many wildlife species have surprised many scientists, but there are also reports of significant effects of chronic irradiation at relatively very low doses [11][12]. This contradiction has been observed at doses of environmental radioactivity and has yet to be fully understood. It is important to do this because ionizing radiation, although present in nature, can also derive from human activities. The exposure of a biological system to ionizing radiation activates a series of signals that start with the absorption of energy and continue in biological lesions [13]. There are two types of interactions, direct and indirect. In direct, the energy of the radiation is deposited directly in the targets. In indirect actions, on the other hand, the energy is first absorbed by an external medium, leading to the production of diffusible intermediates which subsequently attack the targets. The main target in both interactions of ionizing radiation is the H2O molecule, present in all organisms [11][14]. The primary reactions are excitation and ionization, which produce ionized water molecules (H2O•+) and the radicals H• and •OH (Figure 1) [11][14]. In addition, in a biological system this type of ionization is induced along the entire path of the radiation, triggering chain reactions, which produce secondary reactive oxygen species (ROS) because of H• and eaq− becoming trapped [15][16]. The most important ROS is H2O2; O2−• is produced in low doses, based on molecular oxygen levels. The •OH radical can react very quickly with different types of macromolecules including lipids and proteins, but especially with Deoxyribonucleic Acid (DNA) [11][15][16]. However, depending on the dose, many of the resulting injuries can be readily mitigated and repaired. Radio-induced DNA damage is mainly caused by indirect effects and is considered the most important, although direct effects may contribute to the damage [17]. Consequently, based on the dose and radio-sensitivity of the species, both genomic and chromosomal aberrations are generated. Generally, DNA is a candidate to be the primary site of radiation damage, thus explaining the resulting radiation-induced mitotic arrest [17][18]. Furthermore, there are cellular mechanisms that allow damaged cells to repair the damage; however, errors in DNA repair naturally occur and this leads to aberrations and subsequent transmission to descent [19][20]. Thus, cell division in the meristem or germline responds drastically to ionizing radiation. It is difficult to compare current data on plant responses to ionizing radiation as the conditions of the models and parameters of past experiments are very different [21]. Therefore, the type of irradiation (acute or chronic), the dose rate or dose applied, the physiological parameters such as species, varieties, cultivars considered, the stage of development at the time of irradiation and the variations of the individual could be different between studies. The current inhomogeneity is further aggravated by the coexistence of different experimental data, data applied by the food industry and data relating to accidents [22][23]. Consequently, in the experimental field of plant radiation, doses can vary from a few Gy to several hundred Gy but can also reach the level of kGy. Furthermore, the dose range response strongly depends on the species studied. It is difficult to predict a standard response to ionizing radiation in plants even though promising standardization schemes are emerging [11][22][24].
Plants 11 00222 g001 550
Figure 1. Primary (•OH, H•) and secondary (H2O2, O2•−) ROS involved in the oxidative stress produced by Ionizing Radiation (eaq−: solvated electron; H2O*: excited water molecule).
The effects of ionizing radiation in higher plants are of interest to agriculture, ecology, health, and new space frontiers [11][12]. In general, four fundamental aspects of plant biology need to be considered as they provide a vital context for analyzing the effects of ionizing radiation [22]. The light reactions of photosynthesis start with photolysis of H2O; consequently, this process produces large quantities of ROS, the same products of H2O radiolysis that plants are generally able to block thanks to the large production of antioxidants [25]. It must be considered that in multicellular plants, cells divide into meristematic tissues, and they have quiescent centers with the same functionality as stem cells but are not identical to each other [11][22]. For example, they lack the apoptotic response of animal stem cells, mediated by the p53-oncoprotein. Meristems in plants are a biologically distinct product resulting from an independent evolution of multicellularity; the effects of ionizing radiation on them are not yet well known [22]. It is important to note that the meiotic divisions that produce the generation of gametophytes in the reproductive organs of plants are separated in each generation by many divisions of vegetative cells in the generation of sporophytes; in detail, the plants alternate between generations and no dedicated germ line [26]. Although tumors can occur in plant tissues, thanks to multiple controls on dividing plant cells and the low probability of metastasis, being organisms without a circulatory system, plants do not suffer the oncogenesis effects, typical of many animals [22]. Hence, plants are unlikely to have the same stochastic effects as ionizing radiation in animals, where in many cases they cause carcinogenesis [22]. Therefore, the current information on the effects of ionizing radiation on multicellular organisms is provided by the knowledge of the effects on organisms with lower antioxidant capacity than plants, which possess stem cells and germ lines without equivalent in plants and which suffer from stochastic effects that probably do not occur in the plant kingdom [11][22][27].

3. Current and Future Perspectives: Management in the Medical Field

Research into the long-term effects on flora, fauna and human health following the FNPP incident is constantly evolving and proceeding relentlessly [7]. The CNPP and FNPP accidents provide unique opportunities on the investigation of radiological consequences and radiation effects on environment in a large scale that cannot be observed in the laboratory, not only for the health of higher plants but also for human health [28][29][30][31][32]. Prior to the Fukushima incident, few decision makers paid attention to the need to plan health investigations following a large-scale radiation scattering incident [32][33][34]. Following the FNPP incident, despite the serious initial difficulties, the basic concept of the Fukushima Health Management Survey (FHMS) was developed, which defined not only the health effects of ionizing radiation, but also other problems, such as example mental health and the consequences of long-term relocation [35][36]. To cope with this emergency, SHAMISEN (Nuclear Emergency Situations—Improvement of Medical and Health Surveillance) was founded in 2016 and its activities are still largely active today [35]. It is a project funded by the Open Project For European Radiation Research Area, that aims to develop recommendations for medical and health surveillance of populations affected by previous and future radioactive incidents based on lessons learned from past incidents, including the CNPP and FNPP incidents [35][37][38]. SHAMISEN recommendations state that: “The management of radiological incidents also raises important ethical questions. Although most radiation protection actions, including health surveillance, are aimed at reducing the impacts of exposure to ionizing radiation, most of them lead to with it a multitude of direct and indirect consequences that can have a great impact on the well-being of the affected populations. Ethical considerations are also important for the design and implementation of health surveillance and epidemiological studies” [35][37][38]. In conclusion, given the purpose and current activity of SHAMISEN, it is currently, proving to be a useful tool to better manage future large-scale incidents with dispersion of ionizing radiation [37][38]. Ensuring a precise, targeted, and fast intervention in the management of emergencies is the ultimate goal of decision makers. Learning from past incidents and implementing this knowledge can make a significant difference in terms of lives and costs in healthcare management [39].

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