Introduction

With the advancement of science and technology in the 21^st century, there is growing concern about the risks posed by radioactive nuclides released during accidents at nuclear facilities. Such events can lead to severe contamination across terrestrial and aquatic ecosystems. When radioactive nuclides accumulate in the soil, plants can absorb them through their roots. Consequently, when humans consume these contaminated plants, the radioactive materials can transfer into the human body, leading to internal exposure (With et al., 2021).

Key radioactive nuclides released during nuclear power plant accidents include iodine(¹³¹I), cesium(¹³⁷Cs), strontium (⁹⁰Sr), plutonium(²³⁹Pu), and rhenium(¹⁸⁶Re). These nuclides have long half-lives and high mobility in the environment, posing long-term and widespread risks to various ecological components, particularly plants and humans. Following major nuclear accidents such as the Chernobyl disaster in 1986 and the Fukushima disaster in 2011, extensive research has been conducted, especially in nuclear-advanced countries, to understand the environmental impact of radioactive nuclides (Doe et al., 2024; Wang and Yu, 2015; Yanagisawa et al., 1992).

In dose assessment models for radioactive ingestion, the ratio of nuclide concentration in plants to that in soil is referred to as the transfer factor (TF). This value can vary significantly depending on the cultivation environment and crop type (Banuelos et al., 2010; Lee et al., 2018; Nedelkoska and Doran, 2000; Wang and Chen, 2000). However, applying data from advanced countries to domestic crops has limitations due to differences in crop types and cultivation conditions. Although studies on the soil-to-plant transfer factors of radioactive nuclides have been conducted in South Korea since the early 1980s, research on rhenium remains insufficient (Baek et al., 2010).

To enhance the reliability of radiation dose assessments for rhenium, it is essential to generate data specific to major domestic crops and varieties. Rhenium, including its primary radioactive isotopes ¹⁸⁸Re and ¹⁸⁶Re, is a fission product with high mobility and chemical stability in the environment. If released, it can cause severe ecological damage (Zhu and Smolders, 2000). The effects of rhenium on plant growth vary depending on plant species, soil characteristics, and rhenium concentration (Banuelos et al., 2010; Wang and Chen, 2000). High concentrations of rhenium have been reported to inhibit chlorophyll production, thereby reducing photosynthetic capacity (Hodkinson and Sheppard, 1994; Vandenhove and Gil-García, 2007), and to negatively affect overall plant growth by inhibiting root division and elongation (Seregin and Ivanov, 2001). Moreover, rhenium can damage DNA and cell structures, leading to genetic mutations and reduced plant survival and reproductive efficiency (Vandenhove and Gil-García, 2007).

Therefore, the addition of organic matter to soil has been proposed as a strategy to mitigate the absorption of radioactive nuclides by plants during nuclear power plant accidents (Doe et al., 2024; Fedorkova et al., 2012). This approach can potentially reduce radioactive exposure to animals and humans by immobilizing radioactive materials in the soil and limiting their uptake by plants (Rafferty et al., 1994).

This study aims to investigate the effects of radioactive rhenium on the growth and damage symptoms of two representative vegetable crops in South Korea, Chinese cabbage (Brassica rapa subsp. pekinensis) and radish (Raphanus sativus), and to examine the impact of organic matter on the behavior of rhenium in the soil. The findings are expected to contribute to ecological restoration in rhenium-contaminated areas and provide fundamental data for improving agricultural productivity.

Materials and Methods

1. Experimental design

The experiments were conducted in a glasshouse at Pusan National University. The Chinese cabbage cultivar used was ‘Danobom’ (Brassica rapa subsp. pekinensis), and the radish cultivar was ‘Cheongjamoo’ (Raphanus sativus), both obtained from Jinheung Co., Gyeonggi-do, Korea. The experiments were performed in a Venlo-type glasshouse (86.4 m², 28.8 m × 3 m) using raised beds for pot cultivation. The temperature was maintained at 20°C throughout the study.

2. Radioactive rhenium treatment and soil organic matter concentration in mixed soil.

To investigate the effects of soil organic matter on the behavior of radioactive nuclides and crop growth, a mixed soil was prepared using peat moss (PRO-MOSS TBK), potting soil mix (Seonghwa, Boseong, Korea), and organic fertilizer (Hyundae Teugsan, Gimhae, Korea). The soil was adjusted to organic matter contents of 5% (peat moss 12 : organic matter 2 : bed soil 240 v/v), 10% (peat moss 24 : organic matter 2 : bed soil 240 v/v), and 15% (peat moss 36 : organic matter 2 : bed soil 240 v/v).

Ammonium perrhenate (NH₄ReO₄, Sigma, CAS No. 13598-65-7) was used as a substitute for the radioactive isotope ¹⁸⁶Re, which requires specific handling and a dedicated laboratory. The ammonium perrhenate was mixed into the soil at concentrations of 0, 5, 10, and 20 mg·kg^-1, and growth and damage symptoms were monitored over time. After stabilizing the soil in 1/5,000 Wagner pots filled to 80% with the growing medium, Chinese cabbage and radish seedlings with two fully expanded true leaves were transplanted. Each treatment was replicated three times, with three plants per replicate. Pots of control group consisted of soil mixed with organic matter at varying concentrations of 5%, 10%, 15% but without rhenium treatment.

3. Crop management

Crop management followed the cultivation guidelines provided by the Rural Development Administration (RDA, 2017). Growth assessments were conducted 50 and 80 days after rhenium treatment. Parameters measured included number of leaves, leaf area, leaf length, leaf width, plant height, root length, fresh weight, dry weight, and chlorophyll content. Three plants per replicate were sampled for these assessments. The number of leaves was counted if the leaf area exceeded 1 cm², and leaf area was measured using a leaf area meter (LI-3000, LI-COR, USA). Chlorophyll content was measured on three points of the third fully expanded leaf using a chlorophyll meter (SPAD-502, Minolta Co., Ltd., Japan), and the average value was recorded.

Leaf length and width were measured on the third fully expanded leaf, plant height was measured from the hypocotyl to the highest point of the shoot, and root length was measured from the base to the tip of the root. Fresh weight was recorded as the total biomass, and dry weight was determined after drying the plants at 70°C for 24 hours.

Plants with dead apical meristems were considered as deceased individuals.

4. Statistical analysis

The radioactive nuclide Re was treated with 4 repetitions per concentration, and the experimental plots were arranged using a randomized block design. Statistical analyses were performed using the SAS program (Version 9.4, SAS Institute Inc., Cary, NC, USA). Data were analyzed using the Least Significant Difference (LSD) test at a 95% confidence level.

Results and Discussion

1. Effect of rhenium treatment concentrations and soil organic matter content on the growth of Chinese cabbage and radish

This study investigated the effects of varying concentrations of ammonium perrhenate (NH₄ReO₄), a surrogate for the radioactive isotope ¹⁸⁶Re, on the growth of Chinese cabbage (Brassica rapa subsp. pekinensis) and radish (Raphanus sativus). The interaction between rhenium and soil organic matter was analyzed to determine its impact on plant growth (Table 1).

For Chinese cabbage, significant growth changes were observed with different rhenium treatment concentrations (p < 0.001). However, increasing the soil organic matter content did not significantly influence growth. At the 50-day growth stage, the control group (no rhenium treatment) exhibited the best growth, with a higher number of leaves, larger leaf area, and greater plant height and root length. In contrast, increasing rhenium concentrations led to reductions in these parameters. Similar results were observed at the 80-day growth stage, where higher rhenium concentrations significantly reduced leaf area, leaf length, plant height, and root length. Conversely, soil organic matter levels did not significantly affect these growth indicators.

For radish, growth patterns were similar to those observed in Chinese cabbage, with rhenium concentration being the primary factor influencing growth (Figs. 4 and 5). No significant differences in growth were observed due to soil organic matter alone or the interaction between soil organic matter and rhenium concentration. At the 50-day growth stage, the control group showed the best growth, with an increase in rhenium concentration leading to a decline in the number of leaves, leaf area, leaf length, plant height, and root length. By the 80-day growth stage, radishes exposed to rhenium concentrations of 10 mg·kg^-1 or higher displayed symptoms of plant death (Fig. 6). The control group exhibited the most vigorous root growth, while root growth decreased significantly with increasing rhenium concentrations.

Rhenium, as a radioactive nuclide released into the environment, is absorbed by plants through their roots, accumulating primarily in the roots before moving to the shoot parts. The mobility of rhenium within plants can vary depending on its chemical state and the plant species (Doe and Smith, 2022). Within the plant, rhenium negatively impacts growth by causing DNA damage, inhibiting cell division, disrupting hormone balance, and hindering the absorption of essential nutrients. Symptoms of plant damage include abnormal leaf growth, discoloration, tissue necrosis, and other physiological changes, such as increased reactive oxygen species (ROS) production and altered antioxidant enzyme activity (Baek et al., 2010).

Generally, plants exposed to relatively low concentrations of radioactive nuclides for short periods may exhibit growth inhibition and physiological changes but can recover over time (Page et al., 2006). However, in this study, even at a low concentration of 5 mg·kg^-1, symptoms of damage were evident. By the 80-day growth stage, the damaged tissue in rhenium-exposed plants did not recover, leading to plant necrosis (Tables 1 and 3). Plants exposed to higher concentrations of rhenium exhibited severe damage, including tissue necrosis and reduced survival rates, likely due to metabolic imbalances (Figs. 1 and 2).

Fig. 1.

Growth changes of Chinese cabbage seedlings cultivated for 50 days after transplanting in radionuclide ammonium perrhenate treated soil with varying organic matter levels.

Fig. 2.

Growth changes of Chinese cabbage seedlings cultivated for 80 days after transplanting in radionuclide ammonium perrhenate treated soil with varying organic matter levels.

Fig. 3.

Changes in growth of Chinese cabbage seedlings by radionuclide Re treatment with 5, 10, 15% of organic matter at 50 and 80 days after rhenium treatment.

This study focused on the impact of radioactive nuclides, particularly rhenium, on the growth of Chinese cabbage and radish, considering the effect of soil organic matter content. Previous studies have suggested that soil organic matter can adsorb or immobilize radioactive nuclides, thereby limiting their movement within the soil and reducing their uptake by plants (Cornell, 1993). Organic matter is also known to promote microbial activity, which plays a crucial role in enhancing plant growth (Chibowski and Zygmunt, 2002). However, in this study, increasing the organic matter content did not mitigate the inhibitory effects of rhenium on plant growth (Tables 1 and 3, Fig. 3).

It was anticipated that higher organic matter content would lead to greater adsorption and immobilization of rhenium in the soil, resulting in improved crop growth. However, this was not observed, suggesting that the ability of organic matter to adsorb and immobilize radioactive nuclides may vary depending on the specific nuclide involved. While Cornell (1993) demonstrated that organic matter could effectively limit the movement of certain radioactive nuclides, the results for rhenium were less conclusive.

These findings indicate that managing radioactive nuclide-contaminated soils and developing remediation strategies should involve a tailored approach that considers the specific characteristics of the radioactive nuclides involved (Murashov et al., 2012).

2. Effect of rhenium treatment concentrations and soil organic matter content on dry matter production of Chinese cabbage and radish

The effects of varying concentrations of the radioactive isotope rhenium and different soil organic matter contents on the fresh and dry weight of Chinese cabbage and radish were investigated at different growth stages (Tables 3 and 4). The growth of Chinese cabbage was assessed at two growth stages: 50 days and 80 days. As the rhenium concentration increased, both the fresh and dry weights of Chinese cabbage decreased. Notably, at a high rhenium concentration of 20 mg·kg^-1, the fresh weights of the shoot and root parts of Chinese cabbage were 4.7 g and 0.6 g, respectively, which were significantly lower than those of the control group, which were 135.2 g and 4.9 g, respectively. The dry weight also tended to decrease as the rhenium concentration increased, with the shoot parts dry weight decreasing from 1.0 g to 0.5 g.

At the 80-day growth stage, the fresh and dry weights of Chinese cabbage continued to decrease as the rhenium concentration increased. In the group with a high rhenium concentration of 20 mg·kg^-1, the fresh weight of the shoot parts was only 0.6 g, which was significantly lower than the 259.3 g observed in the control group. These results suggest that high concentrations of rhenium severely inhibit the growth of Chinese cabbage (p < 0.001). In contrast, the soil organic matter concentration had little effect on the fresh and dry weights of Chinese cabbage.

The effects of rhenium concentration and organic matter content on the fresh and dry weights of radish were also evaluated (Table 4). At the 50-day growth stage, as the rhenium concentration increased, both the fresh and dry weights of radish decreased. Particularly at the high rhenium concentration of 20 mg·kg^-1, the fresh weights of the shoot and root parts of radish were 3.7 g and 0.3 g, respectively, which were significantly lower than the 28.6 g and 2.1 g observed in the control group. The dry weight also showed a decreasing trend with increasing rhenium concentration.

Fig. 4.

Growth changes of radish seedlings cultivated for 50 days in radionuclide ammonium perrhenate treated soil with varying levels of organic matter.

Fig. 5.

Growth changes of radish seedlings cultivated for 80 days in radionuclide ammonium perrhenate treated soil with varying levels of organic matter.

Fig. 6.

Changes in growth of radish seedlings by radionuclide Re treatment with 5, 10, 15% of organic matter at 50 and 80 days after rhenium treatment.

At the 80-day growth stage, the fresh and dry weights of radish continued to decrease as the rhenium concentration increased, with symptoms of necrosis appearing even at the low concentration of 5 mg·kg^-1. This indicates that rhenium, absorbed through the roots and accumulated in the plant, has a detrimental effect on the growth of both Chinese cabbage and radish (Lefèvre et al., 2009; Murashov et al., 2012).

In contrast, the soil organic matter content did not significantly affect the growth of Chinese cabbage and radish, nor was there any interaction between organic matter and rhenium concentration. This suggests that organic matter does not mitigate the toxic effects of rhenium on plants (Tables 3 and 4).

When rhenium contaminates the soil, it inhibits plant growth, and at high concentrations, it disrupts the physiological functions of plants, reducing dry matter production. The process by which radioactive isotopes transfer from soil to plants is influenced by various factors (Banuelos et al., 2010; Delannoy et al., 2012; Sheppard et al., 1996), and organic matter plays a particularly important role in this process. Rhenium can exist in various chemical forms in soil, and these forms greatly influence its mobility to plants. When rhenium is adsorbed to soil particles or bound to organic matter, its uptake by plants is limited. Conversely, if rhenium’s binding with organic matter is weak, it can easily transfer to plants, increasing its transfer coefficient. Generally, higher organic matter concentrations tend to reduce the absorption of radioactive isotopes, indicating that organic matter inhibits the movement of radioactive substances (Chibowski and Zygmunt, 2002).

While organic matter acts as a crucial regulator in the transfer of radioactive isotopes to plants, its effect may vary depending on the type of radioactive isotope and soil characteristics (Doe et al., 2024). In this study, rhenium inhibited plant growth, and with prolonged exposure even at low concentrations, plants failed to grow normally and eventually died. Furthermore, increasing organic matter concentration did not alleviate the inhibitory effect of rhenium on plant growth.

However, the effect of organic matter may vary depending on the type of radioactive isotope, crop type, and cultivation environment, necessitating further in-depth research. The impact of rhenium on plant growth can vary depending on factors such as plant species, soil characteristics, and rhenium concentration. As the rhenium concentration increases, the generation of reactive oxygen species (ROS) is promoted, leading to oxidative stress, which can cause damage to plant cell lipids, proteins, and DNA, resulting in apoptosis or necrosis.

Additionally, rhenium interferes with the uptake of mineral nutrients by plants, causing nutritional imbalances. Plants exposed to rhenium experience inhibited chlorophyll synthesis, reducing photosynthetic capacity (Hodkinson and Sheppard, 1994; Vandenhove and Gil-García, 2007), and root division and elongation growth are suppressed, leading to overall growth decline (Seregin and Ivanov, 2001). Moreover, genetic mutations may occur due to DNA and cellular structure damage, reducing plant survival and reproductive efficiency.

In this study, even low concentrations of rhenium inhibited the growth of Chinese cabbage and radish, and at high concentrations, necrotic individuals increased, leading to a rapid decline in survival rates. These results highlight the severity of rhenium contamination and suggest the need for various management strategies to reduce rhenium concentrations in soil.

3. Effects of rhenium treatment concentration and soil organic matter content on chlorophyll content in chinese cabbage and radish

Chlorophyll is a key factor in photosynthesis, and its content in plants is an important indicator of photosynthetic efficiency, growth status, and overall plant health. To investigate the effect of varying soil organic matter concentrations and different concentrations of the radioactive isotope rhenium on the chlorophyll content of Chinese cabbage and radish we conducted a chlorophyll content analysis at different growth stages (Table 5).

At the 50-day growth stage of Chinese cabbage, a statistical analysis of the interaction between soil organic matter concentration and rhenium treatment concentration on chlorophyll content revealed no significant interaction effect. However, the rhenium treatment concentration did significantly affect chlorophyll content. In contrast, soil organic matter concentration had little impact on the chlorophyll content of both Chinese cabbage and radish. For Chinese cabbage, the highest chlorophyll content was observed in the control group without rhenium treatment across all soil organic matter concentrations of 5%, 10%, and 15%. As the rhenium treatment concentration increased, a decreasing trend in chlorophyll content was noted.

For radish, the impact of rhenium treatment on chlorophyll content at the 50-day growth stage was minimal. Although chlorophyll content in rhenium-treated radishes was lower than in the control group, the difference was not statistically significant. However, at the 80-day growth stage, both crops exhibited necrotic symptoms regardless of the rhenium treatment concentration. Consequently, chlorophyll content measurements were not possible in any of the rhenium- treated groups except for the control. These findings suggest that exposure to rhenium inhibited chlorophyll synthesis in both Chinese cabbage and radish, resulting in reduced photosynthetic capacity, overall growth deterioration, and eventual plant death (Seregin and Ivanov, 2001).

Overall, these results indicate that rhenium not only stunted plant growth but also significantly reduced crop survival rates. Thus, rhenium contamination could pose extensive risks to the entire ecosystem, not just individual plants (Chai et al., 2014; Delannoy et al., 2012). When rhenium contaminates soil, plants absorb it, and it can eventually transfer through the food chain to animals and humans (Chai et al., 2014). Therefore, the transfer characteristics of radioactive isotopes may vary depending on the specific isotope and crop type, necessitating ongoing research. Such studies play a crucial role in providing scientific evidence to protect ecosystems from radioactive contamination and safeguard human health.