Verification of Uptake and Transport Properties of Cesium in Hydroponically Cultivated Quercus serrata

Abstract

Several studies have shown that the radiocesium content of trees growing in the forest and open fields is lower when soil potassium content is higher. This is comparable to what has been observed in many crops, including rice. Then, is the cesium uptake mechanism of trees the same as that of rice and other herbaceous plants? To investigate the basic properties of potassium (K⁺) and cesium (Cs⁺) uptake and transport in Konara oak (Quercus serrata. Thumb.), which is widely grown in Fukushima Prefecture, elemental analysis and radiotracer experiments similar to those conducted on rice plants were carried out using hydroponically grown young oak seedlings. The results showed that oak roots do not selectively uptake K⁺ over Cs⁺ from hydroponic solution. The changes in the root K⁺–Cs⁺ uptake mechanism in response to the K supply during cultivation were minimal, and the suppression of Cs⁺ uptake by K⁺ supply can be mainly due to the competition between the two ions.

5.1 Introduction

Under natural environment where various incidents such as drought, flooding, mycorrhizal symbiosis, and pathogen invasion can occur, plants acquire minerals and thrive. Such a complex and fluctuating growing environment cannot be reproduced by an artificial hydroponic cultivation system, which is generally employed in plant science laboratories. However, it is also true that experiment using hydroponic medium containing essential minerals contributes to the investigation of some fundamental physiological characteristics of plants. The mechanism by which plant roots selectively uptake some ions from soils containing various concentrations of many elements has been investigated actively in 1900s. For potassium uptake, detailed kinetic analysis using liquid medium suggested that it is the carrier site present in the roots that allow for potassium ion (K⁺)-selective uptake (Epstein et al. 1963). For the K⁺ transporters that were subsequently identified, molecular biological studies are now progressing to understand the mechanisms that confer K⁺ selectivity on them.

Since the recognition of radiocesium as an environmental contaminant, the relationship between K⁺ and Cs⁺ absorption has been investigated using hydroponics, with the goal of reducing the absorption of radiocesium by plants (Zhu and Smolders 2000). The results of many previous studies, including the inverse proportional relationship between K⁺ supply and Cs⁺ uptake, and the identification of the Cs⁺-permeable K⁺ transporter Arabidopsis HAK5 (Qi et al. 2008), have provided an important scientific basis for the effect of potassium fertilization in crop production on contaminated agricultural lands after the Fukushima accident. Indeed, potassium fertilization has been very successful in reducing radiocesium contamination in agricultural products (Kato et al. 2015; Kubo et al. 2017).

However, studies that have contributed to accumulating basic knowledge through hydroponic systems have mainly been conducted using herbaceous plants that are easy to handle in the laboratory. Do the trees contaminated with radiocesium in the forests of Fukushima have the same mechanisms of uptake and transport of cesium that we know about? One way to answer this question would be to apply the classic experiment of growing plants in an environment with controlled K⁺ and Cs⁺ concentrations to investigate the content of both elements in the plant body to trees. Therefore, we conducted a study using hydroponically grown Konara oak (Quercus serrata Thunb.) to analyze the characteristics of Cs⁺ uptake in the context of its relationship with K⁺ (Kobayashi et al. 2019).

5.2 The Effect of K Nutrition on the Cs Content

Oak seeds harvested from a single Konara oak tree were incubated in vermiculite for germination and further cultivated in the hydroponic medium for raising seedlings. Then, the 4-seek-old oak seedlings were transferred to the hydroponic medium containing 50, 200, or 3000 μM K⁺ and 0.1 μM Cs⁺ (Table 5.1, Kobayashi et al. 2019). After 4 weeks, the content of K and Cs in the seedling developing 4 leaves at the top (upper leaves) and another 4 at the bottom node (lower leaves) was analyzed using the inductively coupled plasma-mass spectrometry, and it was found that Cs content in the seedlings cultivated with 3000 μM K⁺ was lower than other seedlings (Table 5.1, Kobayashi et al. 2019). The difference, however, was not as large as we had expected. The Cs content in the shoot tissues of the seedlings grown with 50 μM K⁺ was only twice as high as that in the seedlings grown with 3000 μM K⁺ (Table 5.1, Kobayashi et al. 2019). This is markedly different from what we know from similar experiments conducted on several herbaceous plants, which showed that plant Cs contents can drop below one-tenth when the K concentration in the nutrient solution increases from 50 to 1000 μM (Zhu and Smolders 2000). The K content in oak seedlings was increased as the K⁺ concentration in the hydroponic medium increased (Table 5.1, Kobayashi et al. 2019). The difference in the shoot K content between cultivating under 3000 μM K⁺ condition and under 50 μM K⁺ condition was almost twofold (Table 5.1, Kobayashi et al. 2019). Similar observation was reported for the soybean plants grown under 30 and 3000 μM K⁺; fourfold difference in shoots and twofold difference in roots (Nihei et al. 2018). Plants in general will have a mechanism to keep K content stable under conditions in which the K⁺ concentration in the culture medium fluctuates in the 100-fold range.

Table 5.1 Growth and elemental content of 8-week-old oak seedlings grown in the preculture medium containing K⁺ at various concentrations and 0.1 μM Cs⁺ (n = 6–11) (Kobayashi et al. 2019)

To understand the characteristics of Cs⁺ dynamics in plants, the discrimination factor (DF) based on the ratio between K and Cs has been frequently referred. Here, we calculated K/Cs discrimination factor for oak seedlings by dividing K/Cs value in plant tissue by K/Cs value in the culture medium (Table 5.2). The result was what could be called unexpected. Under 200 μM K⁺ condition, the K/Cs DF was almost 1 in all tissues (Table 5.2), which means that Cs⁺ and K⁺ are absorbed and accumulated without discrimination. The K/Cs DF was further lowered when oak seedlings were cultivated under 3000 μM K⁺ condition (Table 5.2). Oak seedlings never preferentially absorb K⁺ than Cs⁺ under K-rich environments. Such results have not been observed for herbaceous plants (Zhu and Smolders 2000). In addition, the result that the lower the K⁺ concentration in the medium, the higher the K/Cs DF value also does not fit with previous findings. In Arabidopsis, K-starvation activates HAK5-mediated Cs⁺ and K⁺ absorption, which causes increased Cs⁺ and K⁺ uptake rate while decreasing K⁺ selectivity in root uptake process (Qi et al. 2008). In K-rich environment, HAK5 expression is reduced and K⁺ uptake comes to largely depend on the K⁺-selective channel, AKT1 (Rubio et al. 2010), resulting in the elevated K/Cs DF value. To investigate whether such a shift in the K⁺–Cs⁺ uptake mechanism in response to K nutrition occurs in oak seedlings, we conducted an ion uptake experiment using radiotracers (Fig. 5.1). The result was that neither K⁺ nor Cs⁺ uptake in 2 h from the medium containing 50 μM K⁺ and 0.1 μM Cs⁺ differed significantly by the K nutrition in the hydroponic medium in which they were grown for more than 3 months (Table 5.3, Kobayashi et al. 2019). However, when the ratio between the K⁺ uptake amount and Cs⁺ uptake amount was calculated for each individual sample, it was found the value was slightly lower in the seedlings grown under 3000 μM K⁺ condition (Kobayashi et al. 2019). Therefore, it seems that the K⁺–Cs⁺ uptake mechanism changes a little in response to K nutrition in the opposite direction of that seen in Arabidopsis.

Table 5.2 K/Cs discrimination factor (DF) values in hydroponically grown oak seedlings

Fig. 5.1

Experimental procedures. Oak seedlings were grown hydroponically in the plant growth chamber with a controlled day/night cycle of 12 h/12 h and 25 °C/20 °C (Kobayashi et al. 2019)

Table 5.3 The effect of K concentration in the preculture medium on the uptake and transport of Cs⁺ and K⁺

5.3 Inhibition of Cs⁺ Uptake Through Competition Between K⁺ and Cs⁺

Increased K⁺ concentration in the medium can competitively reduce Cs⁺ uptake (Handley and Overstreet 1961), which can be another mechanism in which the effect of K nutrition on decreasing Cs accumulation is achieved. The competition between Cs⁺ and K⁺ in uptake process was evaluated using oak seedlings grown under 200 μM K⁺ and 0.1 μM Cs⁺. We incubated these seedlings with ⁴²K and ¹³⁷Cs radiotracers with either 50, 200, or 3000 μM K⁺ and 0.1 μM Cs⁺ for 2 h, and calculated the amount of Cs⁺ and K⁺ uptake based on the radioactivity in each tissue (Table 5.4, Kobayashi et al. 2019). This experiment showed that 3000 μM K⁺ in the medium can competitively reduce Cs⁺ uptake by one-third (Table 5.4, Kobayashi et al. 2019). The intensity of competition between the two ions in oak seems to be roughly comparable to that in rice (Oryza sativa). Similar radiotracer experiment conducted with rice seedlings demonstrated that Cs⁺ uptake rate drops by one-fifth as the K⁺ concentration in the medium increased from 100 to 1000 μM (Kobayashi et al. 2016).

Table 5.4 The effect of K concentration in the uptake medium on the uptake and transport of Cs⁺ and K⁺

5.4 Uptake and Transport of Cs⁺ in Oak and Rice Plants

Radiotracer experiment can describe the characteristics of ion uptake and further transport inside the plant quantitatively. Application of ⁴²K and ¹³⁷Cs at the same time allows simultaneous tracking of K⁺ and Cs⁺ and calculating the K⁺:Cs⁺ ratio for each tissue of each individual plant. This is very effective to detect any differences in the transport process of both ions. Consequently, a selective K⁺ uptake and root-to-shoot transport over Cs⁺ was clearly demonstrated for the rice plant grown under 274 μM K⁺ and 0.1 μM Cs⁺. The K⁺:Cs⁺ ratios for the rice root and leaves were approximately 5-times and 20-times higher than in the medium, respectively (Fig. 5.2). In contrast, the K⁺:Cs⁺ ratio for the oak roots was similar to that in the medium (Fig. 5.2). This result supports the possibility that K⁺ and Cs⁺ are not so discriminated during the root uptake. The value slightly increased in the shoot, indicating that oak has a mechanism to filter out K⁺ during the root-to-shoot transport process.

Fig. 5.2

Comparison of ion selectivity on uptake and transport processes between rice and oak. The values for rice were cited from the precious report (Kobayashi et al. 2016) and those for oak seedlings were calculated using the values in Table 5.3

5.5 Conclusions

The results of the experiment in hydroponically grown oak seedlings are consistent with precious findings in herbaceous plants in terms of the general theory that K⁺ supply reduces Cs⁺ uptake. However, there are noticeable differences in each aspect of the mechanism. It is suggested that oak root tissue uptakes K⁺ and Cs⁺ without apparent discrimination, and that the effect of K⁺ supply in reducing Cs⁺ accumulation is mainly due to ion competition. The property of the molecule functioning in K⁺–Cs⁺ uptake in oak root can be different from the known K⁺–Cs⁺ uptake transporters found in herbaceous plants.