Abstract
Research carried out by me and my group over the last almost four decades are summarized here. The main emphasis of my work was and continues to be on plant physiology using radiation and radioisotopes. Plants live on water and inorganic elements. In the case of water, we developed neutron imaging methods and produced 15O-labeled water (half-life 2 min) and applied them to understand water circulation pattern in the plant. In the case of elements, we developed neutron activation analysis methods to analyze a large number of plant tissues to follow element specific distribution. Then, we developed real-time imaging system using conventional radioisotopes for the macroscopic and microscopic observation of element movement. After the accident in Fukushima Daiichi nuclear power plant, we, the academic staff of Graduate School, have been studying agricultural effects of radioactive fallout; the main results are summarized in two books published by Springer.
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Introduction
First of all, I would like to express my sincere thanks to the members of Hevesy Medal Award Selection Panel 2016 as well as to all the people who supported me for the Hevesy Medal Award. I would like to summarize in this paper the kind of research I have been doing in my life.
After determining the half-lives of long-lived nuclides, namely 91Nb and 92Nb for my PhD thesis, I have been targeting plant physiology for many years and applying radiochemical approaches. Though plants live on only inorganic elements and water, little is known about the distribution or movement of these and water in a living plant. For example, photosynthesis is known to produce sucrose out of CO2 and water but water itself has not been gathering the attention. Water was simply granted to exist there but is playing an important role for the chemical process of the energy conversion, form light to chemical energy. However, we do not know how water is absorbed and transferred in the plant.
Therefore, my first interest was water, in particular how water is distributed and move within a living plant as well as its absorption from roots. To study water distribution in plants, neutron beam was applied which produced water-specific image of the plant. The neutron beam allowed imaging not only water itself but also the morphological development of the plant tissue which was not visualized earlier, such as seed formation in pods or roots imbedded in soil. Then, 15O labeled water was used to trace the water movement in detail and we found that tremendous amount of water was always flowing out from xylem and re-entered the xylem again, indicating that there is a circulation of water flow in the stem of a soybean plant.
Element was another target of my research. There are 17 essential elements for plant growth, but little is known about the overall accumulation or movement manner of the elements. Activation analysis was performed for a large number of plant tissues and the element-specific accumulation pattern in the plant was found; and this pattern was maintained in the same way throughout the developmental stage. When flowering was induced, Mg-specific distribution pattern disappeared which led later to develop the production of the radioactive magnesium tracer, 28Mg (half-life: 21 h).
Since each element showed its specific distribution pattern in a plant, next step was the development of real-time imaging of the elements. Though imaging using positron emitters has been developed especially in medical field known as PET (Positron Emission Tomography), its resolution cannot be less than mm because of the relatively high positron energy. In the case of fluorescence imaging, imaging under light condition is not possible and the amount of the element in the image could not be measured. Therefore, the real-time RI imaging system was developed by us using not only gamma-ray but also beta emitters which are commercially available so that other people can also use them. We have been successful in developing the systems both for macroscopic and microscopic imaging. The real-time movement of the elements can now be visualized and analyzed using 14C, 22Na, 28Mg, 32P, 33P, 35S, 42K, 45Ca, 54Mn, 55Fe, 59Fe,65Zn, 86Rb, 109Cd, 137Cs, etc. The image of 14CO2 gas fixation provided that the photosynthate was moved quickly to produce the meristem of the root tip.
After Fukushima nuclear accident, our group studied the agricultural consequences of radioactive contamination from the Fukushima Daiichi reactors. I was able to edit two English books summarizing our data, published by Springer. The on-line version of the first book was accessed more than 50,000 times a year and the third book is now going to be published next year.
A brief survey of many research projects carried out over the years by me and my group is given below.
Research topics: methodology, results and discussion
Since plants live on water and inorganic elements, the applied radiation or radioisotope method used in presented in Fig. 1.
Neutron beam imaging: water distribution [1–27]
Flower, wood disk and seeds
Neutron beam was applied to observe water-specific image in a living plant, since neutron imaging provided the highest resolution yet attainable for water in tissue. With high specificity for water, neutron beam could image water movement in seeds or in roots imbedded in soil as well as in wood disks and meristems during the development. Through neutron image analysis, we were able to analyze the activity of intact cells or tissue.
Since more than 80% of the living plant is consisted of water, water image indicates the tissue image itself. Figure 2 is one of the examples of the water-specific image of lily and from this image we can estimate how the pistil and stamen inside the bud are developing. Similar to the flower bud, the seed formation inside the pod became visible. In the case of agricultural industry, to create the sterile plant which does not develop the seed, is one of the important feature to be able to provide the seeds every year. (not clear what the author wants to say in the last sentence).
Another requirement from flower industry is to keep the flowering time of the cult flowers longer. One of the solutions is to change the viscosity of water supplied to the cut flowers. When the xenon gas was dissolved in water under pressure and supplied to the cut flowers, it was found to slower the senescence, which was studied using a carnation flower. To analyze the effect of the water prepared, spatial distribution of the water within the flower was constructed (Fig. 3). When the water decreasing part in flower was visualized, keeping water around the ovule was crucial and applying water with high viscosity, such as Xe gas dissolved water, was found to be effective. In the case of roses, there was a specific problem called “bent neck” phenomenon. When the flower is bent, it never comes back again to the straight position even with fresh supply of plenty of water. The neutron beam imaging suggested that there are two types of tissues in the stem below the flower, which allows water absorption after draught condition.
Several kinds of tree were downed and water distribution within the bark disk was investigated. The water in the bark is liable to evaporate fast from the cut surface, therefore, the trees were cut down as a log on the day of neutron irradiation and was further cut to the wood disk just before the imaging. In the case of acacia, although the heartwood formation was not observed from outside, water distribution showed as if heartwood was developed where water amount was very low. The color of the Japanese cypress did not show any heartwood formation like acacia, from the water distribution image of the disk (Fig. 4). Lumber of cedar trees are very popular materials in Japan for constructing houses or making furniture but water amount at heartwood cannot be known until downed. Even the same kind of the trees growing in the neighbor, the amount of the water at heartwood is different. When the water amount in the heartwood is high, it is hardly possible to remove all of the water from the log by drying process and remaining water causes distortion or cracks after construction. When neutron beam was applied, water was found to distribute according to the annual ring inside the cedar disk. Neutron images were taken periodically during the drying process and water amount and distribution of the disk were analyzed (Fig. 5).
The water absorption in seeds is still not known well, though it was estimated that the seeds do not absorb and accumulate water homogeneously. Neutron beam imaging provided some clue to this question. Figure 6 shows the water image in 5 different seeds, broad bean, corn, morning-glory, wheat and rice. They were put in water container and they were taken out every 2 h to take water images. It was shown that water was accumulated to the embryo, meristem part, after absorption. The specific accumulation of water in the seed was well visualized for the corn seed, where water was hardly accumulated to the other parts but to embryo (Fig. 6).
Roots imbedded in soil
When a thin container packed with soil was prepared to grow the plants, neutron image provided not only water movement in soil but also morphological development of the roots, nondestructively (Fig. 7). Therefore, at first a thin Al container (3–5 mm in thickness) was prepared to analyze how the roots are absorbing water from the soil, since there was not any report about water movement within 1 mm form the surface of the roots. Then we prepared two kinds of water absorbing polymers, namely poly-vinyl alcohol polymer and poly-acrylic polymer, and compared the water supplying activity to the roots (Fig. 8). When the soybean was grown in the soil mixed with the polymers, the roots grown in poly-acrylic polymer did not grow well and changed the color, since the roots could not absorb water from the polymer. The size of the polymer did not change during the week and the color of the soil turned darker, indicating that most of the water in soil was absorbed by the plant. On the other hand, the size of the poly-vinyl alcohol polymer decreased, indicating that the water in the polymer was supplied to the roots. The study of the polymer function was further developed to apply these polymers in semiarid area to maintain water in soil of farming land.
The neutron imaging method was developed from two dimensional to spatial one. To get the spatial image, the plant was grown in a pipe containing soil and was rotated to get neutron images from different angles. The 180 projection images were taken while rotating the sample from 0° to 180°, and the one line image at the same height was taken out of 180 images to construct one dissection image. Then the dissection images constructed every 50 μm were piled up to produce the spatial image (Figs. 9, 10). To our great surprise, most of the adjacent place of the root in soil was an empty space (Fig. 10), which suggested that water was not absorbed from the root as solution but as vapor. There has been a long discrepancy between the soil scientist and plant physiologist for the plant growth in soil. It was suggested that the case of the water culture, which was the base of the plant physiology, might be different from that of soil culture, at least in the way, how to absorb water. The spatial image of the plant root growing in a pipe provided much information on not only water absorption movement but also the relationship between the water absorption site and side root emerging site. The application of the neutron imaging provided the way to develop the in situ physiology of the plant. Especially, the morphological development of the roots shown by neutron imaging provided the fundamental data for soil condition including the effect of fertilizer.
Water movement using RI: water movement [28–35]
The first trial to analyze water absorption manner was studied using the cowpea plant supplied with 18F (half-life: 110 min) containing water, which was produced by irradiation of water with He+ beam. Cowpea plant was selected, since this is a draught tolerant plant and is an important crop among Asian and African countries. The plant developed a special tissue in the stem, which was suggested to function as water storage tissue but was not studied well. The water supplying function of the specific tissue under draught condition was shown using both neutron imaging and 18F tracer.
Then, two kinds of the cowpea plants selected in Africa were used. When water absorption manner of naturally divested cowpea plants, draught tolerant and sensitive one, were compared, it was very interesting to note that the amount of water absorbed by the draught tolerant cowpea, under normal condition, was far less than that of the sensitive one. But under draught condition, the tolerant one began to absorb much more water while the sensitive one could not absorb water. We take it as granted that the water absorption activity is very strong in draught tolerant plant so that it can survive under less amount of water in soil. Therefore, in the laboratory when we try to produce draught tolerant plant, we apt to add the water absorbing ability of the tolerant one to the sensitive one to convert the nature of the weak plant. However, naturally diverted cowpea plant showed that in usual case, water absorbing activity of the tolerant one is far behind to the sensitive one. It seems that they are saving the energy for the emergency.
Applying 18F-labeled water, water storage function of the tissue in the draught resistant cowpea plant was demonstrated. However, the different movement of 18F in water from that of 15O-labeled water was shown. Therefore, 15O-labeled water (half-life: 2 min) was used to measure trace amount of water movement. The measuring system was prepared and the performance of the system was developed so that the trace amount of the water moving in the soybean could be measured. Since 15O is a positron emitter, a pair of BGO detector was used and their performance was studied. Using the system, circulation of water movement within the stem was found for the first time. That is, tremendous amount of water was leaking out horizontally form the xylem tissue, which has been regarded as a mere pipe to transport water from root to the up-ground part (Fig. 11). After examining every possibility of the route for the water flowing out from the xylem, it was found that the leaked water from the xylem entered the xylem tissue again and then transferred to the upper part (Fig. 12). Because of the constant returning manner of the leaked water to the xylem, the water velocity in the xylem tissue was kept constant. The constant flow rate of the water in xylem tissue was measured by preparing three pairs of BGO detectors. From the known distance between the detectors and from the time when they first detected the radiation from 15O-labeled water coming up from the root, the flow rate between the detectors was found to be constant.
This water movement was also confirmed using 3H-labeled water (Fig. 13). In the case of 3H, the beta-ray energy from 3H was too low to be detected from outside the plant. Therefore, 3H-water was supplied for 5 s and the stem was cut after, 0, 10 and 20 s, 1 and 2 min and autoradiography was taken by an imaging plate. As is shown in the figure, the flowing out water from xylem tissue was spread out to all of the stem tissue after 20 s and gradually returning to the xylem. Through simulation, the newly absorbed water from the root replaced half of the water already existed in the stem within 20 min. This water circulation movement was also found even under high humidity when the transpiration, water loss from leaves, was hardly expected.
Neutron activation analysis (NAA): inorganic element distribution [36–60]
NAA was performed for many kinds of plant tissues. An element specific distribution profile was found in barley leaves. Each leaf showed gradient of the elemental concentration and the profile hardly changed throughout the developmental stage. Similarly, when morning-glory was analyzed from germination to seed formation by NAA, specific distribution pattern was found for each inorganic element and the tendency was confirmed in a 7-day seedling (Fig. 14). The result showed that K concentration in the leaf stem was always high and meristems are free from toxic elements. In the case of Cl and Br, their concentrations in the plant decreased during the developmental stage, suggesting that these elements evaporated and lost from the plant. Most of the heavy elements tended to accumulate in roots except for Cr and Mn (Fig. 15), suggesting that the element which has many chemical valences can easily move. Cobalt concentration in stem was higher compared to those of the other tissues in above-ground parts of the plant. Although the concentration of the Co is important to grow excellent grass in meadows, the reason is still not clear.
Sensitivity of Al in NAA is extremely high making it easy to detect in any sample. The Al concentration in root tip was measured in a morning-glory plant after 4 and 5 days of germination. The light period lasted during 07:00–19:00 for 4 days but on day 5 it ended at 15:00. There was a high fluctuation of Al concentration in root tip and was highest just before the end of the dark period, though the total amount of Al decreased gradually (Fig. 16). The fluctuation of Al indicates absorption and secretion of the Al from roots. Though the absorption and secretion of other elements such as K are known, Al has not been studied well since there are not any suitable and easily available radioisotopes for Al.
Using a large set of data on elemental profile of a whole plant, attempts were made to construct an element recycling scenario in soil–plant system since it is known that more than half of the N contained in a leaf was recovered by the plant before falling off to the ground. Before 28Mg tracer was available, Mg content in plant tissue was analyzed mainly by NAA. It was found that the amount of Mg absorption increased during the day time and drastic change in Mg concentration at the center cells of meristem appeared when flowering was induced; these results prompted further study to prepare 28Mg tracer.
NAA as well as prompt gamma-ray analysis was applied to food samples too, such as onions and beefs produced at different places, to measure levels of as many elements as possible. The elemental profile of the food showed specific features of elemental ratios, representing the production sites of the vegetables and cows.
Development of radioisotope imaging (RI) systems [61–80]
Two types of the real-time RI imaging system, namely macroscopic and microscopic, were developed. Both systems are designed to use conventional RI so that it is possible to perform imaging in our laboratory and not at special laboratory with special facilities. Although it is possible to produce RIs by an accelerator and use the RI at special facilities, the frequency of doing experiment is limited and the environment for plant research including growth chamber has to be installed. Therefore, it was much preferable and efficient to develop the imaging system if we could use the conventionally available RI. Plant sample has another problem: the up-ground part requires light and the root does not. To perform imaging from root in dark to up-ground part in light, fluorescent imaging method cannot be employed since it requires dark condition only. It was our first trial to develop real-time imaging system using conventional RI.
Macroscopic real-time RI imaging
The principle of RI imaging is shown in Fig. 17. Radioisotopes were supplied to the plant sample and the radiation emitted from the plant was converted to weak light by Cs(Tl)I scintillator deposited on a fiber optic plate (FOS). The light image was then collected by a highly sensitive CCD camera and processed through computer. The kind and thickness of the scintillator was studied and 100 μm deposition of Cs(Tl)I on FOS surface and was found to be suitable for this study. The sensitivity was more than ten times higher than that of an imaging plate (IP) which is now widely used for autoradiography. The higher sensitivity of the imaging means the accumulation time to get imaging is short, indicating that the successive images can be recorded smoothly and made to a movie.
The 3rd generation (Fig. 18) macroscopic imaging system has now been developed. Everything had to be kept in dark in the 1st generation; then the plant box was introduced in the 2nd generation where only the upper part of the plant was irradiated with light. Finally, the light was off only when camera was on in the 3rd generation so that weak radiation energy from radioisotopes such as 14C, 35S and 45Ca, etc. could be easily detected. With the 3rd generation system, it is possible to image relatively large plants which cannot grow in a plant box. Now we are able to image how CO2 gas is fixed by the plant by introducing 14CO2 gas to the plant.
The first trial was to image 32P-phosphate uptake movement in a soybean plant using the 1st generation system, since phosphate is reported to move fast in a plant and it sometimes represent the movement of water. When phosphate was supplied, it was transferred fast to the youngest leaves and then gradually transferred to the older tissue. It was interesting that the accumulation pattern of 32P-phosphate was different among the leaves and there was high accumulation of phosphate between the vain, shown as dots, in one kind of the leaf (Fig. 19). Since the image is based on radiation counts, we could treat the image numerically. For example how 32P was accumulated in seeds in pod was analyzed, and found that there was no difference of the transferring speed as well as the phosphate delivery between the two seeds (Fig. 20 bottom right). The lap-time images shown in Fig. 20 was treated with pseudo color. From the radiation image, we could not know the chemical form of the 32P. Therefore, we performed a chemical separation of the phosphate in the tissue and found that the chemical form of 32P was maintained as phosphate at least during the first 30 min after the treatment. Birdsfoot trefoil or Arabidopsis were used to image 32P-phosphate movement, which led to the phosphate transporter research in root tips using both macroscopic and microscopic imaging system (Fig. 21).
In order to show the difference between water and soil cultures, 32P-phosphate was supplied to rice seedlings and the movement of 32P was traced for 60 h. The uptake amount of 32P-phosphate was more than ten times higher for the plants grown in water culture compared to that in the soil culture. The growth of the plant is much faster when grown in water culture. Phosphate is known to adsorb on to the soil and plant root was able to take up the phosphate from the soil in the vicinity of the root which is shown as remaining root shape in the soil (Fig. 22). However, the yield of the rice grown in water culture is low compared to that grown in soil, which might suggest that when the nutrient was supplied in ion form it was easy to absorb and the plants become rather inactive in generating the next generation under too comfortable condition.
Tracer production (28Mg, 42K) [81–98]
28Mg (half-life: 21 h) and 42K (half-life: 12 h) were produced and applied for the first time in plant study. Aluminum film target was irradiated with He+ beam and 28Mg produced in the target was radiochemically separated (Fig. 23). Magnesium is a very rare element among the other inorganic elements, keeping constant concentration in the plant species, suggesting that the role of Mg is to keep the homeostasis of the plant activity. Sometimes it has been reported that the behavior of Mg is similar to that of Ca because of they belong to the same group in the periodic table. However, because of the overwhelming amount of Ca compared to that of Mg in plants, popular fluorescence staining method could not distinguish Mg from Ca for any meaningful conclusion. Using 28Mg tracer, basic properties of Mg movement in plants were clarified and the trial to find Mg specific transporter is now being developed.
42K is another nuclide applied to plant study for the first time. It was prepared from 42Ar (half-life: 30 a) gas by a milking method (Fig. 24). Potassium study was rather behind in plant research because of the lack of suitable tracers. Instead of K tracers, 86Rb was used to trace the K movement but there was a difference in movement. When all the K in the plant was replaced with Rb, the plants could not grow further. 42K tracer was useful, after Fukushima Daiichi nuclear accident, to study the relationship between the behavior of K and Cs, such as to analyze the reduction mechanism of 137Cs absorption by applying K to the plants.
Imaging the movement of various RI tracers including 14CO2 gas
As mentioned earlier, some of the radioactive nuclides applied to our systems were: 14C, 22Na, 28Mg, 32P, 33P, 35S, 42K, 45Ca, 54Mn, 55 Fe, 59Fe, 65Zn, 86Rb, 109Cd, 137Cs, etc. In the case of macroscopic system, tracer movement from root to up-ground part of Arabidopsis was visualized using various radioactive nuclides. Figure 25 shows how the inorganic elements, namely 22Na, 28Mg, 32P, 35S, 42K, 45Ca, 54Mn, and 137Cs, absorbed from roots were transferred to the up-ground part of Arabidopsis within 24 h. Among the nuclides, 28Mg, 45Ca, and 54Mn were very slow to move up-ground part from the root.
We were even able to image 14CO2 gas fixation manner. Figure 26 shows the 14CO2 gas application from different sites of the plant. When 14CO2 gas was supplied to the rosette of the Arabidopsis, the photosynthate was moved swiftly to the meristem of the main stem and roots, whereas when applied from the other part of the plant, the photosynthate moved to the meristem of the branch stem only and was not transferred to the roots. What derives the different route of the photosynthate is not presently known.
The 4-h application of 14CO2 gas to the up-ground part of the Arabidopsis seedling showed that the photosythate went downward and accumulated to the certain part of the root tip, indicating that the photosythate produced from this 4-h application of 14CO2 gas actually produced certain part of the root tissue. Besides 14CO2 gas, downward movements of the inorganic elements were observed. Among the elements, 32P or 35S moved toward the root tip when supplied from the leaf, whereas most of 59Fe or 45Ca stayed on the leaf. Figure 27 shows the movement of the two representative nuclides, namely 32P and 59Fe supplied from the leaf during 24 h with the microscopic image of 32P in root tips.
Microscopic real-time RI imaging system
The microscopic system was prepared by modifying a fluorescence microscope so that three kinds of images, namely light image, radiation image and fluorescence image, could be obtained at the same time (Fig. 28). The representative images of Arabidopsis applying 32P, 35S, 45Ca and 59Fe are shown in Fig. 29. In the case of Arabidopsis root, 32P movement even in a fine root of diameter of about 100 μm was analyzed.
There were many mutants created in Arabidopsis plants which limit the expression site of the transporter gene, such as phosphate. A mutant (C) was produced to express the transporter gene only at root tip. The gene expression of the mutant (C) was shown by fluorescent staining method. But at the same time, it is important to know whether phosphate is also transported at the same site or not. Therefore, 32P-phosphate image was taken for the mutant (C), and was shown that the site where transporter was expressed was also the site transporting the 32P-phosphate (Fig. 29). Though the expression of element specific transporter gene is gathering attention, gene expression shows only the gene activity and does not necessarily mean the element movement itself; therefore radioactive tracer imaging could provide the direct evidence of gene function.
Fukushima accident related work [99–133]
Immediately after the accident, a faculty specialist team was organized in a wide variety of areas including soil, vegetation, animal life, fishery, forestry, etc. to carry out the research studies, where my role was making up the team and the research projects. It was important that the results of these studies were useful for the recovery of the affected area; so my other role was to work for official announcement of the results. Twelve meetings were held to report research results since November 2011 and an easy-to-understand book was published in Japanese (Nakanishi, 2013). This book was published to allow a wide range of ordinary people to have a correct understanding of the impact of radioactive materials on agriculture. In addition, a collection of papers was published as a series of book (titled Agricultural implications of Fukushima nuclear accident) by Springer in 2013 and in 2016 and they were made available for free download on the web so that the results of the research and studies could be widely shared with foreign and domestic researchers. A follow-up 3rd book is going to be published early in 2017 by Springer.
Radioactive cesium distribution in a rice plant
The gaseous fission products released from Fukushima Daiichi nuclear accident was adsorbed on anything when they first touched, which was exposed to the air at the time of the accident. In the case of soil, the soil surface was the most contaminated part and the depth movement of the radioactivity is now about 1–2 mm/year which is the similar to that of the Chernobyl accident. About a month later, radioactive cesium (137Cs + 134Cs) was found to be the main radioactive nuclides remained in the environment. Therefore, the study was focused on the movement of radiocesium.
Since a lot of rice is produced in the Fukushima Prefecture, one of the main concerns of the people was how much radiocesium is absorbed by the rice plant. Applying K to the farming land was an excellent measure to reduce the absorption of radiocesium by the plant. Since Cs and K can behave in a similar fashion, any possible difference was studied in detail using 137Cs and 42K produced from the 42Ar generator. Real-time RI imaging system showed that when the rice is grown in contaminated soil, most of the radiocesium adsorbed on the soil was not picked up by the rice plants; this was different from the result of water culture where most of the radiocesium in culture medium was absorbed by the plant. Figure 30 shows the difference between the radiocesium uptake manner between water and soil cultures.
Micro-autography (MAR)
One of the important objectives of our study was to find out how and at what site the radioactive cesium was accumulated in the rice grain using radioactive tracer 137Cs. After supplying the 137Cs, the rice grain during the developmental stage was harvested and the distribution of 137Cs was recorded. Figure 31 shows the trial to construct a three dimensional image of 137Cs in a brown rice gain 15 days after flowering. Sliced grain was placed on an imaging plate for exposure and a three dimensional image was constructed from all these images. It is evident from the figure that 137Cs was distributed in the embryo and the periphery of endosperm; outer skin of the grain was removed as bran when milled.
To examine the 137Cs accumulated part in more detail, a micro-autoradiography (MAR) method was developed, where nuclear emulsion film was prepared and attached to the thin section of the sample (Fig. 32). The figures in the middle show the comparison of 109Cd distribution image obtained by an imaging plate (IP) to that by MAR at the connection part of the root and up-ground part in a rice plant demonstrating that MAR provides an image with higher resolution. In the case of 137Cs distribution of the rice grain, the accumulation site was also studied. Although 137Cs was accumulated at embryo indicated by an IP, 137Cs concentration was low at the specific site of plumule and radicle, and the surrounding part of these tissue contained higher amount of 137Cs.
Conclusions
Application of radiation and radioisotopes provide new findings what no other methods could do. Especially, in the case of water, only neutron beam provides the water specific image and for water movement itself; without using 15O it is hardly possible to calculate the small amount of water.
The relationship between the element and the plant is one of the important issues, since plants live on inorganic elements. However, the study of the element whose radioactive isotope is not available has not been developed well. For example, the research on B or Si has been far behind compared to the other elements such as P or S. Therefore, it is very important to prepare and apply the radioisotopes for research which are not commercially available even if they have relatively short half-lives, such as 15O, 38 Mg and 42K.
There are many advantages of using radioactive tracers especially for the real-time imaging. Image is the direct evidence to show the activities of living plants. When RI imaging is referred, only static imaging using an imaging plate is widely performed and the real-time imaging using the conventional radioisotopes has not been studied well. The wide application of RI imaging is expected not only in plant study but also for foods, industrial materials, etc.
In the case of Fukushima, after the accident the local government had set up the system to measure the radioactivity of all agricultural products. In the case of rice, radioactivity of all the rice grains produced in Fukushima was measured prior to marketing. Now no bag was found showing higher radioactivity than the regulation value of 100 Bq/kg. However, since the half-life of 137Cs is 30 years, we still have to continue the study to understand the radio-contamination effect in agricultural environment including forests.
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Ohya T, Tanoi K, Iikura H, Rai H, Nakanishi TM (2008) Effect of rhizosphere pH condition on cadmium movement in a soybean plant. J Radioanal Nucl Chem 275:247–251
Tanoi K, Saito T, Iwata N, Ohmae Y, Hirose A, Kobayashi NI, Iwata R, Nakanishi TM (2011) The preparation of 28Mg and analysis of Mg uptake in rice plant. Radioisotopeps 60(8):299–304 (Japanese)
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Fujimura S, Yaginuma T, Sakuma Y, Satou M, Tanoi K, Nakanishi TM (2012) Decontamination of radioactive caesium in the paddy field in Fukushima prefecture by removal of the surface soil using an earth moving equipment (Rear Grader Blade). Radioisotopes 61(6):327–330 (Japanese)
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Acknowledgements
For neutron radiography and neutron activation analysis experiments, I would like to thank deeply all the people in charge of using JRR3 facilities at the Japan Atomic Energy Agency for helping to get our work done smoothly. I also want to express my gratitude to the people at National Institute for Quantum and Radiological Science and Technology and, special thanks to Prof. Ren Iwata at Tohoku University to help us to produce 28Mg tracer. I would like to express my sincere gratitude to Mr. Kazumasa Ikushima, the president of Musashi Engineering, Inc. for the continuous financial support of my research.
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Nakanishi, T.M. Research with radiation and radioisotopes to better understand plant physiology and agricultural consequences of radioactive contamination from the Fukushima Daiichi nuclear accident. J Radioanal Nucl Chem 311, 947–971 (2017). https://doi.org/10.1007/s10967-016-5148-z
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DOI: https://doi.org/10.1007/s10967-016-5148-z