Effectiveness of medium pruning on the reduction of radioactive cesium concentration in new shoots of tea plants in Ibaraki Prefecture, Japan

Abstract

Tea (Camellia sinensis (L.) Kuntze) plantations in the northern part of Ibaraki Prefecture in Japan, approximately 100 km southwest of the Fukushima Daiichi Nuclear Power Station (FDNPS), were contaminated with radioactive cesium (Cs) released from the power station during the Fukushima Daiichi Nuclear Disaster in 2011. Radioactive Cs concentrations in new shoots were monitored over the 3 years since June 2011. The use of conventional skiffing management resulted in 64% lower radioactive Cs concentration in October 2011 compared with that in June 2011, and the concentration continued to decrease gradually thereafter. Medium pruning to approximately 40 cm from the ground decreased radioactive Cs concentrations in the first tea crop of 2012 and 2013 by 24–25% more than conventional skiffing management.

Key words:

• nuclear disaster
• radioactive cesium
• tea
• new shoot
• prune

INTRODUCTION

Tea (Camellia sinensis (L.) Kuntze) districts in Ibaraki Prefecture were contaminated with radioactive cesium (Cs) released from the Fukushima Daiichi Nuclear Power Station (FDNPS), Tokyo Electric Power Co. (TEPCO) during the nuclear disaster that followed The Great East Japan Earthquake that occurred in northeastern Japan on March 11, 2011. Radioactive Cs concentrations in the first tea crop in May 2011 were above the permissible limit in food (500 Bq kg⁻¹ fresh weight of raw tea leaves), and distribution of this batch was restricted (Ministry of Health, Labor and Welfare 2011). Hirono (2011) reviewed reports on the impact of the Chernobyl nuclear accident on tea plantations in Turkey. Additionally, Nonaka (2014) reviewed reports on the development of techniques that decreased radioactive Cs concentration in tea plants in Japan after the nuclear disaster in 2011. Matsumoto et al. (2012) and Shiraki et al. (2012b) introduced medium pruning of tea plants as a method to decrease radioactive Cs concentration in new shoots; however, radioactive Cs concentration in tea plants and the effectiveness of pruning varied across geographical locations based on their distance from FDNPS and plantations. Therefore, we investigated radioactive Cs concentration after the nuclear disaster in 2011 in a major tea plantation located in the northern part of Ibaraki Prefecture. In addition, we monitored radioactive Cs concentration in new shoots over 3 years and examined the effectiveness of medium pruning in reducing radioactive Cs concentration in new shoots of tea plants.

MATERIALS AND METHODS

Changes in radioactive Cs concentration in new shoots of tea plants under conventional skiffing management

Our study was conducted in a tea plantation in a major tea district located in the northern part of Ibaraki Prefecture, approximately 100 km southwest of the FDNPS. We continuously monitored the changes in radioactive Cs concentration in new shoots of 16-year-old tea plants (Camellia sinensis cv. ‘Yabukita’) in 2011 under conventional skiffing management. Conventional skiffing management included the following practices: (1) light trimming of the canopy on June 8, 2011, (2) cutting of rapidly grown shoots on November 22, 2011, (3) spring skiffing on March 30, 2012, (4) skiffing after the first tea crop on June 8, 2012, (5) spring skiffing on March 30, 2013, and (6) skiffing after the first tea crop on June 11, 2013.

Tea plants were grown 30 cm apart within a 1.8-m row. The soil type in the plantation is classified as Haplic Andosol. New shoots were periodically trimmed from tea plants that were 110 cm wide, with a 30–70 cm plucking surface length.

Leaves were collected on June 3, 2011, 3 months after the nuclear disaster, and subsequently on July 21, 2011, October 11, 2011, May 25, 2012, August 1, 2012, May 26, 2013, and July 26, 2013. New shoots were freshly collected or kept frozen for approximately 5–24 months and naturally thawed, before they were placed in Marinelli beakers for the measurement of radioactive Cs concentration. Radioactive Cs concentrations were measured by gamma-ray spectrometry using a germanium semiconductor detector; the equipment that was used included GEM40-S (ORTEC, USA) and GX3019 R (Canberra, USA) in 2011, INSP-14 (Canberra, USA), MFG-DSPEC-JR 2.0 (SEIKO EG&G, Japan), GEM-30 (ORTEC, USA) in 2012, and GEM40P4-76 (AMETEK, USA) and GEM-30 (ORTEC, USA) in 2013. Radioactive Cs concentrations were calculated as the sum of ¹³⁴Cs and ¹³⁷Cs on each sampling date, corrected in the decay rate, and expressed in Bq kg⁻¹ fresh weight of tea leaves.

Effectiveness of medium pruning in reducing radioactive cesium concentration in new shoots of tea plants

To examine the effectiveness of medium pruning in reducing radioactive Cs concentration in new shoots of tea plants, we used a second tea plantation also located in the northern part of Ibaraki Prefecture, and having the same soil type. We compared the effectiveness of medium pruning with conventional skiffing management. Medium pruning included trimming of branches to approximately 40 cm from the ground after the first tea crop, on June 8, 2011. In the medium pruning plot, new shoots of the second tea crop were not plucked in the same year. Conventional skiffing management involved trimming of the canopy for regeneration after the first tea crop on June 8, 2011, while new shoots of the second tea crop were plucked on July 14, 2011. Additionally, some 9-year-old (in 2011) tea plants (cv. ‘Yabukita’) in a separate area were subjected to medium pruning or trimming on June 8, 2011, and skiffing after the second tea crop on July 30, 2012, while others in both plots, that were under conventional skiffing management, were subjected to trimming, cutting and skiffing as described in the preceding paragraph. Leaf plucking was carried out on May 31, 2012 (first tea crop), on July 26, 2012 (second tea crop) and on May 26, 2013 (first tea crop). Radioactive Cs concentrations were measured in new shoots collected on July 14, 2011 (second tea crop), May 31, 2012 (first tea crop) and May 26, 2013 (first tea crop), but the branches, skiffed leaves and new shoots of the second tea crop of 2012 and 2013 were not measured. New shoots were plucked from tea plants that were 100–131 cm wide, with a 100-cm-long ridge. Raw leaf yield, the number of new shoots and radioactive Cs concentration were recorded per m² of farmland. Additionally, the presence or absence of pruning resulted in differences in ridge width; the ridge in the plot that was subjected to pruning was narrower than that under conventional skiffing management. Radioactive Cs concentrations were measured and calculated as described in the preceding paragraph.

RESULTS AND DISCUSSION

Changes in radioactive Cs concentration in new shoots of tea plants under conventional skiffing management

Radioactive Cs concentration in new shoots was 195 Bq kg⁻¹ on June 3, 2011, 3 months after the nuclear disaster. Conventional skiffing management decreased radioactive Cs concentration in new shoots substantially, to 70 Bq kg⁻¹, 7 months after the nuclear disaster, and the concentration continued to decrease gradually thereafter, to 24 and 10 Bq kg⁻¹, 14 and 26 months after the nuclear disaster, respectively (Fig. 1). A rapid decline in radioactive Cs concentration was also reported in tea plantations in Turkey after the Chernobyl nuclear accident (Hirono 2011; Şaziye 2012), as well as in Kanagawa Prefecture (Shiraki et al. 2012a, 2012b, 2013) and Shizuoka Prefecture (Matsumoto et al. 2012) in Japan after the nuclear disaster at FDNPS.

Figure 1 Changes in radioactive cesium (Cs) concentration in new shoots of tea plants.

All samples were periodically collected from 16-year-old (in 2011) tea plants (Camellia sinensis cv. ‘Yabukita’). The bar on July 26, 2013, shows only ¹³⁷Cs, because ¹³⁴Cs was not detected in the sample (< 4.0 Bq kg⁻¹ fresh weight). Error bars indicate ± standard error.

Shiraki et al. (2013) reported that the rapid decline of radioactive Cs concentrations in old leaves, twigs and branches until October 2011 could be partly attributed to the removal of tea leaves, which were directly exposed to radioactive Cs, during skiffing and harvesting. We suggest that the rapid decline of radioactive Cs concentration in new shoots until October 2011 could also be partly attributed to the removal of tea leaves and twigs by skiffing, although radioactive Cs adhering to the surface of old leaves and twigs might be absorbed and translocated to new shoots. Therefore, the rapid decline could also be partly attributed to the removal of new shoots, which were already contaminated from old leaves and twigs, by plucking. Shiraki et al. (2013) reported that the rapid decline of radioactive Cs concentrations in old leaves, twigs and branches could also be partly attributed to washing by rain and to the falling of 2010–2011 winter leaves until the end of September 2011. We also believe that the rapid decline of radioactive Cs concentration in new shoots in the present work could be attributed to the same mechanisms.

Effectiveness of medium pruning in reducing radioactive cesium concentration in new shoots of tea plants

The radioactive Cs concentration in the new shoots decreased, irrespective of pruning, as shown in Table 1. Further, medium pruning in 2011 decreased the radioactive Cs concentration (Bq kg⁻¹ fresh weight) in new shoots by 24–25% more than conventional skiffing management in 2012 and 2013 (Table 1). And the radioactive Cs content (Bq m⁻² of farmland) was reduced in 2012. A similar trend was also observed in 2013. Additionally, medium pruning in 2011 significantly decreased the raw leaf yield and number of new shoots in 2012; however, the difference between the plots in raw leaf yield in 2013 was less than that in 2012.

Table 1 Effect of medium pruning and conventional skiffing management on raw leaf yield, the number of new shoots and radioactive cesium (Cs) concentrations in new shoots of 9-year-old (in 2011) tea plants (Camellia sinensis cv. ‘Yabukita’)

					Radioactive Cs concentraion in new shoots of tea plants
Date and type of management	Raw leaf yield (g fresh weight m^–^2)		Number of new shoots (number m^–^2)		(Bq m^–^2)		(Bq kg^–^1 fresh weight)		Index ^b
July 14, 2011 (second tea crop)
Conventional skiffing management ^a	255		−		24.9		176.0		−
May 31, 2012 (first tea crop)
Medium pruning ^a	154		161		2.6		17.0		76
		*		*		*		ns
Conventional skiffing management	322		394		7.2		22.4		100
May 26, 2013 (first tea crop)
Medium pruning	205		470		1.1		5.2		75
		*		*		ns		ns
Conventional skiffing management	233		728		1.6		7.0		100

Plants in both plots were under conventional skiffing management after June 9, 2011. Both plots were n = 3, except for the plot under conventional skiffing management that was n = 2 on July 14, 2011. The significance of the difference between the treatments was examined with Student’s t-test (*: p < 0.05; ns: not significant).

^a Medium pruning: approximately 40 cm from the ground after the first tea crop on June 8, 2011, 3 months after the nuclear disaster.

^b Radioactive Cs concentration in new shoots under medium pruning compared to those under conventional skiffing management (100).

Shiraki et al. (2012b) reported that medium pruning after the first tea crop in 2011 decreased radioactive Cs concentration in new shoots by 55% in the second tea crop in 2011, 4 months after the nuclear disaster, compared to conventional skiffing management. They also reported that medium pruning decreased radioactive Cs concentration by 67% in the first tea crop in 2012 compared to conventional skiffing management. In our study, medium pruning decreased radioactive Cs concentrations in the first tea crop of 2012 and 2013 by 24–25% more than did conventional skiffing management.

The differences between the two studies could be attributed to the magnitude of radionuclide contamination in tea plantations. Firstly, we assumed that since the tea plantation in our study was exposed to higher radioactive Cs concentration than that in the study by Shiraki et al. (2012b), radioactive Cs concentration would also be higher after medium pruning. Secondly, we believe that radioactive Cs was concentrated in the few new shoots after medium pruning. Climatic differences may have influenced the results, since the temperature in our study area was lower than that in the study by Shiraki et al. (2012b). The number of new shoots after medium pruning in our study might also be lower than that in their study. The radioactive Cs that moved from the branches and twigs would be distributed in a smaller number of new shoots. Therefore, the concentration of radioactive Cs in individual new shoots after medium pruning in our study might be higher than that in the study by Shiraki et al. (2012b). Thirdly, the differences in the plucking and pruning times between the two studies could also explain some of the differences between the results obtained. Overall, the plucking and pruning times in the study by Shiraki et al. (2012b) were earlier than those chosen based on climatic factors in our study. However, in 2011, tea plants in our study were exposed to plucking and pruning 6–10 days earlier than those in the study by Shiraki et al. (2012b) for the same year. Lastly, we assumed that the shoot architecture of tea plants might be different between the two studies because of the different ages of the tea plantations. Overall, our study demonstrated that medium pruning is effective in reducing radioactive Cs concentration in new shoots by at least 25% and, consequently, the risk of radioactive Cs contamination in tea.

ACKNOWLEDGMENTS

This study was supported by the research project “Development of Decontamination Technologies for Radioactive Substances in Agricultural Land” conducted by the Ministry of Agriculture, Forestry and Fisheries of Japan. The authors are grateful to Mr. K. Nonaka and other staff members of the National Institute of Vegetable and Tea Science of the National Agriculture of Food Research Organization (NARO). Additionally, all of the other cooperating NARO Institutes and Universities and nearby Prefectures are gratefully acknowledged.