The Transition of Radiocesium in Peach Trees After the Fukushima Nuclear Accident

Abstract

In this chapter, we introduce the effects of radiocesium released by the Fukushima Daiichi nuclear accident on fruit trees, especially the change of radiocesium in fruit during the past 6 years. We investigated radiocesium and 40 K in peach during the maturity of its fruit chronologically for 6 years. In the investigation during one crop period, the concentration of radiocesium in young fruit 15 days after the full bloom was the highest, and this result was common in all the investigated years. After that, the concentration of radiocesium decreased as the fruit became bigger; the decrease until 60 days after the full bloom was considerable. This tendency was also common among all investigations conducted until 2016. Though the concentration of 40 K during the same period also decreased in the same way as radiocesium, the rate of the decrease from 15 to 30 days after the full bloom was different. When looking at the chronological transition, the concentration of radiocesium in harvested fruit decreased by one third every year from 2011 to 2013. However, such decrease could not be seen from 2014 to 2016. While the concentration in the harvested fruit tended to stop decreasing, the concentration in fruit 15 days after the full bloom tended to decrease over the years from 2012 to 2016. During the past 6 years, there was no year-over-year decrease in the concentration of 40 K in fruit. The reason why the transition of radiocesium in fruit varied according to their stage of maturity was because the difference in timing to use the tree's nutrient reserves. To understand the year-over-year transition of radiocesium in peach, the amount of 137 Cs in every part of the tree was measured. When comparing the distribution of 137 Cs and 40 K in the peach trees, it was found that 137 Cs was existing in the body of the tree, which was contaminated by fallout, while 40 K was distributed more in the leaves and fruit. Also, while the weight of the trees and the amount of 40 K in the tree body increased with time, the amount of 137 Cs decreased over the years. It is considered that radiocesium stored in the mature woody parts such as stem and branches had been transferred to fruit, leaves, or young branches.