Transport of 137Cs into Fruits After External Deposition onto Japanese Persimmon Trees
Authors: Sato Mamoru 
Published: August 2023

Prior to the Fukushima Daiichi accident, radiocesium released during previous nuclear accidents was deposited in periods when fruit was growing on fruit trees and radiocaesium deposited onto surface of soil and leaf was assumed to be the main route of transfer into fruit trees. In contrast, fruit trees in Japan were in their dormancy phase, so radiocesium was deposited onto external branches of the trees and onto orchard soils after the Fukushima Daiichi nuclear power plant accident in March 2011. While data had previously been compiled on root to fruit transfer, there were few studies identifying and quantifying the relative importance of external deposition and translocation compared with root to fruit transfer. This study quantified the transfer rate (TRf) of 137Cs applied to leaves and calyx into Japanese persimmon, which is an important fruit crop in Fukushima prefecture. The study examined the effects of leaf position on branches where radiocesium was administered and of fruit load on the transport of 137Cs into fruit before bud burst. When 137Cs was applied to leaves, derived TRf were higher after radiocesium application at the fruit growing stage than at the young fruit stage. There was no significant difference in TRf in mature fruit when contamination occurred via calyx or via leaves. However, the TRf via calyx contamination was higher at the young fruit stage, whereas the TRf via leaves result in an opposite trend. The TRf was dependent on the distance between fruit and the contaminated leaves. The effect of fruit thinning was verified contaminating the central of 5 consecutive fruiting shoots (bearing one fruit per each). The TRf of 137Cs from leaves into fruit on the contaminated fruiting shoot was higher where fruits on the shoots adjacent to the central one were picked off (fruit thinning) than where no fruit thinning was applied. The TRf of 137Cs into fruit on the uncontaminated fruiting shoot was lower than in fruits of contaminated shoots. There was a significant correlation between the amount of 137Cs and 40K as well as the concentration in the fruit on the contaminated fruiting shoot. TRf and aggregated transfer factors (Tagf-b) of 137Cs into fruits were also studied after application of radiocesium at dormancy to apical fruiting mother shoots or to 2-year-old lateral branches. When the apical fruiting mother shoot was contaminated, the TRf of the combined mature fruits collected from uncontaminated fruiting mother shoots was less than a quarter of the TRf of fruit growing on the contaminated apical mother shoot (on the same 2-year-old lateral branch). In contrast, when the 2-year-old lateral branch was contaminated, the TRf and Tagf-b into fruit were similar for both the fruit on both the apical and other fruiting mother shoots. These results indicate that the transfer of 137Cs to other parts of the fruit from the apical fruiting mother shoot is limited, whereas from the 2-year-old lateral branches, the transfer of 137Cs to fruit is similar on all the fruiting mother shoots. The results as well as the comparative experiment of fruit loads were consistent with the hypothesis that the transfer of 137Cs was controlled by the sink strength and activity of fruits. Tagf-b values of 137Cs applied to apical fruiting mother shoot and 2-year-old lateral branch into fruits before bud burst were about 10−3 to 10−4 m2 kg−1 based on dry and fresh weights, respectively. A significantly higher concentration of 137Cs and Tagf-b values were derived when fruit thinning was carried out than when fruits were not thinned. Refraining from intensive fruit thinning may be a useful remediation option that could reduce 137Cs activity concentrations in Japanese persimmon fruits.