Behavior of Trace Elements on Leaf Surfaces of Crop Plants
Hitoshi KAWABATA, Hidenao HASEGAWA, Hirofumi TSUKADA,
Yuichi TAKAKU, Shun'ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides released into the atmosphere are deposited on the leaf surfaces of crop plants, taken up
by the plants, and translocated to other parts from the leaves. Some amount of the radionuclides deposited
onto the leaves is removed from the surfaces by the environmental process called weathering, i.e. removal
by rain, wind, etc. Although weathering and translocation are important processes involved in the radiationdose assessment of radionuclides from plants, site-specific parameters to describe those processes have notyet been elucidated. This work aims to establish site-specific parameters for those processes for Cs, Sr, and I
using stable elements in a climate chamber in which meteorological conditions are controlled. The effect ofrainfall on the behaviors of Cs and Sr on leaf surfaces was studied in FY 2009.
After applying solid aerosols of NaCl containing Cs and 86Sr onto the leaf surfaces of radish,Raphanus sativus, the plants were treated at different rainfall intensities and duration time of rainfall using arain simulator. After plant samples were collected, leaf surfaces were washed with acidified water containing
detergent and the washing solutions for each sample were collected. Each washing solution and plant sample
was analyzed for Cs and Sr.
Since results for Cs and 86Sr were similar to each other, those for Cs are described hereafter. Analysis
results showed that more Cs was removed from the leaf surfaces as the rainfall intensity and time increased.
The remaining proportion of Cs on the leaf surfaces for rainfall intensity was approximated by a function
with two exponential terms of time. The fraction of the compartment with faster decreasing rate was 0.7 0.9 except for the rain intensity of 1.2 mm h-1.
To examine the aftereffect of rain, plants loaded with the target elements on the leaf surfaces weretreated for 1 or 3 h at the rainfall intensity of 0.7 mm h-1, and then were maintained for 1 or 4 d in thechamber at 20°C and 70% relative humidity with 12 h light cycle at 20,000 lx. The surface absorption,which was defined as the ratio of the amount of Cs in the plant to the sum of that on leaf surfaces and in theplant, just after the rainfall treatment was equal to or larger than the control value without the treatment.
Although surface absorptions for 1 or 4 d after the treatment showed a relatively large variation, they wereaffected by rainfall, and tended to increase with rainfall intensity.
The absorption ratio was defined here as the ratio of the amount of Cs in the plant to that loaded
initially on the leaf surfaces, and used as an index of Cs remaining in the plant. The absorption ratio of Cs
for the plant treated with rainfall did not significantly differ from that for the control plant. Although part ofthe Cs on the leaf surfaces was removed by the rainfall, the surface absorption tended to increase more thanfor the control. Therefore, the overall effect of rainfall on the absorption ratio became unclear and further
analysis of the results is required for elucidating the weathering effect of rain.

0.71

Absorption ratio

Remaining proportion

0.6

0.1
Rainfall intensity（mm h-1）
0.71.22.0
3.3

0.5

Duration of rainfall (h)
:0:1
:3

0.4
0.3
0.2
0.1

0.01
048

1216Fig. 1

Remaining proportion of Cs on leaf surfaces
and rainfall duration. Vertical bars indicate thestandard deviation of 3 samples. The lines
show a least square approximation by afunction with two exponential terms ofduration time.

1
0.9Surface absorption

0.8
0.7

＊

Duration of rainfall (h)
:0:1
:3

＊ ＊

0.6
0.5

＊

0.4
0.3
0.2
0.1
001
Days after rainfall treatment (d)00

Duration of rainfall （h）

4Fig. 2 Surface absorption defined as the ratio of
the amount of Cs in plant to the sum ofthat on leaf surfaces and in plant.
Vertical bars indicate the standard
deviation of 3 samples.
* indicates a statistically significant
difference from the control with no raintreatment.

1
Days after rainfall treatment (d)4Fig. 3 Absorption ratio of Cs defined as the ratio of
amount of Cs in plant to that loaded initiallyon the leaf surfaces. Vertical bars indicate thestandard deviation of 3 samples.