Behavior of Trace Elements on Leaf Surface 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 surface by the environmental process called weathering, i.e. removal byrain, wind, etc. Although weathering and translocation are important processes involved in the radiation dose
assessment of radionuclides from plants, site-specific parameters to describe those processes have not yet
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 ofrelative humidity on the behaviors of I- and IO3- on leaf surfaces and in plants was studied in FY 2008.
After applying solid aerosols of NaCl containing NaI or NaIO3 onto the leaf surfaces of radish plants,Raphanus sativus, the plants were grown for 14 d in the chamber at different relative humidity levels. Thefoliar uptake and translocation to root tubers of I were periodically obtained by analyzing plant samples andthe solutions obtained by washing the leaf surface. Fraction of volatilized I was estimated by subtracting thesum of the I fraction in the plant and the washed amount from the amount of I initially deposited on the leaf
surfaces. The deposited amount of I was estimated by using sample plants collected just after the applying
the aerosols.
Absorption of I- into the plant on 14 d after the applying the aerosol was several times higher than thatof IO3-. Translocations of both of I- and IO3- to root tuber were lower than 10% of the deposited I. Volatilized
fraction of I- was 23 - 31% of the deposited I, while IO3- was hard to volatilize. To analyze the behavior of I
applied on leaf surfaces and in the plant, a dynamic model with four compartments was constructed. Twocompartments were adopted as compartments of I on leaf surface for describing two phases of foliar
absorption with rapid and slow rates. The fraction of I- partitioned into the first leaf surface compartment
was approximately seven times larger than that of IO3-. The first leaf surface compartment had a large rate
constant of 2.2 - 7.2 d-1 to transfer I into the leaf, while the rate constant of the second one was a few order ofmagnitude smaller than that of the first one. The ratio of the rate constant of I- translocated from root tuber tothe leaf to that of the reverse route was approximately seven times larger than that of IO3-. This indicatedthat I was absorbed from the leaf surface with different chemical forms depending on the applied forms.
Rate constants and partition constants to the first leaf surface component were not affected by the relative
humidity levels examined.

◆：Leaf surface, ■：Leaf, △：Root tuber, ○：Volatilization
10I

1-

Relative distribution

Relative distribution

100.1
0.010.001

IO3-

10.1
0.010.001
0510
Days after application of I （d）

Fig. 1150510Days after application of I （d）

15

Relative distribution of I applied on leaf surface of radish plant.
Vertical bar indicates a standard deviation of three samples.
Volatilization
RV1

PLS1

Deposition

Leafsurface 1（LS1）

UptakeRLS1

Leaf
surface 2（LS2）1－PLS1

Translocation
RL

Leaf
（L）

Roottuber
（R）

RR

RLS2

RV2
Volatilization

Fig. 2Compartment model describing foliar uptake, translocation and volatilization of I in plant.

Table 1 Absorption, translocation and volatilization of I applied on leaf surface of radish plants up to 14 d after theapplication.
Translocation
to root tuber（%）

Humidity

Absorption
into plants(%）（% RH）

I-

IO3-

I-

IO3-

I-

5073 ±614 ±57.6 ±1.6

6.0 ±3.9

25 ±610 ±107062 ±11

14 ±44.8 ±1.8

3.8 ±1.4

23 ±140.2 ±0.29056 ±319 ±3

5.9 ±2.7

3.3 ±1.0

31 ±1Volatilization（%）
IO3-0Table 2 Partition constant to the first leaf surface component, and rate constants of foliar uptake, translocation
between leaf and root tuber and volatilization to atmosphere of I in compartment model.
Chemical Humidity
formI-

IO3-（% RH）
507090507090

Partition
PLS1
0.78
0.74
0.69
6.5×10-2
0.12
0.12

RV1RV200.12
1.5 1.8×10-2
1.0 7.6×10-2
0.77
00.77
000Rate constant （d-1）
RLS1
RLS2
5.2
0
4.5 2.9×10-2
7.2
0
3.0 2.6×10-3
4.2 7.0×10-4
2.20RL
1.1
0.26
1.4
10
9.7
0RR
15
2.9
14
1220
1.1

RR/RL
13.7
11.4
9.8
1.2
2.1－