1989 Volume 38 Issue 2 Pages 57-62
In an atmospheric discharge of radioiodines, direct deposition of the nuclides onto leaf surface must be the most significant pathway from the environment to man. However, 129Ireaches man through several pathways because of its long half life of 1.6×107 years. Root uptake of 129I is one of the most important pathways of this nuclide. In Japan, rice is thought to be the most critical crop on the pathway.
In this paper, uptake of radioiodine from irrigation water by rice plant was investigated. Rice plants, Oryza sativa ev. Nihonbare, were grown under flooded condition in Wagner pots containing soil collected in Tokai-mura. Iodine-131 was added as a tracer into the surface water in the pots at three different growing stages, heading, dough-ripe and yellowripe stages, respectively, and the plants were cultivated until the harvest time in a plant growth chamber.
At the harvest time, concentration of 131I in each organ of rice plant was measured with a NaI scintillation counter. The profile of 131I in the soil was also investigated. The results obtained are as follows;
(1) Activities of 131I in leaf blade and sheath of lower part were generally higher than those of upper part. Compared to the 131I activity of the flag leaf, the ratios of the activity in raehis-branch, hull and brown rice were 1.0-0. 5, 0.1 and 1-5×10-3, respectively. These may suggest that iodine taken up by the roots scarcely re-transloeated into rice.
(2) Ratio of 131I in brown rice and hull was about 1: 4.
(3) Activity ratio (“concentration of 131I in brown rice”/“average concentration of that in the soil” during 6 days uptake experiment.) was 4-5×10-4.
