Rice plants at different seed-developing stages were exposed to HTO vapor in an exposure box for 1 h during daytime and nighttime to investigate the levels of tissue free water $^3H$ (TFWT) and organically bound $^3H$ (OBT) in different plant parts. In the daytime experiment, TFWT concentrations in leaves at the end of exposure $(h_0)$ were around 100% of the 1 hour mean HTO concentrations in air moisture whereas in the nighttime experiment, they were as low as $30{sim}40%$ of the air concentration. TFWT concentrations in both experiments decreased very rapidly in the beginning but much mote slowly later and those at harvest were hundreds to hundred thousands times lower than those at $h_0$. OBT concentrations varied with time in different manners depending on plant parts and exposure times and differed between at $h_0$ and at harvest by factors of less than 10 on the whole. Even during nighttime exposures, OBT was produced at about a third the rate for daytime exposures. The degree of the conversion of airborne HTO into OBT in mature rice seeds, being several times higher in the daytime experiment than in the nighttime experiment, was highest after the exposure peformed at the most actively seed-developing stage for both experiments. It is estimated that OBT would contribute much more to the ingestion radiation dose than TFWT if rice plants are exposed to HTO vapor for the seed-developing period.
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