Tritium concentration in soybean plants exposed to atmospheric HTO during nighttime and daytime

Potted soybean plants were exposed to evaporate HTO for 1 h inside an exposure box at two different growth stage（flowering and podding stage, filling stage）during nighttime and daytime. The concentration of tissuefree water tritium（TFWT） and organically bound tritium（OBT） in leaves and beans was measured at the end of the exposure. The results indicated that the quasi-equilibrium between the TFWT concentrations in the soybean leaves and the HTO concentration of the ambient air moisture required more than 1 h to be reached. The relative TFWT concentrations in the nighttime represent about 1/2 to 2/3of the concentrations obtained in the day in the leaves compared to about 1/2 to 4/3 in the beans. The relative OBT concentrations under night conditions were about 2/5 to 1/2 of those under day conditions in the leaves, contrary to 1/2–7/10 for the beans. By developing tritium concentration assessment model with a short-term release of atmospheric HTO, we comprehensively considered the plant growth stage and the environmental conditions.