As a substitute of tritium, deuterated water (D2O) vapor release experiments were performed in a greenhouse to estimate the different formation and subsequent retention of organically bound deuterium in rice plants between daytime and nighttime exposure. Potted rice plants were exposed to D2O vapor in the greenhouse for 8 h, under day or night conditions. Deuterium concentrations in free water and organic matter in rice leaves and ears were investigated until harvest time. The formation of organically bound deuterium in the daytime was higher than during the nighttime by the factors of 2.4 for the ear and 2.9 for the leaf. The decrease of the organically bound deuterium concentration in the ear after the nighttime exposure was faster than that after the daytime exposure. Data analysis was carried out using a compartment model in which different generating processes of organic matter were considered. The calculated organically bound deuterium retention in rice agreed with the measured value.
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