Formation of Organically Bound Deuterium at Each Growing Stage of Rice

Potted rice plants were exposed to deuterated water vapor, as a substitute of tritium, for 4 hours at five different times during the grain-ripening period to estimate the influence of the growth stage on the formation and retention of organically bound deuterium (OBD) in rice. The plants were grown outside before and after the exposure experiments and were exposed to deuterated water vapor in a laboratory in a small chamber equipped with controllers of temperature, humidity and light intensity. Deuterium concentrations in free water and organic matter in rice leaves, stems and grains were investigated up to the harvest time. The deuterium in free water in the grains remained for a longer time after the end of exposure than in the leaves and stems. The mass of OBD in grain at harvest showed the highest value when the exposure was carried out in the early stage of the ripening period. When the exposure was carried out after 26 days from the heading, the increase of OBD in the grain was small.