Tritium levels in Chinese cabbage and radish plants acutely exposed to HTO vapor at different growth stages

To simulate an acute exposure of Chinese cabbage and radish plants to airborne HTO, the potted plants were exposed to HTO vapor under semi-outdoor conditions for 1 h at different times from the early to late growth stages. The plants were grown outdoors and the plant tritium was measured at the end of an exposure (h0) and at harvest. The leaf tissue free water tritium (TFWT) concentrations at h0 were considerably lower than estimated equilibrium concentrations. In the leaves of Chinese cabbage, the exposure at the earlier growth stage generally ended with a higher TFWT concentration. Such a tendency was not apparent either in the leaves or roots of radish. On the other hand, the earlier stage exposure gave rise to lower TFWT concentrations at the harvest of both crops. For the OBT (organically bound tritium), however, the same occurred only in the Chinese cabbage leaves. During the period between the exposure and harvest, the TFWT concentrations reduced by factors of up to 1.1 × 106 for the Chinese cabbage leaves and 1.3 × 104 for the radish roots. Based on the activity ratios of OBT to TFWT at harvest, it is estimated that OBT mostly contributes much more to the ingestion dose than TFWT does.