Movement and Distribution of THO in Tissue Water and Vapor Transpired by Shoots of Helianthus and Nicotiana

Abstract

The distribution of tritium in a sunflower plant in light with roots in tritiated water was studied as a function of time. During one 12-hour light period, concentrations of tritium in the stem tissue and petioles of mature leaves approached equality with the concentration of tritium in the nutrient solution. The approach to equality was much slower in the terminal nodes and the internodes that were still elongating and in leaf tissue was still expanding. The 2 apical nodes and internodes, bracts, inflorescence parts, and seeds did not reach equality even when the plants had been grown from seed to seed maturity in tritiated water. The tritium concentration in the leaves of intact plants growing in tritiated water did not reach the concentration of the external medium over a protracted period. Although the veins themselves equilibrated in a manner similar to that of the mature stem and petiole, the interveinal tissue did not reach tritium equality with major veins, the stem, or the external solution. Water of the interveinal tissue maintained a stable tritium content at 60-70% of external concentration for long periods. The pattern of results obtained was similar in sunflower and tobacco. Tritium content of leaf-tissue water vapor transpired by the canopy of an intact sunflower plant raised from seed in THO and treated with a stream of dry air inside a closed chamber increased over a 12-hour period. At the end of this period, interveinal tissue water and transpired vapor reached at least 95% of external tritium concentration about the roots. It was shown that the lack of equilibration in interveinal tissue should be attributed to exchange of tritium between the leaf and the surrounding unlabeled water vapor of the atmosphere. The results reported here contradict the hypothetical concept that a sizable fraction of tissue water is inaccessible for turnover.