Transpiration stream & ascension of calcium.

Since Cline's report (5) of a very slow equilibration in the leaves of tritiated water absorbed by the roots of bean plants, no resolution has been made between his expressed alternative views that this was due A, to failure of tissue water to exchange with the incoming tritium-labeled water, or B, to a large isotope effect. During the preparation of the present manuscript Vartapetyan and Kursanov (14), using HO'8-enriched water in the nutrient solution, have also reported a slow equilibration of this tracer in the leaves (& roots) of the bean plant. In addition, they grew sunflower plants from seed with H.018enriched water and observed almost complete equilibration in all parts of the plant and thereby showed that the failure to equilibrate was due to failure of mixing, and that there remained the possibility of only an insignificant isotope effect. Even this was explainable by unlabeled water brought into the experiment in the seed, or to the formation of metabolic water in which there was a slight isotope effect. Our report confirms the slow equilibration in leaves of the labeled water delivered in the transpiration stream, and in addition shows the progress of the labeled water through the stem and into the leaves in a way which discloses the path and manner of movement of the transpiration stream. The Ca45 isotope was used with the tracer water in order to compare the ascent of a mineral marker. Calcium was selected as it is not subject to a significant redistribution from leaves and could be expected to reside in the leaves into which it was delivered (4). The degree of dependence between the movement of a mineral component and the movement of the transpiration stream has never been satisfactorily investigated. The traditional concept is that mineral nutrients move upward in the xylem vessels in conjunction with the transpiration stream; once started the water and minerals move together and, with the exception of those salts removed by absorption along the way, reach their final destination in the leaves (6). The data obtained in the present study indicate that neither