Vacuolar Na/K Exchange, its Occurrence in Root Cells ofHordeum,AtriplexandZeaand its Significance for K/Na Discrimination in Roots

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Abstract
Using excised low-salt roots of barley and Atriplex hortensls the transport of endogenous potassium through the xylem vessels was studied It was enhanced by nitrate and additionally by sodium ions which apparently replaced vacuolar potassium which was then available in the symplasm of root cells for transport to the shoot Vacuolar Na/K exchange also has been investigated by measurements of longitudinal ion profiles in single roots of both species. In Atriplex roots a change in the external solution from K+ to Na+ induced an exchange of vacuolar K+ for Na+, in particular in the subapical root tissues and led to increased K+ transport and loss of K+ from the cortex. In inverse experiments a change from Na+ to K+ did not induce an exchange of vacuolar Na+; merely in meristematic tissues Na+—apparently from the cytoplasm—was extruded in exchange for K+. In roots of barley seedlings without caryopsis, as in excised roots, a massive exchange of K+ for Na+ was observed in the continuous presence of external 1.0 mM Na and 0.2 mM K. This exchange also was attributed to the vacuole and was most pronounced in the young subapical tissues. It did not occur, however, in the corresponding tissues in roots of fully intact barley seedlings. In these, the young tissues retained a relatively high K/Na ratio also in their vacuoles. Similarly, contrasting results were obtained with intact and excised roots of Zea mays L. Based on these results a scheme of the events that lead to selective cation uptake in intact barley roots is proposed. In this scheme a crucial factor of selectivity is sufficient phloem recirculation of K+ by the aid of which K+ rich cortical cells are formed near the root tip. When matured these cells are suggested to maintain a high cytoplasmic K/Na ratio due to K+ dependent sodium extrusion at the plasmalemma and due to recovery of vacuolar K+ by Na/K exchange across the tonoplast.