Extrusion of Sodium Ions by Barley Roots: II. Localization of the Extrusion Mechanism and its Relation to Long-distance Sodium Ion Transport

An active sodium effux hasbeen demonstrated in excised barleyroots. Isolated cortical and stelar tissues from the same materialdo not show any evidence of such an efflux mechanism. When intactbarley plantswere allowed to absorb ²²Na active sodium extrusionwas not observed, although the percentage of ²²Na transportedto the shoot was stimulated 20 to 40 percent by ouabain at 5x 10^–4 M. Absorption and long-distance transport of phosphatewas unaffected by ouabain at this concentration. Interpretationof these data lead to the conclusion.that the sodium-effluxpump in barley roots is located at or near the endodermis externalto the Casparian band. This pump thus provides a mechanism wherebysodium can be partially excluded from the shoots of barley plants.