Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues3

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Abstract
Certain aspects of the secretion of solutes into, and removal from, the sieve tubes of isolated stem segments and rooted cuttings of Salix viminalis have been studied. Sieve-tube sap was obtained either as honeydew from whole individuals or via the severed stylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemically unrelated solutes, sucrose and the cations potassium and rubidium. On raising the potassium concentration in the sieve-tube sap by passing a solution of this ion through the xylem, the sucrose concentration declined. When the sucrose concentration fell over a period of days due to respiratory loss of carbohydrates from an isolated stem segment, a concomitant rise in either the potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, the concentration of this ion in the sieve-tube sap rose, whilst that of potassium fell at first, but later rose higher than its initial value, indicating that both antagonism and synergy can occur between these ions. On introducing both these cations into the xylem simultaneously, more sodium than potassium was taken up by the segment, though the increase in the sodium concentration in the sieve-tube sap was less than that of the potassium. Perfusing the xylem with a calcium solution had no effect upon the concentration of potassium in the sieve tube. It has been shown that the rate of translocation of a solute along the sieve tube, as measured by the two colony technique, depends upon the rate of removal of this solute from the sieve tube. The amount of such lateral loss from the sieve tube is related to the potential gradient for a solute between the sieve tube and surrounding cells.