Modeling of Sodium and Potassium Flows via Phloem and Xylem in the Shoot of Salt-stressed Barley

Abstract

Summary Hordeum vulgare, cv. California Mariout was grown in liquid culture in presence of NaCl (1to 100 mol m-3). 14 and 19 days after sowing, i.e. at the three to four leaf stage, plants were harvested, and increments of K+ and Na+ during this interval were determined in individual organs. In parallel, K/Na ratios of phloem exudate and xylem sap were measured. Phloem exudation was induced by employing Li2EDTA; xylem sap was obtained by applying pressure to the root system. Efficient control of ion flows across the roots was checked by measuring solute vs. volume flow. Using ion increments and K/Na ratios of the sap net flows and partitioning of K+ and Na+ within the shoots were calculated. Although all K+ flows in salt-treated barley (100 mol m-3 NaCl) were greatly reduced relative to the control, there was substantial retranslocation of K+ from mature to young leaves and recirculation of K+ via phloem from shoot to roots. Part of this recirculated K+ was transferred from phloem to xylem within the root and returned to the shoot via xylem. Translocation of K+ in the phloem exceeded that of Na+ despite high external Na+ concentrations. The relation of K+ translocation and of low phloem mobility of Na+ to salt tolerance of barley is discussed.