Na⁺ and Cl^- Transport in the Phloem from Leaves of NaCl-treated Barley

Abstract

Sieve tube sap was collected from fully expanded leaves of NaCl-treated barley plants through aphid stylets. The sap was analysed for Na⁺, CI^-, K⁺, phosphate, sucrose and osmolality. Na⁺ and Cl^- appeared in the sap within a day of NaCl being added to the nutrient solution. When plants were exposed to various NaCl concentrations for 1-2 weeks, the sap contained about 15 mol m^-3 Na⁺ and 25 mol m^-3 Cl⁺, irrespective of whether the external NaCl concentration was 25, 100 or 200 mol m^-3. When plants were kept for more than 2 weeks at 200 mol m^-3 NaCl, the Cl^- concentration in the sap increased but the Na⁺ concentration remained the same. Na⁺ and Cl^- in the sap decreased very slowly when NaCl was removed from the nutrient solution. The calculated efflux of Na⁺ and Cl^- in the phloem from the leaves was only 10% of the influx in the xylem, so the phloem can play little part in controlling the NaCl content of the leaves of these plants.