Na+, K+and Cl−in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Abstract

Munns, R. 1985. Na⁺, K⁺ and Cl⁻ in xylem sap flowing to shoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl⁻ and K⁺ concentrations were measured in xylem sap obtained by applying pressure to the roots of decapitated barley plants grown at external [NaCl] of 0, 25, 50, 100, 150 and 200 mol m⁻³. For any given NaCl treatment, ion concentrations in the xylem sap were hyperbolically related to the flux of water. Ion concentrations in sap collected at very low volume fluxes (without applied pressure) were 5–10 times higher than in sap collected at moderate fluxes (under pressure). For a given moderate volume flux, Na+ concentration in the xylem sap, [Na⁺]_x, was only 4.0 mol m⁻³ at external [NaCl] of 25–150 mol m⁻³, and increased to 7.0 mol m⁻³ at 200 mol m⁻³. [Cl-]x showed a similar pattern. This shows there would be little difference in the rate of uptake to the shoot of plants at 25–150 mol m⁻³ external NaCl and indicates little change even at 200 mol m^-3 NaCl because transpiration rates would be much lower. Thus the reduced growth of the shoot of plants at high NaCl concentrations is not due to higher uptake rates of Na⁺ or Cl⁻. The fluxes of Na⁺, Cl⁻ and K⁻ increased non-linearly with increasing volume flux indicating little movement of salt in the apoplast. The flux of K⁺ increased even when [K⁺]_x was greater than external [K⁺], indicating that membrane transport processes modify the K⁺ concentration in the transpiration stream as it flows through the root system.