Ion Transport inSuaeda maritima: Its Relation to Growth and Implications for the Pathway of Radial Transport of Ions across the Root

Abstract

The ion relations of the halophytc Suaeda maritima are described. When plants grew in 340 mol m−3 sodium chloride (—1⋅76 MPa) leaf solute potentials decreased, and were sustained around —2⋅5 MPa Inorganic ion concentration (mostly of sodium chloride) accounted for this. Comparable shoot ion concentrations of potassium, nitrate and sulphate occurred when plants grew on different salinity types characterized by these ions. Net sodium transport and shoot sodium concentration increased dramatically with increases in external sodium chloride concentration up to 85 mol m−3; thereafter, further increases in external sodium chloride concentration had relatively little effect upon either shoot sodium concentration or upon net transport of sodium to the shoot. The net transport of sodium plus potassium only doubled when the external concentration of sodium plus potassium increased from 24 to 687 mol m−3 Shoot ion concentrations were remarkably constant with time, external concentration and salinity type. The membrane flux rates and symplasmic ion concentrations needed to sustain the observed net transport of sodium (of some 10 mmol g−1 dry wt. of roots d−1) are calculated from anatomical and stereological data for the root system of this species. The minimum net sodium chloride flux to load the symplasm would be 260 nmol m−2s−1 if the whole cortical and epidermal plasmalemmal surface area were used uniformly, but the flux rate required would be 3000 nmol m−2s−1 if uptake took place only at the root surface. A flux rate of at least 1000 nmol m−2s−1 is needed between symplasm and xylem. The symplasmic concentration of NaCl would be at least 80 mol m−3. It is argued (1), that both symplasmic and xylem loading are likely to be passive processes mediated by ion channels rather than active carriers, (2), that net ion transport at 340 mol m−3 sodium chloride is close to the maximum which is physiologically sustainable and (3), that growth of this halophyte is limited by NaCl supply from the root.