| Abstract: | The ion relations of the halophytc Suaeda maritima are described.When plants grew in 340 mol m–3 sodium chloride (—1•76MPa) leaf solute potentials decreased, and were sustained around—2•5 MPa Inorganic ion concentration (mostly of sodiumchloride) accounted for this. Comparable shoot ion concentrationsof potassium, nitrate and sulphate occurred when plants grewon different salinity types characterized by these ions. Netsodium transport and shoot sodium concentration increased dramaticallywith increases in external sodium chloride concentration upto 85 mol m–3; thereafter, further increases in externalsodium chloride concentration had relatively little effect uponeither shoot sodium concentration or upon net transport of sodiumto the shoot. The net transport of sodium plus potassium onlydoubled when the external concentration of sodium plus potassiumincreased from 24 to 687 mol m–3 Shoot ion concentrationswere remarkably constant with time, external concentration andsalinity type. The membrane flux rates and symplasmic ion concentrations neededto sustain the observed net transport of sodium (of some 10mmol g–1 dry wt. of roots d–1) are calculated fromanatomical and stereological data for the root system of thisspecies. The minimum net sodium chloride flux to load the symplasmwould be 260 nmol m–2s–1 if the whole cortical andepidermal plasmalemmal surface area were used uniformly, butthe flux rate required would be 3000 nmol m–2s–1if uptake took place only at the root surface. A flux rate ofat least 1000 nmol m–2s–1 is needed between symplasmand xylem. The symplasmic concentration of NaCl would be atleast 80 mol m–3. It is argued (1), that both symplasmicand xylem loading are likely to be passive processes mediatedby ion channels rather than active carriers, (2), that net iontransport at 340 mol m–3 sodium chloride is close to themaximum which is physiologically sustainable and (3), that growthof this halophyte is limited by NaCl supply from the root. Key words: Suaeda maritima, halophyte, salinity, roots, radial ion transport |
