The effect of divalent cations on Na⁺ tolerance in Charophytes. I: Char a buckellii

Abstract The comparative Na⁺ tolerance of Chora buckellii cultured in freshwater (FW) or artificial Waldsea water (AWW, which contains about 110 mol m⁻³ each Na ⁺, Mg²⁺, Cl⁻ and SO^2‐₄ was tested with respect to the external Na⁺ to Ca²⁺ ratio (Na: Ca). Fifty per cent of FW cells subjected to 70 mol m⁻³ NaCl, which raised Na:Ca from 10: 1 to 700: 1 and the external osmotic pressure from 0.024 to 0.402 MPa, died within 6 d. Death was associated with the loss of Na/K selectivity, H⁺ ‐pump activity and turgor. Restoration of Na:Ca to 10:1 in high Na⁺ medium with CaCl₂ ensured 100% survival and maintained H⁺‐pump activity and Na/K selectivity of FW cells. Turgor was regulated within 3 d with net uptake of Na ⁺, K⁺ and Cl⁻ in the vacuolc. Mg²⁺ was not as effective as Ca²⁺ in enhancing survival or maintaining H⁺ ‐pump activity and Na/K selectivity of FW cells in the presence of elevated Na⁺. However, turgor was regulated within 3 d by accumulation of Cl⁻ and an unknown cation in the vacuole. All AWW cells subjected to an increase of 70 mol m ⁻³ NaCl, which raised Na: Ca from 16:1 to 25: 1 and the external osmotic pressure from 0.915 to 1.22 MPa, survived and maintained H ⁺ ‐pump activity. Turgor was regulated within 6d by accumulating Na ⁺, K⁺ and Cl⁻ in the vacuole. All AWW cells subjected to 70molm⁻³ NaCl in a medium in which Na:Ca was equal to 700:1 survived and maintained H ⁺ ‐pump activity, but showed loss of Na/K selectivity. Turgor was regulated with an unknown osmoticum(a) within 6 d.