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

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 SO2‐4 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.