Alterations in Ca²⁺-binding on plasma membrane after adaptation to salt stress of tobacco cells in suspension

Electrophoresis was used to study effects of salinity on the characteristics of Ca²⁺ binding to the outer surface of plasma membrane (PM) of protoplasts isolated from two types of tobacco (Nicotiana tabacum L., cv. Bright Yellow) cultured cells that were adapted (tolerant) and unadapted (sensitive) to 50 mM NaCl stress. Electrophoretic analysis of salt-sensitive NaCl-unadapted cells shows that Na⁺ induced an appreciably higher degree of reduction in the amount of Ca²⁺ bound to PM compared with K⁺ with increasing concentration from 0.1 to 30 mM. In salt-tolerant NaCl-adapted cells, however, both Na⁺ and K⁺ ions induced almost the same degree of reduction in the amount of Ca²⁺ bound to PM in the physiological concentration range of Ca²⁺ in the medium between 2 and 4 mM. These results suggest that, under the physiological conditions, PM of salt-sensitive NaCl-unadapted cells has an appreciable amount of PM-bound Ca²⁺ that is desorbed much easier by Na⁺ than K⁺, whereas PM of salt-tolerant NaCl-adapted cells has the PM-bound Ca²⁺ that can be equally desorbed by Na⁺ and K⁺.