Plasma membrane Na⁺ transport in a salt-tolerant charophyte: Isotopic fluxes, electrophysiology, and thermodynamics in plants adapted to saltwater and freshwater

In salt-tolerant Chara longifolia, enhanced Na⁺ efflux plays an important role in maintaining low cytoplasmic Na⁺. When it is cultured in fresh water (FW), C. longifolia has a higher Na⁺ efflux than the obligate FW Chara corallina, although pH dependence and inhibitor profiles are similar for both species (J. Whittington and M.A. Bisson [1994] J Exp Bot 45: 657- 665). When it is cultured in saltwater, C. longifolia has a Na⁺ efflux of 264 ± 14 nmol m^-2 s^- at pH 7, 13 times higher than FW-adapted cultures and 31 times higher than C. corallina. As in FW-adapted plants, efflux is highest at pH 5, but pH dependence is less steep and more linear in cells adapted to saltwater. In plants of both species from FW cultures, Na⁺ efflux is inhibited by Li⁺ at pH 5 but not at pH 7 or 9, whereas in the salt- adapted C. longifolia, Li⁺ inhibits Na⁺ efflux at pH 7 and 9 but not at pH 5. Amiloride inhibits Na⁺ efflux in salt-adapted cells but not in FW cells. We conclude that a new type of Na⁺ efflux system is induced in salt-adapted plants, although both systems have characteristics suggestive of a Na⁺/H⁺ antiport. In all cases, a 1:1 Na⁺/H⁺ antiport would have sufficient energy to maintain the cytoplasmic Na⁺ activities measured at pH 5 and 7 but not at pH 9, which suggests that another efflux system must be operating at pH 9.