lsotopic 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 (1. Whittington and M.A. Bisson 119941 j Exp Bot 45: 657-665). When it is cultured in saltwater, C. longifolia has a Na+ efflux of 264 f 14 nmol m-'s-' at pH 7, 13 times higher than FW-adapted cultures and 31 times higher than C. corallina. As in FW-adapted plants, efflux i s highest at pH 5, but pH dependence i s less steep and more linear in cells adapted to saltwater. In plants of both species from FW cultures, Na+ efflux i s 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.