PLANT PHYSIOLOGY , Vol 111, Issue 4 1191-1197, Copyright © 1996 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Plasma Membrane Na+ Transport in a Salt-Tolerant Charophyte (Isotopic Fluxes, Electrophysiology, and Thermodynamics in Plants Adapted to Saltwater and Freshwater)
E. A. Kiegle and M. A. Bisson
Department of Biological Sciences, Cooke Hall 109, State University of New York, Buffalo, New York 14260-1300
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
[plus or minus] 14 nmol m-2 s-1 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.
