SummaryThe halophyte Aster tripolium, unlike well-studied nonhalophytic species, partially closes its stomata in response to high Na ⍣ concentrations. Since A. tripolium possesses no specific morphological adaptation to salinity, this stomatal response, preventing excessive accumulation of Na ⍣ within the shoot via control of the transpiration rate, is probably a principal feature of its salt tolerance within the shoot. The ionic basis of the stomatal response to Na ⍣ was studied in guard cell protoplasts from A. tripolium and from a non-halophytic relative, Aster amellus, which exhibits classical stomatal opening on Na ⍣ . Patch-clamp studies revealed that plasma membrane K ⍣ channels (inward and outward rectifiers) of the halophytic and the non-halophytic species are highly selective for K ⍣ against Na ⍣ , and are very similar with respect to unitary conductance and direct sensitivity to Na ⍣ . On the other hand, both species possess a significant permeability to Na ⍣ through non-rectifying cation channels activated by low (physiological) external Ca 2⍣ concentrations. Finally, it appeared that the differential stomatal response between the two species is achieved, at least in part, by a Na ⍣ -sensing system in the halophyte which downregulates K ⍣ uptake. Thus, increases in guard cell cytosolic Na ⍣ concentration in A. tripolium but not in A. amellus, lead to a delayed (20-30 min) and dramatic deactivation of the K ⍣ inward rectifier. This deactivation is probably mediated by an increase in cytosolic Ca 2⍣ since buffering it abolishes the response. The possible role of K ⍣ inward rectifiers in the response of A. tripolium's stomata to Na ⍣ , suggested by patch-clamp studies, was confirmed by experiments demonstrating that specific blockade of inward rectifying channels mimics Na ϩ effects on stomatal aperture, and renders aperture refractory to Na ⍣ .