Guard cell cation channels are involved in Na + –induced stomatal closure in a halophyte

Summary The halophyte Aster tripolium , unlike well‐studied non‐halophytic 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 + effects on stomatal aperture, and renders aperture refractory to Na + .