Avoidance of sodium accumulation by the stomatal guard cells of the halophyte Aster tripolium

X-ray microanalysis has revealed that the sodium content of the stomatal guard cells of Aster tripolium remains much lower than that of other leaf cells when the plants are grown at high salinity. Large amounts of sodium did, in contrast, accumulate in epidermal and subsidiary cells, and particularly in the mesophyll tissue, suggesting that a mechanism exists to limit the extent of its entry into guard cells. Even in plants grown at high salinity, the content of potassium was much higher than that of sodium in the guard cells, consistent with the view that this is a major ion involved in determining stomatal movements in this halophyte. Determinations were also made for the nonhalophyte Commelina communis, and it was found that the guard cells accumulated large amounts of sodium when it was presented to them as an alternative to potassium. It is suggested that the acquisition by the guard cells of some ability to restrict the intake of sodium ions may be an important component of sodium-driven regulation of transpiration, and hence of salinity tolerance, in A. tripolium.