Reduced stomatal responses to light, carbon dioxide and abscisic acid in the presence of sodium ions

Responses of stomata to light and CO 2 were smaller when detached epidermis of Commelina communis L. was incubated on a medium containing 50 mol m −3 NaCl than when an equimolar KCl solution was used. Although opening in the light in the absence of CO 2 seemed to be the same whichever salt was present, apertures on KCl solutions were smaller in the dark or with CO 2 ‐containing air. The response to 10 −7 mol dm −3 ABA was similarly reduced in the presence of NaCl. If there is an optimal NaCl concentration for stomatal CO 2 and light responses it is at or below 25 mol m −3 . These findings point towards control of stomatal movements by light, CO 2 and ABA at the level of cation uptake or extrusion.