Do stomatal movements have a limited dynamic range?

Stomatal movements are driven through the uptake and release of potassium (K+) salts by guard cells, which surround a central pore. The extrusion of K+ from guard cells occurs via GORK K+-efflux channels, but potentially the AtKUP2, 6 and 8 encoded K+- transporters also play role. To test the roles of AtKUP and GORK proteins, gas-exchange experiments were conducted with mature Arabidopsis leaves. These experiments revealed that loss of KUP2, 6 and 8 lowered the stomatal conductance, while it increases in the GORK loss-of function mutant. Despite the difference in stomatal conductance, the changes in transpiration induced by light and ABA had the same amplitude in wild type and mutant lines. Our data suggest that stomata have a limited dynamic range that is not affected by mutations in KUP2, 6 and 8, or GORK. We propose that these rapid stomatal movements depend on uptake and release of K+, inorganic and small organic anions. Consequently, changes in stomatal opening beyond the dynamic range will depend on osmolytes that cannot be rapidly released, such as larger organic anions and amino acids.