Abstract. The stimulation by KCl of stomatal opening in isolated epidermal strips of
Vicia faba was examined. In dark + normal air the opening response was maximal at 100 mM
KCI while in light + CO,-free air it was maximal at about 10 mm KCI. C02-free air was
more influential than light in reducing the KCl concentration required for maximal opening.
K+ was essentia;l while Cl- seemed to be of secondary importance in these processes.
The use of 86Rb+ as a tracer for K+ showed that the increase in stomatal aperture under
various conditions was well correlated with K+ uptake. The estimated amount of K+ taken up
by guard cells, along with a counter ion, was sufficient to account for the changes in solute
potential associated with opening. It is suggested that the absorption of extracellular solutes,
such as K+, may be the primary mechanism of stomatal opening. Both opening and K+
absorption are stimulated by light + C02-free air.
The increase in stomatal aperture was also well correlated with the decrease in stainable
starch in guard cells under all conditions. It is suggested that this is a secondlary change,
although perhaps closely linked to K+ absorption.