Stomatal Opening Quantitatively Related to Potassium Transport

When stomata of Vicia faba opened (from a stomatal aperture of about 2 micrometers to one of 12 micrometers) the solute content of the guard cells increased by 4.8 × 10−12 osmoles per stoma. During the same time an average of 4.0 × 10−12 gram equivalents of K+ were transported into each pair of guard cells. This amount of K+, if associated with dibasic anions, is sufficient to produce the changes in guard cell volume and osmotic pressure associated with stomatal opening. Analysis of Cl, P, and S showed that these elements were not transported in significant amounts during stomatal opening. This finding suggests that the anions balancing K+ were predominantly organic. K+ was specifically required because no other elements, likely to be present as cations, were found to accumulate in appreciable quantities in guard cells of open stomata.