Effects of abscisic acid on potassium uptake and starch content of stomatal guard cells.

Abscisic acid (ABA) at a concentration of 100 μm reduced the mean stomatal aperture on isolated epidermis of Commelina communis from 9.5 to 3.1 μm. This closure resulted from a fall in osmotic pressure of the guard cells from 14.1 to 9.8 bars; the osmotic pressure of the subsidiary cells did not change significantly. Histochemical tests showed that the potassium concentration in guard cells was reduced by ABA-treatment, while the starch content of the chloroplasts increased. ABA was found to exert a significant effect on Rb(86) uptake into leaf discs, but this was relatively small in magnitude. It is concluded that ABA has a greater effect on ion uptake into guard cells than into the leaf tissues as a whole.Recent hypotheses of the stomatal mechanism are discussed in relation to these new observations, and the rejection by some writers of any major role for starch hydrolysis is challenged. Evidence from several sources suggests that starch disappearance occurs simultaneously with K(+) entry into guard cells. Breakdown of starch may lead to formation of organic anions, with which K(+) uptake may be associated. In this case starch breakdown would contribute as much to the increased osmotic pressure as does K(+) uptake.