A Critical Assessment of the Role of Potassium and Osmolarity in Stomatal Opening

Abstract

An attempt has been made to separate the osmotic effect from the ionic effect of KCI in stomatal responses. For this purpose isolated illuminated epidermis from species with and without subsidiary cells were treated with KCI (0-250 mOs kg⁻¹) and with mannitol (0-250 mOs kg⁻¹). Since osmolarity was made the basis of comparison, the effect of mannitol had to be observed immediately, before guard cell contents could have leached into the incubation medium. When plotting apertures against osmolarity sigmoid curves were obtained with KCI, but with mannitol straight lines resulted provided that prior to stripping and incubation leaves were briefly illuminated. Whilst in lower concentrations (60 mOs kg⁻¹ for Vicia faba; 90 mOs kg⁻¹ for Zantedeschia aethiopica; 190 mOs kg⁻¹ for Commelina communis) pores were wider in mannitol than in KCI, in concentrations above these values the situation was reversed. It appeared therefore that KCI had either an inhibitory or a promoting effect. Inhibition was most pronounced when at the beginning of incubation stomata were closed; the inhibitory effect on stomata without subsidiary cells occurred at low concentrations (0-60 mOs kg⁻¹) whereas when subsidiary cells were present inhibition occurred at up to 190mOs kg⁻¹. Other experiments started with KCI solutions of 50 mOs kg⁻¹ for Vicia faba, 85 mOs kg⁻¹ for Zantedeschia aethiopica and 115 mOs kg⁻¹ for Commelina communism; mannitol was additionally used to give the progressive increases in osmolarity. Degrees of opening were then reached which with KCI alone could only be attained at the very highest concentrations. Starch disappearance was followed using the periodic-acid-silver test; by using either ⁸⁶Rb or ⁴³K it was shown that ion uptake was restricted to guard cells alone only at osmolarities exceeding 200 mOs kg⁻¹. On the basis of these observations it was concluded that K transport does not represent the major mechanism of stomatal regulation.