Chloride transport in stomatal guard cells

Abstract

Stomatal guard cells regulate the size of the stomatal pore by the changes in their shape and volume, which are associated with changes in their turgor. Accumulation of potassium salts plays a major role in this process, and frequently chloride, if available, provides the major balancing anion. Measurements have been made of two-way ion fluxes in guard cells, in epidermal strips Commelina communis L. , after treatment at low pH to kill all cells except the guard cells. In such material, opening depends on the ion concentration in the bathing solution, and for this purpose the three salts KC1, KBr and RbCl seem to be equivalent. ⁸² Br ^- and ⁸⁶ Rb+ fluxes have been measured in a range of steady states, with different apertures, and in the transitions between one steady state and another. Analysis of the kinetics of tracer efflux in the steady states allows calculation of the cytoplasmic and vacuolar contents, and their changes with aperture, with wider opening produced by increasing concentration in the bathing solution, or by light incubation compared with dark incubation. The results show that the increases with aperture of the cytoplasmic salt concentration are comparable with the osmotic changes required, but the changes in vacuolar concentration are much less than those required osmotically. Opening must therefore be associated with vacuolar accumulation not only of salt, but also of some other solute. The decrease in aperture on addition of 2 x 10 ^-5 M abscisic acid to the solution bathing ‘isolated’ guard cells, or on their transfer from light to dark, is achieved by marked transient increases in ion efflux, with little change in influx. There are also changes in tonoplast fluxes. The aperture is determined by the level of vacuolar solute accumulation, and thus the results show that this responds to environmental signals by control of plasmalemma efflux rather than influx, and by control of tonoplast fluxes. The ability to transfer salt from cytoplasm to vacuole may be critical for the maintenance of turgor and aperture.