Ion Fluxes in ‘Isolated’ Guard Cells ofCommelina communisL.

Abstract

Ion fluxes have been measured in ‘isolated’ guard cells of Commelina communis L. using ⁸⁶RbCl and K⁸²Br, in epidermal strips in which all cells other than guard cells have been killed by treatment at low pH. To avoid problems of slow free space exchange most fluxes have been measured at pH 3.9, at which stomata open well in K(Rb) Cl(Br) and are stable for many hours. At pH 3.9 the intracellular ⁸⁶Rb exchanged as a single compartment with a half-time of 2–3 h, independent of external concentration (C_o). The influx of ⁸⁶Rb rose with concentration, to a V_max of about 23 pmol mm⁻² h⁻¹. The efflux curve of ⁸²Br could be well fitted by two exponential terms, with half-times of 38 min (independent of C_o), and 5–35 h (falling with increasing C_o). Bromide contents of cytoplasm and vacuole (Q_c and Q_v), and fluxes at plasmalemma and tonoplast, were calculated from the efflux kinetics. Over C_o 20–60 mM, as the aperture increased from 7 μm to 17 μm, the tonoplast flux (0.5–11.5 pmol mm⁻²h⁻¹) was always much less than the plasmalemma flux (7–77 pmol mm⁻² h⁻¹). Q_c and Q_v both increased with aperture. The increase in Q_c of 10.3 pmol mm⁻² μm⁻¹ is adequate to account for the osmotic changes required to change the aperture, as previously estimated. However, the change in vacuolar content of only 5.9 pmol mm⁻² μm⁻¹ is much too small to account for the osmotic changes required, or to balance the cytoplasmic changes. It appears therefore that increasing KBr outside not only increases the cytoplasmic salt content, and the Br flux at the tonoplast, but also stimulates the vacuolar accumulation of some other solute.