Effects of Light/Dark on Anion Fluxes in Isolated Guard Cells ofCommelina communisL.

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Abstract
The effects of light/dark on anion fluxes in isolated guard cells of Commelina communis L. have been studied, using 82Br and 36Cl. Transfer of open guard cells from light to dark has no effect on the 82Br influx, but produces a marked transient stimulation of 82Br or 36Cl efflux, similar to the effect of such transfer on the 86Rb fluxes, and to the effects on both 86Rb and 82Br fluxes of adding ABA. On return of guard cells to light, after the transient, there is a further reduction in Cl/Br efflux. It is argued that control of a specific process of ion extrusion is important in regulating the ability of guard cells to stay open. In three out of four batches of steady-state tissue labelled with 82Br, the plasmalemma fluxes were high enough, relative to the tonoplast fluxes, for the efflux kinetics to be separable into two exponential components, allowing estimation of bromide contents in cytoplasm and vacuole (Qc and Qv), and fluxes at plasmalemma and tonoplast. With opening in light, Qc increased by 3.9 ± 0.4 pmol mm−2 μm−1 and Qy by 5.2 ± 0.6 pmol mm−2 μm−1 (change in content per mm2 of epidermis perμm change in aperture). Using rough estimates for the volumes of cytoplasm and vacuole these figures suggest that at 6.1 μm in the dark the concentrations were about 63 mol m−3 in the cytoplasm and 35 mol m−3 in the vacuole, rising to about 185 mol m−3 in the cytoplasm and 125 mol m−3 in the vacuole, at 16.7 μm aperture in light. Neither increase can provide an adequate increase in salt concentration to account for the osmotic change required, and some solute other than potassium salt must also be involved. In one experiment with 82Br and in the only experiment with 36Cl the plasmalemma flux was lower, and not high enough relative to the tonoplast flux to allow separation of two phases in the efflux curves, and calculation of cytoplasmic and vacuolar contents and fluxes. The effects of transfer from light to dark were, nevertheless, similar in both types of tissue.