Abstract: | The effects of light/dark on anion fluxes in isolated guardcells of Commelina communis L. have been studied, using 82Brand 36Cl. Transfer of open guard cells from light to dark hasno effect on the 82Br influx, but produces a marked transientstimulation of 82Br or 36Cl efflux, similar to the effect ofsuch transfer on the 86Rb fluxes, and to the effects on both86Rb and 82Br fluxes of adding ABA. On return of guard cellsto light, after the transient, there is a further reductionin Cl/Br efflux. It is argued that control of a specific processof ion extrusion is important in regulating the ability of guardcells to stay open. In three out of four batches of steady-statetissue labelled with 82Br, the plasmalemma fluxes were highenough, relative to the tonoplast fluxes, for the efflux kineticsto be separable into two exponential components, allowing estimationof bromide contents in cytoplasm and vacuole (Qc and Qv), andfluxes at plasmalemma and tonoplast. With opening in light,Qc increased by 3.9 ± 0.4 pmol mm2 µm1and Qy by 5.2 ± 0.6 pmol mm2 µm1(change in content per mm2 of epidermis perµm change inaperture). Using rough estimates for the volumes of cytoplasmand vacuole these figures suggest that at 6.1 µm in thedark the concentrations were about 63 mol m3 in the cytoplasmand 35 mol m3 in the vacuole, rising to about 185 molm3 in the cytoplasm and 125 mol m3 in the vacuole,at 16.7 µm aperture in light. Neither increase can providean adequate increase in salt concentration to account for theosmotic change required, and some solute other than potassiumsalt must also be involved. In one experiment with 82Br andin the only experiment with 36Cl the plasmalemma flux was lower,and not high enough relative to the tonoplast flux to allowseparation of two phases in the efflux curves, and calculationof cytoplasmic and vacuolar contents and fluxes. The effectsof transfer from light to dark were, nevertheless, similar inboth types of tissue. Key words: Commelina communis L., Light/dark effects, Anion fluxes, Guard cells |