The Circadian Rhythm of Stomatal Opening: Evidence for the Involvement of Potassium and Chloride Fluxes

Abstract

Epidermal strips were taken from plants of Commelina communis at opposite phases of an entrained circadian rhythm, at times which corresponded to the middle of the day phase and the middle of the night phase. Earlier observations of much reduced opening in response to external stimuli in the night phase were confirmed. Evidence is now presented that uptake or retention of specific ions by the guard cells is affected by the phase of the rhythm. The overt expression of the rhythm was reduced if the epidermis was presented with potassium iminodiacetate instead of potassium chloride. Iminodiacetate is a non-absorbable anion, and this result suggested that chloride uptake is important for the full expression of the rhythm. The use of an anion channel inhibitor (DIDS) gave strong support for this conclusion. Reduced uptake of chloride would explain an earlier observation that formation of malate is greater during the night phase. When the guard cells were presented with sodium chloride instead of potassium chloride there was still an overt expression of the rhythm, but this was of reduced magnitude. This suggests that potassium movements might contribute to the rhythm, and studies of ionic fluxes using ⁸⁶Rb as a tracer showed greater effluxes during the night phase. It is concluded that the circadian rhythm in stomatal opening may be the result of a varying ability of the guard cells to accumulate or retain both chloride and potassium ions.