Responses of Stomata to IAA and Fusicoccin at the Opposite Phases of an Entrained Rhythm

The stomata of Commelma communis showed reduced opening responsesto light and low CO₂ concentrations during the night phase oftheir entrained circadian rhythm. Increased supplies of potassiumions, and treatments with indol-3-ylacetic acid and fusicoccin,failed to promote opening during the night phase to a levelequivalent to that in the day phase. The inability of fusiccocinto overcome the suppression of opening during the night phasecontrasts with its ability to counteract the closure inducedby agents such as CO₂, darkness and abscisic acid. It is concludedthat there are at least two basic mechanisms by which the turgorof guard cells can be regulated, one which is susceptible tooverriding control by fusicoccin and another which is unaffectedby fusicoccin. Several previous studies had shown a positive correlation betweenmalate in the epidermis (mainly located in guard cells) andstomatal opening. In the present experiments the aperture/malatecorrelation was broken in epidermis treated with fusicoccinduring the night phase of the rhythm. The amount of malate presentexceeded that associated with the same stomatal aperture inthe day phase. Possible explanations are (1) that fusicoccinstimulates similar proton fluxes out of the guard cells duringboth phases of the rhythm, but an unknown factor imposes a restrictionon stomatal opening during the night phase; (2) that there arelower proton fluxes in the night phase (limited, for example,by a reduced supply of ATP) but chloride availability or transportis reduced to an even greater extent so that a larger productionof malate in the guard cells is required. Key words: Stomata, IAA, Fusicoccin, Rhythms