Abscisic acid (ABA) induces a transient stimulation of Rb-86(+) efflux from isolated guard cells of Commelina communis L. The form of the efflux transients produced in suboptimal conditions (low concentrations of ABA and/or high external pH at which ABA will penetrate poorly) has been compared with the full transient. In suboptimal conditions the stimulation of efflux is both delayed and reduced. The pH-dependence of the delay before initiation of the efflux transient suggests that a threshold internal concentration of ABA is required. However in suboptimal conditions even when the threshold internal concentration is reached and a transient is triggered, the degree of stimulation is reduced, an effect which also appears to depend on internal ABA. It is suggested that the differences reflect activation of different numbers of tonoplast ion channels for release of vacuolar ions. By contrast, the same end-state seems to be reached in optimal and suboptimal conditions, but after different times. The relative efflux stimulation during the efflux transient tracks the declining ion content; both the peak and the end of the transient are reached at the same ion content, but at different times. It is suggested that this reflects an ABA-induced change in the set-point of a regulated ion channel which is sensitive to ion content, perhaps a stretch-activated channel. This effect is independent of external concentration in the range 0.1-10 mu M, and pH 6 and pH 8 are equally effective, suggesting an external site of action. Thus the results suggest multiple actions of ABA, involving both internal and external receptors. Regulation of both tonoplast ion channels by internal ABA, and of a regulated channel responsive to ion content by external ABA are suggested.
