Experiments on86Rb+fluxes were used to identify the changes in ionic state induced by transferring stomata from light to darkness. Guard-cell fluxes and contents were measured on “isolated” epidermal strips fromVicia fabaL. in which all cells other than the guard cells had been killed by ultrasonic disruption. Closure of stomata in response to darkness was achieved by a large, transient stimulation of86Rb+efflux from the guard cells, combined with a reduction of ion transfer from cytoplasm to vacuole, but there was little significant change in influx. Removal of blue but not red light appeared to trigger the flux responses associated with darkening. Attempts to inhibit the closing response (using methoxyverapamil, nifedipine and bepridil, compounds possessing activity against the calcium channels of animal cells) were mostly unsuccessful and the significance of this result is discus
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