Transmembrane ion exchange and blue-light dependent responses are central processes in plants in general1,2 and stomatal guard cells in particular3. Stimuli, including light, cause H+ extrusion by stomatal guard cells4,5, K+ uptake, osmotic swelling and enlarged stomatal pore apertures, resulting in increased leaf conductance for CO2 influx and water efflux6. The mechanisms under-lying ion movement and the specific response of stomata to blue light 7-9 remain, however, unknown. We have now investigated these phenomena using gas-exchange techniques on leaves and whole-cell patch clamping10,11 of protoplasts. Blue light-stimulated stomatal opening in the leaf is correlated with activation of an electrogenic pump in the guard cell plasmalemma which results in hyperpolarizations as large as 45 mV and outward currents as large as 5.5 pA. The pump functions in the absence of K +; in the presence of K+ an increase in voltage noise upon light-induced hyperpolarization more negative than the K+ equilibrium potential implies K+ flow through K+-selective channels12. The data are in accord with a chemiosmotic model of stomatal response13 whereby active H+ extrusion creates an electrochemical gradient for passive ion fluxes. A 25-35-s delay in the onset of hyperpolarization, and its peak several minutes after application of a blue light pulse suggest that blue light functions as an activator, rather than a direct energy source, for ion pumping.
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