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Abstract

WHEN plants assimilate CO2 from the atmosphere, water vapour escapes. This exchange of gases can be moderated by the plant through volume changes of guard cells, which in pairs surround each stomatal pore in the epidermis and act as turgor-operated valves. When light intensity diminishes or water stress develops, guard cells shrink and stomata close. This process requires the release of ions from the guard cells. Voltage-dependent K+ chan-nels1,2 provide a path for cations. The required corresponding passage for anions, particularly chloride and malate3, is not known at the molecular level. We have applied the patch-clamp technique4 to guard-cell protoplasts of Vicia faba and found strongly voltage-dependent activities of single anion channels in the plasma mem-brane (plasmalemma). These channels have a conductance of 39 pS in 100 mM KC1 and are permeable for chloride and malate. They become active at membrane potentials positive to −80 mV. Their maximum activity occurs at −40 mV. The resulting outward Cl– currents of individual guard cells are at least 25 pA. These voltage–dependent anion channels provide a mechanism for the control of salt efflux from guard cells.