ANION-CHANNEL BLOCKERS INHIBIT S-TYPE ANION CHANNELS AND ABSCISIC-ACID RESPONSES IN GUARD CELLS

The effects of anion-channel blockers on light-mediated stomatal openi ng, on the potassium dependence of stomatal opening, on stomatal respo nses to abscisic acid (ABA), and on current through slow anion channel s in the plasma membrane of guard cells were investigated. The anion-c hannel blockers anthracene-9-carboxylic acid (9-AC) and niflumic acid blocked current through slow anion channels of Vicia faba L. guard cel ls. Both 9-AC and niflumic acid reversed ABA inhibition of stomatal op ening in V. faba L. and Commelina communis L. The anion-channel blocke r probenecid also abolished ABA inhibition of stomatal opening in both species. Additional tests of 9-AC effects on stomatal aperture in Com melina revealed that application of this anion-channel blocker allowed wide stomatal opening under low (1 mM) KCI conditions and increased t he rate of stomatal opening under both low and high (100 mM) KCI condi tions. These results indicate that anion channels can function as a ne gative regulator of stomatal opening, presumably by allowing anion eff lux and depolarization, which prohibits ion uptake in guard cells. Fur thermore, 9-AC prevented ABA induction of stomatal closure. A model in which ABA activation of anion channels contributes a rate-limiting me chanism during ABA-induced stomatal closure and inhibition of stomatal opening is discussed.