Mutual Antagonism of Sulfur Dioxide and Abscisic Acid in Their Effect on Stomatal Aperture in Broad Bean (Vicia faba L.) Epidermal Strips.

Abscisic acid (ABA) was found to counteract the stomatal opening in Vicia faba L. caused by SO(2). The antagonism between SO(2) and ABA was mutual, and their combined effect depended upon which compound was in the greatest concentration. Stomatal apertures were monitored in detached epidermal strips floated in the light on aqueous solutions of SO(2) (sulfurous acid) and/or ABA in 0.01 molar sodium citrate buffer (pH 5.8). Low concentrations of sulfurous acid (10(-10) to 10(-7) molar) increased stomatal aperture, but concentrations greater than 10(-5) molar decreased it. A progressive decrease in aperture size occurred as ABA was increased from 10(-10) to 10(-5) molar.No evidence was found for a direct chemical reaction between the buffered sulfurous acid and ABA (exogenous or endogenous). Extractable, endogenous ABA in the strips remained relatively constant after exposure to several different concentrations of sulfurous acid. A technique for quantitating ABA from methanolic extracts of small samples of epidermis (20 milligrams dry weight) using reverse phase high performance liquid chromatography is described.