Suppression of Stomatal Opening in Leaves Treated with Abscisic Acid

Small doses of abscisic acid (approximately 0.02 µg cm^-2of leaf) applied to the leaf surface as a 10^-4 M solution causedmarked stomatal closure in Xanthium pennsylvanicum, and theeffect persisted for up to 9 days after application. Similareffects were found when 10^-4 M abscisic acid was supplied todetached tobacco leaves via their petioles. CO²-free air didnot cause a reversal of the closure, and it was therefore concludedthat the effect was not due simply to an increase in the intercellularCO²concentration; a more direct effect on the stomatal apparatusis suggested. It is considered that abscisic acid could playan endogenous role in the control of stomatal aperture, andthat this, and/or related substances, might be more useful as‘anti-transpirants’ than the phytotoxic substancescurrently employed for this purpose.