Open stomata of detached leaves of Xanthium strumarium L. closed only when carbon dioxide and abseisic acid (ABA) were presented simultaneously. Three parameters of stomatal closing were determined after additions of ABA to the irrigation water of detached leaves, while the leaves were exposed to various CO 2 concentrations ([C02]s) in the air: a) the delay between addition of ABA and a reduction of stomatal conductance by 5 %, b) the velocity of stomatal closing, and c) the new conductance. Changes in all three parameters showed that stomatal responses to ABA were enhanced by CO~; this effect followed saturation kinetics. Half saturation occurred at an estimated [CO2] in the stomatal pore of 200 ~l 1-1. With respect to ABA, stomata responded in normal air with half their maximal amplitude at [ABA]s between 10 -6 and 10 -~ M(โข The amounts of ABA taken up by the leaves during the delay increased with a power < 1 (on the average, 0.67) of the [ABA] in the transpiration stream. The minimal amount of ABA found to produce a stomatal response was about 1 pmol of (โข per cm ~ leaf area, almost two orders of magnitude smaller than the original content of the leaves in ABA indicating that most of the endogenous ABA was in a compartment isolated from the guard cells.
An interaction between stomatal responses to CO~ and ABA was also found in Gossypium hirsutum L. and Commelina communis L. ; it was however much weaker than in X. strumarium.
Based on earlier findings and on the results of this investigation it is suggested that stomata close if the cytoplasm of the guard cells contains much malate and H +. The acid content in turn is determined by the relative rates of production of malic acid (from endogenous as well as exogenous COs) and its removal (by transport of the anion into the vacuole and exchange of the H + for K + with the environment of the guard cells). The simultaneous requirement of CO s and ABA for stomatal closure leads to the inference that ABA inhibits the expulsion of H+ from guard cells.