Control of stomatal behaviour by abscisic acid which apparently originates in the roots

Two experiments indicate that abscisic acid (ABA) may influence stomatal behaviour of Commelina communis L. Stomatal conductance could not be correlated with bulk leaf ABA content but when the abaxial epidermis was assayed for ABA, small increases in ABA content correlated well with limitations of leaf conductance. Restricted conductance of the abaxial surface of leaves was associated with an increase of approximately 40 amole ABA per stomatal complex. This agrees with previously published figures.When roots of individual plants were split between two containers, drying the soil around one part of the root system restricted leaf conductance, even though leaf water relations were not affected. Increased ABA content of the epidermis coincided with increased ABA content of the roots in drying soil. Other roots of the same plant in moist soil did not show increased ABA content. These results suggest that in drying soil, ABA can move from the roots to the epidermis and restrict stomatal aperture even when leaf water potentials and turgors remain constant. The importance of this mechanism in providing a sensitive foliar response to decreasing soil moisture is discussed.