STOMATAL SENSITIVITY TO ABSCISIC-ACID FOLLOWING WATER-DEFICIT STRESS

Short- and medium-term stresses (1 and 24 h, respectively) were applie d to detached leaves of Commelina communis L., resulting in both cases in a final leaf cell water potential (psi(w)) of about -1.0 MPa and r elative water content (RWC) of about 74%. A long-term stress (14 d), a pplied to intact plants, resulted in a final leaf cell psi(w) of -0.57 MPa and RWC of 90%. All the stresses resulted in lower leaf conductan ces (g(i)) compared to controls, although temporary increases in g(i) were observed early in the short- and medium term stresses. When epide rmal strips were incubated in vitro, the stomatal opening potential wa s reduced by the stresses, possibly due to lower initial apertures cau sed by the stress. Pre-opened stomata on epidermal strips were treated with abscisic acid (ABA) to investigate their sensitivity to this com pound. The short-term stress had no significant effect on the dose-res ponse relationship for ABA and stomatal closure. The medium-term stres s significantly increased the maximum rate of stomatal closure compare d to controls, but had no other significant effect. At the end of the long-term stress, the maximum rate of stomatal closure had significant ly decreased, but other features of the dose-response curve were not s ignificantly affected. This temporal pattern of response was confirmed by observing the rate of closure caused by a near-saturating concentr ation of ABA as the long-term stress developed. In tandem with estimat es of the concentration of ABA in the guard cell apoplasm, our results indicate that control of stomatal aperture by ABA during water defici t stress is feasible; however, the results also suggest that in the me dium to long term, the responding system undergoes changes resulting i nitially in a faster response to the compound, followed by a desensiti zation.