FACTORS THAT REGULATE ABSCISIC-ACID CONCENTRATIONS AT THE PRIMARY SITE OF ACTION AT THE GUARD-CELL

pH gradients in the leaf tissues control the ABA distribution in the l eaf and the ABA concentration at the primary site(s) of action at guar d cell complexes, and hence influence stomatal aperture and transpirat ional water loss. ABA carriers at epidermal plasma membranes, and meta bolism of ABA in the cytosol of mesophyll and epidermal cells are impo rtant for keeping the apoplastic ABA concentration in unstressed leave s low, and for speeding up ABA under stress. stress-dependent ABA in r oots may also regulate the ABA concentration in/at the guard cell comp lex. The significance of an apoplastic bypass flow of ABA in roots, an d of ABA-conjugates in the xylem sap is also discussed.