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 leaf and the ABA concentration at the primary site(s) of action at guard cell complexes, and hence influence stomatal aperture and transpirational water loss. ABA carriers at epidermal plasma membranes, and metabolism of ABA in the cytosol of mesophyll and epidermal cells are important for keeping the apoplastic ABA concentration in unstressed leaves low, and for speeding up ABA redistribution under stress. Stress-dependent ABA redistribution in roots may also regulate the ABA concentration in/at the guard cell complex. The significance of an apoplastic bypass flow of ABA in roots, and of ABA-conjugates in the xylem sap is also discussed.