THE PH GRADIENTS IN THE ROOT-SYSTEM AND THE ABSCISIC-ACID CONCENTRATION IN XYLEM AND APOPLASTIC SAPS

Abscisic acid (ABA) is a stress signal that is transported from the ro ot system to leaves, and induces stomatal closure before water relatio ns of the leaves are affected by soil drying. Xylem vessels are in dir ect contact with the leaf apoplasm, the only leaf compartment that is directly connected with the primary site of ABA action, the outer surf ace of the guard cell plasma membrane (Hartung 1983). ABA distributes among the leaf compartments according to the anion trap concept and th e Henderson-Hasselbalch equation, with the free acid as the permeating and the anion as the nearly non-permeating molecular species. Applyin g this concept, a flattening of the intracellular pH gradients increas es the apoplastic ABA concentration. Indeed, stress increases the apop lastic pH (Hartung et al. 1988) and decreases slightly the cytosolic p H. The validity of this concept has been shown repeatedly and was conf irmed by a mathematical leaf model (Slovik et al. 1992). It is appropr iate to ask whether these mechanisms also contribute to ABA compartmen tation and redistribution in the root system. Therefore, we have incor porated compartmental pH values of unstressed and stressed root cells, the permeability coefficients of root membranes for ABA and anatomica l data into a mathematical model, similar to that of Slovik et al. (19 92). The simulation shows that ABA redistribution in roots caused by c hanging pH gradients can account for up to a 2 to 3-fold accumulation of ABA in the xylem sap of stressed plants.The model also predicts tha t the pH gradient across the cortical plasma membrane has the most dis tinct effects on redistribution of ABA into the xylem sap of stressed plants and, additionally, that the ABA concentration in the rhizospher ic aqueous solution can play an important role in root-to-shoot signal ling.