W. Daeter et al., THE PH GRADIENTS IN THE ROOT-SYSTEM AND THE ABSCISIC-ACID CONCENTRATION IN XYLEM AND APOPLASTIC SAPS, Philosophical transactions-Royal Society of London. Biological sciences, 341(1295), 1993, pp. 49-56
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.