. Some of the ABA stored in the chloroplasts is released
to the apoplast (the cell wall space) of the mesophyll
cell (Hartung et al. 1998). The redistribution of ABA
depends on pH gradients within the leaf, on the
weak-acid properties of the ABA molecule, and on the
permeability properties of cell membranes (Figure
25.3). The redistribution of ABA makes it possible for
the transpiration stream to carry some of the ABA to
the guard cells