Uptake and Release of Abscisic Acid by Runner Bean Root Tip Segments

Uptake o f abscisic acid by 5 mm long decapped root tips is a linear function o f the external ABA concentration in the range o f 2.9 x 1 0 8 m to 10~4 m and decreases dramatically with in­ creasing pH. At pH 8 .0 uptake rate is extremely low, even at high ABA concentrations. This indicated that nearly all o f the ABA is taken up as the undissociated m olecule ABAH. Uptake o f ABA is influenced by agents modifying the pH gradients between the medium and the tissue such as salts o f weak acids incubated at low external pH (inhibition o f uptake and stimulation o f ABA release by abolishing the pH gradients), protonophores such as CCCP (inhibition o f uptake) and fusicoccin (stimulation o f uptake by increasing the pH between m edium and cytoplasm). It is concluded that ABA distributes between the compartments o f the root cells according to the pH gradients with the undissociated molecule as the only penetrating species. Uptake and release occur without participation o f a saturable component by diffusion. In contrast IAA permeates the plasmalemma as both IAAH and IAA.