MOVEMENT OF ABSCISIC-ACID ACROSS THE PLASMA-MEMBRANE OF PHLOEM ELEMENTS OF PLANTAGO-MAJOR

Uptake of ABA into isolated vascular bundles of Plantago major is linearly correlated with the external ABA concentration. The results indicate that uptake occurs mainly by diffusion of the lipophilic undissociated ABAH molecule. The permeability coefficient of the plasma membrane for ABAH is 9 x 10(-8) ms-1. The internal pH was estimated using C-14-DMO as pH probe and was found to be 7.5. Additionally, the effect of osmotic stress on the cytoplasmic pH of the phloem cytosol was investigated. Under osmotic stress conditions the pH decliness slightly by approximately 0.2 units.