The Intracellular Distribution Of Abscisic Acid In Mesophyll Cells — The Role Of The Vacuole

Summary Of the total free endogenous ABA present in barley mesophyll cells 20 to 30 % were found in the vacuolar fraction. In the plant kept in the dark, the vacuolar content was 40 – 50 % higher than in plants which had been illuminated for 24 h. If the volume ratio of vacuole to cytosol is assumed to be 5/1, the concentration of ABA in the vacuoles is about 20 times lower than in the extravacuolar compartments. Whereas the amount of conjugated ABA, which can be hydrolysed by alkaline treatment was very low in isolated vacuoles, the concentration of conjugated non-hydrolysable ABA in the vacuoles was 4.7 times higher than the concentration of vacuolar free ABA; in the protoplasts the amount of these forms of ABA was comparable to the amount of total free ABA. Significant light-dark effects on the content of bound ABA in the vacuoles could not be detected. Light or dark treated protoplasts of Valerianella locusta were exposed to radioactive ABA for three hours and subsequently treated with DEAE-dextran which releases soluble cytoplasmic constituents. Of the labeled ABA which had been taken up, 5 % were released upon treatment of illuminated protoplasts, 20 % when the protoplasts had been kept in the dark. 50 – 80 % of the labeled ABA was found in the chloroplast fraction. Significant uptake of radioactivity by vacuoles could be detected neither in vivo (in the protoplasts) nor in vitro during incubation with 14 C-ABA up to 60 min indicating that the permeability of the tonoplast for ABA is poor. It is discussed whether these data can be taken to indicate complete impermeability of the tonoplast for ABA or ABA-derivatives.