ABSCISIC ACID-LIPID INTERACTIONS - A PHOSPHOLIPID MONOLAYER STUDY

Lipid monolayer studies were performed on a Langmuir trough in the abs ence and in the presence of the plant hormone abscisic acid (ABA). The ABA-induced effects on the lipid monolayers can be summarized as foll ows: (i) ABA as the free acid (pH below 5.3) increased the molecular a rea and slightly decreased the surface pressure in the collapse points of monolayers made of saturated, unsaturated and of mixed lipids; ABA as the anion showed only minor effects. (ii) The ABA-induced area inc rease of the lipid monolayers decreased when the surface pressure incr eased, but some ABA remained in the monolayers made of unsaturated pho spholipids even at collapse pressure. (iii) The incorporation of ABA i nto the monolayers could be inhibited by adding the plant sterol beta- sitosterol to the monolayer forming phospholipids. (iv) There was no s ubstantial difference of ABA action on plant phospholipids as compared with other phospholipids. (v) ABA had a much stronger influence on un saturated phospholipids than on saturated ones. (vi) ABA decreased the phase-transition temperature of saturated phospholipids. These result s, which agree with those obtained from phospholipid vesicle studies, indicate that the physical state of the lipid is important for the abi lity of ABA penetrating into the lipid monolayer. Finally, a possible relevance of these results is discussed in terms of the action of ABA on guard cell membranes of plants.