Plant Growth Substances and the Ionic Permeability of Membranes

Abstract

The concept that plant growth substances might exert their primary effects by altering membrane properties was proposed by Veldstra [20]. In reviewing the effects of growth substances on ion transport Van Steveninck [19] posed a series of questions on their mode of action with respect to ion transport, including (1) whether growth substances directly affect membrane permeability or conductance through interaction with the membrane structure, and (2) whether growth substances affect metabolic processes which involve the establishment of ion gradients. A number of authors have attempted to answer these questions, in particular for ABA [21] and IAA [16,18]. In searching for a simpler experimental system for ABA, Lea and Collins [11] abandoned biological membranes and used artificial phospholipid membranes. They found transient fluctuations in conductance which they associated with the formation of multimeric channels. Their data, therefore, suggest that ABA has an effect on membrane conductance which is independent of receptor molecules, indicating that ABA has a mode of action analogous to uncouplers (compounds which make membranes permeable to H⁺) or ionophores (compounds which increase membrane permeability to certain ions or groups of ions). The evidence from biological systems on whether ABA has ionophoretic properties is conflicting. It has previously been suggested that ABA was an uncoupler [3] or might have properties like those of an uncoupler [13]. However, Boyer [11 found no evidence for uncoupling of chloroplasts by ABA.