Surface Activity as Related to Physiological Activity of Plant Growth-Regulators

Abstract

Among theories proposed to explain the action of plant growth-regulators is one which focuses attention on the altered permeability of cell membranes caused by the adsorption of growth-controlling compound to this phase boundary. This theory, proposed by Veldstra (11), is based in part on a polarographic investigation in which the effects of the compounds on the oxygen maximum were used as a measure of the adsorbability of the compounds and therefore of their physiological activity. Subsequently (12), the concept was modified to allow for a chemical reaction between growth regulator and cellular constituents ; and the results achieved at the surface of the mercury drop according to this hypothesis must then be considered as indicative of chemical reactivity as well as adsorption. The possibility that an examination of substances with respect to their effect on the oxygen maximum might indicate a relative measure of their adsorbability or surface activity was first propounded by Heyrovsky and detailed by Rasch (5). According to this theory, the oxygen maximum is the result of the adsorption of oxygen to the surface of the mercury drop, and any reduction in this maximum (at the same potential) is due to the preferential adsorption of some other substance which replaces oxygen at the mercury surface. Another explanation of the maximum involves the electrokinetic stirring in the mercury drop and the electrolyte solution (7) which brings reducible ions to the mercury surface more rapidly than diffusion alone would. At the maximum potential the density of indifferent cations in the electrical double layer restricts the movement of reducible ions to that of diffusion, whereupon the current drops to the diffusion current. Surface active substances depress the maximum, according to this theory, by lowering the surface tension of the mercury and the subsequent cessation of the stirring effect. According to Kolthoff and Lingane (2) neither one of the theories adequately accounts for all characteristics of the maximum but the complete explanation probably will include aspects of both. The number of compounds reported on by Veldstra was few, and the growth tests with which the results were compared were the rooting of cuttings and the curvature of split stems of peas. It was thought that a more comprehensive investigation of the effects on the oxygen maximum of compounds effecting cell elongation together with several compounds known to