Effects of (22S,23S)-Homobrassinolide and Related Compounds on Membrane Potential and Transport of Egeria Leaf Cells

(22S,23S)-Homobrassinolide was tested for its effect on the electric cell potential, proton extrusion, ferricyanide reduction, and amino acid and sucrose uptake of leaves of Egeria densa Planchon. In the light, (22S,23S)-homobrassinolide and its derivative, 2alpha-3alpha-dihydroxy-5alpha-stigmast-22-en-6-one, were similar to each Other and similar to fusicoccin in causing hyperpolarization and proton extrusion, whereas stigmasterol was less effective. In darkness, the three sterols showed comparable effects. (22S,23S)-Homobrassinolide slightly stimulated ferricyanide reduction and promoted uptake of sucrose and alpha-aminoisobutyric acid. The results are compatible with a stimulation of an electrogenic Proton Pump mechanism at the plasmalemma by (22S,23S)-homobrassinolide.