The fluorescent probe dis-C₃-(5) (3,3'-dipropyl-2,2'-thiodicarbocyanine iodide) was applied to investigate what passive and active cation transport changes occur in the plasmalemma during the different stages of gravitropic reaction. The coleoptiles were oriented horizontally for 30 s, 1 min, 3 min, 30 min. A membrane fraction enriched with the plasmalemma was isolated from such gravitropically irritated coleoptiles. The cation permeability of the plasmalemma was shown to be intensified under 30 s, i. e. the potassium diffusive potential (KDP) was decreased, but 1 • 10^–5 M IAA in vivo reduced the passive cation transport, and the KDP was on the increase. To characterize the gravitropic reaction, the following sequence of its processes can be suggested: gravitation force (g) effect →shift of cell structures →change of passive cation transport or H⁺ potential through the plasmalemma →Ca²⁺ concentration changes →formation of IAA complexes and IAA transport.

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