Influence of Cytoplasmic Streaming and Turgor Pressure Gradient on the Transnodal Transport of Rubidium and Electrical Conductance in Chara corallina

In order to study the transnodal transport of Rb⁺ in internodalcells of Chara corallina, a low-temperature loading system wasestablished to separate the loading process from the transportprocess. Tandem cells, consisting of internode-node-internode,were isolated from algal plants. Treatment of a single internodewith 100 mM RbCl at 5°C for 30 min caused an accumulationof 43 mM Rb⁺ in the cytoplasm of this cell (= source cell),but no Rb⁺ was found in the other internode (= sink cell) ofthe tandem cells. In 40 min after a return to 25°C, about12% of the Rb⁺ loaded in the source cell was transported intothe sink cell. The apparent transnodal permeability of Rb⁺ wascalculated to be 4.6 x 10^–7 m.s^–1. Under the assumptionthat the total cross-sectional area of plasmodesmata occupies10% of the nodal area, the diffusion coefficient of RbCl throughplasmodesmata was calculated to be 2.3 x 10^–11 m².s^–1which is about 1% of the free diffusion coefficient in water(2 x 10^–9m².s^–1). The transnodal transport of Rb⁺ was intimately correlated withthe rate of cytoplasmic streaming. The rate of streaming inboth the source and sink cells was varied either by treatingthe cells with cytochalasin B (CB) or by lowering the temperature.The transport rate correlated with the streaming rate irrespectiveof the method used. Since the level of ATP was not influencedby CB or low temperature, the transnodal transport is assumedto be the result of passive diffusion process through plasmodesmata. A turgor pressure gradient across the node decreased both thenodal electrical conductance and the transnodal transport ofRb⁺. By contrast, the exposure of both internodal cells to asolution of sorbitol had no effect on either of them. A turgorpressure gradient of 240 mOsm decreased the transport of Rb⁺in the first hour to 3% of the control, while it decreased thenodal conductance to about 50%. The increase in the electricalresistance occurred on the junction side between the node andthe internode that was treated with sorbitol. Cytochalasin Ehad no effect on the nodal electrical resistance. It is assumedthat plasmodesmata are equipped with a valve-like mechanismwhich is sensitive to the gradient of turgor pressure acrossthe node and is not regulated by an actomyosin system. (Received February 15, 1989; Accepted April 20, 1989)