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Microelectrodes were used to demonstrate two electrical responses which occur in oat (Avena sativa L.) coleoptile parenchyma-cells during exposure to red light. The membrane potential of these cells depolarized 5-10 mV in several seconds in red light and repolarized more slowly in far-red light. By pulsing current through the cells, it was found that cellular coupling along the longitudinal axis of the coleoptile increased about 2-fold in red light, but that coupling along the lateral axis was not affected. The rapid changes in membrane potential are consistent with the idea of a membrane locale for early phytochrome action. The coupling experiments suggest that phytochrome may also affect plasmodesmata in this system.

Measurements with microelectrodes implanted into Samanea saman (Jacq.) Merrill leaf pulvini showed that membrane potentials were rhythmically sensitive to the application of sucrose. The magnitude of the electrical depolarizations induced by sucrose were dependent on the concentration of H(+) in the medium, yet changes in [H(+)] alone did not greatly affect the potential. During sucrose-induced electrical depolarization, there was a slight increase in the pH of the bathing medium; both effects were abolished by high levels of K(+), Na(+) or Ca(2+) in the medium. These observations indicate that H(+) enter the cells by some cooperative action with sucrose. A model of H(+)-substrate cotransport is proposed in which a sugar carrier in the membrane is made more permeable by the attachment of a proton. The rhythmic nature of this proposed cotransport may be related to circadian leaf-movements in this plant.

The internal electrical potential of protoplasts from six different plant species was positive. Plasmolyzed cells of leaves had positive voltages of similar magnitude. Both Elodea leaf cells and tobacco protoplasts with regenerated cell walls became more electronegative during deplasmolysis. These findings suggest that cell turgor affects membrane components that determine cellular potential.