Electrical evidence for rhythmic changes in the cotransport of sucrose and hydrogen ions in Samanea pulvini

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.