DISTRIBUTION AND ACTIVITY OF THE PLASMA-MEMBRANE H-ATPASE IN MIMOSA-PUDICA L IN RELATION TO IONIC FLUXES AND LEAF MOVEMENTS()

Plasma membrane H+-ATPase was immunolocalized in several cell types of the sensitive plant Mimosa pudica L., and transmembrane potentials we re measured on cortical cells. In comparison with the nonspecialized c ortical cells of the petiole or stem, the proton pump was highly expre ssed in motor cells. These immunological data are in close agreement w ith electrophysiological data, because the active component of the tra nsmembrane potential was low in the nonspecialized cortical cells and high in motor cells. Therefore, motor cells contain the plasma membran e H+-ATPase required to mediate the ionic fluxes that are involved in circadian leaf movements and that are necessary to recover the turgor potential that is considerably affected by the large K+ and Cl- efflux associated with seismonastic movement. With the exception of sieve tu bes, the phloem also had a high density of H+-ATPase. This suggests th at the recovery of the transmembrane ionic gradients (K+ and Cl-), whi ch is affected by various stimuli, is more energized by the companion and parenchyma cells than by the sieve elements. In addition, at the p hloem/cortex interface collocytes displayed the required properties fo r lateral transduction of the action potential toward the pulvinal mot or cells.