Cell walls as reservoirs of potassium ions for reversible volume changes of pulvinar motor cells during rhythmic leaf movements

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Abstract
The laminar pulvinus of primary leaves of Phaseolus coccineus L. was investigated with respect to the total K+ content, the apoplastic K+ content, and the water potential of extensor and flexor sections in relation to the leaf positions in a circadian leaf-movement cycle, as well as the cation-exchange properties of isolated extensor- and flexor-cell walls. Turgid tissue showed a high total but low apoplastic K+ content, shrunken tissue a low total but high apoplastic K+ content. Thus, part of the K+ transported into and out of the swelling or shrinking protoplasts is shuttled between the protoplasts and the surrounding walls, another part between different regions of the pulvinus. The K+ fraction shuttled between protoplasts and walls was found to be 30–40% of the total transported K+ fraction. Furthermore, 15–20% of the total K+ content of the tissue is located in the apoplast when the apoplastic reservoir is filled, 5–10% when the apoplastic reservoir is depleted. The ion-exchange properties of walls of extensor and flexor cells appear identical in situ and in isolated preparations. The walls behave as cation exchangers of hhe weak-acid type with a strong dependence of the activity of fixed negative charges as well as of the K+-storing capacity on pH and [K+] of the equilibration solution. The high apoplastic K+ contents of freshly cut tissues reflect the cation-storing capacity of the isolated walls. We suggest that K+ ions of the Donnan free space are used for the reversible volume changes (mediating the leaf movement) mainly by an electrogenic proton pump which changes the pH and-or the [K+] in the water free space of the apoplast.