W. E. Mayer

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

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.

Effects of cAMP, theophylline, imidazole, and 4-(3,4-dimethoxybenzyl)-2-imidazolidone on the leaf movement rhythm of Trifolium repens—a test of the cAMP-hypothesis of circadian rhythms

The period length of the leaf movement rhythm of Trifolium repens L. is lengthened by continuously offered cAMP (0.5–1.0 mol m-3) and theophylline (0.5–4 mol m-3). At the higher concentrations this effect is more pronounced and the rhythm damps out faster. Imidazole (0.5 and 1 mol m-3) has no effect on the period length; however, after 5 mol m-3 the rhythm is abolished. Offered as 4 h pulses the resulting phase response curves for cAMP and imidazole are similar and show delays of up to 4 h during the day position of the leaves. Theophylline pulses lead to delays of up to 5 h during closure and advances of up to 3 h during opening. No phase shift is brought about by 4-(3,4-dimethoxybenzyl)-2-imidazolidone. The results do not support the cAMP-model of the circadian clock which has been proposed by Cummings (J. theor. Biol. 55, 455–470; 1975). The effect of the substances tested could, however, be based upon influences on the transport of Ca2+.

Mechanics of circadian pulvini movements in Phaseolus coccineus L.

AbstractThe circadian movement of the lamina of primary leaves of Phaseolus coccineus L. is mediated by antagonistic changes in the length of the extensor and flexor cells of the laminar pulvinus. The cortex of the pulvinus is a concentric structure composed of hexagonal disc-like cells, arranged in longitudinal rows around the central stele. Observations with polarization optics indicate that the cellulose microfibrils are oriented in a hoop-like fashion in the longitudinal walls of the motor cells. This micellation is the structural basis of the anisotropic properties of the cells: tangential sections of the extensor and flexor placed in hypotonic mannitol solutions showed changes only in length. As a consequence a linear correlation between length and volume was found in these sections. Based on the relationship between the water potential (which is changed by different concentrations of mannitol) and the relative volume of the sections and on the osmotic pressure at 50% incipient plasmolysis, osmotic diagrams were constructed for extensor and flexor tissues (cut during night position of the pulvinus). The bulk moduli of extensibility,

Protoplasts from Phaseolus coccineus L. Pulvinar Motor Cells Show Circadian Volume Oscillations

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Unequal Distribution of Fixed Negative Charges in Isolated Cell Walls of Various Tissues in Primary Leaves of Phaseolus

, were estimated from these diagrams. Under physiological conditions the

Energy-dependent phases of the circadian clock and the clock-controlled leaf movement in Phaseolus coccineus L.

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