Turgor Pressure Regulation in Valonia utricularis

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Abstract
The electrical membrane resistance .rho. of the marine alga V. utricularis shows a marked maximum in dependence on the turgor pressure. The critical pressure Pc at which the maximum occurs, as well as its absolute value .rho.max are strongly volume-dependent. Both Pc and .rho.max increase with decreasing cell volume. Apparently these relationships reflect the elastic properties of the cell wall, because the volumetric elastic modulus .epsilon. is also volume-dependent, increasing hyperbolically with cell volume. Both Pc and .rho.max can be affected by external application of IAA at concentrations of 2.10-7 M to 2.cntdot.10-5 M. The critical pressure is shifted by 1.2-6 bars toward higher pressures and the maximum membrane resistance increased up to 5.6-fold. During the course of the experiments (up to 4 h) IAA had no effect on the volumetric elastic modulus .epsilon.. The maximum in membrane resistance is discussed in terms of a pressure-dependent change of K fluxes. The volume dependence of Pc and .rho.max suggests that not only turgor pressure but also .epsilon. must be considered as a regulating parameter during turgor pressure regulation. On this basis a hypothesis is presented for the transformation of a pressure signal and of changes in the elastic properties of the cell wall into alterations of ion fluxes. It is assumed that the combined effects of tension and compression of the membranes as well as the interaction between membrane and cell wall opposingly change the number of transport sites for K+, providing a turgor-sensing mechanism that regulates ion fluxes. The IAA effects demonstrated are consistent with this view, suggesting that the basic mechanisms for turgor pressure regulation and growth regulation are similar. Any relation connecting growth rate with turgor pressure should be governed by 2 parameters, i.e., by a yielding pressure at which cell growth starts and by the critical pressure at which it ceases again.