Turgor pressure and cell volume relaxation inHalicystis parvula

Summary

The elastic modulus, ε, for the cell wall ofHalicystis parvula (defined by$\varepsilon = V\frac{{dP}}{{dV}}$) was determined by two different ways:

• 1) By measuring the stationary pressure-volume curve and by calculating the long-term elastic coefficient,ɛ _s, from the slope of the curve at a given volume and

• 2) By measuring differential changes in cell turgor pressure and cell volume using the pressure probe technique and by calculating the short-term elastic coefficient, ε, according to the definition equation. The values of the elastic coefficients differ considerably and show different dependences on cell turgor pressure.ɛ _s is about 0.5 to 2 bar, and is therefore in agreement with measurements of Graves and Gutknecht (Graves, J., Gutknecht, J. (1976)J. Gen. Physiol. 67 ∶ 579) on perfused cells ofH. parvula.ɛ _s is almost pressure independent within the pressure range of 0.05 to 0.9 bar.

On the other hand, ε assumes values of about 1 to 2 bar at a low pressure (about 0.05 to 0.15 bar) and increases to about 16 bar at 0.9 bar turgor pressure. Evidence is presented that the short-term elastic coefficient, ε, determined from differential changes in cell turgor pressure,dP, and cell volume,dV, reflects the true elastic properties of the cell wall, whereas the long-term elastic coefficient,ɛ _s, also includes other mechanical properties of the cell wall, which could not be identified up to now. The hydraulic conductivity,L _p, of the cell membrane ofH. parvula was determined by directly measuring both the turgor pressure relaxation process (pressure probe) and the volume changes (microscope) in response to osmotic stress.L _p was calculated from the slope of volume-time curves without knowledge of the elastic modulus. It has a value of about 0.8 to 2×10⁻⁶ cm sec⁻¹ bar⁻¹. The calculation ofL _p from the turgor pressure relaxation process leads to identical results when using the short-term elastic coefficient. Under these conditionsL _p assumes values of about 1.5 to 2.5×10⁻⁶ cm sec⁻¹ bar⁻¹.L _p increases as the plasmolytic point is approached.

The result demonstrates that the short-term elastic coefficient determined by the pressure probe technique controls the instantaneous water transport between the cell interior and the external medium. The high extensibility of the cell wall (resulting from the low elastic coefficient) is the reason whyH. parvula exhibits a change in cell volume rather than a change in cell turgor. The results are discussed in relation to pressure measurements in individual cells of higher plant tissues.