The Hydraulic Conductivity and Volumetric Elastc Modulus of Cells and Isolated Cell Walls of Nitella and Chara spp.: Pressure and Volume Effects

Abstract

The hydraulic conductivities (L*p) and the volumetric elastic moduli (e) of N. flexilis, C. intermedia and C. fragilis were determined by means of direct cell turgor measurements. For large cell volumes (V) the function e = f(P), where P = pressure, is a hyperbola. For small cell volumes e is nearly independent of pressure and the absolute e values are smaller than those obtained for larger volumes. This volume dependence of the volumetric elastic modulus was also verified by measurements of the elastic properties of isolated cell walls of N. Jlexilis under conditions where the lengths of the cell wall tubes prepared from each cell were varied. The volume dependence of e, which is unexpected within the framework of Hooke's law, can be explained by assuming that two different intrinsic moduli e*1 and e*2 are applicable to different cell regions with volumes V*1 and V*2. The quantities e*1 and V*1 are related to the cylindrical part, and e*2 and V*2 to the small node or end regions of the internode. With this assumption the overall e is then given by: e = (e*1e*2V)/(e*2V*1 + e*1 V*2) ˜ e*1e*2 V /(e*2 V + e*1 V*2) . This indicates that for very large cells e has a saturation value equal to e*1, while for smaller volumes the influence of e*2 will become predominant. The value of e*1 was calculated to be about 7.5 x 10*7 Pa* and that of e*2 to be about 10*6 Pa. Since for small cells the overall volumetric elastic modulus e is mainly determined by e*2, the weak pressure dependence of e in such cells reflects a weak pressure dependence of e*2. On the other hand, the strong pressure dependence of e in large cells points to a strong pressure dependence of e*1. The direct determination of the elastic properties of the end regions of the internodes, which was not possible up to the present, is of great importance for growth and growth regulation. The hydraulic conductivity of Nitella and Chara spp. was found to be independent of cell volume, but dependent on the cell turgor pressure. The L*p values were constant at high pressures, but increased on approaching the plasmolytic points. In contrast, the L*p value of the isolated cell wall of N. flexilis was constant over the whole pressure range 0-8 x 10*5 Pa and amounted to (6.9 ± 1.3) x 10-*l2 ms-¹ Pa-¹. Since the L*p value in the living N. flexilis cell increased to 4 x 10-*12 ms-¹ Pa-¹ at a pressure of 5 x 10*4 Pa it can be concluded that the cell wall becomes the rate-limiting barrier for water flow and that the hydraulic conductivity of the cell membranes must be remarkable at low pressures. The increase in L, in the low pressure range is not caused by artificial leakages or by leakages through the plasmodesmata, since no water flow across plasmodesmata could be detected.