Abstract
The pressure response of (plant) cells to osmotic challenges depends on the reflection coefficient, σ, of osmotically active solutes; it is less than predicted by the van't Hoff equation if σ < 1. In Valonia utricularis, σ is significantly reduced by internal (and, to a lesser extent, by external) unstirred layers, protecting the cytoplasm against vacuolar osmotic fluctuations. As shown by scanning and transmission electron microscopy, diffusion-restricted spaces are formed by innumerable small vacuoles that are interconnected with each other and with the central vacuole. They are embedded in networks of cytoplasmic strands connecting and encircling the organelles. Unstirred layers are also created in the central vacuole by an extensive network of acid mucopolysaccharide filaments (visualized by alcian blue staining). Mucopolysaccharides apparently also affect steady-state turgor by reducing the water activity. When the effective vacuolar osmotic pressure was adjusted to that of the bath by perfusion with an artificial vacuolar sap (AVS), an ‘offset turgor pressure’ of 17 ± 5 kPa was recorded. Consistent with the ultrastructural data, σ values less than unity were calculated from the pressure response upon vacuolar addition of KCl or sucrose by perfusion (σiKCl = 0.63 ± 0.13; σisuc = 0.58 ± 0.17). Dilution of AVS yielded slightly higher σiKCl values (0.73 ± 0.35). External addition to the artificial sea water (ASW) indicated that σe > σi for these osmotica. However, even in this case, σesuc (0.86 ± 0.09) and σePEG (0.58 ± 0.08) were significantly less than σeNaCl (0.94 ± 0.05) and σeKCl (0.91 ± 0.13), presumably due to unstirred layers within the 4 µm thick cell wall. Consistent with the low σ values, a partial replacement of NaCl by osmotically equivalent amounts of sucrose (ASWsuc), PEG and dextran, respectively, as well as replacement of Cl− by the large anion MES− induced an ‘anomalous’ hyposmotic turgor pressure response followed by the usual backregulation of pressure. After a 2-day preincubation in ASWsuc, significantly lower σe values were obtained both hyperosmotically (σeNaCl = 0.78 ± 0.14; σesuc = 0.72 ± 0.15) and hyposmotically (σeNaCl = 0.70 ± 0.17; σesuc = 0.63 ± 0.09), probably due to long-term effects on membrane structure to be elucidated yet. The freshwater alga Chara corallina lacked these apparently closely related structural and biophysical features of Valonia.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Heidecker, M., Mimietz, S., Wegner, L. et al. Structural Peculiarities Dominate the Turgor Pressure Response of the Marine Alga Valonia utricularis upon Osmotic Challenges . J. Membrane Biol. 192, 123–139 (2003). https://doi.org/10.1007/s00232-002-1068-7
Received:
Issue Date:
DOI: https://doi.org/10.1007/s00232-002-1068-7