Method for determination of turgor pressure in macroalgae,with particular reference to the Phaeophyta

A procedure is described for the determination of the internal osmotic pressure and turgor pressure of marine macroalgae, for use in the laboratory and on the shore. A volume-related parameter (either thallus fresh weight, or area) is measured before and after transfer of plant material to a range of hyperosmotic solutions. Plotting the final fresh weight/area as a percentage of the initial value gives a biphasic curve, with an initial component of negative slope due to the change in thallus volume in less extreme hyperosmotic solutions, where the non-rigid thallus contracts in response to decreasing cell turgor pressure. The second component has a shallower slope and represents plasmolysis in more extreme hyperosmotic solutions, i.e., where turgor pressure is reduced to zero and the protoplast shrinks away from the cell wall; the extraprotoplast space created by plasmolysis will be filled with the external solution and thus no further changes in weight occur. These two components intersect at the lowest osmotic pressure at which cell turgor is zero. By correcting for any effects of the cell wall on thallus volume, the relationship can be used to calculate internal osmotic pressure and hence turgor pressure, assuming that the remaining change in thallus volume of the initial component is due entirely to variation in the intraprotoplast volume (approximately equivalent to the intraprotoplast water content, determined by subtraction of the extraprotoplast water and dry weight from the thallus fresh weight). Using this procedure, the turgor pressures of Fucus spiralis L., Ectocarpus siliculosus, (Dillw.) Lyngb. and Laminaria digitata (Huds.) Lamour. (from Fife Ness, Scotland, May–August 1987) in a seawater-based medium were 0.82, 0.58 and 1.34 Osmol kg^–1, respectively. The turgor pressure of F. spiralis on the shore at Fife Ness (June 1987) was 0.74 Osmol kg^–1.