OSMOTIC POTENTIALS OF WATER-SATURATED MOSSES

From 14 species of mosses of the temperate climate osmotic potentials (psi(pi)) at full turgidity were determined cryoscopically by use of both extracts produced from dried materials in consideration of the saturation water content and press sap squeezed out from water-saturated plants. With this techniques psi(pi) values between - 0.5 and - 1.0 MPa were found for most of the mesic species. Osmotic potentials measured via freezing point depressions of press sap were about 10 - 20 % higher (-less negative) than those obtained with extracts from dry mass. In contrast, psi(pi) values achieved by use of the conventional plasmolytic method were usually 2 to 3 times lower. It is maintained that these values do not reflect osmolalities at a defined water status and, thus, the plasmolytic method is not qualified for determination of osmotic potentials of leafy bryophytes. Comparison of mosses that were harvested in summer and winter revealed that some species exhibited significantly lower osmotic potentials during the cold season. This can be explained in part by higher sugar concentrations of the more frost-hardy tissues. Generally, the sugars constitute roughly 20 - 30 % of the total osmolality of the cell solution.