Apoplasmic water fractions and osmotic potentials at full turgidity of some Bryidae

The fractions of apoplasmic water of six moss species were estimated by comparison of the osmotic potentials of fully turgid living tissues (Ψ _π(s)) and of killed shoots (Ψ _π(k)). The values of Ψ _π(s) were determined by pressure-volume analysis using thermocouple hygrometry, whereas those of Ψ _π(k) were obtained by cryoscopy using extracts produced from dried materials and taking into consideration the original saturation water content of the shoots. Most of the tissues had Ψ _π(s) values around-1 to -1.5 MPa and non-osmotic water fractions of roughly 20% of the total water content at full turgor. Quantitative analysis of a number of osmotically active cell constituents showed that about one-third of the osmotic potential resulted from the accumulation of sugars. The total free amino acids accounted for about 15–20% of the osmolality of the cell solution. Malic- and citricacid contents varied much more with species but, in general, both carboxylic acids together contributed nearly 10–20% to the osmotic potential. The contents of inorganic anions such as free chloride and phosphate in mosses were low.