The Importance of Water Activity and Water Structure during Hyperosmotic Stress in Algae and Higher Plants

Abstract

Summary The concept of osmotic regulation in plant cells, including both turgor pressure regulation by transport of inorganic ions into giant-celled algae and the accumulation of organic compounds in several microscopic algae and tissue cells is critically investigated. The importance of the cellular water activity and water structure and their influence on the solubility and hydration sphere of the biopolymers is strengthened. The hyperosmotic conditions, causing the synthesis of polyols or proline in several plant cells, include the state of plasmolysis. Under these conditions, a reduction of the cellular water activity is assumed to impair the hydration sphere of the cellular constituents. The function of the polyols and proline is interpreted to improve the solubility of the cellular constituents. Until now, there is not suficient experimental evidence that the accumulated organic compounds act by an osmotic balance. During turgor pressure regulation in giant-celled luarine algae plasmolysis is not involved. It is suggested that changes of sea water salinities, occurring under the natural condition and inducing the corresponding alterations of the internal osmotic pressure, do not impair the hydration of the cytoplasmic components. Probably they are adapted to a certain range of water activities by a different structure. Therefore, the solubility-improving mechanism with organic compounds is not necessary, but the maintenance of turgor pressure is the aim of this regulatory mechanism. These considerations led to the conclusion that the concept of an osmotic regulation is only true for turgor pressure regulation in giant algal cells.