Plant Physiology - Musienko M. M. 2001

Plant Water Regime
State and Fractional Composition of Intracellular Water. Hydration

When ions interact with Water molecules, the water dipoles orient themselves within the electric field of the ion. Some ions increase the mobility of water molecules, while others decrease it. An increase in the translational motion of water is referred to as negative Hydration, whereas a decrease is called positive hydration. Negative hydration is caused by potassium, cesium, ammonium, chloride, iodide, and nitrate ions; positive hydration is caused by magnesium, calcium, sodium, and sulfate ions. Furthermore, the energy of interaction between an ion and water dipoles significantly exceeds the energy of interaction between water dipoles and Hydrogen Bonds. Cations interact with the negatively charged oxygen atoms of water, while anions interact with the positively charged H⁺ of the water dipole. Such shielding of ionic fields reduces the interaction between them and allows them to remain in solution. This principle forms the Molecular Basis of hydration.

Slightly soluble substances that do not form bonds with water molecules are also characterized by The ability to alter the physical properties and Structure of water. Hydration of macromolecular substances is classified into ionic and electroneutral. It is based on the electrostatic attraction of water molecules. In ionic hydration, water molecules interact with NH₃⁺ and -COO groups, whereas in electroneutral hydration, water molecules are bound by heteropolar groups: -COOH, -OH, -CHO, -NH₂, and others. It is likely that in the Cytoplasm, which contains ions as well as low- and high-molecular-weight compounds, hydration of all its components takes place simultaneously.

Water bound by ions and low-molecular-weight compounds is referred to as osmotically bound water.

Cell walls and the Vessels of the conducting system contain capillary-bound water.

Additionally, one should distinguish immobilized or structurally bound water. Immobilization represents the mechanical entrapment of water during conformational changes of molecules, As a result of which the molecules end up in a confined space. Most of this water retains The properties of pure water; however, it is quite difficult to remove, and in this sense, it should be considered "bound." We know very little about its physiological role. Confined to a closed space, it evaporates with difficulty, despite being characterized by significant mobility. It is believed that seeds retain their viability for a long time precisely due to the presence of Immobilized Water in their embryos.

As is known, CARBOHYDRATES do not dissociate into ions and carry no charge. Therefore, their hydration occurs primarily via hydroxyl groups due to the electronegative oxygen atom. A glucose molecule contains five hydroxyl groups and one aldehyde group. Glucose is readily soluble in water, whereas its polymer (Cellulose) is insoluble because about 50% of the cellulose hydroxyl groups are shielded by ions or located inside the micelle. The binding of water molecules by these concealed groups is called permutoid hydration.

Water bound to both internal groups and those located On the surface is termed colloid-bound water.

Amino Acids contain two electronegative elements: nitrogen and oxygen. They are capable of forming hydrogen bonds with H₂O through unshared electron pairs. There are no fundamental differences between the hydration of Amino Acids and that of carbohydrates. Protein hydration is accomplished through the interaction of water molecules with hydrophilic and hydrophobic radicals, as well as through water immobilization in confined regions.

As is well known, protoplasm contains significant amounts of Lipids, which feature A large number of hydrophobic groups whose effects on water remain insufficiently studied.

Thus, water as a constituent of protoplasm in plants exists predominantly in a stabilized state, meaning its activity is reduced. The cause of this stabilization is its binding via hydration and through the barrier function of membranes. This creates the fraction of colloid-bound water—the most stable water reserve of the Organism, which ensures resilience in stressful situations.

The fraction of water categorized as osmotically bound and so-called free water acts as the most active participant in physiological processes.