Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

3. Water metabolism in plants. Diffusion, osmosis, chemical and water potentials, osmotic pressure
3.1 The role of water in plant life activity

Water is the principal component of plants. Its content varies across different plant Organs (for instance, it accounts for 95% in lettuce leaves, yet no more than 10% of tissue mass in dry seeds) and depends on environmental conditions, as well as the species and age of the plant. For normal physiological functioning, a plant must maintain a specific water content. The dual processes of Water uptake and evaporation are collectively referred to as the water balance.

Water serves as the medium in which metabolic processes take place. All Hydrolysis and redox reactions occur with the participation of water. Water acts as a source of oxygen, which is released during Photosynthesis, and hydrogen, which is utilized for the reduction of carbon dioxide. Water maintains the conformation (the specific spatial arrangement of micro- and macromolecules arising without the Cleavage of chemical bonds due to the rotation of individual molecular chains) of protein molecules, as well as the structural Stability of the Cytoplasm and Cell walls in a turgid state. Changes in turgor pressure are also associated with certain movements of plant parts.

Charges within a water molecule are distributed unevenly, as the oxygen atom attracts electrons away from the hydrogen atoms. Consequently, a water molecule Functions as a dipole: one pole of the molecule carries a positive charge, while the other is negative. This polarity enables water molecules to associate with one another, as well as with ions and protein molecules. Water plays a key role in the uptake and Transport of substances because it acts as an excellent solvent. The Hydration shells surrounding ions restrict their interaction.

Water possesses a high heat capacity—1 cal/g·°C—which allows plants to buffer and mitigate fluctuations in ambient Temperature. The evaporation of water by plants, known as Transpiration, serves as the primary mechanism for plant thermoregulation. Plants transpire vast amounts of water. This high water consumption is due to the fact that plants possess a substantial leaf surface area, which is necessary for absorbing carbon dioxide, whose concentration in the atmosphere is low (0.032%).