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

3. Water relations in plants. Diffusion, osmosis, chemical and water potentials, osmotic pressure
3.7 Water movement through the vascular system

Measurement Methods and Transpiration indices. Transpiration rate is usually expressed in grams of evaporated Water per hour per unit area or per 1 gram of dry mass (daytime: 15-250 g/m² per hour; nighttime: 1-20 g/m² per hour).

Transpiration ratio - the number of grams of dry matter produced per 1,000 grams of water lost (1-8 g per 1,000 g of water).

Transpiration coefficient - the number of grams of water expended to produce one gram of dry matter (120-150 g per 1 g of dry matter).

The upward water flow moves predominantly through the xylem, a distribution system that supplies all Plant Tissues and Organs with water. The xylem develops from ROOT and stem procambial Cells, initially containing Cytoplasm, but reaching maturity consisting solely of lignified Cell walls. About 1-10% of the upward water flow occurs along The Cell walls of living cells and is also driven by The water potential gradient created by transpiration. Simultaneously with the upward Movement of water, water molecules are exchanged with all Cells of the stem. Several theories explain The Mechanism of the upward water stream.

The driving force of the upward water flow within the xylem conductive elements is the water potential gradient across the plant from the soil to the atmosphere. It is maintained by:

1) the osmotic potential gradient in root cells (from the soil to the xylem vessels) resulting from Active ion transport in living cells, including young living xylem elements;

2) transpiration.

Maintaining the first gradient requires metabolic energy, whereas transpiration relies on the energy of solar radiation.

The former ensures water absorption by the root, while the latter serves as the primary driving force for the upward water flow. This force creates a steep negative hydrostatic pressure gradient in the xylem, which manifests as water tension within the xylem vessels.

According to the cohesion-tension theory (19th century), water in the capillary tubes of xylem vessels ascends in response to the suction pull of transpiration, driven by the mutual attraction (cohesion) of water molecules and the adhesion of the water Column to the hydrophilic walls of the vessels. These forces also prevent The formation of air cavities capable of blocking 65

vessels. In the event of a blockage (embolism), a sufficient number of intact water strands always remains in other vessels. Furthermore, mechanisms exist to restore the continuity of water columns. It has been demonstrated that the tensile strength of water is 30 MPa, which is ample to drive water ascent to heights of 120-130 meters.

The absolute velocity of water movement through the xylem is relatively low: 20 cm³/h per 1 cm² for deciduous trees; 5 cm³/h per 1 cm² for conifers (by comparison, Blood velocity in Arteries is 40-50 cm³/s). Nevertheless, such a velocity minimizes resistance to water flow.

Significance of the upward flow for the plant:

1. The upward stream from The Root System to the aerial parts serves as a vehicle for transporting and accumulating Mineral Substances and root-synthesized chemical compounds in above-ground organs.

2. Transpiration is intrinsically linked to CO₂ assimilation. To acquire CO₂, a plant must inevitably release water, and minimizing H₂O loss (via stomatal closure) likewise restricts the influx of CO₂. In agriculture and forestry, understanding the ratio between photosynthetic yield and plant water loss is crucial for regulating these processes to achieve maximum harvests.

3. The upward flow is essential for ensuring an adequate water supply to all cells and maintaining turgor pressure. Water scarcity in cells triggers various physiological disorders. Consequently, developing irrigated agriculture is vital for securing high crop yields in arid regions.

4.  Transpiration can also act as a mechanism protecting the plant against overheating.