Cohesion Tension Theory for Transpiration Pull

This idea, on the other hand, describes the transfer of water from a plant’s roots to its leaves. Osmosis allows water from the soil to reach the xylem of a plant’s roots. Because water molecules are hydrogen bound to one another, they create a string of molecules as they migrate toward the xylem. The water molecules adhere to one another and are drawn upward by the force known as tension. This force is generated by evaporation at the leaf’s surface.

Cohesion Tension Theory

A negative pressure pulls water up the plant, according to Irish plant physiologists H. H. Dixon and J. Joly. As we’ve seen, plants constantly lose water through transpiration. Dixon and Joly hypothesised that water loss in leaves attracts water from the xylem ducts back into the leaf. But even the finest vacuum pump can only lift water 10.4 m (34 ft) or so. This is because a 1.03 MPa water column is just counterbalanced by the atmospheric pressure. How does water get to the summit of a 113 m (370 ft) sequoia? Taking everything into consideration, a pull of 1.9 MPa is likely required.

The answer to the conundrum is water molecule cohesion, or the ability of water molecules to form hydrogen bonds. The force of cohesion between water molecules gives ultrapure water confined in narrow tubes considerable strength. The column is said to be broken by stresses of up to 21 MPa, roughly the same as steel wires of the same diameter. Water molecules have the physical qualities of solid wires due to their cohesion.

The cohesion-tension hypothesis states that transpiration drives xylem water flow. It causes –2 MPa negative pressure (tension) at the leaf surface. This strain draws up water from the roots. Negative water potential attracts soil water into root hairs and xylem. Water is drawn up the xylem by cohesion. Transpiration removes water from the leaf. From the root cells through the stem, to the topmost leaves, and eventually to the atmosphere, water potential decreases.

Features

Water molecules’ cohesive and adhesive characteristics contribute to the formation of an unbroken continuous water column in the xylem. The force or strain produced on this water column by transpiration. Xylem vessels are tubular structures that extend from the roots to the plant’s crown. Cells are stacked one on top of the other with perforated end walls to make a continuous tube. These are supported by xylem tracheids, which have porous walls. The xylem tube is linked to the root hairs at one end by the pericycle, endodermis, and cortex, and to the sub stomatal cavity in the leaves at the other end via mesophyll cells. This tube is half-full of water. Water is absorbed into the xylem capillaries and creates a continuous water column due to the cohesion and adhesion qualities of water. Due to the cohesion and adhesion of water, the water column cannot be broken or dragged away from the xylem walls.

A significant element that might cause the water column to break down is the entry of air bubbles into the xylem. The tensile strength of the water column is weakened by air bubbles entering the xylem, however Scholander et al. (1957) demonstrated that air does not completely obstruct the conducting system. Even when air bubbles were injected, the separate water columns remained intact and connected vertically and laterally via the holes in the cell wall.

Transpiration

Plants absorb a significant amount of water from the soil via their roots and root hairs. However, plants utilise just a small percentage (1.2 percent) of the absorbed water for metabolic processes.

The remaining water (98-99 percent) is lost to the atmosphere via the physiological process known as transpiration. It is a worldwide occurrence. In the presence of sunlight, transpiration is described as the loss of water in the form of water vapor from the internal tissues of live plants via the aerial portions such as leaves, green shoots, and so on.

Stages in Transpiration

Transpiration happens in two stages:

1. Water is initially lost from the leaf’s mesophyll cells into the intercellular gaps.
2. Water then diffuses from the intercellular gaps into the outside environment in the form of water vapour via the stomata or the general surface of the epidermis of leaves.

Proofs for Cohesion Adhesion Tension Theory

• Scholander established the continuous freely movable sap column and the lack of a metabolic pump.
• The sum of all forces has been determined to be 50 atm. Although the tallest tree poses an impediment, water has an adhesive force of up to 350 atm. Which stops the column from fracturing.

Conclusion

As previously stated, the water column in the xylem is continuous. They stretch from the leaf’s sub stomatal chambers to the roots. These uninterrupted water columns resemble steel cables that go from leaves to roots.