Q.Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?

The transpiration pull (cohesion-tension) theory, proposed by Dixon and Jolly, explains the ascent of xylem sap as follows: water evaporates from the moist cell walls of mesophyll cells into the sub-stomatal air spaces and out through open stomata (transpiration); this loss of water creates a negative pressure (tension) at the air–water interface in the leaf. Because water molecules are strongly attracted to one another by hydrogen bonding (cohesion) and to the walls of the narrow xylem vessels (adhesion), the entire water column from the leaves down to the roots forms a continuous, unbroken chain. The tension created at the top is therefore transmitted all the way down this column, pulling water upward through the xylem, ultimately drawing more water into the roots to replace what is lost. …

Transpiration pull model (Cohesion–Tension theory), Dixon and Jolly:

This widely accepted theory explains how water is drawn up through the xylem of even the tallest trees, entirely by physical forces, without requiring continuous input of metabolic energy along the length of the stem.

• Evaporation at the leaf: Water evaporates from the moist walls of mesophyll cells into the intercellular (sub-stomatal) air spaces within the leaf, and then diffuses out into the atmosphere through the open stomata — this loss of water vapour is transpiration.
• Generation of tension: As water evaporates from the surface of mesophyll cell walls, it creates a negative pressure, or tension, at the air–water interface within the leaf tissue. This tension effectively "pulls" more water out of the leaf's xylem (veins) to replace what has evaporated.
• The unbroken water column — cohesion and adhesion: Water molecules exhibit very strong mutual attraction through hydrogen bonding, called cohesion, which holds them together as a continuous, essentially unbreakable column inside the narrow xylem vessels/tracheids. Water molecules also adhere strongly to the cellulose walls of the xylem elements through adhesion, which helps counteract the pull of gravity on the water column and prevents it from simply falling back down.
• Transmission of the pull down to the roots: Because the water column is continuous and held together by cohesion, the tension generated at the leaf is transmitted as a pull all the way down through the xylem to the roots, drawing water upward passively. As water is pulled out of the root xylem, this in turn lowers the water potential there, causing more water to be drawn into the root from the soil by osmosis.
• Role of the root: In this model, the root plays a largely passive role, simply serving as the point of entry through which water (pulled from above) replenishes the column, rather than actively pumping water up (that being the separate, minor contribution of root pressure).

Factors influencing the rate of transpiration:

External (environmental) factors:

• Light: Higher light intensity generally causes stomata to open wider, increasing transpiration.
• Temperature: Higher temperature increases the rate of evaporation of water from cell surfaces, increasing transpiration.
• Humidity: Higher atmospheric humidity reduces the water-potential gradient between the leaf's internal air spaces and the outside air, decreasing transpiration; low humidity increases it.
• Wind speed: Wind removes the humid air layer immediately around the leaf surface (boundary layer), which can increase transpiration, though very strong wind may also cause stomatal closure.
• Availability of soil water: Adequate soil moisture is needed to sustain high transpiration; water-stressed plants reduce transpiration (often via stomatal closure) to conserve water.
• Atmospheric CO₂ concentration: Elevated CO₂ often causes partial stomatal closure, reducing transpiration.

Internal (plant) factors:

• Number, distribution, and aperture (opening) of stomata: More numerous or more widely open stomata increase transpiration.
• Leaf area and morphology: Larger leaf surface area generally means more transpiration; thick cuticles reduce it. …

Present the cohesion-tension model as a chain of cause and effect: evaporation at the leaf creates tension → cohesion/adhesion keep the water column unbroken → tension is transmitted down to the roots, pulling water up. Then separately list external and internal f …

• Describing transpiration pull as an active, energy-consuming process rather than a passive physical consequence of evaporation and water cohesion.
• Omitting either cohesion or adhesion — both are needed: cohesion keeps the column unbroken, adhesion resists the column falling under gravity within the vessel walls. …