Transpiration is the process of water movement through a plant, culminating in its evaporation from the aerial parts, primarily the leaves. While only a small fraction of the water absorbed is used for growth and metabolism, the vast majority, often over 97%, is lost to the atmosphere as vapor. This passive process requires no direct energy from the plant and serves several important functions. The evaporation creates a continuous pulling force that draws water and dissolved mineral nutrients from the soil up through the plant structure. Furthermore, this continuous evaporation helps to cool the plant’s surface, similar to how sweating cools the human body.
The Stomata: Regulating Water Vapor Loss
The tiny, microscopic pores on the surface of leaves, called stomata, are the primary sites where water loss occurs. Each stoma is surrounded by a pair of specialized cells known as guard cells, which function as the control mechanism for gas exchange and water vapor escape. These cells control the size of the stomatal pore, thereby regulating the rate of transpiration.
Stomatal opening is driven by changes in the guard cells’ turgor pressure. When water is plentiful, guard cells actively take up solutes, causing water to rush in by osmosis, making the cells swell. Due to the uneven thickness of their walls, the swelling causes the guard cells to bow outward, opening the pore and allowing carbon dioxide (CO2) to enter for photosynthesis.
The plant faces a constant compromise between maximizing CO2 intake and minimizing the loss of water vapor. When the plant experiences water stress or high temperatures, the guard cells lose solutes, causing water to leave the cells and close the stoma. This closing action reduces the rate of water vapor diffusion out of the leaf, conserving water but restricting the CO2 supply for photosynthesis. The rapid diffusion of water vapor from the leaf’s interior air spaces through the open stomata to the drier external air is the initial event that powers the entire water transport system.
Xylem: The Plant’s Water Pipeline
The water lost through the stomata is replaced by water drawn up from the roots through the xylem tissue, the specialized vascular tissue responsible for conveying water and dissolved minerals throughout the plant. It forms a continuous system of tubes extending from the roots, through the stem, and into the veins of the leaves.
The main water-conducting cells within the xylem are the tracheids and the vessel elements, both of which are dead and hollow at maturity. Tracheids are elongated, thin cells with tapered ends, and water moves between them through small porous regions called pits. Vessel elements are generally wider and shorter, forming long, continuous tubes called vessels, where water moves relatively unimpeded through perforated end walls.
The movement of water through the xylem is characterized as bulk flow, meaning water molecules move together in a stream. This movement is entirely passive, allowing for the rapid, long-distance transport of water. The thick, lignified walls of the xylem cells provide the necessary structural support to keep the conduits open, which is especially important given the forces exerted during water transport.
Integrating the System: Cohesion-Tension Theory
The involvement of the stomata and the xylem is explained by the Cohesion-Tension Theory, which describes how water is pulled from the roots to the leaves. The evaporation of water vapor through the open stomata creates a negative pressure, or tension, on the water column within the leaf, initiating the upward movement of water (transpiration pull).
As water molecules evaporate from the moist surfaces inside the leaf, they pull on the adjacent water molecules. This pulling action is possible because of cohesion, the strong attraction between individual water molecules due to hydrogen bonding. Cohesion gives the water column a high tensile strength, preventing it from breaking apart under the negative pressure created by the leaf tension.
Adhesion is the attraction between the polar water molecules and the walls of the xylem conduits. This adhesive force helps counteract the downward pull of gravity and prevents the water column from pulling away from the sides of the vessels and tracheids. The combined effect of cohesion and adhesion maintains an unbroken column of water from the roots to the leaves, allowing the tension generated at the stomata to be transmitted downward. The tension can be substantial, reaching pressures as low as –2 MegaPascals (MPa) at the leaf surface, which is enough to draw water to the top of the tallest trees.
External Variables Controlling Transpiration Rate
The rate of transpiration is controlled by external variables that influence the water vapor concentration gradient around the stomata. Light is a primary regulator, as it triggers the opening of the stomata to facilitate CO2 uptake for photosynthesis, thereby increasing the rate of water loss.
Humidity affects the concentration gradient between the leaf’s interior and the outside air. When the surrounding air is dry (low humidity), the concentration gradient is steep, causing water vapor to diffuse out of the stomata more quickly. Conversely, high humidity slows the rate of water loss.
Temperature increases the rate of transpiration because it causes water molecules to evaporate more rapidly, and warmer air has a greater capacity to hold water vapor. Additionally, wind accelerates the process by moving the humid air layer, known as the boundary layer, away from the leaf surface. This action maintains a steep concentration gradient, ensuring the escape of water vapor.