Transpiration is a fundamental biological process involving the movement of water through a plant and its subsequent release into the atmosphere as vapor. Plants absorb large volumes of water through their roots, but only a small fraction is used for growth and metabolism. The majority of this water travels up through the plant structure and is ultimately lost from the aerial parts, primarily the leaves. This continuous stream of water loss is a passive, physical process that drives many functions necessary for the plant’s survival and growth.
The Definition and Types of Transpiration
Transpiration is the physiological loss of water vapor from the plant’s body to the outside atmosphere. This evaporation occurs because the air outside the plant is typically drier than the air inside the leaf tissue. The process begins when water moves from the moist surfaces within the leaf into the surrounding air spaces.
Transpiration is categorized into three types based on where the evaporation occurs. Stomatal transpiration is the most significant, accounting for approximately 90 to 95 percent of total water loss, where vapor diffuses through stomata—microscopic pores usually located on the underside of leaves. Cuticular transpiration involves water escaping through the waxy layer (cuticle) covering the leaf epidermis, typically 5 to 10 percent. The least significant form is lenticular transpiration, which occurs through lenticels, small openings found in the bark of woody stems and twigs.
The Mechanism Driving Water Upward
The evaporation of water from the leaves creates tension, or negative pressure, which is the driving force for water transport, commonly called the transpirational pull. As water vapor exits the leaf, it pulls the next water molecule along, creating a continuous column of water moving upward from the roots.
This physical phenomenon is explained by the Cohesion-Tension Theory. Water molecules exhibit cohesion—a strong mutual attraction due to their polarity—allowing them to stick tightly together and form an unbroken column within the plant’s xylem vessels. They are also attracted to the cellulose walls of the xylem vessels, a property called adhesion. The combination of cohesion and adhesion gives the water column high tensile strength, meaning it resists being broken by the pull of gravity. The tension generated by evaporation at the leaf surface is sufficient to lift water hundreds of feet to the tops of the tallest trees without the plant expending metabolic energy.
Essential Functions for Plant Survival
Transpiration plays a role in the transport of dissolved mineral nutrients absorbed by the roots. As water is pulled up through the xylem in the transpiration stream, it carries essential minerals and salts from the soil to the leaves and other growing parts of the plant. This mass flow ensures that all cells receive the necessary elements for metabolism and growth.
Another function of this water loss is thermal regulation for the plant. The evaporation of water from the leaf surface absorbs heat energy, providing an evaporative cooling effect. This mechanism is comparable to how sweating cools the human body, preventing the leaves from overheating when exposed to intense sunlight.
The continuous uptake of water, driven by transpiration, also helps maintain turgor pressure within the plant cells. Turgor pressure is the internal water pressure that pushes the cell membrane against the cell wall, giving the plant its rigidity and structural support. Maintaining this pressure is necessary for keeping stems upright and leaves extended to maximize light absorption for photosynthesis.
Environmental Factors Controlling Water Loss
The rate at which a plant transpires is sensitive to the surrounding environmental conditions. Temperature is a major factor; as the air temperature increases, the rate of water evaporation from the leaf surface rises. Higher temperatures also cause the stomata to open wider, further increasing water loss.
Atmospheric humidity also influences the rate of water loss. When the air is humid, the concentration gradient of water vapor between the inside of the leaf and the outside air is low, which slows down the diffusion of water vapor. Conversely, dry air creates a steep gradient, leading to a much faster rate of transpiration.
Air movement, or wind, affects transpiration by removing the layer of humid air that accumulates immediately around the leaf surface. This action replaces the moist air with drier air, maintaining a steep concentration gradient and increasing the speed of water loss.
Light intensity also plays an indirect role. Stomata generally open in the presence of light to allow carbon dioxide uptake for photosynthesis, which then facilitates the simultaneous escape of water vapor.