Transpiration is the biological process where plants absorb liquid water through their roots and then release it into the atmosphere as water vapor, primarily through their leaves. This movement helps regulate internal conditions and facilitates other processes. It establishes a continuous column of water from the soil, through the plant, and up into the air.
The Mechanism of Water Movement
The upward movement of water in a plant is explained by the Cohesion-Tension Theory, a model that describes how water is pulled from the roots to the leaves against the force of gravity. This movement is powered by the difference in water potential, creating a gradient from the high potential in the wet soil to the low potential in the dry air. The evaporation of water from the leaf surface creates a strong negative pressure, or tension, that acts like a suction pump.
This tension is transmitted down the plant’s vascular tissue, the xylem, because of two physical properties of water: cohesion and adhesion. Cohesion refers to the strong attraction between individual water molecules, allowing them to stick together and form an uninterrupted chain. Adhesion is the attraction between water molecules and the hydrophilic walls of the xylem vessels, preventing the water column from breaking or falling back down. The tension generated by the evaporating water at the top pulls this continuous, cohesive column upward from the roots.
Types and Control Points
Water loss through transpiration occurs via three primary pathways. Stomatal transpiration accounts for the vast majority of water loss, typically between 80% and 95%. This occurs through microscopic pores called stomata, which are found mainly on the underside of leaves.
The stomata are regulated by specialized guard cells, which act as the plant’s main control valve for water loss and gas exchange. When guard cells absorb water, they become turgid and bow outward, which opens the pore. Conversely, when they lose water, they become flaccid, causing the pore to close. Stomatal movement is primarily triggered by light, low carbon dioxide concentrations inside the leaf, and the availability of water signaled by hormones.
Cuticular transpiration, the second type, involves water vapor diffusing directly through the waxy layer covering the leaf epidermis, which is called the cuticle. Since this layer is relatively impermeable to water, cuticular loss usually accounts for only 5% to 10% of total transpiration. The third type is lenticular transpiration, which is the loss of water through tiny pores called lenticels found in the bark of woody stems and fruits. This pathway is responsible for a negligible amount of water loss.
Environmental Influences on Rate
The rate at which water vapor leaves the plant is heavily governed by the environmental conditions surrounding the leaves. Temperature directly influences the rate of evaporation, meaning that higher air temperatures lead to a faster rate of water loss. High humidity in the air significantly slows transpiration because it reduces the concentration gradient of water vapor between the moist leaf interior and the surrounding air.
Wind also affects the rate by disrupting the “boundary layer,” which is a thin layer of relatively still, humid air that naturally forms around the leaf surface. Moving air sweeps this humid layer away, replacing it with drier air and accelerating transpiration. Finally, the availability of water in the soil dictates the plant’s ability to maintain the process. When the soil dries out, the roots struggle to absorb water, causing the guard cells to close the stomata, which dramatically lowers the transpiration rate.
The Essential Role in Plant Life
While transpiration results in a loss of water, it performs two functions that are necessary for the plant’s survival and growth. The sustained flow of water from root to leaf, driven by transpirational pull, provides the mechanism for nutrient transport. This upward movement carries dissolved mineral nutrients and salts absorbed from the soil through the xylem to the rest of the plant in a process known as mass flow.
The second major benefit is evaporative cooling, which helps the plant regulate its internal temperature. As water changes from a liquid to a gas during evaporation, it absorbs latent heat from the leaf, drawing excess energy away from the plant tissue. This cooling effect is especially important for leaves exposed to direct sunlight, preventing the enzymes involved in photosynthesis from overheating and becoming damaged.