Most of the water a plant absorbs from the soil is not used to build plant tissue but is released back into the atmosphere as vapor. Plants manage large volumes of water, drawing it up from the ground and releasing it from their aerial parts in a continuous exchange with the environment. This cycle of water absorption, transport, and release provides the driving force for various life-sustaining functions within the plant.
Transpiration: The Scientific Term
The process where plants give off water vapor is scientifically known as transpiration. This involves the movement of water through the plant from the roots and its subsequent evaporation from the plant’s above-ground parts, such as the leaves, stems, and flowers. Approximately 97 to 99.5 percent of the water absorbed by the roots is lost through this process.
The primary exit points for this water vapor are microscopic pores called stomata, found predominantly on the underside of leaves. These pores are bordered by specialized guard cells that regulate their opening and closing. Stomata are necessary for gas exchange, allowing carbon dioxide to enter for photosynthesis, but they also create the pathway for water loss.
How Water Moves Through the Plant
The ascent of water from the roots to the leaves, often against gravity, is explained by the cohesion-tension theory. This theory states that the evaporation of water from the leaf surface creates a negative pressure, or tension, that pulls the entire column of water upward. This pulling force, called the transpiration pull, is the main driver of water movement in the plant’s vascular tissue.
The physical properties of water allow this continuous column to remain intact. Water molecules exhibit strong cohesion, meaning they stick tightly to one another due to hydrogen bonding. This cohesive force prevents the water column within the xylem vessels from breaking under negative pressure. Adhesion, the attraction of water molecules to the hydrophilic walls of the xylem, also helps counteract gravity, ensuring the uninterrupted flow of water.
Water initially enters the roots from the soil via osmosis. This uptake can sometimes generate a slight positive pressure in the roots, known as root pressure, which contributes a minor upward push. However, the powerful evaporative tension generated at the leaves is responsible for pulling water to the tops of even the tallest trees.
The Essential Functions of Transpiration
Transpiration serves at least two fundamental biological functions for the plant. The continuous flow of water from the roots to the shoots ensures the mass transport of essential nutrients and minerals. These dissolved substances are absorbed by the roots and travel upward with the water stream, reaching all parts of the plant necessary for growth and metabolism.
The second major function is thermal regulation, similar to how sweating cools the human body. As water changes from a liquid to a gas during evaporation, it absorbs heat energy from the leaf. This evaporative cooling effect keeps the leaf surface temperature within an optimal range for photosynthesis and other biochemical processes. Maintaining this temperature balance is important because excessive heat can damage necessary enzymes.
External Controls on Water Loss
The rate at which a plant transpires is heavily influenced by the environment, as plants actively adjust water loss in response to external conditions. Light is a primary external factor because it stimulates the guard cells to open the stomata, allowing carbon dioxide in for photosynthesis and simultaneously increasing water vapor release. Conversely, in darkness, the stomata close to conserve moisture.
Temperature also plays a role; warmer air increases the rate of evaporation and can hold more water vapor, which drives faster transpiration. If the temperature becomes excessively high, plants may close their stomata to prevent water loss, which can temporarily decrease the rate. Atmospheric humidity affects the concentration gradient for water vapor diffusion. Low relative humidity creates a steep gradient, causing water vapor to diffuse out of the leaf more quickly, while high humidity slows the process.
Wind speed also influences the rate of transpiration by sweeping away the layer of moist air that accumulates near the leaf surface. This removal of humid air maintains a steep diffusion gradient, accelerating the release of water vapor. Ultimately, the plant balances the need for carbon dioxide intake against the risk of dehydration by managing the opening and closing of its stomata based on these environmental signals.