Transpiration is the movement of water through a plant and its subsequent evaporation as vapor, primarily from the leaves. This continuous stream of water drives the transport of dissolved mineral nutrients throughout the plant body. Evaporation also cools the plant’s surface, which is important under direct sunlight. Although water loss is necessary for nutrient distribution, the plant must strictly control the rate of evaporation to maintain its internal water balance. Stopping this process completely is an extreme measure taken in response to certain environmental or internal conditions.
The Plant’s Control Valve
The primary points of water vapor exit are microscopic pores on the leaf surface called stomata, which function as the plant’s control valves for gas exchange. Each stoma is flanked by a pair of specialized guard cells responsible for regulating the size of the pore opening. Regulation of these stomatal apertures is the most effective way for a plant to manage its water economy.
The opening and closing mechanism is driven by changes in the water pressure, or turgor, within the guard cells. When water flows into the guard cells, they swell and become turgid, pulling the stoma open. Conversely, when the guard cells lose water, they become flaccid and collapse inward, causing the stomatal pore to close completely. This total stomatal closure is the plant’s active, physiological method for halting transpiration.
The Impact of Severe Drought
The most common condition that causes a complete halt to transpiration is severe water deficit in the soil, often called drought. When a plant experiences a lack of available water, it shifts its priority from growth and nutrient uptake to immediate survival. The plant must prevent dehydration by shutting down the water loss pathway.
This survival strategy is coordinated by Abscisic Acid (ABA), which acts as the plant’s internal stress hormone. As the soil dries out and water potential drops, ABA is synthesized and transported to the guard cells. The arrival of ABA initiates a biochemical cascade within the guard cells that overrides signals to keep the stomata open.
ABA promotes the movement of ions, including potassium ions (K+) and certain anions, out of the guard cells. This efflux of solutes lowers the internal concentration. Water quickly follows the ions out of the cells by osmosis, resulting in an immediate loss of turgor pressure. This loss of turgor causes the guard cells to become flaccid, physically sealing the stomatal pore and stopping transpiration.
When the Air is Saturated
Transpiration can also physically stop, or drop to a near-zero rate, when the surrounding air becomes completely saturated with moisture. This can happen even if the plant has plenty of water and open stomata. The physical driving force for transpiration is the water vapor concentration gradient, which is the difference in water vapor content between the leaf’s moist internal air spaces and the ambient air.
The air spaces inside a fully hydrated leaf are always close to 100% relative humidity. For water vapor to diffuse out, the air outside must have a lower water vapor concentration than the air inside. When the relative humidity of the surrounding air approaches 100%, the difference in water vapor concentration diminishes.
At the point of air saturation (100% relative humidity), the water vapor gradient effectively disappears. Without this concentration gradient, the net diffusion of water vapor from the leaf physically stops, regardless of the plant’s internal regulatory signals. This condition is distinct from the drought response because it is a passive, physical phenomenon driven by the environment. In a fully saturated atmosphere, water cannot evaporate, thus halting transpiration even with open stomata.