Transpiration is the biological process of water movement through a plant and its subsequent evaporation from aerial parts, such as leaves, stems, and flowers. This water loss, primarily as vapor, is an unavoidable consequence of a plant opening its pores to take in carbon dioxide for photosynthesis. The vast majority—up to 99%—of the water absorbed by the roots is released back into the atmosphere through this mechanism. Transpiration acts as the engine that pulls water and dissolved nutrients from the soil up through the plant’s vascular system. The timing of this water loss is highly regulated by internal plant mechanisms and external environmental conditions.
The Primary Timing Mechanism: Stomatal Regulation
The timing of transpiration is fundamentally linked to the behavior of small pores on the plant surface called stomata. These openings, typically most numerous on the underside of leaves, must be open for carbon dioxide to enter the leaf’s interior for photosynthesis. Water vapor diffuses out through these same openings whenever the stomata are open and the outside air is drier than the air inside the leaf.
Stomata are flanked by specialized guard cells, which control the aperture of the pore. In most plants, these guard cells operate on a circadian rhythm, opening in the presence of light and closing in the dark. During daylight, light stimulates the guard cells to actively take up ions, such as potassium, which draws water into the cells by osmosis. This influx of water makes the guard cells turgid, causing them to bow outward and open the stomatal pore.
Conversely, the absence of light or an increase in internal carbon dioxide concentration causes the guard cells to lose ions and water. The resulting loss of turgor causes the guard cells to become flaccid, pulling the stomatal pore closed and halting the bulk of transpiration. Therefore, for most plant species, transpiration is largely confined to the daytime hours when light is available.
External Conditions That Drive Transpiration Rates
Once the stomata are open, the rate at which water vapor escapes is dictated by the surrounding atmospheric conditions. The driving force for transpiration is the difference in water vapor concentration, or water potential, between the leaf’s interior and the outside air. Any external factor that increases this concentration gradient will increase the rate of water loss.
Higher air temperatures significantly increase the rate of transpiration. Warmer air holds substantially more moisture than cooler air, which decreases relative humidity. This lower humidity steepens the water potential gradient between the moist air inside the leaf and the drier air outside, accelerating water vapor diffusion.
Low relative humidity also directly promotes faster transpiration. When the atmosphere is dry, the low concentration of water vapor maximizes the difference in vapor pressure compared to the saturated air within the leaf. Furthermore, wind continuously removes the layer of humid air that naturally accumulates around the leaf surface. By replacing this moist boundary layer with drier air, wind maintains a steep vapor pressure gradient, driving a high rate of water loss.
The Limiting Factor: Water Availability
While stomatal opening and atmospheric conditions determine the potential for transpiration, the actual occurrence of the process is ultimately limited by the supply of water from the soil. A plant cannot sustain a high rate of water loss if its roots cannot absorb water fast enough to replace what is being evaporated. The availability of soil water represents the final condition that dictates if transpiration can proceed.
If the soil becomes too dry, the plant will experience water stress, triggering a protective mechanism that overrides the typical light-driven opening of the stomata. Under severe drought, the plant releases a hormone called abscisic acid (ABA), which signals the guard cells to close the stomata, even during the day. This emergency closure prevents the plant from wilting and suffering permanent damage by conserving what little water remains.
Even on a sunny, hot, and windy day when atmospheric conditions are ideal for rapid water loss, transpiration will drastically slow or cease entirely if the soil water content falls too low. This survival mechanism ensures that the plant prioritizes self-preservation over gas exchange for photosynthesis. The process of water movement and evaporation occurs most readily when the internal timing aligns with favorable external conditions, provided the soil moisture reservoir is adequately full.