Transpiration is the biological process of water movement through a plant and its subsequent evaporation from aerial parts, primarily the leaves. This water loss occurs as vapor released from tiny pores on the leaf surface into the surrounding atmosphere. Temperature is one of the most significant environmental factors dictating the speed of this process. The relationship between heat and evaporation is direct and foundational. Generally, as the temperature increases, the rate of transpiration accelerates, driving faster water movement through the plant.
The Direct Physical Effect of Heat
Increased temperature directly accelerates transpiration by influencing the physical state and movement of water molecules. Higher temperatures cause an increase in the kinetic energy of the water molecules within the leaf tissue. This enhanced movement makes it easier for liquid water molecules to overcome the forces holding them together and transition into the gaseous state of water vapor.
This physical acceleration of evaporation is magnified by changes in the atmospheric environment surrounding the leaf. Temperature steeply influences the air’s capacity to hold moisture, with warmer air able to hold significantly more water vapor than cooler air.
If the air around the leaf is warm but not saturated with moisture, a strong concentration gradient is created. This difference between the saturated air inside the leaf and the drier air outside is known as the Vapor Pressure Deficit (VPD). Because the air spaces inside a hydrated leaf are nearly 100% relative humidity, an increase in external temperature drastically increases the VPD.
A higher VPD acts as a powerful suction force, driving the rapid diffusion of water vapor from the leaf interior into the atmosphere. The rate of transpiration is directly proportional to this vapor pressure difference. Consequently, temperature primarily governs the magnitude of the driving force for water movement out of the plant.
Stomatal Regulation and Temperature Thresholds
While physical forces drive the potential rate of transpiration, the plant exerts biological control over water loss through structures called stomata. These microscopic pores are surrounded by specialized guard cells that regulate the opening and closing of the aperture. Stomatal movement is governed by changes in the turgor pressure of these guard cells, which swell or shrink to open or close the pore.
Under conditions of moderate temperature increase, plants typically respond by increasing stomatal opening. This action facilitates the increased evaporation rate, allowing for greater gas exchange. The mechanism involves the guard cells optimizing their aperture to balance the need for carbon dioxide uptake with the increased water vapor loss.
However, this responsive opening only occurs up to a certain point, after which a high-temperature stress response is triggered. When temperatures exceed a specific, high threshold—often above 30°C to 40°C in many species—the plant initiates a survival mechanism: stomatal closure. This response overrides the initial tendency to open, reducing the stomatal aperture or closing it entirely.
The closure under extreme heat is a measure to conserve water and prevent desiccation, even though it limits the intake of carbon dioxide. This behavior represents a trade-off, where the plant chooses water conservation over the immediate need for gas exchange and cooling. By closing the stomata, the plant reduces the volume of water vapor escaping, but this action also decreases the ability to dissipate heat, leading to higher leaf temperatures.
Biological Functions of Temperature-Driven Transpiration
The temperature-driven process of water loss serves two major biological outcomes that support plant function and survival. One primary function is evaporative cooling, which acts similarly to sweating in animals. As water changes from liquid to vapor inside the leaf, it absorbs a large amount of energy known as the latent heat of vaporization.
This energy transfer effectively dissipates excess thermal energy from the leaf surface, helping to maintain leaf temperature within a functional range. During periods of high solar radiation and elevated air temperature, this mechanism prevents overheating. The process provides a significant cooling effect.
The second major function is the continuous upward movement of water and dissolved substances, often referred to as the transpiration pull. As water vapor is lost from the leaves, a negative pressure, or tension, is created within the plant’s vascular system. This tension pulls the water column upward from the roots through the xylem vessels, drawing new water from the soil.
The continuous flow of water driven by temperature-induced evaporation facilitates the transport of essential mineral nutrients absorbed by the roots. These dissolved nutrients are distributed throughout the plant body, maintaining the necessary circulation for metabolic and growth processes.