Plants primarily control the rate of transpiration by carefully managing the opening and closing of tiny pores on their leaves called stomata, while external environmental factors also play a significant role.
Stomatal Control: The Main Regulator
The primary mechanism plants use to control transpiration is by controlling the size of the stomatal apertures. Stomata are tiny pores, mostly located on the underside of leaves, that allow for gas exchange (uptake of carbon dioxide for photosynthesis and release of oxygen). However, water vapor also escapes through these pores during transpiration.
- Guard Cells: Specialized cells called guard cells surround each stoma. These guard cells regulate the opening and closing of the stomata in response to various environmental and internal signals.
- Turgor Pressure: Changes in the turgor pressure (water pressure) within the guard cells cause them to either swell (opening the stoma) or become flaccid (closing the stoma).
- Opening Mechanisms: When water is plentiful, guard cells take up water, increasing their turgor pressure, causing them to bow outwards and open the stoma.
- Closing Mechanisms: When water is scarce, guard cells lose water, reducing their turgor pressure, causing them to become flaccid and close the stoma.
Environmental Factors Influencing Transpiration
Beyond stomatal control, the rate of transpiration is also significantly affected by the evaporative demand of the atmosphere surrounding the leaf. These factors include:
- Boundary Layer Conductance: The layer of still air surrounding the leaf affects how easily water vapor can move away.
- Humidity: Lower humidity increases the rate of transpiration as there is a greater difference in water vapor concentration between the leaf and the air.
- Temperature: Higher temperatures increase the rate of transpiration by increasing the rate of evaporation.
- Wind: Wind removes the humid air surrounding the leaf, increasing the rate of transpiration.
- Incident Sunlight: Sunlight provides the energy for evaporation and also can increase leaf temperature, thus increasing transpiration.
Summary of Factors Affecting Transpiration
| Factor | Effect on Transpiration Rate | How it Works |
|---|---|---|
| Stomatal Aperture | Direct control | Guard cells regulate opening and closing based on turgor pressure, influenced by water availability and other signals. |
| Humidity | Inverse relationship | Lower humidity increases the water vapor concentration gradient between the leaf and the air. |
| Temperature | Direct relationship | Higher temperature increases the rate of evaporation. |
| Wind | Direct relationship | Wind removes humid air around the leaf, increasing the water vapor gradient. |
| Incident Sunlight | Direct relationship | Provides energy for evaporation and increases leaf temperature. |
| Boundary Layer Conductance | Direct relationship | Increased boundary layer conductance allows for greater water vapor diffusion from the leaf surface. |
