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[[File:transpiration Overview.svg|thumb|Overview of transpiration: ]]
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaf, Plant stem and . It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of Plant nutrition. When water uptake by the roots is less than the water lost to the atmosphere by evaporation, plants close small pores called stomata to decrease water loss, which slows down nutrient uptake and decreases CO2 absorption from the atmosphere limiting metabolic processes, photosynthesis, and growth.
Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of the soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem. Mass flow of liquid water from the roots to the leaves is driven in part by capillary action, but primarily driven by water potential differences. If the water potential in the ambient air is lower than that in the leaf airspace of the stomatal pore, water vapor will travel down the gradient and move from the leaf airspace to the atmosphere. This movement lowers the water potential in the leaf airspace and causes evaporation of liquid water from the mesophyll cell walls. This evaporation increases the tension on the water in the cell walls and decreases their radius, thus exerting tension in the cells' water. Because of the cohesive properties of water, the tension travels through the leaf cells to the leaf and stem xylem, where a momentary negative pressure is created as water is pulled up the xylem from the roots. (2025). 9780321743671, Pearson. ISBN 9780321743671 In taller plants and trees, the force of gravity pulling the water inside can only be overcome by the decrease in hydrostatics in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere.
| More leaves (or spines, or other photosynthesizing organs) means a bigger surface area and more stomata for gaseous exchange. This will result in greater water loss. |
| More stomata will provide more pores for transpiration. |
| A leaf with a bigger surface area will transpire faster than a leaf with a smaller surface area. |
| A waxy cuticle is relatively impermeable to water and water vapor and reduces evaporation from the plant surface except via the stomata. A reflective cuticle will reduce solar heating and temperature rise of the leaf, helping to reduce the rate of evaporation. Tiny hair-like structures called on the surface of leaves also can inhibit water loss by creating a high humidity environment at the surface of leaves. These are some examples of the adaptations of plants for the conservation of water that may be found on many xerophytes. |
| The rate of transpiration is controlled by the stomatal aperture, and these small pores open especially for photosynthesis. While there are exceptions to this (such as night or CAM photosynthesis), in general, a light supply will encourage open stomata. |
| Temperature affects the rate in two ways:
1) An increased rate of evaporation due to a temperature rise will hasten the loss of water.
2) Decreased relative humidity outside the leaf will increase the water potential gradient. |
| Drier surroundings give a steeper water potential gradient, and so increase the rate of transpiration. |
| In still air, water lost due to transpiration can accumulate in the form of vapor close to the leaf surface. This will reduce the rate of water loss, as the water potential gradient from inside to outside of the leaf is then slightly less. The wind blows away much of this water vapor near the leaf surface, making the potential gradient steeper and speeding up the diffusion of water molecules into the surrounding air. Even in wind, though, there may be some accumulation of water vapor in a thin boundary layer of slower moving air next to the leaf surface. The stronger the wind, the thinner this layer will tend to be, and the steeper the water potential gradient. |
| Water stress caused by restricted water supply from the soil may result in stomatal closure and reduce the rates of transpiration. |
Transpiration rates of plants can be measured by a number of techniques, including , , porometers, photosynthesis systems and thermometric sap flow sensors. Isotope measurements indicate transpiration is the larger component of evapotranspiration. Recent evidence from a global study of water stable isotopes shows that transpired water is isotopically different from groundwater and streams. This suggests that soil water is not as well mixed as widely assumed.
Desert plants have specially adapted structures, such as thick , reduced leaf areas, sunken stomata and to reduce transpiration and conserve water. Many cactus conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis, in which the stomata are closed during the day and open at night when transpiration will be lower.
Scientists have begun using magnetic resonance imaging (MRI) to monitor the internal status of the xylem during transpiration, in a non invasive manner. This method of imaging allows for scientists to visualize the movement of water throughout the entirety of the plant. It also is capable of viewing what phase the water is in while in the xylem, which makes it possible to visualize cavitation events. Scientists were able to see that over the course of 20 hours of sunlight more than 10 xylem vessels began filling with gas particles becoming cavitated. MRI technology also made it possible to view the process by which these xylem structures are repaired in the plant. After three hours in darkness it was seen that the vascular tissue was resupplied with liquid water. This was possible because in darkness the stomates of the plant are closed and transpiration no longer occurs. When transpiration is halted the cavitation bubbles are destroyed by the pressure generated by the roots. These observations suggest that MRIs are capable of monitoring the functional status of xylem and allows scientists to view cavitation events for the first time.
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