The Process of Transpiration
- Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)
- Around 99% of the water absorbed by a plant is lost through evaporation from the plant’s stem and its leaves in a process called transpiration
- Transpiration refers to the loss of water vapour from a plant to its environment by evaporation and diffusion
- Transpiration is a consequence of gaseous exchange at the stomata
- The advantage of transpiration is that:
- It provides a means of cooling the plant via evaporative cooling
- The transpiration stream is helpful in the uptake of mineral ions
- The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants
- The transpiration stream refers to the movement of water from the roots to the leaves
- The evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules causes the movement of water through a plants xylem
- It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells
Transpiration only occurs when there is a concentration gradient. There is usually a lower concentration of water molecules in the air outside the leaf.
The loss of water vapour from the leaves of plants (transpiration) results in a lower water potential creating a concentration gradient between the roots and leaves causing water to move upwards
Factors affecting the rate of transpiration
- The transpiration rate is dependent on the concentration gradient of water vapour between the inside of the leaf and the surrounding air
- A larger concentration gradient results in a faster rate of diffusion
- Air movement, temperature, light intensity and humidity all affect the rate of transpiration in a plant
- Air movement:
- There is usually a lower concentration of water molecules in the air outside the leaf
- When the air is relatively still water molecules can accumulate near the leaf surface. This creates a local area of high humidity which lowers the concentration gradient and the rate of transpiration
- Air currents can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration
- Temperature:
- An increase in temperature results in an increase in the kinetic energy of molecules. Therefore an increase in temperature will increase the rate of transpiration as water molecules move out of the leaf (down the concentration gradient) at a faster rate
- If the temperature gets too high the stomata close to prevent excess water loss. This dramatically reduces the rate of transpiration
- Light intensity:
- Stomata close in the dark, their closure greatly reduces the rate of transpiration
- When the light is sufficient for the stomata to open, the rate of transpiration increases
- Once the stomata are open any increase in light intensity has no effect on the rate of transpiration
- Stomata will remain open at relatively low light intensities
- Humidity:
- If the humidity is high that means there is a large concentration of water molecules in the air surrounding the leaf surface
- This reduces the concentration gradient between inside the leaf and the outside air which causes the rate of transpiration to decrease
- At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves
The different factors have different effects on the rate of transpiration
