3.3.4: Transpiration Flashcards
What is transpiration?
The loss of water vapour from the upper parts of the plant, particularly the leaves.
What is water loss from the upper leaf surface limited by?
The waxy cuticle.
Where does most water vapour evaporate from the leaves?
Through the stomata which open to allow gaseous exchange for photosynthesis.
When is the majority of water lost?
During the day as this is when there is sufficient light for photosynthesis to occur.
Where does the water enter the leaf?
Through the xylem and moves by osmosis into the cells of the spongy mesophyll. It may also pass along the cell walls via the apoplast pathway.
Where does the water evaporate from?
The cell walls of the spongy mesophyll.
What does water vapour diffuse out of and what does this rely on?
Though the open stomata, this relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf. This is known as the water vapour potential gradient.
Describe the water vapour potential gradient needed for water vapour to move by diffusion out of the leaf.
There must be a higher water vapour potential inside the leaf than outside.
What happens when water is lost from the leaf?
It must be replaced from below. This draws water up the stem as a transpiration stream.
Why is transpiration important?
- Transports useful mineral ions up the plant.
- Maintains cell turgidity.
- Supplies water for growth, cell elongation and photosynthesis.
- Supplies water that, as it evaporates, keeps the plant cool on a hot day.
How does light intensity affect transpiration?
In light, the stomata open to allow gaseous exchange for photosynthesis. Higher light intensity increases the transpiration rate.
How does temperature affect transpiration?
Higher temperature will increase transpiration by…
- Increasing the rate of evaporation from the cell surfaces so that the water-vapour potential in the leaf rises.
- Increases the rate of diffusion through the stomata because the water molecules have more kinetic energy.
- Decreases the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf.
How does relative humidity affect transpiration?
Higher levels of humidity in the air will decease the rate of water loss because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.
How does air movement affect transpiration?
Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.
How does water availability affect transpiration?
If there is little water in the soil, then the plant cannot replace the water that is lost. If there is insufficient water in the soil, then the stomata close and the leaves wilt.
