3.3.6 The Process of Transpiration Flashcards
How much water is lost through transpiration from the plants leaves and stems?
Around 99%
What is transpiration?
Transpiration refers to the loss of water vapour from a plant to its environment by evaporation and diffusion
What is transpiration a consequence of?
Is a consequence of gaseous exchange at the stomata
What are the advantages of transpiration?
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
What is the transpiration stream?
The transpiration stream refers to the movement of water from the roots to the leaves
What causes the movement of water through the xylem?
The evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
What permits the movement of water from the soil to the atmosphere?
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
What does the transpiration rate depend on?
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
How does air movement affect transpiration?
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
How does temperature affect transpiration?
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
How does light intensity affect transpiration?
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
How does humidity affect transpiration?
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
