Process Of Water Loss In Plants Flashcards
Define transpiration
Removal of water from leaves and other aerial parts of the plant body as water vapour by diffusion is known as transpiration.
Mechanism of transpiration
In the day time, air in the intercellular air spaces is saturated with water vapour because they are in contact with the moist cell walls. Normally air outside the plant is drier than inside.
Hence water potential of outside air is lower than that of inside.
Therefore water vapour in the air spaces of the plant diffuses down its water potential gradient and exits the plant.
Mechanism of stomatal transpiration
Water is brought to the leaf in the xylem of vascular bundle and subsequently spread from a fine branching network throughout the leaf.
These branches end in one or few xylem vessels or tracheids possessing little lignification.
Therefore water can release easily through their cellulose walls to mesophyll cells.
Water moves through the mesophyll cells by apoplast, symplast and transmembrane pathways according to water potential gradient.
Then water evaporate from the wet walls of the mesophyll cells into the intercellular air spaces particularly into the large substomatal air spaces. From here water vapour diffuses through the stomata to the atmosphere.
Formation of a diffusion shell around the stomata
Immediately next to the leaf is a thin layer of stationary air through which water vapour diffuses out and swept away by moving air.
There is a diffusion gradient from the stationary layer back to the mesophyll cells.
Each stomata has a diffusion gradient or diffusion shell around it.
The diffusion shell of neighbouring stomata overlap in still air to form one overall diffusion shell (Layer).
Thickness of the diffusion shell depends on the surface features of the leaf and wind speed.
Factors affect the formation of a diffusion shell
Surface feature of leaf ( no of stomata in leaf surface ) Wind speed ( wind speed higher it distrusts the formation of a diffusion shell )
What would the formation of diffusion cause
Diffusion shell creates a diffusion gradient from the stationary layer back to the mesophyll cells.
Factors affecting the rate of transpiration
Light intensity
Stomata usually open in the light and close in darkness.
With the increase of light intensity the rate of transpiration increases.
Temperature
In the presence of light, the external factor which has the greatest effect on transpiration is temperature.
The higher the temperature, the greater the rate of evaporation of water from mesophyll cells which result the greater saturation of the leaf atmosphere with water vapour.
At the same time, a rise in temperature lowers the relative humidity of the air outside the leaf.
Both events result in a steeper concentration gradient of water molecules from leaf to external atmosphere.
The steeper this gradient is the faster the rate of diffusion.
Humidity
Low humidity outside environment of the leaf increases transpiration, because it makes the diffusion gradient of water vapour from the moist leaf atmosphere to drier external atmosphere.
As the concentration of water vapour in the external atmosphere is high when humidity rises, the diffusion gradient becomes less steep result in lower transpiration.
Wind speed
In still air, a shell of highly saturated air builds up around the leaf thus reducing the steepness of the diffusion gradient between leaf atmosphere and external atmosphere which makes the transpiration rate low.
In windy condition, flow of air will generally sweep away the shell. Therefore, windy condition increases transpiration rate.
Availability of soil water
As soil dries out, water usually binds more tightly to soil particles reducing the amount of available water.
The soil solution becomes more concentrated and its water potential decreases.
Therefore, tendency for water to enter by osmosis is lower.
This reduces water uptake by plants and as a result transpiration rate is also reduced.
There is greater resistance to movement of water through the plant due to less steep water potential gradient from the soil through the plant to the atmosphere.
Significance of transpiration to the plants
Distribute minerals and water throughout the plant.
Ascent of water in the xylem.
Uptake of water and minerals by roots from the soil solution.
Define guttation
Due to root pressure, more water enters to the leaves than lost by transpiration.
This results in removal of water droplets from leaf tips or leaf margins of some herbaceous plants.
That process is known as guttation
Describe the process of guttation
At night, when the relative humidity is high approaching 100%, transpiration rate is very low or zero.
Root cells continuously pump water and mineral ions into xylem tissue. The endodermis prevents ions from leaking back into the cortex and soil. Therefore more mineral ions accumulate in the vascular cylinder and reduce water potential.
Therefore water moves from the cortex. It generates a root pressure and upward push of xylem sap.
Due to root pressure, more water enters to the leaves than lost by transpiration.
This results in removal of water droplets from leaf tips or leaf margins of some herbaceous plants.
That process is known as guttation.
Why doesn’t guttation occur during daytime
Even in plants that display guttation, root pressure cannot match the water loss by transpiration after sun rise and therefore no guttation is seen in the day time, because then xylem sap is not pushed but pulled upward by transpiration.
