Ch#14 |Transportation | Lec #11| FACTORS AFFECTING THE RATE OF TRANSPIRATION @riffatjahan

Ch#14 |Transportation | Lec #11| FACTORS AFFECTING THE RATE OF TRANSPIRATION @riffatjahan
FACTORS AFFECTING THE RATE OF TRANSPIRATION:
Following are the important factors which affect the rate of transpiration in a plant.
(i) Light
(ii) Temperature
(iii) CO2 concentration
(v) Humidity and vapour pressure
(v) Wind
(vi) Availability of soil water
(i) Light:
The opening and closing of stomata is directly controlled by the light.
In the strong light the rate of transpiration is much more as compared
to dim light or no light. When light is available, Potassium actively enters the guard cells.
Then water follows and guard cells become turgid.
As a result the stoma opens.
(ii) Temperature:
When the sun-light is strong on a bright and sunny day, the environmental temperature is increased.
The higher temperature reduces the humidity of the surrounding air.
The evaporation of water from the surfaces of the mesophyll cells also increases.
Therefore the rate of transpiration is increased.
The rate of transpiration doubles every rise to 10°C in temperature.
Very high environmental temperature (i.e. 40-45°C) causes closure of stomata.
As a result transpiration is very much reduced.
If higher temperature as maintained in the environment for a longer period and soil water is limited, the plants would wilt and may die.
Hormones are involved in stomatal movement in plants.
At high temperature, when the leaf cells start wilting, a hormone is released by the mesophyll cells.
This hormone is called abscisic acid.
This hormone suppresses the effect of light and CO2 concentration.
Therefore the active transport of Potassium into the guard cells stops and the stomata close.
(iii) Dioxide Concentration:
Low carbon dioxide concentration (when photosynthesis is more than respiration), stimulates the active transport of Potassium ions into the guard cells.
The transport causes to open.
Now the CO₂ diffuses into the mesophyll cells of leaves.
At night, cellular respiration in the absence of photosynthesis raises CO2 levels.
This stops the inward transport of K (and water).
Therefore the guard cells become flaccid and stomata closed.
As a result the transpiration almost stops.
(iv) Humidity and Vapour Pressure:
When air is dry, the rate of diffusion of water, from the surfaces of the
mesophyll cells, air spaces, and through stomata to outside the lead, increases.
Therefore the rate of transpiration is increased.
In humid air, the rate of diffusion of water to outside is reduced.
Therefore the rate of transpiration is decreased very much.
(v) Wind:
The air in motion is called wind. The wind increases the rate of diffusion of water, from the surfaces of the mesophyll cells, air spaces, and through stomata to outside the leaf.
Therefore the rate of transpiration is increased.
When the air is still, the rate of diffusion of water to outside is slowed down.
Therefore the rate of transpiration is decreased.
(vi) Availability of Soil Water:


If there is little water in the soil, less is transported to the leaf cells and less is lost to the environment by transpiration.
Therefore when the rate of absorption of water in the root cells is reduced, the rate of transpiration is reduced.
Transpiration As A Necessary Evil

(Importance Of Transpiration):
Transpiration is called necessary evil because it is compulsory and harmful.
Transpiration occurs form the wet cell surfaces which are necessary for the exchange of gases.
Loss of water from the plant can lead to wilting, desiccation and death of a plant if there is drought.
A mild water stress results in reduced growth rate and less crop production.
Following is the importance of transpiration:
(i) Distribution of Dissolved Mineral Salts:
Minerals dissolved in the water are distributed throughout plant body by transpiration stream.
(ii) Transportation of Water:

Water is transported to photosynthesizing cells of leaves in most tall plants by transpiration pull.
(iii) Cooling Effect:

Evaporation of water from the exposed surface of cells of leaves has cooling effect on plant.
This is especially important in higher temperatures.
(iv) Gaseous Exchange:

Wet surface of the leaf cells allow gaseous, exchange.
(v) Stops Wilting:
If the rate of transpiration is very high.
there will be much loss of water from the plant.
Therefore at high temperatures the almost close and the rate of transpiration is reduced.
This stops wilting of the leaves and herbaceous plants.
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