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The rate of transpiration is influenced by certain factors; these factors may be classified as the (i) External factors or the environmental factors

(ii) Internal factors or plants factor.

(i) External factors:

The rate of transpiration is always affected by a group of environmental factors like light, temperature, wind, atmospheric humidity, atmospheric pressure, availability of soil water, anti-transpirants.

  1. Light: The presence of light favours the opening of stomata and transpiration takes place through them. The rate of transpiration is high during day time since the stomata will be open during day. At night stomata remains closed and transpiration is controlled at night. So we can say that light can influence the transpiration by controlling the stomatal movements.
  2. Temperature: The rate of transpiration is directly proportional to the rate of temperature. The light energy that is absorbed by the leaf is converted into heat energy so that it increases the temperature.
  3. Wind: The rate of flow of wind has varying effects on the rate of transpiration. Gently blowing wind increases the rate of transpiration since it can remove moisture air near by the transpiring parts of the plant. The high velocity wind decreases the rate of transpiration as it closes the stomata in response to the loss of water.
  4. Atmospheric humidity: Highly humid atmosphere decreases the rate of transpiration. During the day time the transpiration is greater due to low humidity.
  5. Atmospheric pressure: The rate of transpiration is increased at lower atmospheric pressure. At low pressure the water vapour moves fast from the transpiring surface.
  6. Availability of soil water: It is an important environmental factor which influences the rate of transpiration. When the soil does not contain enough amount of water for the plant to absorb, plant reduces the water absorption and decreases the transpiration rate. When the rate of transpiration exceeds the rate of water absorption, it results in the wilting of the plants. This temporary wilting of the plants can be regained by sufficient watering of the plant subsequently or else the turgidity of the cell can’t be regained which results in the permanent wilting which is an irreversible process.
  7. Anti-transpirants: The chemical substances which can reduce the transpiration are called anti transpirants. The wilting of plants in dry season is prevented by the application of the anti transpirants. ABA and PMA are the anti transpirants that reduces transpiration by partial closure of stomata.

(ii) Internal factors:

The internal factors that control the rate of transpiration are the leaf structure, root-shoot ratio, leaf area, age of plants.

  1. Leaf structure: The structural modification such as presence of thick cuticle, epidermal hairs, frequency of stomata, sunken stomata, developed palisade tissue etc. can reduce the rate of transpiration. In xerophytic plants the leaves are well reduced or absent.
  2. Root-shoot ratio: The rate of transpiration increases with the increase in the root shoot ratio. It depends upon the effectiveness of the absorbing and the evaporating system.
  3. Leaf area: The rate of transpiration increases with large leaf area. In the xerophytic plants the leaves are reduced or absent, then stem takes up the photosynthetic function of the leaves.
  4. Age of plants: The seedlings shows low rate of transpiration and it increases with the age of the plants. The maximum rate of transpiration is observed in the plants when it attains maturity and it low down when the plants reaches it senescence.
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