Transpiration and factors affecting it

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  • Transpiration and factors affecting it
    • Role of transpiration
      • Transpiration is sometimes referred to as a necessary evil. This is because although transpiration is universal in flowering plants, it is the unavoidable result of plants having leaves adapted for photosynthesis
      • Leaves have a large surface area to absorb light and stomata to allow inward diffusion of CO2
      • Although transpiration helps bring water to the leaves, it is not essential as osmotic processes could achieve this.
      • Less than 1% of water moved in the transpiration stream is used by the plant. Benefits = materials such as mineral ions, sugars and hormones are moved around the plant dissolved in water. This water is carried up the plant by the transpiration pull
    • Light
      • Stomata are the openings in leaves through which the carbon dioxide needed for photosynthesis
      • It follows that the stomata of most plants open in the light and close in the dark
      • When the stomata open water moves out of the leaf into the atmosphere, Consequently an increase in light intensity causes an increase in the rate of transpiration
    • Temperature
      • Temperature changes affect two factors that influence the rate of transpiration; how much water the air can hold and the speed at which water molecules move
      • A rise in temperature
        • Increases the kinetic energy and hence the speed of the movement of water molecules. This increased movement of water molecules increases the rate of evapouration of water. This means that water evaporates more rapidly from leaves and so the rate of transpiration increases
        • Decreases the humidity of the air outside the leaf
        • Increases transpiration rate
    • Humidity
      • Humidity is the number of the number of water molecules in the air. The humidity of the air affects the water potential gradient between the air outside the leaf and the air inside the leaf
      • When the air outside the leaf has a high humidity, the gradient is reduced and the rate of transpiration is lower. Conversely low humidity increases the transpiration rate
    • Air movement
      • As water diffuses through stomata, it accumulates as vapour around the stomata on the outside of the leaf. The water potential around the stomata is therefore increased
      • This reduces the water potential gradient between the moist atmosphere in the air spaces within the leaf and the drier air outside. Transpiration rate is therefore reduced
      • Any movement of air around the leaf will disperse the humid layer at the leaf surface and so decrease the water potential of the air
        • The faster the air movement, the more rapidly the humid air is removed and the greater the rate of transpiration


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