The Potometer
The Potometer

The main cause of water loss from a plant is through the stomata of the leaf. The stomata may be opened or closed in order to limit the amount of water-loss experienced by a plant. The amount of water lost by a plant may be investigated experimentally using a Potometer:

The shoot must be cut and inserted into the reservoir underwater.
Why? > This is to prevent air bubbles getting into the cut ends of the xylem vessels, as air bubbles can block the xylem and prevent the formation of columns of water molecules.

Factors Affecting the Rate of Transpiration
Transpiration is influenced by a number of environmental factors. Can you complete the following table?

Increase or
Low light> decrease> Light affects transpiration as stomata usually open during the day and close at night, therefore limiting transpiration. As the stomata open then transpiration increases.
High temperature> increase> In the presence of light, temperature has the greatest effect on transpirational rates. The higher the temperature, the greater the rate of transpiration. High temperatures also lower the relative humidity of air outside the leaf. These cause a greater concentration gradient from leaf to atmosphere to be created, and the steeper the gradient the faster the rate of transpiration.
High humidity> decrease> High humidity reduces the rate of transpiration as it makes the concentration gradient form the leaf to the atmosphere less steep which decreases the rate of diffusion of water molecules.
Wind> increase> In still air, a shell of highly saturated air builds up around the leaf, thus reducing the steepness of the diffusion gradient. Air movement (wind) will remove this from around the leaf. Windy conditions then cause transpiration to increase.
Less soil Water> decrease> Less soil moisture tends to mean what water there is binds more tightly to the soil, and also it has more concentrated dissolved solutes in it. This loweres its water potential and less enters the root by osmosis. Less uptake results in lower transpirational rates.

What is a xerophyte?
> These are plants that possess modifications to their structure or physiology to reduce water loss

Examples of xeromorphic adaptations include:
  1. Smaller leaves with less surface are (eg pine needles)
  2. Losing leaves in winter when there is less water available
  3. A thicker cuticle
  4. Waxy deposits on leaves
  5. Less stomata
  6. Stomata concentrated on lower leaf areas
  7. Sunken stomata- around which high umidity may develop
  8. Rolled-up leaves
  9. Epidermal hairs or scales- to trap moist air around stomata

Have a go at the Potometer Exam Question here

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