18) Transpiration and Stomata

Factors Affecting Transpiration Rate

1. Light Intensity:

   • Brighter light increases the transpiration rate.

   • Stomata close in the dark because photosynthesis cannot occur, limiting water loss.

2. Temperature:

   • Higher temperatures speed up transpiration.

   • Warm conditions give water particles more energy to evaporate and diffuse out of the stomata.

3. Air Flow:

   • Increased air flow around a leaf enhances transpiration.

   • Strong winds remove water vapor from around the leaf, maintaining a lower concentration of water outside, which promotes diffusion.

   • Poor air flow allows water vapor to accumulate, reducing the concentration gradient and slowing diffusion.

4. Humidity:

   • Drier air increases transpiration rates.

   • High humidity means less difference between water concentration inside and outside the leaf, reducing diffusion rates.

   • Transpiration is fastest when there is a high concentration of water vapor inside the leaf and a low concentration outside.

Measuring Transpiration Rate

• Potometer Setup:

  • You can estimate the transpiration rate by measuring the uptake of water by a plant using a potometer.

  • As the plant takes up water, an air bubble moves along the scale of the apparatus.

  • Record the starting position of the air bubble, start a stopwatch, and measure the distance the bubble moves per unit time (e.g., per hour).

  • Keep conditions constant (temperature, humidity) during the experiment to ensure accurate results.

• Variables

  • Independent variable - time

  • Dependent variable - the distance moved by the bubble along the capillary tube

  • Control variables - temperature, air flow or draughts, adequate supply of water

• Risks

  • Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.

Guard Cells and Stomata

• Function of Guard Cells:

  1. Shape: Guard cells have a kidney shape that allows them to open and close the stomata (pores) in a leaf.

  2. Water Regulation:

     • When the plant has sufficient water, guard cells absorb water, become turgid (plump), and open the stomata for gas exchange.

     • When the plant is low on water, guard cells lose water and become flaccid, causing the stomata to close, reducing water loss.

• Adaptations of Guard Cells:

  • Thin outer walls and thickened inner walls facilitate the opening and closing of the stomata.

  • Guard cells are sensitive to light and typically close at night to conserve water while still allowing photosynthesis during the day.