Method: Potted plants of the same species and size are required. The light intensity can be varied by placing the plants at different distances from a light source (e.g., lamps with varying wattage). Temperature and humidity should be kept constant. Plants should be watered regularly. Transpiration rate can be measured as described above (weight change or potometer). It is important to ensure that the light source provides consistent light quality (e.g., using lamps with a similar spectrum).

Variables:

• Independent Variable: Light intensity (e.g., low, medium, high – measured in lux or similar units).
• Dependent Variable: Transpiration rate (measured as change in mass or water uptake by potometer).
• Controlled Variables:

  • Plant species and size
  • Temperature
  • Humidity
  • Soil type
  • Pot size
  • Watering regime

Explanation: Light intensity affects transpiration rate because light stimulates the opening of stomata. When stomata are open, water vapour can escape from the leaf. Higher light intensity generally leads to a greater rate of stomatal opening and therefore a higher transpiration rate. However, very high light intensities can sometimes lead to stomatal closure due to water stress, which would then reduce transpiration.

Translocation is the movement of sugars (primarily sucrose) and other organic molecules through the phloem. It occurs from source tissues (typically mature leaves where photosynthesis is active) to sink tissues (growing regions, storage organs, etc.).

Source Tissue Role: In the source, sucrose is actively loaded into the sieve tubes by companion cells. This creates a high concentration of sucrose in the phloem, establishing a pressure gradient.

Sink Tissue Role: At the sink, sucrose is actively unloaded from the sieve tubes into the sink cells. This lowers the sucrose concentration in the phloem, maintaining the pressure gradient. The sucrose is then used for energy, growth, or stored as starch.

The pressure gradient drives the flow of phloem sap from the source to the sink. This flow is facilitated by turgor pressure differences between the tissues.

A is primarily responsible for the absorption of water from the soil. Root hairs are extensions of epidermal cells in the root, greatly increasing the surface area available for water and mineral absorption. The structure of the root hair cell, including its thin cell wall and living cytoplasm, is specifically adapted for this function.