Prescribed lab 7: Measuring The Rate of Transpiration

1. The research question

How does the rate of transpiration in plants vary between different environmental conditions?

OR

Which environmental conditions allow for the greatest rate of transpiration by plants?

2. Identification of the independent variable

Independent variable: different environments (mist, wind, bright light)

3. Identification of the dependent variable

Dependent variable: transpiration rates (as measured by potometers)

4. Procedure on a step-by-step basis

1. Gather all materials.

2. Assemble 4 potometers. Potometers include:

• A plant cutting

• A calibrated pipette to measure water loss

• A length of clear plastic tubing

• An air-tight seal between the plant and the water-filled tubing

3. Place each potometer in different environments including, room conditions (control group), wind, mist, and bright light.

4. Measure water loss in each potometer every 3 minutes for a total of 30 minutes.

5. Calculate the leaf surface area for each cutting.

6. Compare surface areas of leafs between different environments.

OR

Compare loss of water in pipettes of potometers between different environments.

5. Sample results in a data table

6. Processed results and data analysis

7. Conclusions including biological explanations

Transpiration describes the movement of water through plants, including the process of water loss. A potometer is a device that measures the rate at which a plant draws up water. Since the plant draws up water as it loses it by transpiration, you are able to measure the rate of transpiration. This rate of transpiration will be affected by different environmental factors, and different environmental conditions can be created for these plants, to determine how the rate of transpiration varies with each.

Evaporation of water through the stomata is a most important factor in pulling water toward the top of a tall tree. Light conditions would generally result in the higher rate of transpiration. Stomata are generally open in the light, and solar energy increases evaporation. A dry environment would result in the higher rate of transpiration because the water potential in the surrounding air would be lowest, resulting in more rapid evaporation. Breezy conditions would increase the rate of evaporation of water from the surface of the leaves, lowering the water potential, and resulting in more rapid evaporation. However, if the plant is subjected to very high wind movement, the stomata may actually close, which prevents water loss. Unless a plant is specially adapted for hot conditions, the rate of transpiration will drop in a hot environment because heat stress may cause the stomata to close, which conserves water. Warm, light-breezy conditions provide an environment for the greatest rate of transpiration in plants. In breezy conditions the rate of transpiration would be very high because of evaporative water loss.