Answer:
To effectively measure the impact of light intensity in an experiment looking at how much water plants take up, the following factors should be held constant:
1. Plant type: To verify that any reported differences in water uptake are caused by light intensity and not variations in the plants themselves, it is crucial to employ the same species of plant for all light intensity treatments.
2. Soil type: To prevent any soil-related impacts on water uptake, the type of soil in which the plants are cultivated should remain constant across all treatments.
3. Water quality: To prevent any effects of water quality on the plants, the quality and chemical composition of the water used to water them should be maintained.
4. Air temperature and humidity: It should be kept constant during the experiment in order to account for environmental conditions that may affect how much water plants take in.
5. Plant size and age: To prevent any growth stage-related effects on water uptake, it is best to maintain the size and age of the plants throughout all treatments.
6. Time: To prevent any treatment-specific effects on water uptake caused by elapsed time, the length of the experiment and the time of day when light is delivered should be kept consistent for all treatments.
Hope it helps!