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Difficulty: MediumDetermining Control Groups and Baseline Conditions

Botanists conducted an experiment to evaluate how varying concentrations of ground-level ozone (O3O_3) affect the stomatal conductance of soybean (*Glycine max*) leaves over a 14-day period. Four identical growth chambers were maintained at constant temperature (25∘C25^\circ\text{C}), relative humidity (65%65\%), and photosynthetic photon flux density. The chambers received the following gaseous treatments:

- Chamber 1: Charcoal-filtered air containing 0 ppb0\text{ ppb} of O3O_3.
- Chamber 2: Purified air containing 30 ppb30\text{ ppb} of O3O_3.
- Chamber 3: Purified air containing 60 ppb60\text{ ppb} of O3O_3.
- Chamber 4: Purified air containing 90 ppb90\text{ ppb} of O3O_3.

Which chamber served as the negative control group in this experiment, and what baseline condition did it establish?

  1. Chamber 1; it established baseline stomatal conductance in the complete absence of ozone exposure.Answer
  2. B
    Chamber 2; it established baseline stomatal conductance under minimal ozone exposure.
  3. C
    Chamber 4; it established the baseline maximum stress response of the soybean plants.
  4. D
    Chambers 2, 3, and 4 combined; they established the average baseline across all ozone concentrations.

Answer

Chamber 1 served as the control group, establishing baseline stomatal conductance in the absence of ozone exposure.
In experimental design, a negative control group establishes a benchmark standard by eliminating the independent variable. Chamber 1 receives charcoal-filtered air with 0 ppb0\text{ ppb} of O3O_3, allowing researchers to measure normal, unexposed stomatal conductance and confirm that any changes in Chambers 2–4 are caused by ozone exposure.

Step-by-Step Solution

1
Identify the independent variable tested in the experiment.
The independent variable is the concentration of ground-level ozone (O3O_3) supplied to the chambers.
Determining the manipulated variable allows identification of which group isolates environmental factors from the specific variable being tested.
2
Identify the group that omits the independent variable while maintaining all controlled variables identical.
Chamber 1 receives charcoal-filtered air (0 ppb0\text{ ppb} O3O_3), while temperature, humidity, and light remain identical across all four chambers.
A negative control group must withhold the independent variable so researchers can establish a benchmark measurement.
3
Match the identified control setup with its intended purpose.
Chamber 1 establishes the baseline stomatal conductance of soybean leaves unexposed to ozone stress.
Comparing Chambers 2, 3, and 4 against Chamber 1 isolates changes in stomatal conductance attributable specifically to ozone concentration.

Key Concept

Determining Control Groups and Baseline Conditions
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Determining Control Groups and Baseline Conditions

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