A plant physiologist formulated the following hypothesis: *Hypothesis*: Under constant hig… — ACT

A plant physiologist formulated the following hypothesis:

*Hypothesis*: Under constant high-light conditions, the rate of transpiration in a particular plant species is determined solely by stomatal conductance. Consequently, any environmental change that reduces stomatal conductance will result in a proportionally identical decrease in the transpiration rate.

To test this hypothesis, the physiologist placed several plants in a controlled chamber under constant high-light conditions. Stomatal conductance and transpiration rate were measured at various relative humidity (RH) levels. Measurements taken at $80\%$ RH served as the baseline ( $100\%$ ). The results are shown in the table below:

Relative Humidity (RH)	Stomatal Conductance (% of baseline)	Transpiration Rate (% of baseline)
$80\%$ (Baseline)	$100\%$	$100\%$
$60\%$	$75\%$	$120\%$
$40\%$	$50\%$	$135\%$
$20\%$	$25\%$	$140\%$

Which of the following modifications to the physiologist's hypothesis is most consistent with these results?

A
The rate of transpiration is independent of stomatal conductance, and is instead determined solely by the relative humidity, which shares a direct, positive relationship with the transpiration rate.
The rate of transpiration is determined by multiple interacting factors; a decrease in relative humidity increases the evaporative gradient between the leaf interior and the air, which can increase transpiration even as stomatal conductance decreases.Answer
C
The rate of transpiration is determined solely by stomatal conductance, but the decrease in relative humidity caused an increase in the leaf's internal water vapor concentration that kept stomatal conductance high.
D
The original hypothesis remains valid because the transpiration rate would begin to decrease proportionally once stomatal conductance falls below a threshold of $25\%$ of the baseline.

Answer

The rate of transpiration is determined by multiple interacting factors; a decrease in relative humidity increases the evaporative gradient between the leaf interior and the air, which can increase transpiration even as stomatal conductance decreases.

The correct option correctly identifies that transpiration is influenced by multiple factors, including the evaporative gradient between the leaf interior and the ambient air. As relative humidity decreases, this gradient becomes steeper, driving more water to evaporate from the leaf. This evaporative demand can override the effect of stomatal closure, explaining why the transpiration rate increases even as stomatal conductance decreases from

100\%

25\%

Step-by-Step Solution

Analyze the original hypothesis to identify the predicted relationship between stomatal conductance and transpiration rate.

The hypothesis predicts that transpiration is solely determined by stomatal conductance in a 1:1 proportional relationship, meaning any decrease in conductance must yield an identical percentage decrease in transpiration.

To evaluate a hypothesis, you must first clarify its specific, testable predictions.

Compare the predicted relationship with the actual data shown in the table.

As relative humidity decreases, stomatal conductance decreases (from

100\%

25\%

), but transpiration rate increases (from

100\%

140\%

Comparing actual data against predictions reveals whether the hypothesis is supported or refuted.

Formulate a modified hypothesis that explains why transpiration rate increases despite the reduction in stomatal conductance.

Transpiration must be controlled by multiple factors. The reduction in relative humidity increases the evaporative demand (driving force) on the leaf, which overrides the limiting effect of the closing stomates.

A modified hypothesis must reconcile the conflicting variables to account for the physical trends in the data.

Key Concept

Formulating and Modifying Hypotheses

Estimated Time:1m 30s

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