In C3 plants, conserving water by closing stomata on hot, dry days tends to promote which physiological process?

Introduction / Context:
This biology question relates to plant physiology, specifically the behaviour of C3 plants under conditions of water stress. C3 plants use the Calvin cycle directly to fix carbon dioxide, but this system has a drawback when stomata close to conserve water. Understanding the link between water conservation, stomatal closure, and photorespiration is important for appreciating how environment affects photosynthetic efficiency in different plant types.

Given Data / Assumptions:

• The question refers to C3 plants, the most common photosynthetic type.
• It states that water conservation involves closing stomata on hot, dry days.
• You must identify which process is promoted under these conditions.

Concept / Approach:
In C3 plants, the enzyme RuBisCO normally fixes carbon dioxide in the Calvin cycle. Stomata are pores in leaves that open to allow gas exchange: carbon dioxide enters, and oxygen and water vapour exit. On hot, dry days, plants often close stomata to reduce water loss. However, when stomata close, carbon dioxide entry is restricted while photosynthesis and respiration continue, causing internal CO2 levels to drop and oxygen levels to rise. Under these conditions, RuBisCO sometimes binds oxygen instead of carbon dioxide, starting a wasteful process called photorespiration. Photorespiration consumes oxygen and organic fuel, releases CO2, and produces no sugar, reducing the efficiency of photosynthesis. Therefore, in C3 plants, water conserving stomatal closure tends to promote photorespiration, not efficient photosynthesis.

Step-by-Step Solution:

Verification / Alternative check:
Plant physiology texts describe how C3 plants suffer from high rates of photorespiration under hot, dry conditions because they must close stomata to conserve water. They contrast this with C4 and CAM plants, which have adaptations to concentrate CO2 around RuBisCO or separate carbon fixation steps in space or time, thereby reducing photorespiration. Diagrams often show internal CO2 falling and O2 rising when stomata close, with arrows indicating increased photorespiration. This matches the description in option C and contradicts the idea that photosynthesis is enhanced under these conditions in C3 plants.

Why Other Options Are Wrong:

Photosynthesis, by increasing the availability of carbon dioxide in leaves, is wrong because closing stomata actually decreases CO2 availability, hurting photosynthetic efficiency.

The opening of stomata to bring in more water vapour from the air is incorrect because stomata close to conserve water, and water vapour actually exits leaves through stomata rather than entering in large amounts.

None of the above processes is wrong because photorespiration is a well established response in C3 plants under water stress conditions when stomata are closed.

Common Pitfalls:
Students may assume that any adaptation for water conservation must improve overall plant performance, leading them to choose photosynthesis instead of photorespiration. Another pitfall is confusing C3 plants with C4 or CAM plants, which handle hot, dry conditions differently. To avoid these errors, remember that in C3 plants, stomatal closure to conserve water lowers CO2 and increases O2 inside the leaf, promoting photorespiration and reducing photosynthetic efficiency.