In C3 plants the conservation of water promotes:
photorespiration
photosynthesis
a shift to C4 photosynthesis
the light reactions
the opening of stomata
The Correct Answer and Explanation is:
Correct Answer: Photorespiration
Explanation:
In C3 plants, the process of conserving water—which typically involves closing the stomata—leads to photorespiration, not increased photosynthesis or a shift to C4 photosynthesis.
Understanding Stomatal Function:
Stomata are tiny openings on the surfaces of leaves that allow for gas exchange. During photosynthesis, stomata open to let in carbon dioxide (CO₂) and release oxygen (O₂). However, open stomata also allow water vapor to escape through transpiration. In hot or dry conditions, plants often close their stomata to conserve water.
What Happens When Stomata Close?
When stomata are closed:
- CO₂ levels inside the leaf drop because no new CO₂ is entering.
- O₂ levels rise due to the light-dependent reactions of photosynthesis producing oxygen.
This change in gas concentrations inside the leaf causes Rubisco, the key enzyme of the Calvin cycle, to start fixing oxygen instead of carbon dioxide. This leads to a process called photorespiration.
What is Photorespiration?
Photorespiration is a wasteful pathway that occurs when Rubisco reacts with O₂ instead of CO₂. This process consumes energy (ATP and NADPH) and releases fixed carbon as CO₂, reducing the overall efficiency of photosynthesis. Photorespiration does not produce sugars like normal photosynthesis does.
Why Not a Shift to C4 Photosynthesis?
C3 plants are genetically and structurally different from C4 plants, which have specialized anatomy (Kranz anatomy) and a different biochemical pathway that minimizes photorespiration. A C3 plant cannot simply “shift” to C4 photosynthesis; this is a trait that evolved separately.
Summary:
So, in C3 plants, when water conservation causes stomata to close, the reduction in internal CO₂ levels leads to increased photorespiration—not better photosynthesis, not light reactions, and not stomatal opening. Therefore, photorespiration is the correct answer.