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, conservation of water—typically through the closing of stomata—leads to a condition that promotes photorespiration, rather than photosynthesis or the other processes listed.
Understanding C3 Plants:
C3 plants are the most common type of plants, and they perform photosynthesis through the Calvin cycle, where CO₂ is fixed by the enzyme Rubisco. The first stable product formed is a 3-carbon compound, 3-phosphoglycerate (hence the name C3 plants).
Water Conservation and Stomatal Closure:
When the plant needs to conserve water, especially under hot or dry conditions, stomata (tiny pores on leaves) close to reduce water loss through transpiration. However, this also limits the entry of CO₂ into the leaf and traps O₂ inside.
Photorespiration:
Rubisco, the key enzyme in the Calvin cycle, has dual affinity: it can bind to either CO₂ or O₂. When CO₂ levels drop (due to stomatal closure) and O₂ levels rise, Rubisco increasingly binds to O₂ instead of CO₂. This triggers photorespiration, a wasteful process in which the plant uses energy and releases CO₂ without producing sugars.
Photorespiration competes with photosynthesis and reduces its efficiency. While it may serve some protective functions under stress, it is generally seen as a loss of productivity for C3 plants.
Why Other Options Are Incorrect:
- Photosynthesis: Declines due to lack of CO₂.
- Shift to C4 photosynthesis: C3 plants cannot suddenly become C4 plants; this is a separate evolutionary adaptation.
- Light reactions: These still occur but are limited in efficiency without CO₂ for the Calvin cycle.
- Opening of stomata: This does not conserve water; it increases water loss.
Conclusion:
In C3 plants, conserving water by closing stomata reduces CO₂ intake and promotes photorespiration, making it the correct answer.