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 conservation of water typically involves the closing of stomata to reduce water loss through transpiration. While this is beneficial in preventing dehydration, it has a significant impact on photosynthesis and leads to an increase in photorespiration.
C3 Photosynthesis Overview:
C3 plants, such as wheat and rice, use the Calvin cycle for carbon fixation, where the enzyme RuBisCO fixes carbon dioxide (CO₂) into a three-carbon compound (3-phosphoglycerate). Under normal conditions, RuBisCO uses CO₂ efficiently. However, it can also bind to oxygen (O₂), especially when CO₂ levels inside the leaf drop — which happens when stomata close to conserve water.
What Happens When Stomata Close:
- CO₂ levels inside the leaf decrease.
- O₂ levels increase due to ongoing light reactions producing O₂.
- RuBisCO begins to bind more O₂ than CO₂, initiating photorespiration.
Photorespiration:
Photorespiration is a wasteful process because it uses oxygen and releases previously fixed CO₂. It does not produce ATP or sugar, and actually consumes energy and organic carbon, reducing the overall efficiency of photosynthesis.
Why Not the Other Options?
- Photosynthesis: Decreases under water stress due to closed stomata and lower CO₂ availability.
- A shift to C4 photosynthesis: C3 plants do not shift to C4 photosynthesis; that is a separate evolutionary adaptation seen in plants like corn and sugarcane.
- The light reactions: These may continue under sunlight, but without sufficient CO₂, the Calvin cycle is limited.
- The opening of stomata: Water conservation leads to closing, not opening, of stomata.
Conclusion:
Thus, water conservation in C3 plants promotes photorespiration, a process that is generally counterproductive to efficient carbon fixation and overall photosynthetic productivity.