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—typically achieved by closing the stomata—leads to photorespiration, not enhanced photosynthesis. Understanding why requires a look into how C3 photosynthesis operates.
C3 plants are the most common type of plants on Earth and include rice, wheat, soybeans, and most trees. They fix carbon through the Calvin cycle, where the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) adds carbon dioxide (CO₂) to a 5-carbon compound called RuBP, producing two molecules of a 3-carbon compound, 3-phosphoglycerate (3-PGA). This is the normal photosynthetic pathway in C3 plants.
However, RuBisCO is not highly selective—it can also bind oxygen (O₂) instead of CO₂. When stomata are closed to conserve water (a common response to hot, dry conditions), less CO₂ enters the leaf, and O₂ accumulates because it continues to be produced by the light reactions of photosynthesis. The CO₂:O₂ ratio decreases, and RuBisCO is more likely to bind O₂.
When this happens, photorespiration occurs. RuBisCO adds O₂ to RuBP instead of CO₂, producing a toxic compound that the plant must break down and recycle through a process that consumes energy and releases previously fixed CO₂. This leads to a net loss of carbon and energy, making photorespiration a wasteful process in terms of photosynthesis.
Why Other Options Are Incorrect:
- Photosynthesis: Is reduced, not promoted, when stomata close due to less CO₂ availability.
- A shift to C4 photosynthesis: Cannot occur in C3 plants because this is a genetically fixed trait.
- The light reactions: Still occur, but they are not promoted by water conservation.
- Opening of stomata: Opposite of what happens during water conservation; stomata close to reduce water loss.
In summary, conservation of water in C3 plants limits CO₂ intake and promotes photorespiration, which is inefficient and detrimental to the plant’s productivity.