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 primary enzyme responsible for carbon fixation during the Calvin cycle is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme can bind to both carbon dioxide (CO₂) and oxygen (O₂). Under optimal conditions—such as cool temperatures and adequate water—RuBisCO preferentially binds to CO₂, enabling efficient photosynthesis.
However, when water conservation becomes critical, such as in hot or dry conditions, C3 plants respond by closing their stomata to reduce water loss. While this is essential for preventing dehydration, it has a negative impact on gas exchange: CO₂ cannot enter the leaf, and O₂ cannot leave. As a result, the internal CO₂ concentration decreases while O₂ levels increase within the leaf tissue.
Under these conditions, RuBisCO is more likely to fix O₂ instead of CO₂, initiating a process called photorespiration. Unlike photosynthesis, photorespiration does not produce glucose and instead consumes energy (ATP) and releases fixed carbon as CO₂, making it an inefficient process for the plant. Essentially, photorespiration is a wasteful pathway that occurs when RuBisCO acts as an oxygenase rather than a carboxylase.
So, in C3 plants, water conservation (via stomatal closure) promotes photorespiration due to the altered ratio of CO₂ to O₂ within the leaf.
Other Options Explained:
Conclusion: In C3 plants, conserving water by closing stomata leads to photorespiration due to reduced CO₂ availability and increased O₂ concentration inside the leaf.